Complete Collection
June 1, 2004
I recently bought a used microwave oven. The enamel coating under the glass turntable tray is rusted in a ring around the
track that the turntable rotates on. Should I repair this or is it ok to just use it as is? – AA, Kettering, Ohio
As long as the oven’s metal bottom is sound underneath the rust, there isn’t a problem. The cooking chamber
walls are so thick and highly conducting that they reflect the microwaves extremely well even when they have a
little rust on them. However, if the metal is so rusted that it loses most of its conductivity in the rust sites, you’ll
get local heating across the rusty patches and eventually leakage of microwaves. If you’re really concerned that
there may be trouble, run the microwave oven empty for about 20 seconds and then (carefully!) touch the rusty
spots. If they aren’t hot, then the metal underneath is doing its job just fine.
March 26, 2004
While shopping for a new microwave I was asking the salesperson at a local store some questions regarding microwaves.
He proceeded to tell me how dangerous they were and that they used to sell some sort of testers to see if the new
microwaves they were selling "leaked radiation". He told me that they all did and that microwaves give off "harmful"
radiation. He said that it affects the food that we cook in it and can cause cancer. He said "Think about it, when you get
an x-ray the tech covers himself with a lead shield and here we are putting our food into this and there is no lead shield.
Needless to say I did not purchase a microwave yesterday, and was wondering if you could please give me some insight
on this and tell me is what this salesperson told me is true. Are microwave ovens really harmful? Do they cause cancer?
What about the food, does it become toxic. A friend of mine is totally into all organic food and she "unplugged" her
microwave years ago and never used it since. She swears it is harmful. Please help. Heating food in a pot is so
inconvenient!! – KO
The salesperson you spoke to was simply wrong. If you’ll allow me to stand on my soapbox for a minute, I’ll tell
you that this is a perfect example of how important it is for everyone to truly learn basic science while they’re in
school and not to simply suffer through the classes as a way to obtain a degree. The salesperson is apparently
oblivious to the differences between types of “radiation,” to the short- and long-term effects of those radiations,
and to the importance of intensity in radiation.
Let’s start with the differences in types of radiation. Basically, anything that moves is radiation, from visible
light, to ultraviolet, to X-rays, to microwaves, to alpha particles, to neutrons, and even to flying pigeons. These
different radiations do different things when they hit you, particularly the pigeons. While “ionizing radiations”
such as X-rays, ultraviolet, alpha particles, and neutrons usually have enough localized energy to do chemical
damage to the molecules they hit, “non-ionizing radiation” such as microwaves and pigeons do not damage
molecules. When you and your organic friend worry about toxic changes in food or precancerous changes in
your tissue, what really worry you are molecular changes. Microwaves and pigeons don’t cause those sorts of
changes. Microwaves effectively heat food or tissue thermally, while pigeons bruise food or tissue on impact.
Wearing a lead apron while working around ionizing radiation makes sense, although a simple layer of fabric or
sunscreen is enough to protect you from most ultraviolet. To protect yourself against pigeons, wear a helmet.
And to protect yourself against microwaves, use metal. The cooking chamber of the microwave oven is a metal
box (including the screened front window). So little microwave “radiation” escapes from this metal box that it’s
usually hard to detect, let alone cause a safety problem. There just isn’t much microwave intensity coming from
the oven and intensity matters. A little microwaves do nothing at all to you; in fact you emit them yourself!
If you want to detect some serious microwaves, put that microwave detector near your cellphone! The
cellphone’s job is to emit microwaves, right next to your ear! Before you give up on microwave ovens, you
should probably give up on cellphones. That said, I think the worst danger about cellphones is driving into a
pedestrian or a tree while you’re under the influence of the conversation. Basically, non-ionizing radiation such
as microwaves is only dangerous if it cooks you. At the intensities emitted by a cellphone next to your ear, it’s
possible that some minor cooking is taking place. However, the cancer risk is almost certainly nil.
Despite all this physics reality, salespeople and con artists are still more than happy to sell you protection against
the dangers of modern life. I chuckle at the shields people sell to install on you cellphones to reduce their
emissions of harmful radiation. The whole point of the cellphone is to emit microwave signals to the receiving
tower, so if you shield it you spoil its operation! It would be like wrapping an X-ray machine in a lead box to
protect the patient. Sure, the patient would be safe but the X-ray machine would barely work any more.
Returning to the microwave cooking issue, once the food comes out of the microwave oven, there are no
lingering effects of its having been cooked with microwaves. There is no convincing evidence of any chemical
changes in the food and certain no residual cooking microwaves around in the food. If you’re worried about toxic
changes to your food, avoid broiling or grilling. Those high-surface-temperature cooking techniques definitely
do chemical damage to the food, making it both tasty and potentially a tiny bit toxic. One of the reasons why
food cooked in the microwave oven is so bland is because those chemical changes don’t happen. As a result,
microwave ovens are better for reheating than for cooking.
December 4, 2003
Is it possible to capture and keep ionized gases or air in a container of some sort? That way they could be sprayed out at
any time just like room deodorant. – CW
No, you cannot store charged gases in any simple container. If you try to store a mixture of positively and
negatively charge gas particles in a single container, those opposite charges will attract and neutralize one
another. And if you try to store only one type of charge in a container, those like charges will repel and push one
another to the walls of the container. If the container itself conducts electricity, the charges will escape to the
outside of the container and from there into the outside world. And if the container is insulating, the charges will
stick to its inside surface and you’ll have trouble getting them to leave. Moreover, you’ll have trouble putting
large numbers of those like-charged gas particles into the container in the first place because the ones that enter
first will repel any like charges that follow.
December 1, 2003
What packing material protects best? When we drop an egg wrapped in various packaging materials, we know the force
that gravity exerts on the egg but how do we know the force of the impact? – DL, Springboro, Ohio
I like to view problems like this one in terms of momentum: when it reaches the pavement, a falling egg has a
large amount of downward momentum and it must get rid of that downward momentum gracefully enough that it
doesn’t break. The whole issue in protecting the egg is in extracting that momentum gracefully.
Momentum is a conserved physical quantity, meaning that it cannot be created or destroyed. It can only be
passed from one object to the other. When you let go of the packaged egg and it begins to fall, the downward
momentum that gravity transfers into the egg begins to accumulate in the egg. Before you let go, your hand was
removing the egg’s downward momentum as fast as gravity was adding it, but now the egg is on its own!
Because momentum is equal to an object’s mass times its velocity, the accumulating downward momentum in
the egg is reflected in its increasing downward speed. With each passing second, the egg receives another dose of
downward momentum from the earth. By the time the egg reaches the pavement, it’s moving downward fast and
has a substantial amount of downward momentum to get rid of. Incidentally, the earth, which has given up this
downward momentum, experiences an opposite response—it has acquired an equal amount of upward
momentum. However, the earth has such a huge mass that there is no noticeable increase in its upward speed.
To stop, the egg must transfer all of its downward momentum into something else, such as the earth. It can
transfer its momentum into the earth by exerting a force on the ground for a certain amount of time. A transfer of
momentum, known as an impulse, is the product of a force times a time. To get rid of its momentum, the egg can
exert a large force on the ground for a short time or a small force for a long time, or anything in between. If you
let it hit the pavement unprotected, the egg will employ a large force for a short time and that will be bad for the
egg. After all, the pavement will push back on the egg with an equally strong but oppositely directed force and
punch a hole in the egg.
To make the transfer of momentum graceful enough to leave the egg intact, the protective package must prolong
the momentum transfer. The longer it takes for the egg to get rid of its downward momentum, the smaller the
forces between the egg and the slowing materials. That’s why landing on a soft surface is a good start: it
prolongs the momentum transfer and thereby reduces the peak force on the egg.
But there is also the issue of distributing the slowing forces uniformly on the egg. Even a small force can break
the egg if it’s exerted only on one tiny spot of the egg. So spreading out the force is important. Probably the best
way of distributing the slowing force would be to float the egg in the middle of a fluid that has the same average
density as the egg. But various foamy or springy materials will distribute the forces nearly as well.
In summary, (1) you want to bring the egg to a stop over as long as period of time as possible so as to prolong
the transfer of momentum and reduce the slowing forces and (2) you want to involve the whole bottom surface of
the egg in this transfer of momentum so that the slowing forces are exerted uniformly on the egg’s bottom
surface. As for the actual impact force on the egg, you can determine this by dividing the egg’s momentum just
before impact (its downward speed times its mass) by the time over which the egg gets rid of its momentum.
July 10, 2003
Can infrared lasers, thermal cameras, digital cameras, or optical fiber cameras be used to see through walls of homes or to
monitor people’s conversations? – CB, Connecticut
I'm beginning to think that movies and television do a huge disservice to modern society by blurring the
distinction between science and fiction. So much of what appears on the big and little screen is just fantasy.
The walls of your home are simply hard to look through. They block visible, infrared, and ultraviolet light nearly
perfectly and that doesn’t leave snoopers many good options. A person sitting outside your home with a thermal
camera--a device that "sees" the infrared light associated with body-temperature objects--or a digital camera is
going to have a nice view of your wall, not you inside. There are materials that, while opaque to visible light, are
relatively transparent to infrared light, such as some plastics and fabrics. However, typical wall materials are too
thick and too opaque for infrared light to penetrate. Sure, someone can put a camera inside your home and access
it via an optical fiber or radio waves, but at that point, they might as well just peer through your window.
The only electromagnetic waves that penetrate walls well are radio waves, microwaves, and X rays. If someone
builds an X ray machine around your home, they’ll be able to see you, or at least your bones. Don’t forget to
wave. And, in principle, they could use the radar technique to look for you with microwaves, but you’d be a
fuzzy blob at best and lost in the jumble of reflections from everything else in your home.
As for using a laser to monitor your conversations from afar, that’s a real possibility. Surfaces vibrate in the
presence of sound and it is possible to observe those vibrations via reflected light. But the technical work
involved is substantial and it's probably easier to just put a bug inside the house or on its surface.
June 4, 2003
Why are physicists so skeptical about peoples’ claims to have invented motors that provide mechanical power without
consuming electric power or generators that produce electric power without consuming mechanical power from the
systems that turns them? -- LB (Yes, I’m asking myself this question)
While it may seem as though there is some grand conspiracy among physicists to deny validation to those
inventors, nothing could be farther from the truth. Physicists generally maintain a healthy skepticism about
whatever they hear and are much less susceptible to dogmatic conservativism than one might think. However,
physicists think long and deep about the laws that govern the universe, especially about their simplicity and selfconsistency. In particular, they learn how even the slightest disagreement between a particular law and the
observed behavior of the universe indicates either a problem with that law (typically an oversimplification, but
occasionally a complete misunderstanding) or a failure in the observation. The law of energy conservation is a
case in point: if it actually failed to work perfect even one time, it would cease to be a meaningful law. The
implications for our understanding of the universe would be enormous. Physicists have looked for over a century
for a failure of energy conservation and have never found one; not a single one. (Note: relativistic energy
conservation involves mass as well as energy, but that doesn't change the present story.)
The laws of both energy conservation and thermodynamics are essentially mathematical laws—they depend
relatively little on the specific details of our universe. Just about the only specific detail that's important is timetranslation symmetry: as far as we can tell, physics doesn't change with time--physics today is the same as it was
yesterday and as it will be tomorrow. That observation leads, amazingly enough, to energy conservation: energy
cannot be created or destroy; it can only change forms or be transferred between objects. Together with statistical
principals, we can derive thermodynamics without any further reference to the universe itself. And having
developed energy conservation and the laws of thermodynamics, the game is over for free-energy motors and
generators. They just can't work. It's not a matter of looking for one special arrangement that works among
millions that don't. There are exactly zero arrangements that work.
It's not a matter of my bias, unless you consider my belief that 2 plus 2 equals 4 to be some sort of bias. You can
look all you like for a 2 that when added to another 2 gives you a 5, but I don't expect you to succeed.
About once every month or two, someone contacts me with a new motor that turns for free or a generator that
creates power out of nowhere. The pattern always repeats: I send them the sad news that their invention will not
work and they respond angrily that I am not listening, that I am biased, and that I am part of the conspiracy. Oh
well. There isn't much else I can do. I suppose I could examine each proposal individually at length to find the
flaw, but I just don’t have the time. I’m a volunteer here and this is time away from my family.
Instead, I suggest that any inventor who believes he or she has a free-energy device build that device and
demonstrate it openly for the physics community. Take it to an American Physical Society conference and
present it there. Let everyone in the audience examine it closely. Since anyone can join the APS and any APS
member can talk at any major APS conference, there is plenty of opportunity. If someone succeeds in convincing
the physics community that they have a true free-energy machine, more power to them (no pun intended). But
given the absence of any observed failure of time-translation symmetry, and therefore the steadfast endurance of
energy conservation laws, I don’t expect any successful devices.
April 28, 2003
My 10-year old son understands that body temperature is related to the speeds/kinetic energies of the molecules inside
you, but does friction play a role as well? -- MR
You're both right about temperature being associated with kinetic energy in molecules: the more kinetic energy
each molecule has, the hotter the substance (e.g. a person) is. But not all kinetic energy "counts" in establishing
temperature. Only the disordered kinetic energy, the tiny chucks of kinetic energy that belong to individual
particles in a material contributes to that material's temperature. Ordered kinetic energy, such as the energy in a
whole person who's running, is not involved in temperature. Whether an ice cube is sitting still on a table or
flying through the air makes no difference to its temperature. It's still quite cold.
Friction's role with respect to temperature is in raising that temperature. Friction is a great disorderer. If a person
running down the track falls and skids along the ground, friction will turn that person's ordered kinetic energy
into disordered kinetic energy and the person will get slightly hotter. No energy was created or destroyed in the
fall and skid, but lots of formerly orderly kinetic energy became disordered kinetic energy--what I often call
"thermal kinetic energy."
The overall story is naturally a bit more complicated, but the basic idea here is correct. Once energy is in the
form of thermal kinetic energy, it's stuck... like a glass vase that has been dropped and shattered into countless
pieces, thermal kinetic energy can't be entirely reconstituted into orderly kinetic energy. Once energy has been
distributed to all the individual molecules and atoms, getting them all to return their chunks of thermal kinetic
energy is hopeless. Friction, even at the molecular level, isn't important at this point because the energy has
already been fragmented and the most that any type of friction can do is pass that fragmented energy about
between particles. So friction creates thermal kinetic energy (out of ordered energies of various types)... in effect,
it makes things hot. It doesn't keep them hot; they do that all by themselves.
February 22, 2003
If you have a deck that is snow covered with a very light, fluffy snow, and no one touches it, but in the next few days,
from the sun, or whatever, the snow becomes "heavier" to move, does it actually weigh more? – PP
As the snow settles and becomes denser, it may feel "heavier", but its total weight doesn't change much. The
same water molecules are simply packing themselves into a smaller space. So while each shovel-full of the dense
stuff really does weigh more than a shovel-full of the light stuff, the total number of water molecules present on
your deck and their associated weight is still the same.
In actually, some of the water molecules have almost certainly left via a form of solid-to-gas evaporation known
technically as "sublimation." You have seen this conversion of ice into gas when you have noticed that old ice
cubes in your freezer are smaller than they used to be or when you see that the snow outside during a cold spell
seems to vanish gradually without ever melting. Sublimation is also the cause of "freezer burn" for frozen foods
left without proper wrapping.
September 10, 2002
About 18 months ago, I saw an episode on "Current Affairs," in Australia, in which this dude made a "free electricity"
machine, using magnets, fixed and non fixed-on a spinning wheel. While I know that I should be skeptical, I can't help
thinking "what if?" Have scientists carefully tested this stuff to see for sure that it does or does work? - P, Australia
Not surprisingly, no "free electricity" machines are ever released to real scientists for testing. That's because the
results of such testing are certain: those machines simply can't work for very fundamental and incontrovertible
reasons.
Like so many "scientific" conmen, the purveyors of this particular scam claim to be victims of a hostile scientific
establishment, which refuses to accept their brilliant discoveries. They typically attack the deepest and most
central tenets of science and claim that a conspiracy is perpetuating belief on those tenets. Their refusal to submit
their work to scientific peer review is supposedly based on a fear that such review will be biased and subjective,
controlled by the conspiracy.
The sad reality is that the "scientific establishment" is more than willing to examine the claims, but those claims
won't survive the process of inspection. In some cases, the authors of the claims are truly self-deluded and are
guilty only of pride and ignorance. But in other cases, the authors are real conmen who are out to make a buck at
public expense. They should be run out of town on a rail. >
Click here for more information about the "free electricity" hoax, sent in by readers of this site.
August 14, 2002
I don't want to sound like I know everything in the world or even like I know quite a lot. But you had a question
regarding "If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP"
Well ..Having the thought process that I have, kinda how should I put it? ...Stupid? or inventive or even in-between. Well,
my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the
hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I
wondered if it would burn fast or would it gradually warm up. I slid my finger between...You probably dying to hear what
happened... But it didn't gradually warm up at all. It was instant heat! It didn't scar me or anything like that, but sure
scared the H*** out of me to find out it got so hot so quick. I didn't get any blisters either. But it just burned like touching
something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, "You learn from
your mistakes", stands true. lol - Anonymous
What a remarkable story! As much as I like to think I can predict what should happen in many cases, there is just
nothing like a good experiment to bring some reality to the situation. Your microwave evidently sent a
significant fraction of its 900 watts of microwave radiation through that crack between cooking chamber and
door and roasted your finger instantly. This is a good cautionary tale for those who are careless or curious with
potentially dangerous household gadgets. While I continue to think that serious injuries are unlikely even in a
leaky microwave oven, you have shown that there are cases of real danger. Fortunately, you had time to snap you
finger away. It's like Class 3 lasers, which are now common in the form of laser pointers and supermarket
checkout systems: they can damage your vision if you stare into them, but your blink reflex is fast enough to
keep you from suffering injury. Thanks for the anecdote and I'm glad your finger recovered.
October 25, 2001
Ever since someone struck and damaged the rear bumper of my SAAB 9-3, the air pressure inside the car has been
unbearable to myself and passengers. It causes ear pain and nausea after around 15 minutes of driving. The only solution
is to open the windows. Can you think of any structural aspect that may cause a problem like this? - TA
I suspect that the air inside the car is vibrating the way it does inside an organ pipe or in a soda bottle when you
blow carefully across the bottle's lip. This resonant effect is common in cars when one rear passenger window is
opened slightly. In that case, air blowing across the opening in the window is easily deflected into or out of the
opening and drives the air in the passenger compartment into vigorous vibration. In short, the car is acting like a
giant whistle and because of its enormous size, its pitch is too low for you to hear. Instead, you feel the vibration
as a sickening pulsation in the air pressure.
For the one-open-window problem, the solution is simple: open another window. That shifts the resonant
frequency of the car's air and also helps to dampen the vibrations. Alternatively, you can close the opened
window. In your case, the resonance appears to involve a less visible opening into the car, perhaps near the rear
bumper. If you can close that leak, you may be able to stop the airflow from driving the air in the car into
resonance. If you are unable to find the leak, your best bet is to do exactly what you've done: open another
window.
October 15, 2001
I teach a class on safety helmets (hard hats) and had a question about one of their specifications. The manufacturer rates
their crown impact energy level at 40 foot-pounds. Would this be equivalent to taking an object that weighs 20 pounds
and dropping it 2 feet onto a hard hat? - AH
Assuming that the wearer doesn't let the helmet move and that the object that hits the helmet is rigid, my answer
is approximately yes. If a 20-pound rigid object hits the hat from a height of 2 feet, that object will transfer just
over 40 foot-pounds of energy to the helmet in the process of coming to a complete stop. The "just over" has to
do with the object's continued downward motion as it dents the hat and the resulting release of additional
gravitational potential energy. Also, the need for a rigid dropped object lies in a softer object's ability to absorb
part of the impact energy itself; a dropped 20-pound sack of flour will cause less damage than a dropped 20pound anvil.
However, the true meaning of the "40 foot-pound" specification is that the safety helmet is capable of absorbing
40 foot-pounds of energy during an impact on its crown. This energy is transferred to the helmet by doing work
on it: by pushing its crown downward as the crown dents downward. The product of the downward force on the
crown times the distance the crown moves downward gives the total work done on the helmet and this product
must not exceed 40 foot-pounds or the helmet may fail to protect the wearer. Since the denting force typically
changes as the helmet dents, this varying force must be accounted for in calculating the total work done on the
helmet. While I'm not particularly familiar with safety helmets, I know that bicycle helmets don't promise to be
useable after absorbing their rated energies. Bicycle helmets contain energy-absorbing foam that crushes
permanently during severe impacts so that they can't be used again. Some safety helmets may behave similarly.
Finally, an object dropped from a certain height acquires an energy of motion (kinetic energy) equal to its weight
times the height from which it was dropped. As long as that dropped object isn't too heavy and the helmet it hits
dents without moving overall, the object's entire kinetic energy will be transferred to the helmet. That means that
a 20-pound object dropped from 2 feet on the helmet will deposit 40 found-pounds of energy in the helmet. But
if the wearer lets the helmet move downward overall, some of the falling object's energy will go into the wearer
rather than the helmet and the helmet will tolerate the impact easily. On the other hand, if the dropped object is
too heavy, the extra gravitational potential energy released as it dents the helmet downward will increase the
energy transferred to the helmet. Thus a 4000-pound object dropped just 1/100th of a foot will transfer much
more than 40 foot-pounds of energy to the helmet.
I have noticed that the more I stir the milk into my coffee, the hotter it gets, even though the milk is cold. How does it
work?
Stirring the coffee involves a transfer of energy from you to the coffee. That's because you are doing physical
work on the coffee by pushing it around as it moves in the direction of your push. What began as chemical
energy in your body becomes thermal energy in the coffee. That said, the amount of thermal energy you can
transfer to the coffee with any reasonable amount of stirring is pretty small and you'd lose patience with the
process long before you achieved any noticeable rise in coffee temperature. I think that the effect you notice is
more one of mixing than of heating. Until you mix the milk into the coffee, you may have hot and cold spots in
your cup and you may notice the cold spots most strongly.
Is it possible to heat up the surface of a stealth aircraft by exposing it to strong microwaves? Also, I heard that local
forces in the recent Balkans conflict used cellular phone technology to down the U.S. stealth aircraft. Is that possible? JG
Stealth aircraft are designed to absorb most of the microwave radiation that hits them and to reflect whatever
they don't absorb away from the microwave source. That way, any radar system that tries to see the aircraft by
way of its microwave reflection is unlikely to detect anything returning from the aircraft. In effect, the stealth
aircraft is "black" to microwaves and to the extent that it has any glossiness to its surfaces, those surfaces are
tipped at angles that don't let radar units see that glossiness. Since most radar units emit bright bursts of
microwaves and look for reflections, stealth aircraft are hard to detect with conventional radar. Just as you can't
see a black bat against the night sky by shining a flashlight at it, you can't see a stealth aircraft against the night
sky by shining microwaves at it.
Like any black object, the stealth aircraft will heat up when exposed to intense electromagnetic waves. But trying
to cook a stealth aircraft with microwaves isn't worth the trouble. If someone can figure out where it is enough to
focus intense microwaves on it, they can surely find something better with which to damage it.
As for detecting the stealth aircraft with the help of cell phones, that brings up the issue of what is invisibility.
Like a black bat against the night sky, it's hard to see a stealth aircraft simply by shining microwaves at it. Those
microwaves don't come back to you so you see no difference between the dark sky and the dark plane. But if you
put the stealth aircraft against the equivalent of a white background, it will become painfully easy to see. Cell
phones provide the microwave equivalent of a white background. If you look for microwave emission near the
ground from high in the sky, you'll see microwaves coming at you from every cell phone and telephone tower. If
you now fly a microwave absorbing aircraft across that microwave-rich background, you'll see the dark image as
it blocks out all these microwave sources. Whether or not this effect was used in the Balkans, I can't say. But it
does point out that invisibility is never perfect and that excellent camouflage in one situation may be terrible in
another.
October 5, 2001
I understand now why the sky is blue, but why are sunsets red and orange? - AB, Oak Ridge, Tennessee
As I discussed previously, the sky is blue because tiny particles in the atmosphere (dust, clumps of air molecules,
microscopic water droplets) are better at deflecting shorter wavelength blue light than they are at deflecting
longer wavelength red light. As sunlight passes through the atmosphere, enough blue light is deflected (or more
technically Rayleigh scattered) by these particles to give the atmosphere an overall blue glow. The sun itself is
slightly reddened by this process because a fraction of its blue light is deflected away before it reaches our eyes.
But at sunrise and sunset, sunlight enters our atmosphere at a shallow angle and travels a long distance before
reaching our eyes. During this long passage, most of the blue light is deflected away and virtually all that we see
coming to us from the sun is its red and orange wavelengths. The missing blue light illuminates the skies far to
our east during sunrise and to our west during sunset. When the loss of blue light is extreme enough, as it is after
a volcanic eruption, so little blue light may reach your location at times that even the sky itself appears deep red.
The particles in air aren't good at deflecting red wavelengths, but if that's all the light there is they will give the
sky a dim, red glow.
Why is it easy to stay on a bike while moving, but impossible once it stops? - AS, Switzerland
A bicycle is my favorite example of a dynamically stable object. Although the bicycle is unstable at rest
(statically unstable), it is wonderfully stable when moving forward (dynamically stable). To understand this
distinction, let's start with the bicycle motionless and then start moving forward.
At rest, the bicycle is unstable because it has no base of support. A base of support is the polygon formed by an
object's contact points with the ground. For example, a table has a square or rectangular base of support defined
by its four legs as they touch the floor. As long as an object's center of gravity (the effective location of its
weight) is above this base of support, the object is statically stable. That stability has to do with the object's
increasing potential (stored) energy as it tips-tipping a statically stable object raises its center of gravity and
gravitational potential energy, so that it naturally accelerates back toward its upright position. Since a bicycle has
only two contact points with the ground, the base of support is a line segment and the bicycle can't have static
stability.
But when the bicycle is heading forward, it automatically steers its wheels underneath its center of gravity. Just
as you can balance a broom on you hand if you keep moving your hand under the broom's center of gravity, a
bicycle can balance if it keeps moving its wheels under its center of gravity. This automatic steering has to do
with two effects: gyroscopic precession and bending of the bicycle about its steering axis.
In the gyroscopic precession steering, the spinning wheel behaves as a gyroscope. It has angular momentum, a
conserved quantity of motion associated with spinning, and this angular momentum points toward the left (a
convention that you can understand by pointing the curved fingers of your right hand around in the direction of
the tire's motion; your thumb will then point to the left). When the bicycle begins to lean to one side, for example
to the left, the ground begins to twist the front wheel. Since the ground pushes upward on the bottom of that
wheel, it tends to twist the wheel counter-clockwise according to the rider. This twist or torque points toward the
rear of the bicycle (again, when the fingers of your right hand arc around counterclockwise, your thumb will
point toward the rear). When a rearward torque is exerted on an object with a leftward angular momentum, that
angular momentum drifts toward the left-rear. In this case, the bicycle wheel steers toward the left. While I know
that this argument is difficult to follow, since angular effects like precession challenge even first-year physics
graduate students, but the basic result is simple: the forward moving bicycle steers in the direction that it leans
and naturally drives under its own center of gravity. You can see this effect by rolling a coin forward on a hard
surface: it will automatically balance itself by driving under its center of gravity.
In the bending effect, the leaning bicycle flexes about its steering axis. If you tip a stationary bicycle to the left,
you see this effect: the bicycle will steer toward the left. That steering is the result of the bicycle's natural
tendency to lower its gravitational potential energy by any means possible. Bending is one such means. Again,
the bicycle steers so as to drive under its own center of gravity.
These two automatic steering effects work together to make a forward moving bicycle surprisingly stable.
Children's bicycles are designed to be especially stable in motion (for obvious reasons) and one consequence is
that children quickly discover that they can ride without hands. Adult bicycles are made less stable because
excessive stability makes it hard to steer the bicycle.
October 1, 2001
I have heard that we "know" the universe is expanding because everything is moving away from everything else. My
question is: if this situation is like ink dots on a balloon, then we should be able to point to the direction of the universe's
center. Which way is that center? - BS
The "ink dots on a balloon" idea provides the answer to your question. In that simple analogy, the ink dots
represent stars and galaxies and the balloon's surface represents the universe. Inflating the balloon is then
equivalent to having the universe expand. As the balloon inflates, the stars and galaxies drift apart so that an ant
walking on the surface of the balloon would have to travel farther to go from one "star" to another. A similar
situation exists in our real universe: everything is drifting farther apart.
The ant lives on the surface of the balloon, a two-dimensional world. The ant is unaware of the third dimension
that you and I can see when we look at the balloon. The only directions that the ant can move in are along the
balloon's surface. The ant can't point toward the center of the balloon because that's not along the surface that the
ant perceives. To the ant, the balloon has no center. It lives in a continuous, homogeneous world, which has the
weird property that if you walk far enough in any direction, you return to where you started.
Similarly, we see our universe as a three-dimensional world. If there are spatial dimensions beyond three, we are
unaware of them. The only directions that we can move in are along the three dimensions of the universe that we
perceive. The overall structure of the universe is still not fully understood, but let's suppose that the universe is a
simple closed structure like the surface of a higher-dimensional balloon. In that case, we wouldn't be able to
point to a center either because that center would exist in a dimension that we don't perceive. To us, the universe
would be a continuous, homogeneous structure with that same weird property: if you traveled far enough in one
direction, you'd return to where you started.
August 17, 2001
I am being assured by very reputable scientists (Professors of Physics in American and European universities) that
centrifugal force is a fictitious force, even though the action of a centrifuge is defined as depending upon it. I would be
very grateful if you could help me explain this apparent contradiction and perhaps outline the physical cause that
underlies the separating action of a centrifuge, since it can hardly be a nonexistent force. - RGT, Portsmouth, UK
While "centrifugal force" is something we all seem to experience, it truly is a fictitious force. By a fictitious
force, I mean that it is a side effect of acceleration and not a cause of acceleration.
There is no true outward force acting on an object that's revolving around a center. Instead, that object's own
inertia is trying to make it travel in a straight-line path that would cause it to drift farther and farther away from
the center. The one true force acting on the revolving object is an inward one-a centripetal force. The object is
trying to go straight and the centripetal force is pulling it inward and bending the object's path into a circle.
To get a feel for the experiences associated with this sort of motion, let's first imagine that you are the revolving
object and that you're swinging around in a circle at the end of a rope. In that case, your inertia is trying to send
you in a straight-line path and the rope is pulling you inward and deflecting your motion so that you go in a
circle. If you are holding the rope with your hands, you'll feel the tension in the rope as the rope pulls on you.
(Note that, in accordance with Newton's third law of motion, you pull back on the rope just as hard as it pulls on
you.) The rope's force makes you accelerate inward and you feel all the mass in your body resisting this inward
acceleration. As the rope's force is conveyed throughout your body via your muscles and bones, you feel your
body resisting this inward acceleration. There's no actual outward force on you; it's just your inertia fighting the
inward acceleration. You'd feel the same experience if you were being yanked forward by a rope-there would be
no real backward force acting on you yet you'd feel your inertia fighting the forward acceleration.
Now let's imagine that you are exerting the inward force on an object and that that object is a heavy bucket of
water that's swinging around in a circle. The water's inertia is trying to make it travel in a straight line and you're
pulling inward on it to bend its path into a circle. The force you exert on the bucket is quite real and it causes the
bucket to accelerate inward, rather than traveling straight ahead. Since you're exerting an inward force on the
bucket, the bucket must exert an inward force on you (Newton's third law again). It pulls outward on your arm.
But there isn't anything pulling outward on the bucket, no mysterious "centrifugal force." Instead, the bucket
accelerates in response to an unbalance force on it: you pull it inward and nothing pulls it outward, so it
accelerates inward. In the process, the bucket exerts only one force on its surroundings: an outward force on your
arm.
As for the operation of a centrifuge, it works by swinging its contents around in a circle and using their inertias
to make them separate. The various items in the centrifuge have different densities and other characteristics that
affect their paths as they revolve around the center of the centrifuge. Inertia tends to make each item go straight
while the centrifuge makes them bend inward. The forces causing this inward bending have to be conveyed from
the centrifuge through its contents and there's a tendency for the denser items in the centrifuge to travel straighter
than the less dense items. As a result, the denser items are found near the outside of the circular path while the
less dense ones are found near the center of that path.
August 9, 2001
When you are defrosting and the magnetron is turning on and off, when it is off, are the microwaves still bouncing around
or is the food just sitting there warming itself up? - LEA, PA
During the defrost cycle, the microwave oven periodically turns off its magnetron so that heat can diffuse
through the food naturally, from hot spots to cold spots. These quiet periods allow frozen parts of the food to
melt the same way an ice cube would melt if you threw it into hot water. While the magnetron is off, it isn't
emitting any microwaves and the food is just sitting there spreading its thermal energy around.
June 18, 2001
I understand how a transformer changes voltage, but how does it regulate the amperage? - DE
A transformer's current regulation involves a beautiful natural feedback process. To begin with, a transformer
consists of two coils of wire that share a common magnetic core. When an alternating current flows through the
primary coil (the one bringing power to the transformer), that current produces an alternating magnetic field
around both coils and this alternating magnetic field is accompanied by an alternating electric field (recall that
changing magnetic fields produce electric fields). This electric field pushes forward on any current passing
through the secondary coil (the one taking power out of the transformer) and pushes backward on the current
passing through the primary coil. The net result is that power is drawn out of the primary coil current and put
into the secondary coil current.
But you are wondering what controls the currents flowing in the two coils. The circuit it is connected to
determines the current in the secondary coil. If that circuit is open, then no current will flow. If it is connected to
a light bulb, then the light bulb will determine the current. What is remarkable about a transformer is that once
the load on the secondary coil establishes the secondary current, the primary current is also determined.
Remember that the current flowing in the secondary coil is itself magnetic and because it is an alternating
current, it is accompanied by its own electric field. The more current that is allowed to flow through the
secondary coil, the stronger its electric field becomes. The secondary coil's electric field opposes the primary
coil's electric field, in accordance with a famous rule of electromagnetism known as Lenz's law. The primary
coil's electric field was pushing backward on current passing through the primary coil, so the secondary coil's
electric field must be pushing forward on that current. Since the backward push is being partially negated, more
current flows through the primary coil.
The current in the primary coil increases until the two electric fields, one from the primary current and one from
the secondary current, work together so that they extract all of the primary current's electrostatic energy during
its trip through the coil. This natural feedback process ensures that when more current is allowed to flow through
the transformer's secondary coil, more current will flow through the primary coil to match.
June 15, 2001
Many of the new cordless phones operate at 2.4GHz like a microwave oven. Are we microwaving our ears when we use
them, or is the wattage so small it doesn't affect us? - R
As far as anyone has been able to determine so far, the wattage is so small that this microwave radiation doesn't
affect us. Not all radiations are the same, and radio or microwave radiation is particularly nondestructive at low
intensities. It can't do direct chemical damage and at low wattage can't cause significant RF (radio frequency)
heating. At present, there is thus no plausible physical mechanism by which these phones can cause injury. I
don't think that one will ever be found, so you're probably just fine.
How does a paper towel absorb water?
Paper towels are made out of finely divided fibers of cellulose, the principal structural chemical in cotton, wood,
and most other plants. Cotton is actually a polymer, which like any other plastic is a giant molecule consisting of
many small molecules linked together in an enormous chain or treelike structure. The small molecules or
"monomers" that make up cellulose are sugar molecules. We can't get any nutritional value out of cellulose
because we don't have the enzymes necessary to split the sugars apart. Cows, on the other hand, have
microorganisms in their stomachs that produce the necessary enzymes and allow the cows to digest cellulose.
Despite the fact that cellulose isn't as tasty as sugar, it does have one important thing in common with sugar: both
chemicals cling tightly to water molecules. The presence of many hydroxyl groups (-OH) on the sugar and
cellulose molecules allow them to form relatively strong bonds with water molecules (HOH). This clinginess
makes normal sugar very soluble in water and makes water very soluble in cellulose fibers. When you dip your
paper towel in water, the water molecules rush into the towel to bind to the cellulose fibers and the towel absorbs
water.
Incidentally, this wonderful solubility of water in cellulose is also what causes shrinkage and wrinkling in cotton
clothing when you launder it. The cotton draws in water so effectively that the cotton fibers swell considerably
when wet and this swelling reshapes the garment. Hot drying chases the water out of the fibers quickly and the
forces between water and cellulose molecules tend to compress the fibers as they dry. The clothes shrink and
wrinkle in the process.
Why do things such as sneakers, T-shirts, and nailpolish change color in the sun? The only explanations I've found simple
state that the molecules get excited in the sun.
Sunlight consists not only of light across the entire visible spectrum, but of invisible infrared and ultraviolet
lights as well. The latter is probably what is causing the color-changing effects you mention.
Ultraviolet light is high-energy light, meaning that whenever it is emitted or absorbed, the amount of energy
involved in the process is relatively large. Although light travels through space as waves, it is emitted and
absorbed as particles known as photons. The energy in a photon of ultraviolet light is larger than in a photon of
visible light and that leads to interesting effects.
First, some molecules can't tolerate the energy in an ultraviolet photon. When these molecules absorb such an
energetic photon, their electrons rearrange so dramatically that the entire molecule changes its structure forever.
Among the organic molecules that are most vulnerable to these ultraviolet-light-induced chemical
rearrangements are the molecules that are responsible for colors. The same electronic structural characteristics
that make these organic molecules colorful also make them fragile and susceptible to ultraviolet damage. As a
result, they tend to bleach white in the sun.
Second, some molecules can tolerate high-energy photons by reemitting part of the photon's energy as new light.
Such molecules absorb ultraviolet or other high-energy photons and use that energy to emit blue, green, or even
red photons. The leftover energy is converted into thermal energy. These fluorescent molecules are the basis for
the "neon" colors that are so popular on swimwear, in colored markers, and on poster boards. When you expose
something dyed with fluorescent molecules to sunlight, the dye molecules absorbs the invisible ultraviolet light
and then emit brilliant visible light.
How do people measure g-forces? I have read articles about roller coasters that report specific numbers, such as 3 g's.
How are these numbers obtained? - T
Whenever you accelerate, you experience a gravity-like sensation in the direction opposite that acceleration.
Thus when you accelerate to the left, you feel as though gravity were pulling you not only downward, but also to
the right. The rightward "pull" isn't a true force; it's just the result of your own inertia trying to prevent you from
accelerating. The amount of that rightward "pull" depends on how quickly you accelerate to the left. If you
accelerate to the left at 9.8 meters/second2, an acceleration equal in amount to what you would experience if you
were falling freely in the earth's gravity, the rightward gravity-like sensation you feel is just as strong as the
downward gravity sensation you would feel when you are standing still. You are experiencing a rightward
"fictitious force" of 1 g. The g-force you experience whenever you accelerate is equal in amount to your
acceleration divided by the acceleration due to gravity (9.8 meters/second2) and points in the direction opposite
your acceleration. Often the true downward force of gravity is added to this figure, so that you start with 1 g in
the downward direction when you're not accelerating and continue from there. If you are on a roller coaster that
is accelerating you upward at 19.6 meters/second2, then your total experience is 3 g's in the downward direction
(1 g from gravity itself and 2 g's from the upward acceleration). And if you are accelerating downward at 9.8
meters/second2, then your total experience is 0 g's (1 g downward for gravity and 1 g upward from the downward
acceleration). In this last case, you feel weightless-the weightlessness of a freely falling object such as an
astronaut, skydiver, or high jumper.
June 14, 2001
In regards to your discussion of superheating water in a microwave oven, I've found that it occurs most often when (1) I
reheat water that has been heated before and (2) I heat water that has sat in the cup overnight. Why does that seem to
reduce the number of seed bubbles? - JS
Both processes allow dissolved gases to escape from the water so that they can't serve as seed bubbles for
boiling. When you heat water and then let it cool, the gases that came out of solution as small bubbles on the
walls of the container escape into the air and are not available when you reheat the water. When you let the water
sit out overnight, those same dissolved gases have time to escape into the air and this also reduces the number
and size of the gas bubbles that form when you finally heat the water. Without those dissolved gases and the
bubbles they form during heating it's much harder for the steam bubbles to form when the water reaches boiling.
The water can then superheat more easily.
How do you calculate how much weight a helium balloon can lift? - C & S
A helium balloon experiences an upward force that is equal to the weight of the air it displaces (the buoyant
force on the balloon) minus its own weight. At sea level, air weighs about 0.078 pounds per cubic foot, so the
upward buoyant force on a cubic foot of helium is about 0.078 pounds. A cubic foot of helium weighs only about
0.011 pounds. The difference between the upward buoyant force on the cubic foot of helium and the weight of
the helium is the amount of extra weight that the helium can lift, which is about 0.067 pounds per cubic foot. To
lift a 100 pound person, you'll need about 1500 cubic feet of helium in your balloon.
I am planning to do an experiment with a microwave oven and want to videotape it. I want to operate the microwave oven
with the door open. Will I be safe if I'm 15 feet away? Will opening the door nullify the "chamber" effect that the oven
normally has? - E
Don't operate the oven open. You're just asking for trouble. The oven will emit between 500 and 1100 watts of
microwaves, depending on its rating, and you don't need to be exposed to such intense microwaves. The chamber
effect is important; without the sealed chamber, the microwaves pass through the food only about once before
heading off into the kitchen and you. The food won't cook well and you'll be bathed in the glow from a kilowatt
source of invisible "light."
Imagine standing in front of a 10-kilowatt light bulb (which emits about 1 kilowatt of visible light and the rest is
other forms of heat) and then imagine that you can't see light at all and can only feel it when it is causing
potential damage. Would you feel safe? Your video camera won't enjoy the microwave exposure, either.
If you want to videotape your experiments without having to view them through the metal mesh on the door, you
can consider drilling a small hole in the side of the cooking chamber. If you keep the hole's diameter to a few
millimeters, the microwaves will not leak out. Then put one of the tiny inexpensive video cameras that widely
available a centimeter or so away from that hole. You should get a nice unobstructed view of the cooking process
without risking life and limb.
I thought microwave ovens were sealed shut to keep the waves inside. Why then can you smell the food as it is being
cooked? - E
The cooking chamber of a microwave oven has mesh-covered holes to permit air to enter and exit. The holes in
the metal mesh are small enough that the microwaves themselves cannot pass through and are instead reflected
back into the cooking chamber. However, those holes are large enough that air (or light in the case of the
viewing window) can pass through easily. Sending air through the cooking chamber keeps the cooking chamber
from turning into a conventional hot oven and it carries food smells out into the kitchen.
Which is more economical: operating our air conditioner at 75 °F or operating it at 78 °F and putting fans in front of the
vents? - T
When you put fans in front of the vents, you are probably causing the air conditioner to pump roughly the same
amount of heat out of the room air as it would at 75 °F without the fans. As a result, the fans probably aren't
making the air conditioner work less and aren't saving much electricity. In fact, the fans themselves consume
electricity and produce heat that the air conditioner must then remove, so in principle the fans are a waste of
energy.
However, if the fans are directing the cold air in a way that makes you more comfortable without having to cool
all the room air or if the fans are creating fast moving air that cools you via evaporation more effectively, then
you may be experiencing a real savings of electricity.
To figure out which is the case, you'd have to log the time the air conditioner cycles on during a certain period
while the fans were off and the thermostat set to 75 °F and then repeat that measurement during a similar period
with the fans on and the thermostat set to 78 °F. If the fans significantly reduce the units runtime while leaving
you just as comfortable, then you're saving power.
I'm rewiring a lamp and didn't make sure that the silver and copper wires in the cord matched the screws on the bulb
socket. What will happen if I got it wrong? - L
The bulb will operate perfectly well, regardless of which way you connected the lamp's two wires. Current will
still flow in through one wire, pass through the bulb's filament, and return to the power company through the
other wire. The only shortcoming of reversing the connections is that you will end up with the "hot" wire
connected to the outside of the socket and bulb, rather than to the central pin of the socket and bulb. That's a
slight safety issue: if you touch the hot wire with one hand and a copper pipe with the other, you'll get a shock.
That's because a large voltage difference generally exists between the hot wire and the earth itself.
In contrast, there should be very little voltage difference between the other wire (known as "neutral") and the
earth. In a properly wired lamp, the large spade on the electric plug (the neutral wire) should connect to the
outside of the bulb socket. That way, when you accidentally touch the bulb's base as you screw it in or out, you'll
only be connecting your hand to the neutral wire and won't receive a shock. If you miswire the lamp and have the
hot wire connected to the outside of the socket, you can get a shock if you accidentally touch the bulb base at any
time.
March 16, 2001
I saw the story on Primetime tonight (Superheated Water Produced in Microwave Ovens on ABC Primetime 3/15/2001),
and at weird timing. Just yesterday, a co-worker and I were standing around the kitchen area talking, while she warmed
up some coffee. All of a sudden, there was a loud POP, which startled both of us. Not knowing exactly what had
happened, we stopped the microwave and opened the door, only to find the contents of the mug (coffee) everywhere on
the inside of the cooking chamber, less a few drops at the bottom of the cup.
The story provided SOME insight into what exactly had happened, however, it was reported that the surface of the superheated liquid had to be broken by something for an explosion to be triggered. In the explosion with the coffee, there were
no other objects in the microwave other than the mug and the coffee it held. What then, caused the explosion if nothing
was present to break the surface? - MM, Denver, CO
Superheated water doesn't always wait until triggered before undergoing sudden boiling. All that's needed to start
an explosion is for something to introduce an initial "seed" bubble into the liquid. Sometimes the container
already has everything necessary to form a seed bubble and it's just a matter of getting the water hot enough to
start that process. Many seed bubbles begin as trapped air in tiny crevices. As the water gets hotter, the size of
any trapped air pocket grows and eventually it may be able to break free as a real seed bubble. When water is
sufficiently superheated, just a single seed bubble is enough to start an explosion and empty the container
completely. In your case, the coffee flash boiled spontaneously after something inside it nucleated the first
bubble.
This sort of accident happens fairly often and we rarely think much about it as we sponge up the spilled liquid
inside the microwave oven. But had your friend been unlucky enough to stop heating the coffee a second or two
before that POP, she might have been injured while taking the coffee out of the oven. The moral of this story is
to avoid overcooking any liquid in the microwave oven. If you must drink your coffee boiling hot, pay attention
to it as it heats up so that it doesn't cook too long and then let it sit for a minute after the oven turns off. If you
don't like your coffee boiling hot, then don't heat it to boiling at all.
You must be busy since last night's broadcast (Superheated Water Produced in Microwave Ovens on ABC Primetime
3/15/2001). Very, very scary as we have certainly done exactly what was shown. I have 3 little girls who love to "cook"
their own soups, heat their dad's coffee water, etc. in the microwave. This report terrified me. I am grateful no harm has
come to them. My question is if we strictly use microwaveable plastic bowls, ceramic mugs, or other heavy mixing type
bowls and avoid the glass, is the potential for the explosion still there?
I'm afraid that there's no easy answer to this question. You can use a microwave oven to superheat water in any
container that doesn't assist bubble formation. How a particular container behaves is hard for me to say without
experimenting. I'd heat a small amount of water (1/2 cup or less) in the container and look at it through the oven's
window to see if the water boils nicely, with lots of steam bubbles streaming upward from many different points
on the inner surface of the container. The more easily water boils in the container, the less likely it is to superheat
when you cook it too long. (If you try this experiment, leave the potentially superheated water in the closed
microwave oven to cool!)
Glass containers are clearly the most likely to superheat water because their surfaces are essentially perfect.
Glasses have the characteristics of frozen liquids and a glass surface is as smooth as... well, glass. When you
overheat water in a clean glass measuring cup, your chances of superheating it at least mildly are surprisingly
high. The spontaneous bubbling that occurs when you add sugar, coffee powder, or a teabag to microwaveheated water is the result of such mild superheating. Fortunately, severe superheating is much less common
because defects, dirt, or other impurities usually help the water boil before it becomes truly dangerous. That's
why most of us avoid serious injuries.
However, even non-transparent microwaveable containers often have glass surfaces. Ceramics are "glazed,"
which means that they are coated with glass for both sealing and decoration. Many heavy mixing bowls are glass
or glass-ceramics. As you can see, it's hard to get away from trouble. I simply don't know how plastic
microwaveable containers behave when heating water; they may be safe or they may be dangerous.
If you're looking for a way out of this hazard, here are my suggestions. First, learn to know how long a given
amount of liquid must be heated in your microwave in order to reach boiling and don't cook it that long. If you
really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. My
microwave oven has a "beverage" setting that senses how hot the water is getting. If the water isn't hot enough
when that setting finishes, I add another 30 seconds and then test again. I never cook the water longer than I need
to. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave
oven may mean that it becomes dangerously superheated. Your children can still "cook" soup in the microwave
if they use the right amount of time. Children don't like boiling hot soup anyway, so if you figure out how long it
takes to heat their soup to eating temperature and have them cook their soup only that long, they'll never
encounter superheating. As for dad's coffee water, same advice. If dad wants his coffee boiling hot, then he
should probably make it himself. Boiling water is a hazard for children even without superheating.
Second, handle liquids that have been heated in a microwave oven with respect. Don't remove a liquid the instant
the oven stops and then hover over it with your face exposed. If the water was bubbling spasmodically or not at
all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no
indications of superheating, it takes no real effort to be careful. If you cooked the water long enough for it to
reach boiling temperature, let it rest for a minute per cup before removing it from the microwave. Never put your
face or body over the container and keep the container at a safe distance when you add things to it for the first
time: powdered coffee, sugar, a teabag, or a spoon.
Finally, it would be great if some entrepreneurs came up with ways to avoid superheating altogether. The makers
of glass containers don't seem to recognize the dangers of superheating in microwave ovens, despite the
mounting evidence for the problem. Absent any efforts on their parts to make the containers intrinsically safer, it
would be nice to have some items to help the water boil: reusable or disposable inserts that you could leave in the
water as it cooked or an edible powder that you could add to the water before cooking. Chemists have used
boiling chips to prevent superheating for decades and making sanitary, nontoxic boiling sticks for microwaves
shouldn't be difficult. Similarly, it should be easy to find edible particles that would help the water boil.
Activated carbon is one possibility.
Last night's report wasn't meant to scare you away from using your microwave oven or keep you from heating
water in it. It was intended to show you that there is a potential hazard that you can avoid if you're informed
about it. Microwave ovens are wonderful devices and they prepare food safely and efficiently as long as you use
them properly. "Using them properly" means not heating liquids too long in smooth-walled containers.
November 15, 2000
Why does water react in a violent and dangerous way when overheated in a microwave oven? CA
Water doesn't always boil when it is heated above its normal boiling temperature (100 °C or 212 °F). The only
thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be
able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply
remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as
"nucleation." If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit
to how hot the water can become.
Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels.
Glass containers have few or no such defects. When you cook water in a smooth glass container, using a
microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water
will superheat. This situation becomes even worse if the top surface of the water is "sealed" by a thin layer of oil
or fat so that evaporation can't occur, either. Superheated water is extremely dangerous and people have been
severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up
the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed
such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or highquality movie (16.2MB)). As you'll hear in my flustered remarks after "Experiment 13," I was a bit shaken up by
the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise,
you'll notice that I became much more concerned about yanking my hand out of the oven before the fork reached
the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don't
superheat more than a few ounces of water. You can easily get burned or worse. For a reader's story about a burn
he received from superheated water in a microwave, touch here.
Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart:
September 8, 2000
I left a spoon in my food and I put it in the microwave by accident. Is it dangerous to eat the food after it was put into the
microwave with a metal object. Does it have any radiation? Could it cause cancer? - SK, Santa Monica, California
The spoon will have essentially no effect at all on the food. Metal left in the microwave oven during cooking will
only cause trouble if (a) it is very thin or (b) it has sharp edges or points. The microwaves push electric charges
back and forth in metal, so if the metal is too thin, it will heat up like the filament of a light bulb and may cause a
fire. And if the metal has sharp edges or points, charges may accumulate on those sharp spots and then leap into
space as a spark. But because your spoon was thick and had rounded edges, the charges that flowed through it
during cooking didn't have any bad effects on the spoon: no heating and no sparks.
As far as the food is concerned, the presence of the spoon redirected the microwaves somewhat, but probably
without causing any noticeable changes in how the food cooked. There is certainly no residual radiation of any
sort and the food is no more likely to cause cancer after being cooked with metal around than had there been no
spoon with it. In general, leaving a spoon in a cup of coffee or bowl of oatmeal isn't going to cause any trouble at
all. I do it all the time. In fact, having a metal spoon in the liquid may reduce the likelihood of superheating the
liquid, a dangerous phenomenon that occurs frequently in microwave cooking. Superheated liquids boil violently
when you disturb them and can cause serious injuries as a result.
August 11, 2000
My mother-in-law feels that by shaking a partially consumed bottle of carbonated beverage after re-sealing it, it will repressurize keeping the carbonation better than just resealing it. I believe that, since the amount of CO2 in the beverage
and the container will stay constant, that either re-sealing or re-sealing and shaking will have the same net effect when it
comes to maintaining carbonation. Is she right? - JK, New Mexico
No, you are right. In the long run, the number of CO2 molecules left in the bottle when you close it is all that
matters. Those molecules will drift in and out of the liquid and gas phases until they reach equilibrium. At the
equilibrium point, there will be enough molecules in the gas phase to pressurize the bottle and enough in the
liquid phase to give the beverage a reasonable amount of bite.
By giving the sealed bottle a shake, your mother-in-law is simply speeding up the approach to equilibrium. She
is helping the CO2 molecules leave the beverage and enter the gas phase. The bottle then pressurizes faster, but at
the expense of dissolved molecules in the beverage itself. If there is any chance that you'll drink more before
equilibrium has been reached, you do best not to shake the bottle. That way, the equilibration process will be
delayed as much as possible and you may still be able to drink a few more of those CO2 molecules rather than
breathing them.
Incidentally, shaking a new bottle of soda just before you open it also speeds up the equilibration process. For an
open bottle, equilibrium is reached when essentially all the CO2 molecules have left and are in the gas phase
(since the gas phase extends over the whole atmosphere). That's not what you want at all. Instead, you try not to
shake the beverage so that it stays away from equilibrium (and flatness) as long as possible. For most opened
beverages, equilibrium is not a tasty situation.
July 30, 2000
My roommate and I heard that it's possible to project the picture from our TV set onto the wall. We'd love to sit on our
porch and watch TV while drinking a beer. Any ideas? - JK
The simple answer to your question is yes, you can do it. But you'll encounter two significant problems with
trying to turn your ordinary TV into a projection system. First, the lens you'll need to do the projection will be
extremely large and expensive. Second, the image you'll see will be flipped horizontally and vertically. You'll
have to hang upside-down from your porch railing, which will make drinking a beer rather difficult.
About the lens: in principle, all you need is one convex lens. A giant magnifying glass will do. But it has a
couple of constraints. Because your television screen is pretty large, the lens diameter must also be pretty large.
If it is significantly smaller than the TV screen, it won't project enough light onto your wall. And to control the
size of the image it projects on the wall, you'll need to pick just the right focal length (curvature) of the lens.
You'll be projecting a real image on the wall, a pattern of light that exactly matches the pattern of light appearing
on the TV screen. The size and location of that real image depends on the lens's focal length and on its distance
from the TV screen. You'll have to get these right or you'll see only a blur. Unfortunately, single lenses tend to
have color problems and edge distortions. Projection lenses need to be multi-element carefully designed systems.
Getting a good quality, large lens with the right focal length is going to cost you.
The other big problem is more humorous. Real images are flipped horizontally and vertically relative to the light
source from which they originate. Unless you turn your TV set upside-down, your wall image will be inverted.
And, without a mirror, you can't solve the left-right reversal problem. All the writing will appear backward.
Projection television systems flip their screen image to start with so that the projected image has the right
orientation. Unless you want to rewire your TV set, that's not going to happen for you. Good luck.
July 20, 2000
Is it true that the buoyancy of an incompressible bathysphere doesn't change when it plunges to great depths in the ocean,
even though the pressure exerted on it increases enormously? - AM
A submerged object's buoyancy (the upward force exerted on it by a fluid) is exactly equal to the weight of the
fluid it displaces. In this case, the upward buoyant force on the bathysphere is equal in amount to the weight of
the water it displaces. Since the bathysphere is essentially incompressible, it always displaces the same volume
of water. And since water is essentially incompressible, that fixed volume of water always weighs the same
amount. That's why the bathysphere experiences a constant upward force on it due to the surrounding water. To
sink the bathysphere, they weight it down with heavy metal particles. And to allow the bathysphere to float back
up, they release those particles and reduce the bathysphere's total weight.
May 31, 2000
If a microwave oven door were to open while it was still on, what would happen? Could it hurt you? - JP
The microwaves would flow out of the oven's cooking chamber like light streaming out of a brightly illuminated
mirrored box. If you were nearby, some of those microwaves would pass through you and your body would
absorb some of them during their passage. This absorption would heat your tissue so that you would feel the
warmth. In parts of your body that have rapid blood circulation, that heat would be distributed quickly to the rest
of your body and you probably wouldn't suffer any rapid injuries. But in parts of your body that don't have good
blood flow, such as the corneas of your eyes, tissue could heat quickly enough to be permanently damaged. In
any case, you'd probably feel the warmth and realize that something was wrong before you suffered any
substantial permanent injuries.
April 28, 2000
My teacher said that if you lift a 5 pound sack, you are doing work but if you carry the sack, you aren't doing any work.
Why is that?
When you lift the sack, you are pushing it upward (to support its weight) and it is moving upward. Since the
force you exert on the sack and the distance it is traveling are in the same direction, you are doing work on the
sack. As a result, the sack's energy is increasing, as evidenced by the fact that it is becoming more and more
dangerous to a dog sitting beneath it.
But when you carry the sack horizontally at a steady pace, the upward force you exert on the sack and the
horizontal distance it travels are at right angles to one another. You don't do any work on the sack in that case.
The evidence here is that the sack doesn't become any more dangerous; its speed doesn't increase and neither
does its altitude. It just shifts from one place to an equivalent one to its side.
March 30, 2000
I am currently working on a physics project, the magnetic levitation train. How can I make this train move on the track
without it crashing? I only have a few days to make it work so I can present it in the science fair. - VC
I'm afraid that you're facing a difficult problem. Magnetic levitation involving permanent magnets is inherently
and unavoidably unstable for fundamental reasons. One permanent magnet suspended above another permanent
magnet will always crash. That's why all practical maglev trains use either electromagnets with feedback
circuitry (magnets that can be changed electronically to correct for their tendencies to crash) or
magnetoelectrodynamic levitation (induced magnetism in a conducting track, created by a very fast moving
(>100 mph) magnetized train). There are no simple fixes if what you have built so far is based on permanent
magnets alone. Unfortunately, you have chosen a very challenging science fair project.
February 25, 2000
I am in 4th grade, and working on a science fair project using a basketball and have it pumped with 0 psi, 3 psi, 6 psi, 9
psi and 12 psi of air. Why is it that the 9psi ball bounces the highest when dropped from 6ft? - T
The more pressure a basketball has inside it, the less its surface dents during a bounce and the more of its
original energy it stores in the compressed air. Air stores and returns energy relatively efficiently during a rapid
bounce, so the pressurized ball bounces high. But an underinflated ball dents deeply and its skin flexes
inefficiently. Much of the ball's original energy is wasted in heating the bending skin and it doesn't bounce very
high. In general, the higher the internal pressure in the ball, the better it will bounce.
However, the ball doesn't bounce all by itself when you drop it on a flexible surface. In that case, the surface also
dents and is responsible for part of the ball's rebound. If that surface handles energy inefficiently, it may weaken
the ball's bounce. For example, if you drop the ball on carpeting, the carpeting will do much of the denting, will
receive much of the ball's original energy, and will waste its share as heat. The ball won't rebound well. My
guess is that you dropped the ball on a reasonably hard surface, but one that began to dent significantly when the
ball's pressure reached 12psi. At that point, the ball was extremely bouncy, but it was also so hard that it dented
the surface and let the surface participate strongly in the bouncing. The surface probably wasn't as bouncy as the
ball, so it threw the ball relatively weakly into the air.
I'd suggest repeating your experiment on the hardest, most massive surface you can find. A smooth cement or
thick metal surface would be best. The ball will then do virtually all of the denting and will be responsible for
virtually all of the rebounding. In that case, I'll bet that the 12psi ball will bounce highest.
February 15, 2000
What everyday household chemicals (cleaners, paints, detergents, etc.) contain large enough amounts of phosphor to
glow under black light?
Fluorescent paints and many laundry detergents contain fluorescent chemicals-chemicals that absorb ultraviolet
light and use its energy to produce visible light. Fluorescent paints are designed to do exactly that, so they
certainly contain enough "phosphor" for that purpose. Detergents have fluorescent dyes or "brighteners" added
because it helps to make fabrics appear whiter. Aging fabric appears yellowish because it absorbs some blue
light. To replace the missing blue light, the brighteners absorb invisible ultraviolet and use its energy to emit blue
light.
Is it better to use warm or cold air to defrost your windshield?
If you can't alter the air's humidity, warm air will definitely heat up your window faster and defrost it faster than
cold air. The only problem with using hot air is that rapid heating can cause stresses on the window and its frame
because the temperature will rise somewhat unevenly and lead to uneven thermal expansion. Such thermal stress
can actually break the window, as a reader informed me recently: "On one of the coldest days of this Boston
winter, I turned up the heat full blast to defrost the windshield. The outside of the window was still covered with
ice, which I figured would melt from the heat. After about 10 minutes of heating, the windshield "popped" and a
fracture about 8 inches long developed. The windshield replacement company said I would have to wait a day for
service, since this happened to so many people over the cold evening that they were completely booked." If
you're nervous about breaking the windshield, use cooler air.
About the humidity caveat: if you can blow dry air across your windshield, that will defrost it faster than just
about anything else, even if that air is cold. The water molecules on your windshield are constantly shifting back
and forth between the solid phase (ice) and the gaseous phase (steam or water vapor). Heating the ice will help
more water molecules leave the ice for the water vapor, but dropping the density of the water vapor will reduce
the number of water molecules leaving the water vapor for the ice. Either way, the ice decreases and the water
vapor increases. Since you car's air condition begins drying the air much soon after you start the car than its
heater begins warming the air, many modern cars concentrate first on drying the air rather than on heating it.
February 10, 2000
When a device uses two batteries, why do they have to be place positive to negative? Are there any exceptions? - MS
Batteries are "pumps" for electric charge. A battery takes an electric current (moving charge) entering its
negative terminal and pumps that current to its positive terminal. In the process, the battery adds energy to the
current and raises its voltage (voltage is the measure of energy per unit of electric charge). A typical battery adds
1.5 volts to the current passing through it. As it pumps current, the battery consumes its store of chemical
potential energy so that it eventually runs out and "dies."
If you send a current backward through a battery, the battery extracts energy from the current and lowers its
voltage. As it takes energy from the current, the battery adds to its store of chemical potential energy so that it
recharges. Battery charges do exactly that: they push current backward through the batteries to recharge them.
This recharging only works well on batteries that are designed to be recharged since many common batteries
undergo structural damage as their energy is consumed and this damage can't be undone during recharging.
When you use a chain of batteries to power an electric device, you must arrange them so that each one pumps
charge the same direction. Otherwise, one will pump and add energy to the current while the other extracts
energy from the current. If all the batteries are aligned positive terminal to negative terminal, then they all pump
the same direction and the current experiences a 1.5 volt (typically) voltage rise in passing through each battery.
After passing through 2 batteries, its voltage is up by 3 volts, after passing through 3 batteries, its voltage is up
by 4.5 volts, and so on.
February 9, 2000
How does a parabolic sound collecting dish work? - C
A parabolic dish microphone is essentially a mirror telescope for sound. A parabolic surface has the interesting
property that all sound waves that propagate parallel its central axis travel the same distance to get to its focus.
That means that when you aim the dish at a distant sound source, all of the sound from that object bounces off
the dish and converges toward the focus in phase--with its pressure peaks and troughs synchronized so that they
work together to make the loudest possible sound vibrations. The sound is thus enhanced at the focus, but only if
it originated from the source you're aiming at. Sound from other sources misses the focus. If you put a sensitive
microphone in the parabolic dish's focus, you'll hear the sound from the distant object loud and clear.
Are microwaves attenuated in air?
Not significantly. Air doesn't absorb them well, which is why the air in a microwave oven doesn't get hot and
why satellite and cellular communication systems work so well. The molecules in air are poor antennas for this
long-wavelength electromagnetic radiation. They mostly just ignore it.
February 3, 2000
How do the automatic doors at a supermarket know when to open and close? How do they work? -- KL
Devices that sense your presence are either bouncing some wave off you or they are passively detecting waves
that you emit or reflect. The wave-bouncing detectors emit high frequency (ultrasonic) sound waves or radio
waves and then look for reflections. If they detect changes in the intensity or frequency pattern of the reflected
waves, they know that something has moved nearby and open the door. The passive detectors look for changes in
the infrared or visible light patterns reaching a detector and open the door when they detect such changes.
I have a digital camera and when I put an IR remote control in front of the lens and press a button, a bluish white light is
visible on the camera's monitor. Why is that? -- MC
What a neat observation! Digital cameras based on CCD imaging chips are sensitive to infrared light. Even
though you can't see the infrared light streaming out of the remote control when you push its buttons, the
camera's chip can. This behavior is typical of semiconductor light sensors such as photodiodes and
phototransistors: they often detect near infrared light even better than visible light. In fact, a semiconductor
infrared sensor is exactly what your television set uses to collect instructions from the remote control.
The color filters that the camera employs to obtain color information misbehave when they're dealing with
infrared light and so the camera is fooled into thinking that it's viewing white light. That's why your camera
shows a white spot where the remote's infrared source is located.
I just tried taking some pictures through infrared filters, glass plates that block visible light completely, and my
digital camera worked just fine. The images were as sharp and clear as usual, although the colors were odd. I had
to use incandescent illumination because fluorescent light doesn't contain enough infrared. It would be easy to
take pictures in complete darkness if you just illuminated a scene with bright infrared sources. No doubt there are
"spy" cameras that do exactly that.
February 2, 2000
Is there sound in space? If so, what is the speed of sound there? -- MH
No, there is no sound in space. That's because sound has to travel as a vibration in some material such as air or
water or even stone. Since space is essentially empty, it cannot carry sound, at least not the sorts of sound that we
are used to.
Does ice melt faster in air or in water? -- BP
Ice will melt fastest in whatever delivers heat to it fastest. In general that will be water because water conducts
heat and carries heat better than air. But extremely hot air, such as that from a torch, will beat out very cold
water, such as ice water, in melting the ice.
I work in a company shop that uses a 600-watt laser with a wavelength of 1064 nm. How safe is this machine? What is
the radiation hazard, if any? I've noticed that my eyes feel strange after working with it for 4-5 hours. It also has an
uncomfortable smell. -- EC
The laser you're using is a neodymium-YAG laser. It uses a crystal of YAG (yttrium aluminum garnet), a
synthetic gem that was once sold as an imitation diamond, that has been treated with neodymium atoms to give it
a purple color. When placed in a laser cavity and exposed to intense visible light, this crystal gives off the
infrared light you describe. You can't see this light but, at up to 600 watts, it is actually incredibly bright. You
don't want to look at it or even at its reflection from a surface that you're machining. That's because the lens of
your eye focuses it onto your retina and even though your retina won't see any light, it will experience the heat.
It's possible to injure your eyes by looking at this light, particularly if you catch a direct reflection of the laser
beam in your eye.
In all likelihood, the manufacturer of this unit has shielded all the light so that none of it reaches your eyes. If
that's not the case, you should wear laser safety glasses that block 1064 nm light. But it's also possible that the
irritation you're experiencing is coming from the burned material that you are machining. Better ventilation
should help. High voltage power supplies, which may be present in the laser, could also produce ozone. Ozone
has a spicy fresh smell, like the smell after a lightning storm, and it is quite irritating to eyes and nose.
How come planets are spherical, albeit with somewhat flattened poles? -- DB
The answer is gravity. Gravity smashes the planets into spheres. To understand this, imagine trying to build a
huge mountain on the earth's surface. As you begin to heap up the material for your mountain, the weight of the
material at the top begins to crush the material at the bottom. Eventually the weight and pressure become so great
that the material at the bottom squeezes out and you can't build any taller. Every time you put new stuff on top,
the stuff below simply sinks downward and spreads out. You can't build bumps bigger than a few dozen miles
high on earth because there aren't any materials that can tolerate the pressure. In fact, the earth's liquid core won't
support mountains much higher than the Himalayas--taller mountains would just sink into the liquid. So even if a
planet starts out non-spherical, the weight of its bumps will smash them downward until the planet is essentially
spherical.
The flattened poles are the result of rotation--as the planet spins, the need for centripetal (centrally directed)
acceleration at its equator causes its equatorial surface to shift outward slightly, away from the planet's axis of
rotation. The planet is therefore wider at its equator than it is at its poles.
January 17, 2000
There is a story circulating by email about a 26 year old man who heated a cup of water in a microwave oven and had it
"explode in his face" when he took it out. He suffered serious burns as a result. Is this possible and, if so, how did it
happen? -- JJ, Kirksville, Missouri
Yes, this sort of accident can and does happen. The water superheated and then boiled violently when disturbed.
Here's how it works:
Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave
the surface faster than they return, the quantity of liquid water gradually diminishes. That's ordinary evaporation.
However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but
also from within. If a steam bubble forms inside the hot water, water molecules can evaporate into that steam
bubble and make it grow larger and larger. The high temperature is necessary because the pressure inside the
bubble depends on the temperature. At low temperature, the bubble pressure is too low and the surrounding
atmospheric pressure smashes it. That's why boiling only occurs at or above water's boiling temperature. Since
pressure is involved, boiling temperature depends on air pressure. At high altitude, boiling occurs at lower
temperature than at sea level.
But pay attention to the phrase "If a steam bubble forms" in the previous paragraph. That's easier said than done.
Forming the initial steam bubble into which water molecules can evaporate is a process known as "nucleation." It
requires a good number of water molecules to spontaneously and simultaneously break apart from one another to
form a gas. That's an extraordinarily rare event. Even in a cup of water many degrees above the boiling
temperature, it might never happen. In reality, nucleation usually occurs at a defect in the cup or an impurity in
the water--anything that can help those first few water molecules form the seed bubble. When you heat water on
the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist nucleation so that
boiling occurs soon after the boiling temperature is reached. But when you heat pure water in a smooth cup using
a microwave oven, there may be nothing present to help nucleation occur. The water can heat right past its
boiling temperature without boiling. The water then superheats--its temperature rising above its boiling
temperature. When you shake the cup or sprinkle something like sugar or salt into it, you initiate nucleation and
the water then boils violently.
Fortunately, serious microwave superheating accidents are fairly unusual. However, they occur regularly and
some of the worst victims require hospital treatment. I have heard of extreme cases in which people received
serious eye injuries and third degree burns that required skin grafts and plastic surgery.
You can minimize the chance of this sort of problem by not overcooking water or any other liquid in the
microwave oven, by waiting about 1 minute per cup for that liquid to cool before removing it from the
microwave if there is any possibility that you have superheated it, and by being cautious when you first introduce
utensils, powders, teabags, or otherwise disturb very hot liquid that has been cooked in a microwave oven. Keep
the water away from your face and body until you're sure it's safe and don't ever hover over the top of the
container. Finally, it's better to have the liquid boil violently while it's inside the microwave oven than when it's
outside on your counter and can splatter all over you. Once you're pretty certain that the water is no longer
superheated, you can ensure that it's safe by deliberately nucleating boiling before removing the cup from the
microwave. Inserting a metal spoon or almost any food into the water should trigger boiling in superheated
water. A pinch of sugar will do the trick, something I've often noticed when I heat tea in the microwave.
However, don't mess around with large quantities of superheated water. If you have more than 1 cup of
potentially superheated water, don't try to nucleate boiling until you've waited quite a while for it to cool down.
I've been scalded by the stuff several times even when I was prepared for an explosion. It's really dangerous.
For a reader's story about a burn he received from superheated water in a microwave, touch here.
I always thought that pure water cannot exceed 100° Celsius at atmospheric pressure without first turning into its gaseous
state. How is it that the water heated in the microwave oven can superheat and exceed 100° Celsius? -- AC
The relative stabilities of liquid and gaseous water depend on both temperature and pressure. To understand this,
consider what is going on at the surface of a glass of water. Water molecules in the liquid water are leaving the
water's surface to become gas above it and water molecules in the gas are landing and joining the liquid water
below. It's like a busy airport, with lots of take-offs and landings. If the glass of water is sitting in an enclosed
space, the arrangement will eventually reach equilibrium--the point at which there is no net transfer of molecules
between the liquid in the glass and the gas above it. In that case, there will be enough water molecules in the gas
to ensure that they land as often as they leave.
The leaving rate (the rate at which molecules break free from the liquid water) depends on the temperature. The
hotter the water is, the more frequently water molecules will be able to break away from their buddies and float
off into the gas. The landing rate (the rate at which molecules land on the water's surface and stick) depends on
the density of molecules in the gas. The more dense the water vapor, the more frequently water molecules will
bump into the liquid's surface and land.
As you raise the temperature of the water in your glass, the leaving rate increases and the equilibrium shifts
toward higher vapor density and less liquid water. By the time you reach 100° Celsius, the equilibrium vapor
pressure is atmospheric pressure, which is why water tends to boil at this temperature (it can form and sustain
steam bubbles). Above this temperature the equilibrium vapor pressure exceeds atmospheric pressure. The liquid
water and the gas above it can reach equilibrium, but only if you allow the pressure in your enclosed system to
exceed atmospheric pressure. However, if you open up your enclosed system, the water vapor will spread out
into the atmosphere as a whole and there will be a never-ending stream of gaseous water molecules leaving the
glass. Above 100° C, liquid water can't exist in equilibrium with atmospheric pressure gas, even if that gas is
pure water vapor.
So how can you superheat water? Don't wait for equilibrium! The road to equilibrium may be slow; it may take
minutes or hours for the liquid water to evaporate away to nothing. In the meantime, the system will be out of
equilibrium, but that's ok. It happens all the time: a snowman can't exist in equilibrium on a hot summer day, but
that doesn't mean that you can't have a snowman at the beach... for a while. Superheated water isn't in
equilibrium and, if you're patient, something will change. But in the short run, you can have strange
arrangements like this without any problem.
January 3, 2000
I am twelve years old and weigh 85 pounds. How much helium would it take to lift me off the ground?
While helium itself doesn't actually defy gravity, it is lighter than air and floats upward as descending air pushes
it out of the way. Like a bubble in water, the helium goes up to make room for the air going down. The buoyant
force that acts on the helium is equal to the weight of air that the helium displaces.
A cubic foot of air weighs about 0.078 pounds so the upward buoyant force on a cubic foot of helium is about
0.078 pounds. A cubic foot of helium weighs only about 0.011 pounds. The difference between the upward
buoyant force on the cubic foot of helium and the weight of the helium is the amount of extra weight that the
helium can lift; about 0.067 pounds. Since you weigh 85 pounds, it would take about 1300 cubic feet of helium
to lift you and a thin balloon up into the air. That's a balloon about 13.5 feet in diameter.
Why does a shave that looks great under incandescent light look terrible under fluorescent light? And, for a woman, what
light is best for putting on makeup? -- JE
Illumination matters because your skin only reflects light to which it's exposed. When you step into a room
illuminated only by red light your skin appears red, not because it's truly red but because there is only red light to
reflect.
Ordinary incandescent bulbs produce a thermal spectrum of light with a "color temperature" of about 2800° C. A
thermal light spectrum is a broad, featureless mixture of colors that peaks at a particular wavelength that's
determined only by the temperature of the object emitting it. Since the bulb's color temperature is much cooler
than that of the sun's (5800° C), the bulb appears much redder than the sun and emits relatively little blue light. A
fluorescent lamp, however, synthesizes its light spectrum from the emissions of various fluorescent phosphors.
Its light spectrum is broad but structured and depends on the lamp's phosphor mixture. The four most important
phosphor mixtures are cool white, deluxe cool white, warm white, and deluxe warm white. These mixtures all
produce more blue than an incandescent bulb, but the warm white and particularly the deluxe warm white tone
down the blue emission to give a richer, warmer glow at the expense of a little energy efficiency. Cool white
fluorescents are closer to natural sunlight than either warm white fluorescents or incandescent bulbs.
To answer your question about shaves: without blue light in the illumination, it's not that easy to distinguish
beard from skin. Since incandescent illumination is lacking in blue light, a shave looks good even when it isn't.
But in bright fluorescent lighting, beard and skin appear sharply different and it's easy to see spots shaving has
missed. As for makeup illumination, it's important to apply makeup in the light in which it will be worn. Bluepoor incandescent lighting downplays blue colors so it's easy to overapply them. When the lighting then shifts to
blue-rich fluorescents, the blue makeup will look heavy handed. Some makeup mirrors provide both kinds of
illumination so that these kinds of mistakes can be avoided.
December 23, 1999
What is terminal velocity? -- EW, Fisher, Australia
After falling for a long time, an object will descend at a steady speed known as its "terminal velocity." This
terminal velocity exists because an object moving through air experiences drag forces (air resistance). These drag
forces become stronger with speed so that as a falling object picks up speed, the upward air resistance it
experiences gradually becomes stronger. Eventually the object reaches a speed at which the upward drag forces
exactly balance its downward weight and the object stops accelerating. It is then at "terminal velocity" and
descends at a steady pace.
The terminal velocity of an object depends on the object's size, shape, and density. A fluffy object (a feather, a
parachute, or a sheet of paper) has a small terminal velocity while a compact, large, heavy object (a cannonball, a
rock, or a bowling ball) has a large terminal velocity. An aerodynamic object such as an arrow also has a very
large terminal velocity. A person has a terminal velocity of about 200 mph when balled up and about 125 mph
with arms and feet fully extended to catch the wind.
December 22, 1999
How does a Tesla coil work? -- EK
Popular in movies as a source of long glowing sparks, a Tesla coil is basically a high-frequency, very highvoltage transformer. Like most transformers, the Tesla coil has two circuits: a primary circuit and a secondary
circuit. The primary circuit consists of a capacitor and an inductor, fashioned together to form a system known as
a "tank circuit". A capacitor stores energy in its electric field while an inductor stores energy in its magnetic
field. When the two are wired together in parallel, their combined energy sloshes back and forth from capacitor
to inductor to capacitor at a rate that's determined by various characteristics of the two devices. Powering the
primary of the Tesla coil is a charge delivery system that keeps energy sloshing back and forth in the tank circuit.
This delivery system has both a source of moderately high voltage electric current and a pulsed transfer system
to periodically move charge and energy to the tank. The delivery system may consist of a high voltage
transformer and a spark gap, or it may use vacuum tubes or transistors.
The secondary circuit consists of little more than a huge coil of wire and some electrodes. This coil of wire is
located around the same region of space occupied by the inductor of the primary circuit. As the magnetic field
inside that inductor fluctuates up and down in strength, it induces current in the secondary coil. That's because a
changing magnetic field produces an electric field and the electric field surrounding the inductor pushes charges
around and around the secondary coil. By the time the charges in the secondary coil emerge from the coil, they
have enormous amounts of energy; making them very high voltage charges. They accumulate in vast numbers on
the electrodes of the secondary circuit and push one another off into the air as sparks.
While most circuits must form complete loops, the Tesla coil's secondary circuit doesn't. Its end electrodes just
spit charges off into space and let those charges fend for themselves. Many of them eventually work their ways
from one electrode to the other by flowing through the air or through objects. But even when they don't, there is
little net build up of charge anywhere. That's because the direction of current flow through the secondary coil
reverses frequently and the sign of the charge on each electrode reverses, too. The Tesla coil is a high-frequency
device and its top electrode goes from positively charged to negatively charge to positively charged millions of
times a second. This rapid reversal of charge, together with reversing electric and magnetic fields means that a
Tesla coil radiates strong electromagnetic waves. It therefore interferes with nearby radio reception.
Finally, it has been pointed out to me by readers that a properly built Tesla coil is resonant--that the high-voltage
coil has a natural resonance at the same frequency that it is being excited by the lower voltage circuit. The highvoltage coil's resonance is determined by its wire length, shape, and natural capacitance.
December 2, 1999
If a microwave oven with painted inside walls has some of the paint removed due to a very small fire caused by arcing, is
it still safe to use?
Yes. The paint is simply decoration on the metal walls. The cooking chamber of the microwave has metal walls
so that the microwaves will reflect around inside the chamber. Thick metal surfaces are mirrors for microwaves
and they work perfectly well with or without thin, non-conducting coatings of paint.
November 24, 1999
What is the difference between spark ignition engines and diesel engines? -- JC
Just before burning their fuels, both engines compress air inside a sealed cylinder. This compression process
adds energy to the air and causes its temperature to skyrocket. In a spark ignition engine, the air that's being
compressed already contains fuel so this rising temperature is a potential problem. If the fuel and air ignite
spontaneously, the engine will "knock" and won't operate at maximum efficiency. The fuel and air mixture is
expected to wait until it's ignited at the proper instant by the spark plug. That's why gasoline is formulated to
resist ignition below a certain temperature. The higher the "octane" of the gasoline, the higher its certified
ignition temperature. Virtually all modern cars operate properly with regular gasoline. Nonetheless, people
frequently put high-octane (high-test or premium) gasoline in their cars under the mistaken impression that their
cars will be better for it. If your car doesn't knock significantly with regular gasoline, use regular gasoline.
A diesel engine doesn't have spark ignition. Instead, it uses the high temperature caused by extreme compression
to ignite its fuel. It compresses pure air to high temperature and pressure, and then injects fuel into this air.
Timed to arrive at the proper instant, the fuel bursts into flames and burns quickly in the superheated compressed
air. In contrast to gasoline, diesel fuel is formulated to ignite easily as soon as it enters hot air.
November 23, 1999
What is the function of a magnet in an audio speaker? -- EB
An audio speaker generates sound by moving a surface back and forth through the air. Each time the surface
moves toward you, it compresses the air in front of it and each time the surface moves away from you, it rarefies
that air. By doing this repetitively, the speaker forms patterns of compressions and rarefactions in the air that
propagate forward as sound.
The magnet is part of the system that makes the surface move. Attached to the surface itself is a cylindrical coil
of wire and this coil fits into a cylindrical channel cut into the speaker's permanent magnet. That magnet is
carefully designed so that its magnetic field lines radiate outward from the inside of the channel to the outside of
the channel and thus pass through the cylindrical coil the way bicycle spokes pass through the rim of the wheel.
When an electric current is present in the wire, the moving electric charges circulate around this cylinder and cut
across the magnetic field lines. But whenever a charge moves across a magnetic field line, it experiences a force
known as the Lorenz force. In this case, the charges are pushed either into or out of the channel slot, depending
on which way they are circulating around the coil. The charges drag the coil and surface with them, so that as
current flows back and forth through the coil, the coil and surface pop in and out of the magnet channel. This
motion produces sound.
November 2, 1999
My science book said that a microwave oven uses a laser resonating at the natural frequency of water. Does such a laser
exist or was that a major typo?
It's a common misconception that the microwaves in a microwave oven excite a natural resonance in water. The
frequency of a microwave oven is well below any natural resonance in an isolated water molecule, and in liquid
water those resonances are so smeared out that they're barely noticeable anyway. It's kind of like playing a violin
under water--the strings won't emit well-defined tones in water because the water impedes their vibrations.
Similarly, water molecules don't emit (or absorb) well-defined tones in liquid water because their clinging
neighbors impede their vibrations.
Instead of trying to interact through a natural resonance in water, a microwave oven just exposes the water
molecules to the intense electromagnetic fields in strong, non-resonant microwaves. The frequency used in
microwave ovens (2,450,000,000 cycles per second or 2.45 GHz) is a sensible but not unique choice. Waves of
that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the
foods. Since leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications,
the frequency was chosen in part because it would not interfere with existing communication systems.
As for there being a laser in a microwave oven, there isn't. Lasers are not the answer to all problems and so the
source for microwaves in a microwave oven is a magnetron. This high-powered vacuum tube emits a beam of
coherent microwaves while a laser emits a beam of coherent light waves. While microwaves and light waves are
both electromagnetic waves, they have quite different frequencies. A laser produces much higher frequency
waves than the magnetron. And the techniques these devices use to create their electromagnetic waves are
entirely different. Both are wonderful inventions, but they work in very different ways.
The fact that this misleading information appears in a science book, presumably used in schools, is a bit
discouraging. It just goes to show you that you shouldn't believe everything read in books or on the web (even
this web site, because I make mistakes, too).
October 19, 1999
My four-year-old son was fooling around with a magnet, and when I was turned away, put it right on our TV screen. I
then saw him doing this, and before I could bring myself to think consequences, we were both mollified by the amazing
and colorful patterns it created on the screen. He sort of moved it around the screen, like you would an eraser on a black
board. Well, when he removed the magnet, the screen had been drained of its normally saturated colors, and what we now
have left is a color TV with only three colors, basically green, blue, and red. And they are not solid and deep like they
were before. They are rather faded, and arranged in three distinct blotches, if you will. Are we stuck with this situation
forever, or will this aberration fade with time, back to normal? And, why did this happen? -- E-S.B.
Your son has magnetized the shadow mask that's located just inside the screen of your color television. It's a
common problem and one that can easily be fixed by "degaussing" the mask (It'll take years or longer to fade on
its own, so you're going to have to actively demagnetize the mask). You can have it done professionally or you
can buy a degaussing coil yourself and give it a try (Try a local electronics store or contact MCM Electronics,
(800) 543-4330, 6" coil is item #72-785 for $19.95 and 12" coil is item #72-790 for $32.95).
Color sets create the impression of full color by mixing the three primary colors of light--blue, green, and red-right there on the inside surface of the picture tube. A set does the mixing by turning on and off three separate
electron beams to control the relative brightnesses of the three primary colors at each location on the screen. The
shadow mask is a metal grillwork that allows the three electrons beams to hit only specific phosphor dots on the
inside of the tube's front surface. That way, electrons in the "blue" electron beam can only hit blue-glowing
phosphors, while those in the "green" beam hit green-glowing phosphors and those in the "red" beam hit redglowing phosphors. The three beams originate at slightly different locations in the back of the picture tube and
reach the screen at slightly different angles. After passing through the holes in the shadow mask, these three
beams can only hit the phosphors of their color.
Since the shadow mask's grillwork and the phosphor dots must stay perfectly aligned relative to one another, the
shadow mask must be made of a metal that has the same thermal expansion characteristics as glass. The only
reasonable choice for the shadow mask is Invar metal, an alloy that unfortunately is easily magnetized. Your son
has magnetized the mask inside your set and because moving charged particles are deflected by magnetic fields,
the electron beams in your television are being steered by the magnetized shadow mask so that they hit the
wrong phosphors. That's why the colors are all washed out and rearranged.
To demagnetize the shadow mask, you should expose it to a rapidly fluctuating magnetic field that gradually
decreases in strength until it vanishes altogether. The degaussing coils I mentioned above plug directly into the
AC power line and act as large, alternating-field electromagnets. As you wave one of these coils around in front
of the screen, you flip the magnetization of the Invar shadow mask back and forth rapidly. By slowly moving this
coil farther and farther away from the screen, you gradually scramble the magnetizations of the mask's
microscopic magnetic domains. The mask still has magnetic structures at the microscopic level (this is
unavoidable and a basic characteristic of all ferromagnetic metals such as steel and Invar). But those domains
will all point randomly and ultimately cancel each other out once you have demagnetized the mask. By the time
you have the coil a couple of feet away from the television, the mask will have no significant magnetization left
at the macroscopic scale and the colors of the set will be back to normal.
Incidentally, I did exactly this trick to my family's brand new color television set in 1965. I had enjoyed watching
baseball games and deflecting the pitches wildly on our old black-and-white set. With only one electron beam, a
black-and-white set needs no shadow mask and has nothing inside the screen to magnetize. My giant super
alnico magnet left no lingering effect on it. But when the new set arrived, I promptly magnetized its shadow
mask and when my parent watched the "African Queen" that night, the colors were not what you'd call "natural."
The service person came out to degauss the picture tube the next day and I remember denying any knowledge of
what might have caused such an intense magnetization. He and I agreed that someone must have started a
vacuum cleaner very close to the set and thus magnetized its surface. I was only 8, so what did I know anyway.
Finally, as many readers have pointed out, many modern televisions and computer monitors have built-in
degaussing coils. Each time you turn on one of these units, the degaussing circuitry exposes the shadow mask to
a fluctuating magnetic field in order to demagnetize it. If your television set or monitor has such a system, then
turning it on and off a couple of times should clear up most or all of the magnetization problems. However, you
may have to wait about 15 minutes between power on/off cycles because the built-in degaussing units have
thermal protection that makes sure they cool down properly between uses.
I was recently riding as a passenger in a van and there was a housefly buzzing around in the van. While trying to squash
the fly, I was wondering why was the fly traveling the same speed as the van at 70 mph as it was hovering in mid air.
Shouldn't it have smashed into the rear window of the van just like so many bugs would have been, on the grill of the
vehicle?? -- DS
Flies travel at modest speeds relative to the air that surrounds them. Since the outside air is nearly motionless
relative to the ground (usually), a fly outside the van is also nearly motionless. When the fast-moving van
collides with the nearly motionless fly, the fly's inertia holds it in place while the van squashes it.
But when the fly is inside the van, the fly travels about in air that is moving with the van. If the van is moving at
70 mph, then so is the air inside it and so is the fly. In fact, everything inside the van moves more or less together
and from the perspective of the van and its contents, the whole world outside is what is doing the moving--the
van itself can be considered stationary and the van's contents are then also stationary.
As long as the fly and the air it is in are protected inside the van, the movement of the outside world doesn't
matter. The fly buzzes around in its little protected world. But if the van's window is open and the fly ventures
outside just as a signpost passes the car, the fly may get creamed by a collision with the "moving" sign.
Everything is relative and if you consider the van as stationary, then it is undesirable for the van's contents to get
hit by the moving items in the world outside (passing trees, bridge abutments, or oncoming vehicles.
October 12, 1999
If I knew the initial (exact) conditions of the throw of a die, could I throw a 6 with certainty? How does the Heisenberg
principle affect my ability to control the outcome? -- TW
In the classical view of the world, the view before the advent of quantum theory, nature seemed entirely
deterministic and mechanical. If you knew exactly where every molecule and atom was and how fast it was
moving, you could perfectly predict where it would be later on. In principle, this classical world would allow you
to throw a 6 every time. Of course, you'd have to know everything about the air's motion, the thermal energy in
the die, and even the pattern of light in the room. But the need for enormous amounts of information just means
that controlling the dice will be incredibly hard, not that it will be impossible. For simple throws, you could
probably get by without knowing all that much about the initial conditions. As the throws became more
complicated and more sensitive to initial conditions, you'd have to know more and more.
However, quantum mechanics makes controlling the die truly impossible. The problem stems from the fact that
position and velocity information are not fully defined at the same time in our quantum mechanical universe. In
short, you can't know exactly where a die is and how fast it is moving at the same time. And that doesn't mean
that you can't perform these measurements well. It means that the precise values don't exist together; they are
limited by Heisenberg uncertainty. So quantum physics imposes a fundamental limit on how well you can know
the initial conditions before your throw and it thus limits your ability to control the outcome of that throw. How
much quantum physics affects your ability to throw a 6 depends on the complexity of the throw. If you just drop
a die a few inches onto a table, you can probably get a 6 most of the time, despite quantum mechanics and
without even knowing much classical information. But as you begin throwing the die farther, you'll begin to lose
control of it because of quantum mechanics and uncertainty. In reality, you'll find classical physics so limiting
that you'll probably never observe the quantum physics problem. Knowing everything about a system is already
unrealistic, even in a classical universe. The problems arising from quantum mechanics are really just icing on
the cake for this situation.
September 25, 1999
I recently read a full-page ad for FREE ELECTRICITY from a company called United Services Company of America.
Their Website is at http://UCSofA.com/Free%20Electricity.htm. I walked through their site and viewed some of their
videos "demonstrating" clear violations of the well-known and well-founded Laws of Thermodynamics, and listened to
the description of the new Fourth Law of Motion (following Newton's other well known three). Are these people the
same who were denied patent approval for a Perpetual Motion Machine? Have any reputable independent test labs
reviewed their products under controlled conditions? Do they publish, even at a price, the fundamental mathematical and
physical processes that allow for the claims that seem to be shown? I realize you're not a "debunker", but maybe you can
shed some light on this. They have scheduled dozens of seminars across the country at considerable cost (and most likely
considerable profit to them), and taken out full-page ads in national newspapers. The speakers do not comment on their
academic training or experience, but tend to speak of hidden conspiracies from the power industry to stop their
proliferation of free power. -- DH
What a great find! This site is filled with pseudo-science at its best. I don't know the history or training of these
people, but it's pure garbage. They use the words of science but without any meaningful content. Just as putting
on a crown doesn't make you a king, using phrases like "action and reaction" and "Newton's third law" doesn't
mean that you are discussing real science.
I watched the video on the "Counter Rotation Device" and found the discussion of "Newton's Fourth Law of
Motion" quite amusing. The speaker claims that this fourth law was discovered about 30 years ago by a person
now at their research lab. It is based on Newton's third law, which the speaker simplifies to "for every action
there is an equal and opposite reaction." In a nutshell, his fourth law claims that you can take the reaction caused
by a particular action and apply it to the action in the same direction--action causes reaction which causes more
action which causes more reaction and so on. Pretty soon you have so much action and reaction that anything
becomes possible. The video goes on to show devices that yield more power than they consume and that can
easily become net sources of energy--by using part of the output energy from one of these energy multiplying
devices to power that device, you can create endless energy from nothing at all.
Sadly enough, it's all just nonsense. Newton's third law is not as flexible as the speaker supposes and this endless
feedback process in which reaction is used as action to produce more reaction is ridiculous. A more accurate
version of Newton's third law is: "Whenever one object pushes on a second object, the second object pushes back
on the first object equally hard but in the opposite direction". Thus when you push on the handle of a water
pump, that handle pushes back on you with an equal but oppositely directed force. The speaker's claim is that
there is a way to use the handle's push on you as part of your push on the handle so that, with your help, the
handle essentially pushes itself through action and reaction. You can then pump water almost without effort.
Sorry, this is just nonsense. It's mostly just playing with the words action and reaction in their common language
form: if you scare me, I react by jumping. That action and reaction has nothing to do with physics.
The speaker uses at least three clever techniques to make his claims more compelling and palatable. First, he
refers frequently to a power-company conspiracy that is out to destroy his company and its products. Conspiracy
theories are so popular these days that having a conspiracy against you makes you more believable. Second, he
describes the fellow who discovered the fourth law of motion as a basement inventor who has taken on the rigid
scientific establishment. Ordinary people love to see pompous, highly educated academics brought low by other
ordinary people; it's kind of a team spirit issue. And third, he makes casual use of technical looking equipment
and jargon, as though he is completely at ease in the world of advanced technology. Movies have made it easier
to trust characters like Doc Brown from "Back to the Future" than to trust real scientists.
In fact, there is no power-company conspiracy because there is no free electricity. The proof is in the pudding: if
these guys really could make energy from nothing, they'd be doing it every day and making a fortune. They
would be the power companies. If they were interested in public welfare rather than money, they'd have given
their techniques away already. If they were interested in proving the scientific establishment wrong, they'd have
accepted challenges by scientific organization and demonstrated their devices in controlled situations (where
they can't cheat). The fact is, they're just frauds and of no more interest to the power companies than snake oil
salespeople are to doctors. No decent people want to see others defrauded of money, property, or health, but the
free electricity people present no real threat to the power companies.
The popular notion that an ordinary person is likely to upset established science is an unfortunate product of the
anti-intellectual climate of our present world. Becoming a competent scientist is generally hard work and
requires dedication, time, and an enormous amount of serious thinking. Physics is hard, even for most physicists.
The laws governing the universe are slowly being exposed but it has taken very smart, very hardworking people
almost half a millennium to get to the current state of understanding. Each new step requires enormous effort and
a detailed understanding of a good part of the physics that is already known. Still, there is a common myth that
some clever and lucky individual with essentially no training or knowledge of what has been discovered before
will make some monumental breakthrough. The movies are filled with such events. Unfortunately, it won't
happen. In new or immature fields or subfields, it is possible for an essentially untrained or self-trained genius to
jump in and discover something important. Galileo and Newton probably fit this category in physics and Galois
and Ramanujan probably fit it in mathematics. But most of physics is now so mature that broad new discoveries
are rare, and accessible only to those with extremely good understandings of what is already known. A basement
tinkerer hasn't got a prayer.
Finally, real scientists don't always walk around in white lab coats looking serious, ridiculing the less educated,
and trying to figure out how to trick the government into funding yet another silly, fraudulent, or unethical
research project. In fact, most scientists wear practical clothes, have considerable humor, enjoy speaking with
ordinary folk about their science, and conduct that science because they love and believe in it rather than as a
means to some diabolic end. These scientists use the words of science in their conversations because it is the
appropriate language for their work and there is meaning in each word and each sentence. The gibberish spoken
by "scientists" in movies is often offensive to scientists in the same way that immigrant groups find it offensive
when people mock their native languages.
I don't know about any patent history for the free electricity organization but everyone should be aware that not
all patented items actually do what they're supposed to. In principle, the U.S. Patent Office only awards a patent
when it determines that a concept has not been patented previously, is not already known, is not obvious, and is
useful. The utility requirement should eliminate items that don't actually work. One of my readers, a patent
attorney, reports that he regularly invokes the utility regulation while escorting the "inventors" of impossible
devices such as "free electricity" to the door. They consider him part of the conspiracy against them, but he is
doing us all a service by keeping foolishness out of the patent system. However, proving that something doesn't
work often takes time and money, so sometimes nonfunctional items get patented. Thus a patent isn't always a
guarantee of efficacy. Patented nonsense is exactly that: nonsense.
Finally, how do I know that Free Electricity is really not possible? Couldn't I have missed something somewhere
in the details? No. The impossibility of this scheme is rooted in the very groundwork of physics; at the deepest
level where there is no possibility of mistake. For the counter rotation device to generate 15 kilowatts of
electricity out of nothing, it would have to be a net source of energy--the device would be creating energy from
nothing. That process would violate the conservation of energy, whereby energy cannot be created or destroyed
but can only be transferred from one object to another or converted from one form to another. Recognizing that
our universe is relativistic (it obeys the laws of special relativity), the actual conserved quantity is mass/energy,
but the concept is the same: you can't make mass/energy from nothing.
The origin of this conservation law lies in a mathematical theorem noted first by C. G. J. Jacobi and fully
developed by Emmy Noether, that each symmetry in the laws of physics gives rise to a conserved quantity. The
fact that a translation in space--shifting yourself from one place to another--does not change the laws of physics
gives rise to a conserved quantity: momentum. The fact that a rotation--changing the direction in which you are
facing--does not change the laws of physics gives rise to another conserved quantity: angular momentum. And
the fact that waiting a few minutes--changing the time at which you are--does not change the laws of physics
gives rise to a third conserved quantity: energy. The conservation of energy is thus intimately connected with the
fact that the laws of physics are the same today as they were yesterday and as they will be tomorrow.
Scientists have been looking for over a century for any changes in the laws of physics with translations and
rotations in space and with movement through time, and have never found any evidence for such changes. Thus
momentum, angular momentum, and energy are strictly conserved in our universe. For the counter rotation
device to create energy from nothing, all of physics would have to be thrown in the trashcan. The upset would be
almost as severe as discovering that 1+1 = 3. Furthermore, a universe in which physics was time-dependent and
energy was not conserved would be a dangerous place. Free electricity devices would become the weapons of the
future--bombs and missiles that released energy from nothing. Moreover, as the free electricity devices produced
energy from nothing, the mass/energy of the earth would increase and thus its gravitational field would also
increase. Eventually, the gravity would become strong enough to cause gravitational collapse and the earth
would become a black hole. Fortunately, this is all just science fiction because free electricity isn't real.
For more information about the "free electricity" hoax, sent in by readers of this site, touch here.
September 24, 1999
How can I make an electric generator from scratch? -- OD
Generators and motors are very closely related and many motors that contain permanent magnets can also act as
generators. If you move a permanent magnet past a coil of wire that is part of an electric circuit, you will cause
current to flow through that coil and circuit. That's because a changing magnetic field, such as that near a moving
magnet, is always accompanied in nature by an electric field. While magnetic fields push on magnetic poles,
electric fields push on electric charges. With a coil of wire near the moving magnet, the moving magnet's electric
field pushes charges through the coil and eventually through the entire circuit.
A convenient arrangement for generating electricity endlessly is to mount a permanent magnet on a spindle and
to place a coil of wire nearby. Then as the magnet spins, it will turn past the coil of wire and propel currents
through that coil. With a little more engineering, you'll have a system that looks remarkably like the guts of a
typical permanent magnet based motor. In fact, if you take a common DC motor out of a toy and connect its two
electrical terminals to a 1.5 V light bulb or a light emitting diode (try both directions with an LED because it can
only carry current in one direction), you'll probably be able to light that bulb or LED by spinning the motor's
shaft rapidly. A DC motor has a special switching system that converts the AC produced in the motor's coils into
DC for delivery to the motor's terminals, but it's still a generator. So the easiest answer to your question is: "find
a nice DC motor and turn its shaft".
If I wanted to magnetize a screwdriver, what would be the best way of doing this? I know it can be done by rubbing
magnets across the screwdriver's tip, but I would like to know a way of doing it with a piece of coiled wire and a battery.
I have heard that this can be done with a car battery. -- MS, West Virginia
Iron and most steels are intrinsically magnetic. By that, I mean that they contain intensely magnetic microscopic
domains that are randomly oriented in the unmagnetized metal but that can be aligned by exposure to an external
magnetic field. In pure iron, this alignment vanishes quickly after the external field is removed, but in the
medium carbon steel of a typical screwdriver, the alignment persists days, weeks, years, or even centuries after
the external field is gone.
To magnetize a screwdriver permanently, you should expose it briefly to a very strong magnetic field. Touching
the screwdriver's tip to one pole of a strong magnet will cause some permanent magnetization. Rubbing or
tapping the screwdriver also helps to free up its domains so that they can align with this external field. But the
better approach is to put the screwdriver in a coil of wire that carries a very large DC electric current.
The current only needs to flow for a fraction of a second--just long enough for the domains to align. A car
battery is a possibility, but it has safety problems: it can deliver an incredible current (400 amperes or more) for a
long time (minutes) and can overheat or even explode your coil of wire. Moreover, it may leak hydrogen gas,
which can be ignited by the sparks that will inevitably occur while you are magnetizing your screwdriver.
A safer choice for the current source is a charged electrolytic capacitor--a device that stores large quantities of
separated electric charge. A charged capacitor can deliver an even larger current than a battery can, but only for a
fraction of a second--only until the capacitor's store of separated charge is exhausted. Looking at one of my
hobbyist electronics catalogs, Marlin P. Jones, 800-652-6733, I'd pick a filter capacitor with a capacity of 10,000
microfarads and a maximum voltage of 35 volts (Item 12104-CR, cost: $1.50). Charging this device with three
little 9V batteries clipped together in a series (27 volts overall) will leave it with about 0.25 coulombs of
separated charge and just over 3.5 joules (3.5 watt-seconds or 3.5 newton-meters) of energy.
Make sure that you get the polarity right--electrolytic filter capacitors store separated electric charge nicely but
you have to put the positive charges and negative charges on the proper sides. [To be safe, work with rubber
gloves and, as a general rule, never touch anything electrical with more than one hand at a time. Remember that a
shock across your heart is much more dangerous than a shock across you hand. And while 27 volts is not a lot
and is unlikely to give you a shock under any reasonable circumstances, I can't accept responsibility for any
injuries. If you're not willing to accept responsibility yourself, don't try any of this.]
If you wrap about 100 turns of reasonably thick insulated wire (at least 18 gauge, but 12 gauge solid-copper
home wiring would be better) around the screwdriver and then connect one end of the coil to the positively
charged side of the capacitor and the other end of the coil to the negatively charged side, you'll get a small spark
(wear gloves and safety glasses) and a huge current will flow through the coil. The screwdriver should become
magnetized. If the magnetization isn't enough, repeat the charging-discharging procedure a couple of times,
always with the same connections so that the magnetization is in the same direction.
September 16, 1999
How fast do the electrons in copper flow when that copper is carrying electricity? -- LH, North Hollywood
It turns out that the electrons in copper travel quite slowly even though "electricity" travels at almost the speed of
light. That's because there are so many mobile electrons in copper (and other conductors) that even if those
electrons move only an inch per second, they comprise a large electric current. Picture the electrons as water
flowing through a pipe or river and now consider the Mississippi River. Even if the Mississippi is flowing only
inches per second, it sure carries lots of water past St. Louis each second.
The fact that electricity itself travels at almost the speed of light just means that when you start the electrons
moving at one end of a long wire, the electrons at the other end of the wire also begin moving almost
immediately. But that doesn't mean that an electron from your end of the wire actually reaches the far end any
time soon. Instead, the electrons behave like water in a long hose. When you start the water moving at one end, it
pushes on water in front of it, which pushes on water in front of it, and so on so that water at the far end of the
hose begins to leave the hose almost immediately. In the case of water, the motion proceeds forward at the speed
of sound. In a wire, the motion proceeds forward at the speed of light in the wire (actually the speed at which
electromagnetic waves propagate along the wire), which is only slightly less than the speed of light in vacuum.
September 13, 1999
Why do faster moving fluids have lower pressure? -- JH
Actually, faster moving fluids don't necessarily have lower pressure. For example, a bottle of compressed air in
the back of a pickup truck is still high-pressure air, even though it's moving fast. The real issue here is that when
fluid speeds up in passing through stationary obstacles, its pressure drops. For example, when air rushes into the
open but stationary mouth of a vacuum cleaner, that air experiences not only a rise in speed, it also experiences a
drop in pressure. Similarly, when water rushes out of the nozzle of a hose, its speed increases and its pressure
drops. This is simply conservation of energy: as the fluid gains kinetic energy, it must lose pressure energy.
However, if there are sources of energy around--fans, pumps, or moving surfaces--then these exchanges of
pressure for speed may no longer be present. That's why I put in the qualifier of there being only stationary
obstacles.
September 9, 1999
When you open your eyes underwater everything is blurry, but when you wear a mask, you can see clearly. Why can't the
eye focus underwater unless it has an air space, provided by the mask, in front of it? -- DW, Cork City, Ireland
Just as most good camera lenses have more than one optical element inside them, so your eye has more than one
optical element inside it. The outside surface of your eye is curved and actually acts as a lens itself. Without this
surface lens, your eye can't bring the light passing through it to a focus on your retina. The component in your
eye that is called "the lens" is actually the fine adjustment rather than the whole optical system.
When you put your eye in water, the eye's curved outer surface stops acting as a lens. That's because light travels
at roughly the same speed in water as it does in your eye and that light no longer bends as it enters your eye.
Everything looks blurry because the light doesn't focus on your retina anymore. But by inserting an air space
between your eye and a flat plate of glass or plastic, you recover the bending at your eye's surface and everything
appears sharp again.
September 3, 1999
I will be teaching first graders how to use simple magnifiers. What are the basic safety rules for magnifiers that I should
share with them with regard to sunlight, heat, etc. -- JR
The only source of common light source that presents any real danger to a child with a magnifying glass is the
sun. If you let sunlight pass through an ordinary magnifying glass, the convex lens of the magnifier will cause
the rays of sunlight to converge and they will form a real image of the sun a short distance after the magnifying
glass. This focused image will appear as a small, circular light spot of enormous brilliance when you let it fall
onto a sheet of white paper. It's truly an image--it's round because the sun is round and it has all the spatial
features that the sun does. If the image weren't so bright and the sun had visible marks on its surface, you'd see
those marks nicely in the real image.
The problem with this real image of the sun is simply that it's dazzlingly bright and that it delivers lots of thermal
power in a small area. The real image is there in space, whether or not you put any object into that space. If you
put paper or some other flammable substance in this focused region, it may catch on fire. Putting your skin in the
focus would also be a bad idea. And if you put your eye there, you're in serious trouble.
So my suggestion with first graders is to stay in the shade when you're working with magnifying glasses. As
soon as you go out in direct sunlight, that brilliant real image will begin hovering in space just beyond the
magnifying glass, waiting for someone to put something into it. And many first graders just can't resist the
opportunity to do just that.
September 1, 1999
How do you convert a measurement in liters per second into one in gallons per minute? -- MG
Converting units is always a matter of multiplying by 1. But you must use very fancy versions of 1, such as 60
seconds/1 minute and 1 gallon/3.7854 liters. Since 60 seconds and 1 minute are the same amount of time, 60
seconds/1 minute is 1. Similarly, since 1 gallon (U.S. liquid) and 3.7854 liters are the same amount of volume, 1
gallon/3.7854 liters is 1. So suppose that you have measured the flow of water through a pipe as 283
liters/second. You can convert to gallons/minute by multiplying 283 liters/second by 1 twice: (283
liters/second)(60 seconds/1 minute)(1 gallon/3.7854 liters). When you complete this multiplication, the liter units
cancel, the second units cancel, and you're left with 4,486 gallons/minute.
August 4, 1999
What is the device called in some watches that transforms the kinetic energy created by the watch's motion into energy to
help power the watch's battery? And how does such a device work? -- KW, Washington, DC
As a number of readers have informed me, the watches you're referring to generate electricity that then powers a
conventional electronic watch. These electromechanical watches use mechanical work done by wrist motions on
small weights inside the watches to generate electricity. Seiko's watch spins a tiny generator--a coil of wire
moves relative to a magnetic field and electric charges are pushed through the coil as a result. I have been told
that other watches exist that use piezoelectricity--the electricity that flows when certain mechanical objects are
deformed or strained--to generate their electricity. In any case, your wrist motion is providing the energy that
becomes electric power.
These electromechanical watches are the modern descendants of the automatic mechanical watches. An
automatic watch had a main spring that was wound by the motion of the wearer's hand. A small mass inside the
watch swung back and forth on the end of a lever. Because of its inertia, this mass resisted changes in velocity
and it moved relative to the watch body whenever the watch accelerated. If you like, you can picture the mass as
a ball that rolls about inside a wagon as you roll the wagon around an obstacle course. When the lever turned
back and forth relative to the watch body, the watch was able to extract energy from it. Gears attached to the
lever allowed the watch to use the mass's energy to wind its mainspring. The energy extracted from the mass
with each swing was very small, but it was enough to keep the mainspring fully wound. Ultimately, this energy
came from your hand--you did work on the watch in shaking it about and some of this energy eventually wound
up in the mainspring.
These same sorts of motions are what power the electromechanical watches of today. Instead of winding a
spring, your wrist motions swing weights about inside the watches and these moving weights spin generators to
produce electric power.
July 29, 1999
Is it possible to construct a capacitor capable of storing the energy in lightning, then allowing that energy to flow
gradually into the power grid?
Actually, the system of cloud and ground that produces lightning is itself a giant capacitor and the lightning is a
failure of that capacitor. Like all capacitors, the system consists of two charged surfaces separated by an
insulating material. In this case, the charged surfaces are the cloud bottom and the ground, and the insulating
material is the air. During charging, vast amounts of separated electric charge accumulate on the two surfaces-the cloud bottom usually becomes negatively charged and the ground below it becomes positively charge. These
opposite charges produce an intense electric field in the region between the cloud and the ground, and eventually
the rising field causes charge to begin flowing through the air: a stroke of lightning.
In principle, you could tap into a cloud and the ground beneath and extract the capacitor's charge directly with
wires. But this would be a heroic engineering project and unlikely to be worth the trouble. And catching a
lightning strike in order to charge a second capacitor is not likely to be very efficient: most of the energy released
during the strike would have to dissipate in the air and relatively little of it could be allowed to enter the
capacitor. That's because no realistic capacitor can handle the voltage in lightning.
Here's the detailed analysis. The power released during the strike is equal to the strike's voltage times its current:
the voltage between clouds and ground and the current flowing between the two during the strike. Voltage is the
measure of how much energy each unit of electric charge has and current is the measure of how many units of
electric charge are flowing each second. Their product is energy per second, which is power. Added up over
time, this power gives you the total energy in the strike. If you want to capture all this energy in your equipment,
it must handle all the current and all the voltage. If it can only handle 1% of the voltage, it can only capture 1%
of the strike's total energy.
While the current flowing in a lightning strike is pretty large, the voltage involved is astonishing: millions and
millions of volts. Devices that can handle the currents associated with lightning are common in the electric
power industry but there's nothing reasonable that can handle lightning's voltage. Your equipment would have to
let the air handle most of that voltage. The air would extract power from the flowing current in the lightning bolt
and turning it into light, heat, and sound. Your equipment would then extract only a token fraction of the stroke's
total energy. Finally, your equipment would have to prepare the energy properly for delivery on the AC power
grid--its voltage would have to be lowered dramatically and a switching system would have to convert the static
charge on the capacitors to an alternating flow of current in the power lines.
July 28, 1999
If I mix water and crushed ice, and allow them to sit in an insulated container for about 3 minutes, will their temperature
be 32 degrees Fahrenheit? -- MP, San Francisco
When he established his temperature scale, Daniel Gabriel Fahrenheit defined 32 degrees "Fahrenheit" (32 F) as
the melting temperature of ice--the temperature at which ice and water can coexist. When you assemble a
mixture of ice and water and allow them to reach equilibrium (by waiting, say, 3 minutes) in a reasonably
insulated container (something that does not allow much heat to flow either into or out of the ice bath), the
mixture will reach and maintain a temperature of 32 F. At that temperature and at atmospheric pressure, ice and
water are both stable and can coexist indefinitely.
To see why this arrangement is stable, consider what would happen if something tried to upset it. For example,
what would happen if this mixture were to begin losing heat to its surroundings? Its temperature would begin to
drop but then the water would begin to freeze and release thermal energy: when water molecules stick together,
they release chemical potential energy as thermal energy. This thermal energy release would raise the
temperature back to 32 F. The bath thus resists attempts at lowering its temperature.
Similarly, what would happen if the mixture were to begin gaining heat from its surroundings? Its temperature
would begin to rise but then the ice would begin to melt and absorb thermal energy: separating water molecules
increases their chemical potential energy and requires an input of thermal energy. This lost thermal energy would
lower the temperature back to 32 F. The bath thus resists attempts at raising its temperature.
So an ice/water bath self-regulates its temperature at 32 F. The only other quantities affecting this temperature
are the air pressure (the bath temperature could shift upward by about 0.003 degrees F during the low pressure of
a hurricane) and dissolved chemicals (half an ounce of table salt per liter of bath water will shift the bath
temperature downward by about 1 degree F).
July 26, 1999
The force of gravity decreases as we go down toward the center of the earth and becomes equalized at the center. So why
does pressure increase with depth, for example in the ocean? -- HN, Vancouver, British Columbia
It's true that the force of gravity decreases with depth, so that if you were to find yourself in a cave at the center
of the earth, you would be completely weightless. However, pressure depends on more than local gravity: it
depends on the weight of everything being supported overhead. So while you might be weightless, you would
still be under enormous pressure. Your body would be pushing outward on everything around you, trying to
prevent those things from squeezing inward and filling the space you occupy. In fact, your body would not
succeed in keeping those things away and you would be crushed by their inward pressure.
More manageable pressures surround us everyday. Our bodies do their part in supporting the weight of the
atmosphere overhead when we're on land or the weight of the atmosphere and a small part of the ocean when
we're swimming at sea. The deeper you go in the ocean, the more weight there is overhead and the harder your
body must push upward. Thus the pressure you exert on the water above you and the pressure that that water
exerts back on you increases with depth. Even though gravity is decreasing as you go deeper and deeper, the
pressure continues to increase. However, it increases a little less rapidly as a result of the decrease in local
gravity.
July 23, 1999
When you create lather from a piece of colored soap, why does it produce a white foam? -- CLV, Brasil
The foam consists of tiny air bubbles surrounded by very thin films of soap and water. When light enters the
foam, it experiences partial reflections from every film surface it enters or exits. That is because light undergoes
a partial reflection whenever it changes speed (hence the reflections from windows) and the speed of light in
soapy water is about 30% less than the speed of light in air. Although only about 4% of the light reflects at each
entry or exit surface, the foam contains so many films that very little light makes it through unscathed. Instead,
virtually all of the light reflects from film surfaces and often does so repeatedly. Since the surfaces are curved,
there is no one special direction for the reflections and the reflected light is scattered everywhere. And while an
individual soap film may exhibit colors because of interference between reflections from its two surfaces, these
interference effects average away to nothing in the dense foam. Overall, the foam appears white--it scatters light
evenly, without any preference for a particular color or direction. White reflections appear whenever light
encounters a dense collection of unoriented transparent particles (e.g. sugar, salt, clouds, sand, and the white
pigment particles in paint).
As for the fact that even colored soaps create only white foam, that's related to the amount of dye in the soaps. It
doesn't take much dye to give bulk soap its color. Since light often travels deep into a solid or liquid soap before
reflecting back to our eyes, even a modest amount of dye will selectively absorb enough light to color the
reflection. But the foam reflects light so effectively with so little soap that the light doesn't encounter much dye
before leaving the lather. The reflection remains white. To produce a colored foam, you would have to add so
much dye to the soap that you'd probably end up with colored hands as well.
July 20, 1999
How certain can I be that modern physics applies to distant places? Shouldn't I wait until reputable scientists have
performed experiments way off in outer space? -- JS
Fortunately, you don't have to wait that long. From astronomical observations, we are fairly certain that the laws
of physics as we know them apply throughout the visible universe. It wouldn't take large changes in the physical
laws to radically change the structures of atoms, molecules, stars, and galaxies. So the fact that the light and
other particles we see coming from distant places is so similar to what we see coming from nearby sources is
pretty strong evidence that the laws of physics don't change with distance. Also, the fact that the light we see
from distant sources has been traveling for a long time means that the laws of physics don't seem to have
changed much (if at all) with time, either. While there are theories that predict subtle but orderly changes in the
laws of physics with time and location, effectively making those laws more complicated, no one seriously thinks
that the laws of physics change radically and randomly from place to place in the Universe.
How can a spring "remember" its position? When I stretch a spring or compress a spring it returns to basically the same
size. What is it about the atoms/molecules that make up a spring that allows it to return to its original state? -- JH
Nearly all metals are crystalline, meaning that their atoms are arranged in neat and orderly stacks, like the piles
of oranges or soup cans at the grocery store or the cannonballs at the courthouse square. When you bend a metal,
its crystals can deform either by changing the spacings between atoms or by letting those atoms slide past one
another as great moving sheets of atoms. When the atoms keep their relative orientations but change their
relative spacings, the deformation is called elastic. When the atom sheets slide about and move, the deformation
is called plastic.
Metals that bend permanently are experiencing plastic deformation. Their atoms change their relative
orientations during the bend and they lose track of where they were. Once plastic deformation has occurred, the
metal can't remember how to get back to its original shape and stays bent.
Metals that bend only temporarily and return to their original shape when freed from stress are experiencing
elastic deformation. Their sheets of atoms aren't sliding about and they can easily spring back to normal when the
stresses go away. Naturally, springs are made from materials that experience only elastic deformation in normal
circumstances. Hardened metals such as spring steel are designed and heat-treated so that the atomic sliding
processes, known technically as "slip," are inhibited. When you bend them and let go, they bounce back to their
original shapes. But if you bend them too far, they either experience plastic deformation or they break.
Non-crystalline materials such as glass also make good springs. But since these amorphous materials have no
orderly rows of atoms, they can't experience plastic deformation at all. They behave as wonderful springs right
up until you bend them too far. Then, instead of experience plastic deformation and bending permanently, they
simply crack in two.
One last detail: there are a few exotic materials that undergo complicated deformations that are neither temporary
nor permanent. With changes in temperature, these shape memory materials can recover from plastic
deformation and spring back to their original shapes.
May 19, 1999
What is a superconductor? -- PG
A superconductor is a material that carries electric current without any loss of energy. Currents lose energy as
they flow through normal wires. This energy loss appears as a voltage drop across the material--the voltage of
the current as it enters the material is higher than the voltage of the current when it leaves the material. But in a
superconductor, the current doesn't lose any voltage at all. As a result, currents can even flow around loops
without stopping. Currents are magnetic and superconducting magnets are based on the fact that once you get a
current flowing around a loop of superconductor, it keeps going forever and so does its magnetism.
May 17, 1999
If light has no mass, then how can it be affected by gravity? What property of light is gravitational force acting on? -- DM
At low speeds, mass and energy appear to be separate quantities. Mass is the measure of inertia and can be
determined by shaking an object. Energy is the measure of how much work an object can do and can be
determined by letting it do that work. Conveniently enough, the object's weight--the force gravity exerts on it--is
exactly proportional to its mass, which is why people carelessly interchange the words "mass" and "weight,"
even though they mean different things.
But when something is moving at speeds approaching the speed of light, mass and kinetic energy no longer
separate so easily. In fact, the relativistic equations of motion are more complicated than those describing slow
objects and the way in which gravity affects fast objects is more complicated than simply giving them "weight."
Overall, you can view the bending of light by gravity in one of two ways. First, you can view it as gravity
affecting not on mass, but also energy so that light falls because its energy gives it something equivalent to a
"weight." Second, you can view the bending of light as caused by a change in the shape of space and time around
a gravitating object. Space is curved, so that light doesn't travel straight as it moves past gravitating objects--it
follows the curves of space itself. The latter view is most correct, but a little disconcerting as well. That's why it
took some time for the theory of general relativity to be widely accepted.
March 30, 1999
After a party at work, a friend tied a helium balloon to his car's gearshift lever and drove off. As he started driving
forward, the balloon first went forward and then backward. That's not what happens to everything else. Why does it
happen for the helium balloon? -- S
The helium balloon is the least dense thing in the car and is responding to forces exerted on it by the air in the
car. To understand this, consider what happens to you, the air, and finally the helium balloon as the car first starts
to accelerate forward.
When the car starts forward, inertia tries to keep all of the objects in the car from moving forward. An object at
rest tends to remain at rest. So the car must push you forward in order to accelerate you forward and keep you
moving with the car. As the car seat pushes forward on you, you push back on the car seat (Newton's third law)
and dent its surface. Your perception is that you are moving backward, but you're not really. You're actually
moving forward; just not quite as quickly as the car itself.
The air in the car undergoes the same forward acceleration process. Its inertia tends to keep it in place, so the car
must push forward on it to make it accelerate forward. Air near the front of the car has nothing to push it forward
except the air near the back of the car, so the air in the front of the car tends to "dent" the air in the back of the
car. In effect, the air shifts slightly toward the rear of the car. Again, you might think that this air is going
backward, but it's not. It's actually moving forward; just not quite as quickly as the car itself.
Now we're ready for the helium balloon. Since helium is so light, the helium balloon is almost a hollow,
weightless shell that displaces the surrounding air. As the car accelerates forward, the air in the car tends to pile
up near the rear of the car because of its inertia. If the air can push something out of its way to get more room
near the rear of the car, it will. The helium balloon is that something. As inertia causes the air to drift toward the
rear of the accelerating car, the nearly massless and inertialess helium balloon is squirted toward the front of the
car to make more room for the air. There is actually a horizontal pressure gradient in the car's air during forward
acceleration, with a higher pressure at the rear of the car than at the front of the car. This pressure gradient is
ultimately what accelerates the air forward with the car and it's also what propels the helium balloon to the front
of the car.
Finally, when the car is up to speed and stops accelerating forward, the pressure gradient vanishes and the air
returns to its normal distribution. The helium balloon is no longer squeezed toward the front of the car and it
floats once again directly above the gear shift.
One last note: OGT from Lystrup, Denmark points out that when you accelerate a glass of beer, the rising
bubbles behave in the same manner. They move toward the front of the glass as you accelerate it forward and
toward the back of the glass as you bring it to rest.
March 29, 1999
My third grade art class was wondering what color things would be if there was no sunlight? -- Mrs. P's class
Most objects make no light of their own and are visible only because they reflect some of the light that strikes
them. Without sunlight (or any other light source), these passive objects would appear black. Black is what we
"see" when there is no light reaching our eyes from a particular direction. The only objects we would see would
be those that made their own light and sent it toward our eyes.
The fact that we see mostly reflected light makes for some interesting experiments. A red object selectively
reflects only red light; a blue object reflects only blue light; a green object reflects only green light. But what
happens if you illuminate a red object with only blue light? The answer is that the object appears black! Since it
is only able to reflect red light, the blue light that illuminates it is absorbed and nothing comes out for us to see.
That's why lighting is so important to art. As you change the illumination in an art gallery, you change the
variety of lighting colors that are available for reflection. Even the change from incandescent lighting to
fluorescent lighting can dramatically change the look of a painting or a person's face. That's why some makeup
mirrors have dual illumination: incandescent and fluorescent.
The one exception to this rule that objects only reflect the light that strikes them is fluorescent objects. These
objects absorb the light that strikes them and then emit new light at new colors. For example, most fluorescent
cards or pens will absorb blue light and then emit green, orange, or red light. Try exposing a mixture of artwork
and fluorescent objects to blue light. The artwork will appear blue and black: blue wherever the art is blue and
black wherever the art is either red, green, or black. But the fluorescent objects will display a richer variety of
colors because those objects can synthesize their own light colors.
March 28, 1999
Please explain the forces that allow one team to win a Tug-O-War contest. -- ES
If we neglect the mass of the rope, the two teams always exert equal forces on one another. That's simply an
example of Newton's third law--for every force team A exerts on team B, there is an equal but oppositely
directed force exerted by team B on team A. While it might seem that these two forces on the two teams should
always balance in some way so that the teams never move, that isn't the case. Each team remains still or
accelerates in response to the total forces on that team alone, and not on the teams as a pair. When you consider
the acceleration of team A, you must ignore all the forces on team B, even though one of those forces on team B
is caused by team A. There are two important forces on team A: (1) the pull from team B and (2) a force of
friction from the ground. That force of friction approximately cancels the pull from the team B because the two
forces are in opposite horizontal directions. As long as the two forces truly cancel, team A won't accelerate. But
if team A doesn't obtain enough friction from the ground, it will begin to accelerate toward team B. The winning
team is the one that obtains more friction from the ground than it needs and accelerates away from the other
team. The losing team is the one that obtains too little friction from the ground and accelerates toward the other
team.
March 27, 1999
How is a diode different from a piece of ordinary wire? -- R
An ordinary wire will carry electric current in either direction, while a diode will only carry current in one
direction. That's because the electric charges in a wire are free to drift in either direction in response to electric
forces but the charges in a diode pass through a one-way structure known as a p-n junction. Charges can only
approach the junction from one side and leave from the other. If they try to approach from the wrong side, they
discover that there are no easily accessible quantum mechanical pathways or "states" in which they can travel.
Sending the charges toward the p-n junction from the wrong side can only occur if something provides the extra
energy needed to reach a class of less accessible quantum mechanical states. Light can provide that extra energy,
which is why many diodes are light sensitive--they will conduct current in the wrong direction when exposed to
light. That is the basis for many light sensitive electronic devices and for most photoelectric or "solar" cells.
March 18, 1999
Can you please tell me why two different amounts of heated water cool at the same rate? My second grade daughter and I
took boiling water from the same pot and placed it in two different size Pyrex bowls. We measured the temperature of the
water in each bowl every five minutes. The temperature drop was the same for each amount of water. -- JT
The amount of hot water that's cooling doesn't necessarily determine which bowl of water will cool fastest. That
depends on how quickly each gram of the hot water loses heat, a rate that depends both on how much hotter the
water is than its surroundings and on how that water is exposed to those surroundings. In general, hot water loses
heat through its surface so the more surface that's exposed, the faster it will lose heat. But surface that's exposed
to air will lose heat via evaporation and will be particularly important in cooling the water.
In answer to your question, my guess is that the larger bowl of water also exposes much more of that water to the
air. Although the larger bowl had more water in it, it allowed that water to exchange heat faster with its
environment. If the larger bowl contained twice as much water but let that water lose heat twice as fast, the two
bowls would maintain equal temperatures. If you want to see the effect of thermal mass in slowing the loss of
temperature, you'll need to control heat loss. Try letting equal amounts of hot water cool in two identical
containers--one wrapped in insulation and covered with clear plastic wrap (to prevent evaporation) and one open
to the air. You'll see a dramatic change in cooling rate. And if you want to compare unequal amounts of water,
use two indentical containers that are only exposed to the cooler environment through a controlled amount of
surface area. For example, try two identical insulated cups, one full of water and one only half full. If both lose
heat only through their open tops, the full cup should cool more slowly than the half full cup.
February 18, 1999
My 5 year old wants to do his kindergarten science project on "why do balls bounce?" His hypothesis is that "balls
bounce because of the stuff inside." Can you advise how best to test this hypothesis and explain this concept on a level
that a bright, but still only 5 year old, can truly understand? -- MS, Bayside, New York
I'd suggest finding a hollow rubber ball with a relatively thin, flexible skin and putting different things inside it.
You can just cut a small hole and tape it over after you put in "the stuff." Compare the ball's bounciness when it
contains air, water, shaving cream, beans, rice, and so on. Just drop it from a consistent height and see how high
it rebounds. The ratio of its rebound height to its drop height is a good measure of how well the ball stores
energy when it hits the ground and how well it uses that energy to rebound. A ball that bounces to full height is
perfect at storing energy while a ball that doesn't bounce at all is completely terrible at storing energy. You'll get
something in between for most of your attempts--indicating that "the stuff" is OK but not perfect at storing
energy during the bounce. The missing energy isn't destroyed, it's just turned into thermal energy. The ball gets a
tiny bit hotter with every bounce.
You won't get any important quantitative results from this sort of experiment, but it'll be fun anyway. I wonder
what fillings will make the ball bounce best or worst?
I saw a magic show where they put a needle through a balloon. I tried this and it worked, but only with latex material
balloons. I want to do my science project on this but my teacher said it was not a good idea. I think that it is because it is
science, not magic. What do you think? -- J, 6th Grade
It is science. The needle is able to enter latex without tearing it because the latex molecules are stretching out of
the way of the needle without breaking. Like all polymers (plastics), latex consists of very large molecules. In
latex, these molecules are basically long chains of atoms that are permanently linked to one another at various
points along their lengths. You can picture a huge pile of spaghetti with each pasta strand representing one latex
molecule. Now picture little links connecting pairs of these strands at random, so that when you try to pick up
one strand, all the other strands come with it. That's the way latex looks microscopically. You can't pull the
strands of latex apart because they are all linked together. But you can push a spoon between the strands.
That is what happens when you carefully weave a needle into a latex balloon--the needle separates the polymer
strands locally, but doesn't actually pull them apart or break them. Since breaking the latex molecules will
probably cause the balloon to tear and burst, you have to be very patient and use a very sharp needle. I usually oil
the needle before I do this and I don't try to insert the needle in the most highly stressed parts of the balloon. The
regions near the tip of the balloon and near where it is filled are the least stressed and thus the easiest to pierce
successfully with a needle. A reader has informed me that coating the needle with Vasoline is particularly
helpful.
One final note: a reader pointed out that it is also possible to put a needle through a balloon with the help of a
small piece of adhesive tape. If you put the tape on a patch of the inflated balloon, it will prevent the balloon
from ripping when you pierce the balloon right through the tape. This "cheaters" approach is more reliable than
trying to thread the needle between the latex molecules, but it's less satisfying as well. But it does point out the
fact that a balloon bursts because of tearing and that if you prevent the balloon from tearing, you can pierce it as
much as you like.
February 17, 1999
How does a dehumidifier work? - S, Hong Kong
A dehumidifier makes use of the fact that water tends to be individual gas molecules in the air at higher
temperatures but condensed liquid molecules on surfaces at lower temperatures. At its heart, a dehumidifier is
basically a heat pump, one that transfers heat from one surface to another. Its components are almost identical to
those in an air conditioner or refrigerator: a compressor, a condenser, and an evaporator. The evaporator acts as
the cold surface, the source of heat, and the condenser acts as the hot surface, the destination for that heat.
When the unit is operating and pumping heat, the evaporator becomes cold and the condenser becomes hot. A
fan blows warm, moist air from the room through the evaporator coils and that air's temperature drops. This
temperature drop changes the behavior of water molecules in the air. When the air and its surroundings were
warm, any water molecule that accidentally bumped into a surface could easily return to the air. Thus while
water molecules were always landing on surfaces or taking off, the balance was in favor of being in the air. But
once the air and its surroundings become cold, any water molecules that bump into a surface tend to stay there.
Water molecules are still landing on surfaces and taking off, but the balance is in favor of staying on the surface
as either liquid water or solid ice. That's why dew or frost form when warm moist air encounters cold ground. In
the dehumidifier, much of the air's water ends up dripping down the coils of the evaporator into a collection
basin.
All that remains is for the dehumidifier to rewarm the air. It does this by passing the air through the condenser
coils. The thermal energy that was removed from the air by the evaporator is returned to it by the condenser. In
fact, the air emerges slightly hotter than before, in part because it now contains all of the energy used to operate
the dehumidifier and in part because condensing moisture into water releases energy. So the dehumidifier is
using temperature changes to separate water and air.
February 9, 1999
As part of Math and Science night at her school, my 4th grade daughter recently made ice cream. How did the milk, ice,
salt, and mechanical motion work together to make ice cream? -- DH
To make good ice cream, you want to freeze the cream in such a way that the water in the cream forms only very
tiny ice crystals. That way the ice cream will taste smooth and creamy. The simplest way to achieve this goal is
to stir the cream hard while lowering its temperature far enough to freeze the water in it and to make the fat
solidify as well. That's where the ice and salt figure in.
By itself, melting ice has a temperature of 0° C (32° F). When heat flows into ice at that temperature, the ice
doesn't get hotter, it just transforms into water at that same temperature. Separating the water molecules in ice to
form liquid water takes energy and so heat must flow into the ice to make it melt.
But if you add salt to the ice, you encourage the melting process so much that the ice begins to use its own
internal thermal energy to transform into water. The temperature of the ice drops well below 0° C (32° F) and yet
it keeps melting. Eventually, the drop in temperature stops and the ice and salt water reach an equilibrium, but
the mixture is then quite cold--perhaps -10° C (14° F) or so. To melt more ice, heat must flow into the mixture.
When you place liquid cream nearby, heat begins to flow out of the cream and into the ice and salt water. More
ice melts and the liquid cream get colder. Eventually, ice cream starts to form. Stirring keeps the ice crystals
small and also ensures that the whole creamy liquid freezes uniformly.
January 28, 1999
What properties of rubber change in order to make one ball bounce better than another? -- JM
During a bounce from a rigid surface, the ball's surface dents. Denting a surface takes energy and virtually all of
the ball's energy of motion (kinetic energy) goes into denting its own surface. For a moment the ball is
motionless and then it begins to rebound. As the ball undents, it releases energy and this energy becomes the
ball's new energy of motion.
The issue is in how well the ball's surface stores and then releases this energy. The ideal ball experiences only
elastic deformation--the molecules within the ball do not reorganize at all, but only change their relative spacings
during the dent. If the molecules reorganize--sliding across one another or pulling apart in places--then some of
the denting energy will be lost due to internal friction-like effects. Even if the molecules slide back to their
original positions, they won't recover all the energy and the ball won't bounce to its original height.
In general, harder rubber bounces more efficiently than softer rubber. That's because the molecules in hard
rubber are too constrained to be able to slide much. A superball is very hard and bounces well. But there are also
sophisticated thermal effects that occur in some seemingly hard rubbers that cause them to lose their stored
energy.
January 21, 1999
We know that ozone can be depleted in the atmosphere as a result of various man-made factors. What would happen if
nitrogen were depleted? What man-made influences, if any, would deplete nitrogen? -- BS, Los Angeles
Ozone is an unstable molecule that consists of three oxygen atoms rather than then usual two. Because of its
added complexity, an ozone molecule can interact with a broader range of light wavelengths and has the
wonderful ability to absorb harmful ultraviolet light. The presence of ozone molecules in our upper atmosphere
makes life on earth possible.
However, because ozone molecules are chemically unstable, they can be depleted by contaminants in the air.
Ozone molecules react with many other molecules or molecular fragments, making ozone useful as a bleach and
a disinfectant. Molecules containing chlorine atoms are particularly destructive of ozone because a single
chlorine atom can facilitate the destruction of many ozone molecules through a chlorine recycling process.
In contrast, nitrogen molecules are extremely stable. They are so stable that there are only a few biological
systems that are capable of separating the two nitrogen atoms in a nitrogen molecule in order to create organic
nitrogen compounds. Without these nitrogen-fixing organisms, life wouldn't exist here. Because nitrogen
molecules are nearly unbreakable, they survive virtually any amount or type of chemical contamination.
January 20, 1999
Is the total energy savings still significant for long tube fluorescent lights, as compared to incandescent lights, when you
consider the energy involved in manufacturing all the components of the lights? -- AB, San Antonio, TX
Yes, fluorescents are more energy efficient overall. To begin with, fluorescent lights have a much longer life than
incandescent lights--the fluorescent tube lasts many thousands of hours and its fixture lasts tens of thousands of
hours. So the small amount of energy spent building an incandescent bulb is deceptive--you have to build a lot of
those bulbs to equal the value of one fluorescent system.
Second, although there is considerable energy consumed in manufacturing the complicated components of a
fluorescent lamp, it's unlikely to more than a few kilowatt-hours--the equivalent of the extra energy a 100 watt
incandescent light uses up in a week or so of typical operation. So it may take a week or two to recover the
energy cost of building the fluorescent light, but after that the energy savings continue to accrue for years and
years.
January 19, 1999
If you were at the back of a bus going the speed of light, and you were to run toward the front, would you be moving
faster than the speed of light or turn into energy? -- TM, Ft. Bragg, NC
First, your bus can't be going at the speed of light because massive objects are strictly forbidden from traveling at
that speed. Even to being traveling near the speed of light would require a fantastic expenditure of energy.
But suppose that the bus were traveling at 99.999999% of the speed of light and you were to run toward its front
at 0.000002% of the speed of light (about 13 mph or just under a 5 minute mile). Now what would happen?
First, the bus speed I quoted is in reference to some outside observer because the seated passengers on the bus
can't determine its speed. After all, if the shades are pulled down on the bus and it's moving at a steady velocity,
no one can tell that it's moving at all. So let's assume that the bus speed I gave is according to a stationary friend
who is watching the bus zoom by from outside.
While you are running toward the front of the bus at 0.000002% of the speed of light, your speed is in reference
to the other passengers in the bus, who see you moving forward. The big question is what does you stationary
friend see? Actually, your friend sees you running toward the front of the bus, but determines that your personal
speed is only barely over 99.999999%. The two speeds haven't added the way you'd expect. Even though you
and the bus passengers determine that you are moving quickly toward the front of the bus, your stationary friend
determines that you are moving just the tiniest bit faster than the bus. How can that be?
The answer lies in the details of special relativity, but here is a simple, albeit bizarre picture. Your stationary
friend sees a deformed bus pass by. Ignoring some peculiar optical effects due to the fact that it takes time for
light to travel from the bus to your friend's eyes so that your friend can see the bus, your friend sees a
foreshortened bus--a bus that is smashed almost into a pancake as it travels by. While you are in that pancake,
running toward the front of the bus, the front is so close to the rear that your speed within the bus is miniscule.
Why the bus becomes so short is another issue of special relativity.
January 18, 1999
How does a heat pipe work? -- SG, Sugar Land TX
Heat pipes use evaporation and condensation to move heat quickly from one place to another. A typical heat pipe
is a sealed tube containing a liquid and a wick. The wick extends from one end of the tube to the other and is
made of a material that attracts the liquid--the liquid "wets" the wick. The liquid is called the "working fluid" and
is chosen so that it tends to be a liquid the temperature of the colder end of the pipe and tends to be a gas at the
temperature of the hotter end of the pipe. Air is removed from the pipe so the only gas it contains is the gaseous
form of the working fluid.
The pipe functions by evaporating the liquid working fluid into gas at its hotter end and allowing that gaseous
working fluid to condense back into a liquid at its colder end. Since it takes thermal energy to convert a liquid to
a gas, heat is absorbed at the hotter end. And because a gas gives up thermal energy when it converts from a gas
to a liquid, heat is released at the colder end.
After a brief start-up period, the heat pipe functions smoothly as a rapid conveyor of heat. The working fluid
cycles around the pipe, evaporating from the wick at the hot end of the pipe, traveling as a gas to the cold end of
the pipe, condensing on the wick, and then traveling as a liquid to the hot end of the pipe.
Near room temperature, heat pipes use working fluids such as HFCs (hydrofluorocarbons, the replacements for
Freons), ammonia, or even water. At elevated temperatures, heat pipes often use liquid metals such as sodium.
January 15, 1999
How is sound picked up on a microphone? -- PB, Marion, MA
Sound consists of small fluctuations in air pressure. We hear sound because these changes in air pressure
produce fluctuating forces on various structures in our ears. Similarly, microphones respond to the changing
forces on their components and produce electric currents that are effectively proportional to those forces.
Two of the most common types of microphones are capacitance microphones and electromagnetic microphones.
In a capacitance microphone, opposite electric charges are placed on two closely spaced surfaces. One of those
surfaces is extremely thin and moves easily in response to changes in air pressure. The other surface is rigid and
fixed. As a sound enters the microphone, the thin surface vibrates with the pressure fluctuations. The electric
charges on the two surfaces pull on one another with forces that depend on the spacing of the surfaces. Thus as
the thin surface vibrates, the charges experience fluctuating forces that cause them to move. Since both surfaces
are connected by wires to audio equipment, charges move back and forth between the surfaces and the audio
equipment. The sound has caused electric currents to flow and the audio equipment uses these currents to record
or process the sound information.
In an electromagnetic microphone, the fluctuating air pressure causes a coil of wire to move back and forth near
a magnet. Since changing or moving magnetic fields produce electric fields, electric charges in the coil of wire
begin to move as a current. This coil is connected to audio equipment and again uses these currents to represent
sound.
January 14, 1999
Why does air speed up as it flows over an airplane wing? -- MS
When air flows past an airplane wing, it breaks into two airstreams. The one that goes under the wing encounters
the wing's surface, which acts as a ramp and pushes the air downward and forward. The air slows somewhat and
its pressure increases. Forces between this lower airstream and the wing's undersurface provide some of the lift
that supports the wing.
But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing
and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper
airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the
curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a
common behavior in fluids--viscosity and friction keep them flowing along surfaces as long as they don't have to
turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly,
blame it on the Coanda effect.)
Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface.
This inward bending involves an inward acceleration that requires an inward force. That force appears as the
result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top
surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the
means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc
inward and follow the plane's top surface).
The low pressure region above the wing helps to support the plane because it allows air pressure below the wing
to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With
more pressure behind it than in front of it, the airstream accelerates--it's pushed forward by the pressure
imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it
approaches the wing's trailing edge--a complicated process that produces a small amount of turbulence on even
the most carefully designed wing.
In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in
air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over
much of the wing.
January 13, 1999
I tried freezing two cups of water, one with salt added and one with sugar added, to see which would freeze first. I
conducted my experiment three times and each time the sugar water froze first. Why? -- AM
Dissolving solids in water always lowers the water's freezing temperature by an amount that's proportional to the
density of dissolved particles. If you double the density of particles in water, you double the amount by which
the freezing temperature is lowered.
While salt and sugar both dissolve in water and thus both lower its freezing temperature, salt is much more
effective than sugar. That's because salt produces far more dissolved particles per pound or per cup than sugar.
First, table salt (sodium chloride) is almost 40% more dense than cane sugar (sucrose), so that a cup of salt
weighs much more than a cup of cane sugar. Second, a salt molecule (NaCl) weighs only about 8.5% as much as
a sucrose molecule (C12H22O11), so there are far more salt molecules in a pound of salt than sugar molecules in a
pound of sugar. Finally, when salt dissolves in water, it decomposes into ions: Na+ and Cl-. That decomposition
doubles the density of dissolved particles produced when salt dissolves. Sugar molecules remain intact when
they dissolve, so there is no doubling effect. Thus salt produces a much higher density of dissolved particles than
sugar, whether you compare them cup for cup or pound for pound, and thus lowers water's freezing temperature
more effectively. That's why the salt water is so slow to freeze.
January 12, 1999
How do the automatic soda dispensers at fast food joints know when the cup is full? -- MB
They measure the volume of liquid they deliver and shut off when they have dispensed enough soda to fill the
cup. Accurate volumetric flowmeters, such as those used in the dispensers, typically have a sophisticated
paddlewheel assembly inside that turns as the liquid goes through a channel. When the paddlewheel has gone
around the right number of times, an electronic valve closes to stop the flow of liquid.
January 11, 1999
Is there any mathematical relevance to the period of motion of a pendulum? For example, if I made a scale model of a
pendulum and then squared it or cubed it, would there be any mathematical correlation between the results?
Yes, there would be a simple relationship between the periods of the three pendulums. That's because the period
of a pendulum depends only on its length and on the strength of gravity. Since a pendulum's period is
proportional to the square root of its length, you would have to make your model four times as long to double the
time it takes to complete a swing. A typical grandfather's clock has a 0.996-meter pendulum that takes 2 seconds
to swing, while a common wall clock has a 0.248-meter pendulum that takes 1 second to swing. Note that the
effective length of the pendulum is from its pivot to its center of mass or center of gravity. A precision pendulum
has special temperature compensating components that make sure that this effective length doesn't change when
the room's temperature changes.
January 8, 1999
Since a typical commercial jetliner cruises at around 30,000 feet (higher than Mt. Everest), where the air is very rarified,
is there a mechanism to concentrate the air around the engine intake? -- P
There certainly is such a mechanism. The air at a jetliner's cruising altitude is much too thin to support life so it
must be compressed before introducing it into the airplane's passenger cabin. The compressed air is actually
extracted from an intermediate segment of the airplane's jet engines. In the course of their normal operations,
these engines collect air entering their intake ducts, compress that air with rotary fans, inject fuel into the
compressed air, burn the mixture, and allow the hot, burned gases to stream out the exhaust duct through a series
of rotary turbines. The turbines provide the power to operate the compressor fans. Producing the stream of
exhaust gas is what pushes the airplane forward.
But before fuel is injected into the engine's compressed air, there is a side duct that allows some of that
compressed air to flow toward the passenger cabin. So the engine is providing the air you breathe during a flight.
There is one last interesting point about this compressed air: It is initially too hot to breathe. Even though air at
30,000 feet is extremely cold, the act of compressing it causes its temperature to rise substantially. This happens
because compressing air takes energy and that energy must go somewhere in the end. It goes into the thermal
energy of the air and raises the air's temperature. Thus the compressed air from the engines must be cooled by air
conditioners before it goes into the passenger cabin.
January 7, 1999
I noticed that in your discussions of salted water in cooking, you never mentioned the main reason why people add salt to
water: it raises the boiling temperature of the water so that foods cook faster -- L
You are right that adding salt to water raises the water's boiling temperature. Contrary to one's intuition, adding
salt to water doesn't make it easier for the water to boil, it makes it harder. As a result, the water must reach a
higher temperature before it begins to boil. Any foods you place in this boiling salt water (e.g. eggs or pasta) find
themselves in contact with somewhat hotter water and should cook faster as a result. That's because most
cooking is limited by the boiling temperature of water in or around food and anything that lowers this boiling
temperature, such as high altitude, slows most cooking while anything that raises the boiling temperature of
water, such as salt or the use of a pressure cooker, speeds most cooking. However, it takes so much salt to raise
the boiling temperature of water enough to affect cooking times that this can't be the main motivation for
cooking in salted water. By the time you've salted the water enough to raise its boiling temperature more than a
few degrees, you've made the water too salty for cooking. It's pretty clear that salting your cooking water is
basically a matter of taste, not temperature.
January 6, 1999
If two planets were really close together and you were between them, how would the gravitational force affect you? -MB & Class
If you were directly between the two planets, their gravitational forces on you would oppose one another and at
least partially cancel. Which planet would exert the stronger force on you would depend on their relative masses
and on your distances from each of them. If one planet pulled on you more strongly than the other, you would
find yourself falling toward that planet even though the other planet's gravity would oppose your descent and
prolong the fall. However, there would also be a special location between the planets at which their gravitational
forces would exactly cancel. If you were to begin motionless at that point in space, you wouldn't begin to fall at
all. While the planets themselves would move and take the special location with them, there would be a brief
moment when you would be able to hover in one place.
But there is something I've neglected: you aren't really at one location in space. Because your body has a finite
size, the forces of gravity on different parts of your body would vary subtly according to their exact locations in
space. Such variations in the strength of gravity are normally insignificant but would become important if you
were extremely big (e.g. the size of the moon) or if the two planets you had in mind were extremely small but
extraordinarily massive (e.g. black holes or neutron stars). In those cases, spatial variations in gravity would tend
to pull unevenly on your body parts and might cause trouble. Such uneven forces are known as tidal forces and
are indeed responsible for the earth's tides. While the tidal forces on a spaceship traveling between the earth and
the moon would be difficult to detect, they would be easy to find if the spaceship were traveling between two
small and nearby black holes. In that case, the tidal forces could become so severe that they could rip apart not
only the spaceship and its occupants, but also their constituent molecules, atoms, and even subatomic particles.
January 5, 1999
I have been trying to get information on what causes strange gravity areas to exist...Walking on walls, water rolling
uphill, etc. There are a number of such places advertised in the United States and elsewhere but are they optical illusions
or for real? -- MW
These purported gravitational anomalies are just illusions. Because gravity is a relatively weak force, enormous
concentrations of mass are required to create significant gravitational fields. Since it takes the entire earth to give
you your normal weight, the mass concentration needed to cancel or oppose the earth's gravitation field in only
one location would have to be extraordinary. While objects capable of causing such bizarre effects do exist
elsewhere in our universe (e.g. black holes and neutron stars), there fortunately aren't any around here. As a
result, the strength of the gravitational field at the earth's surface varies less than 1% over the earth's surface and
always points almost exactly toward the center of the earth. Any tourist attraction that claims to have gravity
pointing in some other direction with some other strength is claiming the impossible.
October 23, 1998
Would it be possible to put a thermometer inside a microwave oven? Would the microwaves have an effect on an
electronic thermometer? Would they have an effect on a mercury thermometer? -- R
This is an interesting question because it brings up the tricky issue of what is the temperature in a microwave
oven. In fact, there is no specific temperature in the oven because the microwaves that do the cooking are not
thermal. Rather than emerging from a hot object with a well-defined temperature, these microwaves are
produced in a coherent fashion by a vacuum tube. Like the light emerging from a laser, these microwaves can
heat objects they encounter as hot as you like, or at least until heat begins to escape from those objects as fast as
it's being added.
So instead of measuring the "temperature of the microwave oven," people normally put thermometers in the food
to measure the food's temperature. This works well as long as the thermometers don't interact with the
microwaves in ways that make them either hotter or inaccurate. Electronic thermometers are common in highend microwaves. There is nothing special about these electronic thermometers except that they are carefully
shielded so that the microwaves don't heat them or affect their readings. By "shielded," I mean that each of these
thermometers has a continuous metallic sheath that reflects the microwaves. This sheath extends from the wall of
the oven's cooking chamber all the way to the thermometer probe's tip so that the microwaves themselves can't
enter the measurement electronics. Since the sheath reflects microwaves, the thermometer isn't heated by the
microwaves and only measures the temperature of the food it contacts.
On the other hand, putting a mercury thermometer in a microwave oven isn't a good idea. While mercury is a
metal and will reflect most of the microwaves that strike it, the microwaves will push a great many electric
charges up and down the narrow column of mercury. This current flow will cause heating of the mercury because
the column is too thin to tolerate the substantial current without becoming warm. The mercury can easily
overheat, turn to gas, and explode the thermometer. (A reader of this web site reported having blown up a
mercury thermometer just this way as a child.) Moreover, as charges slosh up and down the mercury column,
they will periodically accumulate at the upper end. Since there is only a thin vapor of mercury gas above this
upper surface, the accumulated charges will probably ionize this vapor and create a luminous mercury discharge.
The thermometer would then turn into a mercury lamp, emitting ultraviolet light. I used microwave-powered
mercury lamps similar to this in my thesis research fifteen years ago and they work very nicely.
October 12, 1998
I wear glasses for distance vision, but my near vision is good. Why is it that when I use a nearby mirror to view distant
objects, I must wear my glasses to see them clearly? I should be able to see the nearby mirror well without glasses. -- JFJ
When you view something in a flat mirror, you are looking at a virtual image of the object and this virtual image
isn't located on the surface of the mirror. Instead, it's located on the far side of the mirror at a distance exactly
equal to the distance from the mirror to the actual object. In effect, you are looking through a window into a
"looking glass world" and seeing a distant object on the other side of that window. The reflected light reaching
your eyes has all the optical characteristics of having come the full distance from that virtual image, through the
mirror, to your eyes. The total distance between what you are seeing and your eyes is the sum of the distance
from your eyes to the mirror plus the distance from the mirror to the object. That's why you must use your
distance glasses to see most reflected objects clearly. Even when you observe your own face, you are seeing it as
though it were located twice as far from you as the distance from your face to the mirror.
October 1, 1998
I understand that to calculate the heat released or absorbed during a nuclear reaction you find the difference between the
product mass and reactant mass and use the formula (E=mc2). But what about heat released or absorbed during a
chemical reaction? The book I have says that mass is conserved during a chemical reaction, so where does the heat
energy come from? -- TC
While your book's claim is well intended, it's actually incorrect. The author is trying to point out that atoms aren't
created or destroyed during the reaction and that all the reactant atoms are still present in the products. But
equating the conservation of atoms with the conservation of mass overlooks any mass loss associated with
changes in the chemical bonds between atoms. While bond masses are extremely small compared to the masses
of atoms, they do change as the results of chemical reactions. However even the most energy-releasing or
"exothermic" reactions only produce overall mass losses of about one part in a billion and no one has yet
succeeded in weighing matter precisely enough to detect such tiny changes.
September 15, 1998
How do propane or kerosene refrigerators work--ones that require no electricity at all and are called "ice from fire" units?
-- KN
Heater-based refrigerators make use of an absorption cycle in which a refrigerant is driven out of solution as a
gas in a boiler, condenses into a liquid in a condenser, evaporates back into a gas in an evaporator, and finally
goes back into solution in an absorption unit. The cooling effect comes during the evaporation in the evaporator
because converting a liquid to a gas requires energy and thus extracts heat from everything around the
evaporating liquid.
The most effective modern absorption cycle refrigerators use a solution of lithium bromide (LiBr) in water. What
enters the boiler is a relatively dilute solution of LiBr (57.5%) and what leaves is dense, pure water vapor and a
relatively concentrated solution of LiBr (64%). The pure water vapor enters a condenser, where it gives up heat
to its surroundings and turns into liquid water. To convert this liquid water back into gas, all that has to happen is
for its pressure to drop. That pressure drop occurs when the water enters a low-pressure evaporator through a
narrow orifice. As the water evaporates, it draws heat from its surroundings and refrigerates them.
Finally, something must collect this low pressure water vapor and carry it back to the boiler. That "something" is
the concentrated LiBr solution. When the low-pressure water vapor encounters the concentrated LiBr solution in
the absorption unit, it quickly goes back into solution. The solution becomes less concentrated as it draws water
vapor out of the gas above it. This diluted solution then returns to the boiler to begin the process all over again.
Overall, the pure water follows one path and the LiBr solution follows another. The pure water first appears as a
high-pressure gas in the boiler (out of the boiling LiBr solution), converts to a liquid in the condenser, evaporates
back into a low-pressure gas in the evaporator, and finally disappears in the absorption unit (into the cool LiBr
solution). Meanwhile, the LiBr solution shuttles back and forth between the boiler (where it gives up water
vapor) and the absorption unit (where it picks up water vapor). The remarkable thing about this whole cycle is
that its only moving parts are in the pump that moves LiBr solution from the absorption unit to the boiler. Its
only significant power source is the heater that operates the boiler. That heater can use propane, kerosene,
electricity, waste heat from a conventional power plant, and so on.
August 24, 1998
If one metric ton of antimatter comes into contact with one metric ton of matter, how much energy would be released? -TC
Since the discovery of relativity, people have recognized that there is energy associated with rest mass and that
the amount of that energy is given by Einstein's famous equation: E=mc2. However, the energy associated with
rest mass is hard to release and only tiny fractions of it can be obtained through conventional means. Chemical
reactions free only parts per billion of a material's rest mass as energy and even nuclear fission and fusion can
release only about 1% of it. But when equal quantities of matter and antimatter collide, it's possible for 100% of
their combined rest mass to become energy. Since two metric tons is 2000 kilograms and the speed of light is
300,000,000 meters/second, the energy in Einstein's formula is 1.8x1020 kilogram-meters2/second2 or 1.8x1020
joules. To give you an idea of how much energy that is, it could keep a 100-watt light bulb lit for 57 billion
years.
July 24, 1998
You said that microwaves heat food by twisting water molecules back and forth and having those water molecules rub
against one another to experience a molecular form of "friction." Since vibrating molecules are the fundamental
manifestation of heat, why is the friction necessary at all? -- GS, Kanata, Canada
While it's true that microwaves twist water molecules back and forth, this twisting alone doesn't make the water
molecules hot. To understand why, consider the water molecules in gaseous steam: microwaves twist those water
molecules back and forth but they don't get hot. That's because the water molecules beginning twisting back and
forth as the microwaves arrive and then stop twisting back and forth as the microwaves leave. In effect, the
microwaves are only absorbed temporarily and are reemitted without doing anything permanent to the water
molecules. Only by having the water molecules rub against something while they're twisting, as occurs in liquid
water, can they be prevented from remitting the microwaves. That way the microwaves are absorbed and never
remitted--the microwave energy becomes thermal energy and remains behind in the water.
Visualize a boat riding on a passing wave--the boat begins bobbing up and down as the wave arrives but it stops
bobbing as the wave departs. Overall, the boat doesn't absorb any energy from the wave. However, if the boat
rubs against a dock as it bobs up and down, it will converts some of the wave's energy into thermal energy and
the wave will have permanently transferred some of its energy to the boat and dock.
July 22, 1998
Do VCR's work on the same principle as audio tape players? If so, how does a VCR generate a signal while it's on pause?
Yes, VCR's work on the same principle as an audio tape player: as a magnetized tape moves past the playback
head, that tape's changing magnetic field produces a fluctuating electric field. This electric field pushes current
back and forth through a coil of wire and this current is used to generate audio signals (in a tape player) or video
and audio signals (in a VCR).
However, there is one big difference between an audio player and a VCR. In an audio player, the tape moves past
a stationary playback head. In a VCR, the tape moves past a spinning playback head. When you pause an audio
tape player, the tape stops moving and there is no audio signal. But when you pause a VCR, the playback head
continues to spin. As the playback head (actually 2 or even 4 heads that trade off from one another) sweeps
across a few inches of the tape, it experiences the changing magnetic fields and fluctuating electric fields needed
to produce the video and audio signals. That's why you can still see the image from a paused VCR. To prevent
the spinning playback heads from wearing away the tape, most VCRs limit the pause time to about 5 minutes.
July 21, 1998
What does a transformer do?
A transformer transfers power between two or more electrical circuits when each of those circuits is carrying an
alternating electric current. Transfers of this sort are important because many electric power systems have
incompatible circuits--one circuit may use large currents of low voltage electricity while another circuit may use
small currents of high voltage electricity. A transformer can move power from one circuit of the electric power
system to another without any direct connections between those circuits.
Now for the technical details: a transformer is able to make such transfers of power because (1) electric currents
are magnetic, (2) the magnetic fields from an alternating electric current changes with time, (3) a time-varying
magnetic field creates an electric field, and (4) an electric fields pushes on electric charges and electric currents.
Overall, one of the alternating currents flowing through a transformer creates a time-varying magnetic field and
thus an electric field in the transformer. This electric field does work on (transfers power to) another alternating
current flowing through the transformer. At the same time, this electric field does negative work on (saps power
from) the original alternating current. When all is said and done, the first current has lost some of its power and
the second current has gained that missing power.
July 3, 1998
In your discussion of event horizons, you stated that light falls just like everything else. I thought that light does not speed
up when falling but just gains energy--that it is blue-shifted. Conversely, when it rises in a gravitational field, it does not
slow down but just loses energy--that it is red-shifted. Is that correct? -- B
Yes. For very fundamental reasons, light can't change its speed in vacuum; it always travels at the so-called
"speed of light." So light that is traveling straight downward toward a celestial object doesn't speed up; only its
frequency and energy increase. But light that is traveling horizontally past a celestial object will bend in flight,
just as a satellite will bend in flight as it passes the celestial object. This trajectory bending is a consequence of
free fall. While the falling of light as it passes through a gravitational field is a little more complicated than for a
normal satellite--the light's trajectory must be studied with fully relativistic equations of motion--both objects fall
nonetheless.
How does a light-detecting diode create voltage when light hits it? -- T
Diodes are one-way devices for electric current and are thus capable of separating positive charges from negative
charges and keeping them apart. Those charges can separate by moving away from one another in the diode's
allowed direction and then can't get back together because doing so would require them to move through the
diode in the forbidden direction. Given a diode's ability to keep separated charges apart, all that's needed to start
collecting separated charges is a source of energy. This energy is required to drive the positive and negative
charges apart in the first place. One such energy source is a particle of light--a photon. When a photon with the
right amount of energy is absorbed near the one-way junction of the diode, it can produce an electron-hole pair (a
hole is a positively charged quasiparticle that is actually nothing more than a missing electron). The junction will
allow only one of these charged particles to cross it and, having crossed, that particle cannot return. Thus when
the diode is exposed to light, separated charge begins to accumulate on its two ends and a voltage difference
appears between those ends.
In the movie "Back to the Future," Doc Brown completes an electrical circuit with a bolt of lightning as the source and
the "flux capacitor" as the load. In the process, he receives a shock. Would the "flux capacitor" still experience a flow of
electrons if Doc Brown had provided a path to the earth? -- BM, Akron, Ohio
While most of the "science" in that movie is actually nonsense, the use of lightning as a source of power has
some basis in reality. The current in a lightning bolt is enormous, peaking at many thousands of amperes, and the
voltages available are fantastically high. With so much current and voltage available, the flow of current during a
lightning strike can be very complicated. Even though Doc Brown provided one path through which the lightning
current could flow into the ground, he only conducted a fraction of the overall current. The remaining current
flowed through the wire and into the "flux capacitor." This branching of the current is common during a
lightning strike and makes lightning particularly dangerous. You don't have to be struck directly by lightning or
to be in contact with the main conducting pathway between the strike and the earth for you to be injured. Current
from the strike can branch out in complicated ways and follow a variety of unexpected paths to ground. You
don't want to be on any one of them. Doc Brown wasn't seriously hurt because it was only a movie. In real life,
people don't recover so quickly.
July 2, 1998
What is the cause of the power "drop" in my house, that will intermittently (every 5 to 10 minutes) cause my lights to
dim? -- JF
Your lights are dimming because something is reducing the voltage of the electricity in your house. The lights
expect the electric current passing through them to experience a specific voltage drop--that is, they expect each
electric charge to leave behind a certain amount of energy as the result of its passage through the lights. If the
voltage of electricity in your house is less than the expected amount, the lights won't receive enough energy and
will glow dimly.
The most probable cause for this problem is some power-hungry device in or near your house that cycles on
every 5 or 10 minutes. In all likelihood, this device contains a large motor--motors have a tendency to draw
enormous currents while they are first starting to turn, particularly if they are old and in need of maintenance.
The wiring and power transformer systems that deliver electricity to your neighborhood and house have limited
capacities and cannot transfer infinite amounts of power without wasting some of it. In general, wires waste
power in proportion to the square of the current they are carrying. While the amount of power wasted in your
home's wiring is insignificant in normal situations, it can become sizeable when the circuits are overloaded. This
wasted power in the wiring appears as a loss of voltage--a loss of energy per charge--at your lights and
appliances. When the heavy equipment turns on and begins to consume huge amounts of power, the wiring and
other electric supply systems begin to waste much more power than normal and the voltage reaching your lights
is significantly reduced. Your lights dim until the machinery stops using so much power.
To find what device that's making your lights dim, listen carefully the next time your lights fade. You'll probably
hear an air conditioner, a fan, or even an elevator starting up somewhere, either in your house or in your
neighborhood. There may be nothing you can do to fix the problem, but it's possible that replacing a motor or its
bearings will reduce the problem. Another possible culprit is an electric heating system--a hot water heater, a
radiant heater, an oven, a toaster, or even a hair-dryer. These devices also consume large amounts of power and,
in an older house with limited electric services, may dim the lights.
June 30, 1998
To keep soda carbonated, is it best to keep it cold in the refrigerator or outside in the room? Also, why does soda fizz
more when you pour it over ice than when you drop ice into already-poured soda--is that just because the falling liquid
has more kinetic energy? -- DG
To keep soda carbonated, you should minimize the rate at which carbon dioxide molecules leave the soda and
maximize the rate at which those molecules return to it. That way, the net flow of molecules out of the soda will
be small. To reduce the leaving rate, you should cool the soda--as long as ice crystals don't begin to form,
cooling the soda will make it more difficult for carbon dioxide molecules to obtain the energy they need to leave
the soda and will slow the rate at which they're lost. To increase the return rate, you should increase the density
of gaseous carbon dioxide molecules above the soda--sealing the soda container or pressurizing it with extra
carbon dioxide will speed the return of carbon dioxide molecules to the soda. Also, minimizing the volume of
empty bottle above the soda will make it easier for the soda to pressurize that volume itself. The soda will lose
some of its carbon dioxide while filling that volume, but the loss will quickly cease.
One final issue to consider is surface area: the more surface area there is between the liquid soda and the gas
above it, the faster molecules are exchanged between the two phases. Even if you don't keep carbon dioxide gas
trapped above soda, you can slow the loss of carbonation by keeping the soda in a narrow-necked bottle with
little surface between liquid and gas. But you must also be careful not to introduce liquid-gas surface area inside
the liquid. That's what happens when you shake soda or pour it into a glass--you create tiny bubbles inside the
soda and these bubbles grow rapidly as carbon dioxide molecules move from the liquid into the bubbles. Cool
temperatures, minimal surface area, and plenty of carbon dioxide in the gas phases will keep soda from going
flat.
As for pouring the soda over ice causing it to bubble particularly hard, that is partly the result of air stirred into
the soda as it tumbles over the ice cubes and partly the result of adding impurities to the soda as the soda washes
over the rough and impure surfaces of the ice. The air and impurities both nucleate carbon dioxide bubbles-providing the initial impetus for those bubbles to form and grow. Washing the ice to smooth its surfaces and
remove impurities apparently reduces the bubbling when you then pour soda of it.
Is terminal velocity the same for every object of the same mass or can the terminal velocity of two parachutists (same
weight and height) be different? -CV
Terminal velocity is the result of a delicate balance between two forces--an object's downward weight and the
upward drag force that object experiences as it moves downward through the air. Terminal velocity is reached
when those two forces exactly balance one another and the object experiences a net force of zero, stops
accelerating, and simply coasts downward at a constant velocity. Since the upward drag force increases with
downward speed, there is generally a velocity at which this balance occurs--the terminal velocity.
But while a parachutist can't change her weight, she can change the relationship between her downward speed
and the upward drag force she experiences. If she rolls herself into a compact ball, she weakens the drag force
and ultimately increases her terminal velocity. On the other hand, if she spreads her arms and legs wide so as to
catch more air, she strengthens the drag force and decreases her terminal velocity. Popping open her parachute
strengthens the drag force so much that her terminal velocity diminishes almost to zero and she coasts slowly
downward to a comfortable landing. So to answer your question--two twin parachutists will descend at very
different terminal velocities if they adopt different profiles or if only one opens a parachute.
June 22, 1998
I am intrigued by your assertion that the speed of light is the fastest speed in the universe. It seems to me that we wouldn't
be able to determine the fastest speed achievable in the universe, just as we can't find the final number in math. When
we're counting, there will always be x+1 so why would calculating the speed of objects in our universe be any different? - GL
Your comparison between the limitless counting numbers and the limited speeds in the universe is an interesting
one because it points out a fundamental difference between the older Galilean/Newtonian understanding of the
universe and the newer Einsteinian understanding. The older understanding claims that velocities can be added in
the same way that counting numbers can be added and that there is thus no limit to the speeds that can exist in
our universe. For example, if you are jogging eastward at 5 mph and a second runner passes you traveling
eastward 5 mph faster, then a person watching the two of you from a stationary vantage point sees the second
runner traveling eastward at 10 mph. The velocities add, so that 5 mph + 5 mph = 10 mph. If the second runner is
now passed by a third runner, who is traveling eastward 5 mph faster than the second runner, then the stationary
observer sees that third runner traveling eastward at 15 mph. And so it goes. As long as velocities add in this
manner, objects can reach any speed they like.
At this point, you might assert that velocities do add and that objects should be able to reach any speed. But
that's not the case. The modern, relativistic understanding of the universe says that even at these small speeds,
velocities don't quite add. To the stationary observer, the second runner travels at only 9.9999999999999994
mph and the third runner at only 14.9999999999999988 mph. As you can see, when two or more velocities are
combined, the final velocity isn't quite as large as the simple sum. What that means is that the velocity you
observe in another object is inextricably related to your own motion. This interrelatedness is part of the theory of
relativity--that observers who are moving relative to one another will see space and time somewhat differently.
For objects traveling close to the speed of light, the failure of velocity addition becomes quite severe. For
example, if one spaceship travels past the earth at half the speed of light and the people in that spaceship watch a
second spaceship pass them at half the speed of light in the same direction, then a person on earth will see the
second spaceship traveling only four-fifths of the speed of light. As you can see, relativity is making it difficult
to reach the speed of light. In fact, it's impossible to reach the speed of light! No matter how you combine
velocities, no observer will ever see a massive object reach or exceed the speed of light. The only objects that can
reach the speed of light are objects without mass and they can only travel at the speed of light.
So while the counting numbers obey simple addition and go on forever, velocities do not obey simple addition
and have a firm limit--the speed of light. The additive counting numbers are an example of a mathematical group
that extends infinitely in both directions, but there are many examples of groups that do not extend to infinity.
The group that describes relativistic, real-world velocities is one such group. You can visualize another simple
limited group--the one associated with walking around the surface of the earth. No matter how much you try, you
can't walk more than a certain distance northward. While it seems as though steps northward add, so that 5 steps
north plus 5 steps north equals 10 steps north, things aren't quite that simple. Eventually you reach the north pole
and start walking south!
June 16, 1998
How do geysers work? -- SP, Morgantown, WV
While I'm not an expert on geysers and would need to visit the library to verify my ideas, I believe that they
operate the same way a coffee percolator does. Both objects involve a narrow water-filled channel that's heated
from below. As the temperature at the bottom of the water column increases, the water's stability as a liquid
decreases and its tendency to become gaseous steam increases. What prevents this heated water from converting
into gas is the weight of the water and air above it, or more accurately the pressure caused by that weight. But
when the water's temperature reaches a certain elevated level, it begins to turn into steam despite the pressure.
Since steam is less dense than liquid water, the hot water expands as it turns into steam and it lifts the column of
water above it. Water begins to spray out of the top of the channel, decreasing the weight of water in the channel
and the pressure at the bottom of the channel. With less pressure keeping the water liquid, the steam forming
process accelerates and the column of water rushes up the channel and into the air. Once the steam itself reaches
the top of the channel, it escapes freely into the air and the pressure in the channel plummets. Water begins to
reenter the channel and the whole process repeats.
June 9, 1998
If I pinch a sheet of aluminized Mylar between two concentric circular rings and weight the middle of the sheet with
water so that it sags into a curved shape, like a parabola, is there an adhesive such as fiberglass which I can adhere to the
back surface to stiffen it so that I can make a giant reflective surface to serve as a solar collector? -- AM, Weldon, CA
What a great idea! Mylar is DuPont's brand of PET film, where "PET" is Poly(ethylene terephthalate)--the same
plastic used in most plastic beverage containers (look for "PET" or "PETE" in the recycling triangle on the
bottom). PET isn't a particularly inert plastic and you shouldn't have any trouble gluing to it. To form a rigid
structure, you need either a glassy plastic backing (one that is stiff and brittle at room temperature) or a stiff
composite backing. I'd go with fiberglass--mount the Mylar in a large quilting or needlepoint frame, coat the
back of the Mylar with the glass and epoxy mixture, invert it, weight it with water, and let it harden. Mylar
doesn't stretch easily, so you'll get a very shallow curve and a very long focal length mirror. While the mirror
will probably have some imperfections and a non-parabolic shape, it should still do a decent job of concentrating
sunlight.
June 8, 1998
You insist over and over again that it is impossible to go faster than the speed of light. This is completely and entirely
untrue. Tachyons travel faster than light. They also go faster as they exert less and less energy. -- K
I'm afraid that you confuse the hypothetical with the actual. While people have hypothesized about superluminal
particles called tachyons, they have never been observed and probably don't exist. This speculation is based on
an interesting but apparently non-physical class of solutions to the relativistic equations of motion. Although
tachyons make for fun science fiction stories, they don't seem to have a place in the real world.
June 3, 1998
I would like to make high frequency and ultrasonic whistles with tubes. I know the formula for the relationship between
wavelength, speed, and frequency but what is the relationship of these quantities with tube length and diameter? -- AH,
Richmond, British Columbia
If a whistle's tube is relatively narrow, its pitch is determined primarily by its length and by how many of its ends
are open to the air. That's because as you blow the whistle, a "standing" sound wave forms inside it--the same
sound wave that you hear as it "leaks" out of the whistle. If the whistle is open at both ends, almost half a
wavelength of this standing sound wave will fit inside the tube. Since a sound's wavelength times its frequency
must equal the speed of sound (331 meters per second or 1086 feet per second), a double-open whistle's pitch is
approximately the speed of sound divided by twice its length. For example, a whistle that's 0.85 centimeters long
can hold one wavelength of a sound with a frequency near 19,500 cycles per second--at the upper threshold of
hearing for a young person. If the whistle is closed at one end, the air inside it vibrates somewhat different; only
a quarter of a wavelength of the standing sound wave will fit inside the tube. In that case, its pitch is
approximately the speed of sound divided by four times its length. However, if you blow a whistle hard enough,
you can cause more wavelengths of a standing sound wave to fit inside it. A strongly blown double-open whistle
can house any half-integer number of wavelengths (1/2, 1, 3/2, or more), emitting higher pitched tones as it does
so. A strongly blown single-open whistle can house any odd quarter-integer number of wavelengths (1/4, 3/4,
5/4, or more).
In one of your answers, you said that the "water on the earth's surface swells up into two bulges: one on the side of the
earth nearest the moon and one on the side farthest from the moon." Can you explain why the water bulges up on the side
farthest from the moon? -- ST
To understand the two bulges, imagine three objects: the earth, a ball of water on the side of the earth nearest the
moon, and a ball of water on the side of the earth farthest from the moon. Now picture those three objects
orbiting the moon. In orbit, those three objects are falling freely toward the moon but are perpetually missing it
because of their enormous sideways speeds. But the ball of water nearest the moon experiences a somewhat
stronger moon-gravity than the other objects and it falls faster toward the moon. As a result, this ball of water
pulls away from the earth--it bulges outward. Similarly, the ball of water farthest from the moon experiences a
somewhat weaker moon-gravity than the other objects and it falls more slowly toward the moon. As a result, the
earth and the other ball of water pull away from this outer ball so that this ball bulges outward, away from the
earth.
It's interesting to note that the earth itself bulges slightly in response to these tidal forces. However, because the
earth is more rigid than the water, its bulges are rather small compared to those of the water.
June 2, 1998
I want to support a group of bird feeders on a horizontal cable, one end of which will be fastened to my house and the
other end of which will run over an 8 inch pulley attached to a large tree. That end of the cable will be attached to some
concrete blocks which must be heavy enough to keep the horizontal cable taut at all times. The idea is to prevent the cable
from snapping when the tree moves in high winds. It's already done so twice, even though I left what I thought was
adequate slack in the line. I guess this sounds like a Rube Goldberg solution, but I can't think of any other solution. How
much should the concrete blocks weigh? -- HS, Burk's Falls, Ontario
Your solution should work nicely--the pulley and weight system should protect your cable from breaking
because the weights should maintain a constant tension in the line. As the tree swings back and forth, the weights
should rise and fall while the tension in the cord remains almost steady. Obviously, if the rising weights reach the
pulley the cord will pull taut and break, so you must leave enough hanging slack.
However, if the tree's motion is too violent, even this weight and pulley system may not save the cable. As long
as everything moves slowly, the tension in the cord should be equal to the weight of the weights. But if the tree
moves away from the house very suddenly, then the tension in the cord will increase suddenly because the cord
must not only support the weights, it must accelerate them upward as well. Part of the cord's tension acts to
overcome the weights' inertia. Just as a sudden yank on a paper towel will rip it free from the roll, so a sudden
yank on your cable will rip it free from the weights. If sudden yanks of this type cause trouble for you, you can
fix the problem by coupling the cord to the weights via a strong spring. On long timescales, the spring will have
no effect on the tension in the cord--it will still be equal to the weight of the weights. But the spring will stretch
or contract during sudden yanks on the cord and will prevent the tension in the cord from changing abruptly
either up or down. The spring shouldn't be too stiff--the less stiff and the more it stretches while supporting the
weights, the more effectively it will smooth out changes in tension.
As far as the weight of the weights, that depends on how much curvature you want in the cable supporting the
feeders. The more weight you use, the less the cable will sag but the more stress it will experience. You can
determine how much weight you need by pulling on the far end of the cable with your hands and judging how
hard you must pull to get a satisfactory amount of sag.
I am interested in experimenting with colored flames, maybe by adding a substance to the flame. Please tell me how to do
it and with what kind of substances. -- M
You can produce colored flames by adding various metal salts to the burning materials. That's what's done in
fireworks. These metal salts decompose when heated so that individual metal atoms are present in the hot flame.
Thermal energy in the flame then excites those atoms so that their electrons shift among the allowed orbits or
"orbitals" and this shifting can lead to the emission of particles of light or "photons". Since the orbitals
themselves vary according to which chemical element is involved, the emitted photons have specific
wavelengths and colors that are characteristic of that element.
To obtain a wide variety of colors, you'll need a wide variety of metal salts. Sodium salts, including common
table salt, will give you yellow light--the same light that's produced by sodium vapor lamps. Potassium salts
yield purple, copper and barium salts yield green, strontium salts yield red, and so on. The classic way to produce
a colored flame is to dip a platinum wire into a metal salt solution and to hold the wire in the flame. Since
platinum is expensive, you can do the same trick with a piece of steel wire. The only problem is that the steel
wire will burn eventually.
March 23, 1998
Why do only certain orbitals exist in an atom?
Because the electrons in an atom move about as waves, they can follow only certain allowed orbits that we call
orbitals. This limitation is equivalent to the case of a violin string--it can only vibrate at certain frequencies. If
you try to make a violin string vibrate at the wrong frequency, it won't do it. That's because the string vibrates in
a wave-like manner and only certain waves fit properly along the strong. Similarly, the electron in an atom
"vibrates" in a wave-like manner and only certain waves fit properly around the nucleus.
When an electron hits a neon atom, does it transfer its energy to the atom and lose its own forever?
Most of the collisions between an electron and a neon atom are completely elastic--the electron bounces perfectly
from the neon atom and retains essentially all of its kinetic energy. But occasionally the electron induces a
structural change in the neon atom and transfers some of its energy to the neon atom. In such a case, the electron
rebounds weakly and retains only a fraction of its original kinetic energy. The missing energy is left in the neon
atom, which usually releases that energy as light.
You said that some rooms in the physics building are made with metal to specifically keep electromagnetic waves out.
How does that work?
Some experiments are so sensitive to electromagnetic waves that they must be performed inside "Faraday cages".
A Faraday cage is a metal or metal screen box. Its walls conduct electricity and act as mirrors for electromagnetic
waves. As long as a wave has a wavelength significantly longer than the largest hole in the walls, that wave will
be reflected and will not enter the box. This reflection occurs because the wave's electric field pushes charges
inside the metal walls and causes those charges to accelerate. These accelerating charges redirect (absorb and
reemit) the wave in a new direction--a mirror reflection. Just as a box made of metal mirrors will keep light out, a
box made with metal walls will keep electromagnetic waves out.
Can microwave ovens leak microwaves? Is my mother's warning not to stand in front of the microwave while it's on
valid?
A properly built and maintained microwave oven leaks so little microwave power that you needn't worry about
it. There are also inexpensive leakage testers available that you can use at home for a basic check, or for a more
reliable and accurate check--as recommended by both the International Microwave Power Institute (IMPI) and
the FDA--you can take your microwave oven to a service shop and have it checked with an FDA certified meter.
It's only if you have dropped the oven or injured its door in some way that you might have cause to worry about
standing near it. If it were to leak microwaves, their main effect would be to heat your tissue, so you would feel
the leakage.
March 4, 1998
Is a CB radio also an AM radio?
CB or citizens band radio refers to some parts of the electromagnetic spectrum that have been set aside for public
use. You can operate a CB radio without training and without serious legal constraints, although the power of
your transmitted wave is strictly limited. The principal band for CB radio is around 27 MHz and I think that the
transmissions use the AM audio encoding scheme. As you talk, the power of your transmission increases and
decreases to represent the pressure fluctuations in your voice. The receiving CB radio detects the power
fluctuations in the radio wave and moves its speaker accordingly.
March 2, 1998
What kinds of things get stored in read-only memory, as opposed to storing them on the hard drive?
When you first turn on a typical computer, it must run an initial program that sets up the operating system. This
initial program has to run even before the computer is able to interact with its hard drive, so the program must be
available at the very instant the computer's power becomes available. Read-only memory is used for this initial
bootup operation. Unlike normal random access memory, which is usually "volatile" and loses its stored
information when power is removed, read-only memory retains its information without power. When you turn on
the computer, this read-only memory provides the instructions the computer uses to begin loading the operating
system from the hard drive.
February 20, 1998
Why can you force the current from the n-type semiconductor to the p-type after a p-n junction has been created but you
can't force current from the p-type to the n-type?
Actually, you are asking about a current of electrons, which carry a negative charge. It's true that electrons can't
be sent across the p-n junction from the p-type side to the n-type side. There are several things that prevent this
reverse flow of electrons. First, there is an accumulation of negative charge on the p-type side of the p-n junction
and this negative charge repels any electrons that approach the junction from the p-type end. Second, any
electron you add to the p-type material will enter an empty valence level. As it approaches the p-n junction, it
will find itself with no empty valence levels in which to travel the last distance to the junction. It will end up
widening the depletion region--the region of effectively pure semiconductor around the p-n junction; a region
that doesn't conduct electricity.
Is it true that you shouldn't put a speaker near a microwave oven?
A microwave oven that's built properly and not damaged emits so little electromagnetic radiation that the speaker
should never notice. The speaker might have some magnetic field leakage outside its cabinet, and that might
have some effect on a microwave oven. However, most microwaves have steel cases and the steel will shield the
inner workings of the microwave oven from any magnetic fields leaking from the speaker. The two devices
should be independent.
February 19, 1998
How does a phonograph work? -- MS
A phonograph record represents the air pressure fluctuations associated with sound as surface fluctuations in
long, spiral groove. This groove is V-shaped, with two walls cut at right angles to one another--hence the "V".
Silence, the absence of pressure fluctuations in the air, is represented by a smooth portion of the V groove, while
moments of sound are represented by a V-groove with ripples on its two walls. The depths and spacings of the
ripples determine the volume and pitch of the sounds and the two walls represent the two stereo channels on
which sound is recorded and reproduced.
To sense the ripples in the V-groove, a phonograph places a hard stylus in the groove and spins the record. As
the stylus rides along the walls of the moving groove, it vibrates back and forth with each ripple in a wall. Two
transducers attached to this stylus sense its motions and produce electric currents that are related to those
motions. The two most common transduction techniques are electromagnetic (a coil of wire and a magnet move
relative to one another as the stylus moves and this causes current to flow through the coil) and piezoelectric (an
asymmetric crystal is squeezed or unsqueezed as the stylus moves and this causes charge to be transferred
between its surfaces). The transducer current is amplified and used to reproduce the recorded sound.
February 18, 1998
Before you speak into the tape recorder, is the tape non-magnetic because half of the magnets face one way and half the
other way?
Exactly. When you switch your tape recorder to the record mode, it has a special erase head that becomes active.
This erase head deliberately scrambles the magnetic orientations of the tape's magnetic particles. The erase head
does this by flipping the magnetizations back and forth very rapidly as the particles pass by the head, so that they
are left in unpredictable orientations. There are, however, some inexpensive recorders that use permanent
magnets to erase the tapes. This process magnetizes all the magnetic particles in one direction, effectively erasing
a tape. Because it leaves the tape highly magnetized, this second technique isn't as good as the first one. It tends
to leave some noise on the recorded tape.
February 16, 1998
I am a mentor to a 7th grader who is doing a report on Einstein. How do I explain his theory in a way that will be relevant
to her? -- MG
The basis for Einstein's theory of relativity is the idea that everyone sees light moving at the same speed. In fact,
the speed of light is so special that it doesn't really depend on light at all. Even if light didn't exist, the speed of
light would still be a universal standard--the fastest possible speed for anything in our universe.
Once we recognize that the speed of light is special and that everyone sees light traveling at that speed, our views
of space and time have to change. One of the classic "thought experiments" necessitating that change is the
flashbulb in the boxcar experiment. Suppose that you are in a railroad boxcar with a flashbulb in its exact center.
The flashbulb goes off and its light spreads outward rapidly in all directions. Since the bulb is in the center of the
boxcar, its light naturally hits the front and back walls of the boxcar at the same instant and everything seems
simple.
But your boxcar is actually hurtling forward on a track at an enormous speed and your friend is sitting in a
station as the train rushes by. She looks into the boxcar through its window and sees the flashbulb go off. She
watches light from the flashbulb spread out in all directions but it doesn't hit the front and back walls of the
boxcar simultaneously. Because the boxcar is moving forward, the front wall of the boxcar is moving away from
the approaching light while the back wall of the boxcar is moving toward that light. Remarkably, light from the
flashbulb strikes the back wall of the boxcar first, as seen by your stationary friend.
Something is odd here: you see the light strike both walls simultaneously while your stationary friend sees light
strike the back wall first. Who is right? The answer, strangely enough, is that you're both right. However,
because you are moving at different velocities, the two of you perceive time and space somewhat differently.
Because of these differences, you and your friend will not always agree about the distances between points in
space or the intervals between moments in time. Most importantly, the two of you will not always agree about
the distance or time separating two specific events and, in certain cases, may not even agree about which event
happened first!
The remainder of the special theory of relativity builds on this groundwork, always treating the speed of light as
a fundamental constant of nature. Einstein's famous formula, E=mc2, is an unavoidable consequence of this line
of reasoning.
What is the difference between a magnet and an electromagnet? Why are some metals automatically magnetic?
Some metals are composed of microscopic permanent magnets, all lumped together. Such metals include iron,
nickel, and cobalt. This magnetism is often masked by the fact that the tiny magnets in these metals are randomly
oriented and cancel one another on a large scale. But the magnetism is revealed whenever you put one of these
magnetic metals in an external magnetic field. The tiny magnets inside these metals then line up with the external
field and the metal develops large scale magnetism.
However, most metals don't have any internal magnetic order at all and there is nothing to line up with an
external field. Metals such as copper and aluminum have no magnetic order in them--they don't have any tiny
magnets present. The only way to make aluminum or copper magnetic is to run a current through it.
How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it
positive and negative?
An electric current is itself magnetic--it creates a structure in the space around it that exerts forces on any
magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire--the
current's magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the
wire up into a coil, the magnetic field takes on a more familiar shape. The current-carrying coil effectively
develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the
direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your
right hand, then your thumb points to the north pole that develops at one end of the coil.
How do the sizes of two magnets determine how much paper can be held between them? -- D
While the full answer to this question is complicated, the most important issues are the strengths and locations of
the magnetic poles in each magnet. Since each magnet has north poles and south poles of equal strengths, there
are always attractive and repulsive forces at work between a pair of magnets--their opposite poles always attract
and their like poles always repel. You can make two magnets attract one another by turning them so that their
opposite poles are closer together than their like poles (e.g. by turning a north pole toward a south pole).
To maximize the attraction between the magnets, opposite magnetic poles should be as near together as possible
while like magnetic poles are as far apart as possible. With long bar magnets, you align the magnets head to toe
so that you have the north pole of one magnet opposite the south pole of the other magnet and vice versa. But
long magnets also tend to have weaker poles than short stubby magnets because it takes energy to separate a
magnet's north pole from its south pole. With short stubby magnets, the best you can do is to bring the north pole
of one magnet close to the south pole of the other magnet while leaving their other poles pointing away from one
another. Horseshoe magnets combine some of the best of both magnets--they can have the strong poles of short
stubby magnets with more distance separating those poles.
Returning to the paper question, size is less important than pole strength and separation. The stronger the
magnets and the farther apart their poles, the more paper you can hold between them.
February 15, 1998
I live under the flight path that leads into Sydney's International/Domestic Airport. As planes fly over, a sound follows
them (3-4 seconds) like air folding in on itself. A slurping sound similar to sucking air in through your cheeks. This
phenomenon does not happen all the time, but seems to happen when overcast. Any clues as to what is happening? -- TA,
Sydney, Australia
The sound you hear may be related to the vortices that swirl behind a plane's wingtips as it moves through the
air. These vortices form as a consequence of the wing's lift-generating processes. Because the air pressure above
a wing is lower than the air pressure below the wing, air is sucked around the wingtip and creates a swirling
vortex. The two vortices, one at each wingtip, trail behind the plane for miles and gradually descend. You may
be hearing them reach the ground after the airplane has passed low over your home. If someone reading this has
another explanation, please let me know.
How do the automatic soda dispensers at fast food joints know when the cup is full? -- MB, San Diego, CA
Those dispensers measure the volume of liquid they dispense and shut off when they've delivered enough liquid
to fill the cup. They don't monitor where that liquid is going, so if you put the wrong sized cup below them or
press the button twice, you're in trouble.
I've heard that there are only four basic forces in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear.
Is this true, and if so, what are the basic differences? -- SH, Purdue, Indiana
The number of "basic forces" has changed over the years, increasing as new forces are discovered and decreasing
as seemingly separate forces are joined together under a more sophisticated umbrella. A good example of this
evolution of understanding is electromagnetism--electric and magnetic forces were once thought separate but
gradually became unified, particularly as our understanding of time and space improved. More recently, weak
interactions have joined electromagnetic interactions to become electroweak interactions. In all likelihood, strong
and gravitational interactions will eventually join electroweak to give us one grand system of interactions
between objects in our universe.
But regardless of counting scheme, I can still answer your question about how the four basic forces differ.
Gravitational forces are attractive interactions between concentrations of mass/energy. Everything with
mass/energy attracts everything else with mass/energy. Because this gravitational attraction is exceedingly weak,
we only notice it when there are huge objects around to enhance its effects.
Electromagnetic forces are strong interactions between objects carrying electric charge or magnetic pole. While
most of these interactions can be characterized as attractive or repulsive, that's something of an
oversimplification whenever motion is involved.
Weak interactions are too complicated to call "forces" because they almost always do more than simply pull two
objects together or push them apart. Weak interactions often change the very natures of the particles that
experience them. But the weak interactions are rare because they involve the exchange of exotic particles that are
difficult to form and live for exceedingly short times. Weak interactions are responsible for much of natural
radioactivity.
Strong forces are also very complicated, primarily because the particles that convey the strong force themselves
experience the strong force. Strong forces are what hold quarks together to form familiar particles like protons
and neutrons.
Is it true that a person in space doesn't get as old as if he was on the earth? -- ASB, Chiapas, Mexico
The effects you are referring to are extremely subtle, so no one will ever notice them in an astronaut. But with
ultraprecise clocks, it's not hard to see strange effects altering the passage of time in space. There are actually
two competing effects that alter the passage of time on a spaceship--one that slows the passage of time as a
consequence of special relativity and the other that speeds the passage of time as a consequence of general
relativity.
The time slowing effect is acceleration--a person or clock that takes a fast trip around the earth and then returns
to the starting point will experience slightly less time than a person or clock that remained at the starting point.
This effect is a consequence of acceleration and the changing relationships between space and time that come
with different velocities.
The time speeding effect is gravitational redshift--a person or clock that is farther from the earth's center
experiences slightly more time than a person or clock that remains at the earth's surface. This effect is a
consequence of the decreased potential energy that comes with being deeper in the earth's gravitational potential
well.
How does an astronaut get prepared for the long period of antigravity that he is going to be put on? -- ASB, Chiapas,
Mexico
When an astronaut is orbiting the earth, he isn't really weightless. The earth's gravity is still pulling him toward
the center of the earth and his weight is almost as large as it would be on the earth's surface. What makes him
feel weightless is the fact that he is in free fall all the time! He is falling just as he would be if he had jumped off
a diving board or a cliff. If it weren't for the astronaut's enormous sideways velocity, he would plunge toward the
earth faster and faster and soon crash into the earth's surface. But his sideways velocity carries him past the
horizon so fast that he keeps missing the earth as he falls. Instead of crashing into the earth, he orbits it.
During his orbit, the astronaut feels weightless because all of his "pieces" are falling together. Those pieces don't
need to push on one another to keep their relative positions as they fall, so he feels none of the internal forces
that he interprets as weight when he stands on the ground. A falling astronaut can't feel his weight.
To prepare for this weightless feeling, the astronaut needs to fall. Jumping off a diving board or riding a roller
coaster will help, but the classic training technique is a ride on the "Vomit Comet"--an airplane that follows a
parabolic arc through the air that allows everything inside it to fall freely. The airplane's arc is just that of a freely
falling object and everything inside it floats around in free fall, too--including the astronaut trainee. The plane
starts the arc heading upward. It slows its rise until it reaches a peak height and then continues arcing downward
faster and faster. The whole trip lasts at most 20 seconds, during which everyone inside the plane feels
weightless.
February 13, 1998
Is not the current used in Europe direct current? If so, do they use transformers or do their lines get very hot? Why do our
appliances not work there?
Europe uses alternating current, just as we do, however some of the characteristics of that current are slightly
different. First, Europe uses 50 cycle-per-second current, meaning that current there reverses directions 100
times per second. That's somewhat slower than in the U.S., where current reverses 120 times per second (60 full
cycles of reversal each second or 60 Hz). Second, their standard voltage is 230 volts, rather than the 120 volts
used in the U.S.
While some of our appliances won't work in Europe because of the change in cycles-per-second, the biggest
problem is with the increase in voltage. The charges entering a U.S. appliance in Europe carry about twice the
energy per change (i.e. twice the voltage) and this increased "pressure" causes about twice the number of charges
per second (i.e. twice the current) to flow through the appliance. With twice the current flowing through the
appliance and twice as much voltage being lost by this current as it flows through the appliance, the appliance is
receiving about four times its intended power. It will probably burn up.
February 11, 1998
Why are batteries so expensive?
They contain highly purified and refined chemicals and are actually marvels of engineering. It's more surprising
to me that they are so cheap, given how complicated they are to make.
If only electrons move around, why do you keep using positive charges in the demos?
It's useful to describe moving electric charges as a current and for that current to flow in the direction that the
charges are moving. Suppose that we define current as flowing in the direction that electrons take and look at the
result of letting this current of electrons flow into a charge storage device. We would find that as this current
flowed into the storage device, the amount of charge (i.e. positive) charge in that device would decrease! How
awkward! You're "pouring" something into a container and the contents of that container are decreasing! So we
define current as pointing in the direction of positive charge movement or in the direction opposite negative
charge movement. That way, as current flows into a storage device, the charge in that device increases!
How come the flashlight works when you switch the batteries but my walkman or gameboy doesn't?
The bulb in a battery doesn't care which way current flows through it. The metal has no asymmetry that would
treat left-moving charges differently from right-moving charges. That's not true of the transistors in a walkman or
gameboy. They contain specialized pieces of semiconductor that will only allow positive charges to move in one
direction, not the other. When you put the batteries in backward and try to propel current backward through its
parts, the current won't flow and nothing happens.
How are you "shocked"?
Your body is similar to salt water and is thus a reasonably good conductor of electricity. Once current penetrates
your skin (which is insulating), it flows easily through you. At high currents, this electricity can deposit enough
energy in you to cause heating and thermal damage. But at lower currents, it can interfere with normal
electrochemical and neural process so that your muscles and nerves don't work right. It takes about 0.030
amperes of current to cause serious problems for your heart, so that currents of that size can be fatal.
February 9, 1998
If the battery separates charges even while it's off, how come it doesn't light up when it's off?
The battery stops separating charges once enough have accumulated on its terminals. If the flashlight is off, so
that charges build up, then the battery soon stops separating charge and the light bulb doesn't light.
How do rechargeable batteries get recharged?
You can recharge any battery by pushing charge through it backward (pushing positive charge from its positive
terminal to its negative terminal). However, some batteries don't take this charge well or heat up. The ones that
recharge most effectively are those that can rebuild their chemical structures most effectively as they operate
backward.
February 6, 1998
What keeps the earth stable so that it doesn't get pulled up into the "magnet"?
If you are asking why doesn't the earth itself get pulled up toward a large magnet or electromagnet that I'm
holding in my hand, the answer is that the magnetic forces just aren't strong enough to pull the magnet and earth
together. I'm holding the two apart with other forces and preventing them from pulling together. The forces
between poles diminish with distance. Those forces are proportional to the inverse square of the distance
between poles, so they fall off very quickly as the poles move apart. Moreover, each north pole is connected to a
south pole on the same magnet, so the attraction between opposite poles on two separate magnets is mitigated by
the repulsions of the other poles on those same magnets. As a result, the forces between two bar magnets fall
over even faster than the simple inverse square law predicts. It would take an incredible magnet, something like a
spinning neutron star, to exert magnet forces strong enough to damage the earth. But then a neutron star would
exert gravitational forces that would damage the earth, too, so you'd hardly notice the magnetic effects.
Is the earth a huge magnet? If so, how does it do this without being made out of metal?
The earth is a huge magnet and it is made out of metal. The earth's core is mostly iron and nickel, both of which
can be magnetic metals. However, the earth's magnetism doesn't appear to come from the metal itself. Current
theories attribute the earth's magnetism to movements in and around the core. There are either electric currents
associated with this movement or some effects that orient the local magnetization of the metal. I don't think that
there is any general consensus on the matter.
February 4, 1998
Is it physically possible for a baseball player to hit a baseball that has been pitched 60 ft away at 90-95 mph? If so, why
are the highest baseball records between 3 and 4 out of ten?
If the ball was pitched straight and true, the same way every pitch, good batters could hit every one. There is
enough time in the wind-up and pitch for the batter to determine where and when to swing and to hit the ball just
right. But the pitches vary and the balls curve. That limits the batter's ability to predict where the ball is going.
There aren't any physical laws that limit a batter's ability to hit every ball well, but there are physiological and
mental limits that lower everyone's batting average.
If the train track gets bumpier in effect with increasing speed, why is it that your car bumps less when you go over a
speed bump fast instead of slow?
Actually, if you drive fast over a real speed bump, it's not good for your wheels and suspension. The springs in
your car do protect the car from some of the effects of the bump, but not all of them. However, imagine driving
over a speed bump on a traditional bicycle--one that has no spring suspension. The faster you drive over that
bump, the more it will throw you into the air.
Are all metals magnetically charged?
First, magnets don't involve charges, they involve poles. So the question should probably be "are all metals
magnetically poled?" The answer to this question is that they are never poled--they never have a net pole. They
always have an even balance of north and south pole. However, there are some metals that have their north and
south poles separated from one another. A magnetized piece of steel is that way. Only a few metals can support
such separated poles and we will study those metals in a few weeks.
Would placing a blue filter on a Xerox machine prevent it from making copies, since blue light has more energy than red?
No. Blue light causes the photoconductor to conduct. When you use white light in a xerographic copier, it's the
blue and green portions of the light that usually do the copying. The red is wasted.
Why do poles have to come in pairs?
There don't appear to be any isolated poles in our universe, or at least none have been found. That's just the way
it is. As a result of this situation, the only way to create magnetism is through its relationship with electricity.
When you use electricity to create magnetic fields, you effectively create equal pairs of poles--as much north
pole as south pole.
Is the red light effect in xerographic copiers the same concept behind red lights in a darkroom? Does film have the same
sort of properties?
Yes. The light sensitive particles in black-and-white photographic paper don't respond to red light because the
energy in a photon of red light doesn't have enough energy to cause the required chemical change. In effect,
electrons are being asked to shift between levels when the light hits them and red light can't make that happen in
the photographic paper. However, most modern black-and-white films are sensitive to red light because that
makes roses and other red objects appear less dark and more realistic in the photographs.
How do color copiers work?
They assemble 4 colors, yellow, cyan, magenta, and black together to form the final image. The photoconductor
creates charge images using blue, red, green, and white illumination successively and uses those images to form
patterns of yellow, cyan, magenta, and black toner particles. These particles are then superimposed to form the
final image, which appears full color. Naturally, the photoconductor used in such a complicated machine must be
sensitive to the whole visible spectrum of light.
Does this photoconductor stuff have to do with why you can only develop film in the dark?
Yes. Particles of light, photons, cause chemical changes in the film. You can work with some black-and-white
films in red light because red light photons don't have enough energy to cause changes in those films. However,
color film and most modern black-and-white films require complete darkness during processing. If you expose
them to any visible light, you'll cause chemistry to occur.
Are black lights less or more conducive to charging the particles in film?
They are generally more conducive. Black light is actually ultraviolet light and its photons carry more energy
than any visible photon. They can cause chemical changes in many materials, including skin.
How do shampoo and conditioners in one work if shampoos have negative charges on one side and conditioners have
positive charges on one side?
I don't know. That question has puzzled me for years. The mixture should find its molecules clinging together.
They must contain something that keeps the oppositely charged systems separate from one another so that they
don't aggregate.
February 2, 1998
If electrons can't change levels, how can a photoconductor help them change one level to another?
In a metal, electrons can easily shift from one level to another empty level because the levels are close together
in energy. In a full insulator, it's very difficult for the electrons to shift from one level to an empty level because
all of the empty levels are far above the filled levels in energy. In a photoconductor, the empty levels are
modestly above the filled levels in energy, so a modest amount of energy is all that's needed to shift an electron.
This energy can be supplied by a particle or "photon" of light. An illuminated photoconductor conducts
electricity.
How does one create an electric or magnetic field?
The simplest way to make these fields is with electric charges (for an electric field) or with magnets (for a
magnetic field). Charges are naturally surrounded by electric fields and magnets are naturally surrounded by
magnetic fields. But fields themselves can create other fields by changing with time. That's how the fields in a
light wave work--the electric field in the light wave changes with time and creates the magnetic field and the
magnetic field changes with time and creates the electric field. This team of fields can travel through space
without any charge or magnets nearby.
How do you get static out of hair?
If you put a conditioner on your hair, it will attract enough moisture to allow static charge to dissipate.
How do dryer sheets diminish the clothes' static?
They leave a layer of conditioning soap on the clothes and this soap attracts moisture. The moisture conducts
electricity just enough to allow static charge to dissipate.
Does an MRI work in the same way as a copier (or puts you in a magnetic field and copies an image of your body)?
No, an MRI uses a very different technique for imaging your body. A copier uses light to examine the original
document while an MRI machine uses the magnetic responses of hydrogen atoms to map your body.
Can the electric current be taken out of the metal where the charge will not carry?
While charges can move freely through a metal, allowing the metal to carry electric current, it's much harder for
charges to travel outside of a conductor. Charges can move through the air or through plastic or glass, but not
very easily. It takes energy to pull the charges out of a metal and allow them to move through a non-metal. Most
of the time, this energy requirement prevents charges from moving through insulators such as plastic, glass, air,
and even empty space.
How does one "pull up their legs"? Wouldn't you have to jump in some way or another?
It is possible to simply pull up your legs. When you do that, you reduce the downward force your feet exert on
the ground and the ground responds by pushing upward on your feet less strongly. With less upward force to
support you, you begin to fall.
January 30, 1998
In alternating current, current reverses directions rapidly between the two wires, white and black. Why is it that only the
black wire is "hot"?
When you complete a circuit by plugging an appliance into an electrical outlet, current flows out one wire to the
appliance and returns to the electric company through the other wire. With alternating current, the roles of the
two wires reverse rapidly, so that at one moment current flows out the black wire to the appliance and moments
later current flows out the white wire to the appliance. But the power company drives this current through the
wires by treating the black wire specially--it alternately raises and lowers the electrostatic potential or voltage of
the black wire while leaving the voltage of the white wire unchanged with respect to ground. When the voltage
of the black wire is high, current is pushed through the black wire toward the appliance and returns through the
white wire. When the voltage of the black wire is low, current is pulled through the black wire from the
appliance and is replaced by current flowing out through the white wire.
The white wire is rather passive in this process because its voltage is always essentially zero. It never has a net
charge on it. But the black wire is alternately positively charged and then negatively charged. That's what makes
its voltage rise and fall. Since the black wire is capable of pushing or pulling charge from the ground instead of
from the white wire, you don't want to touch the black wire while you're grounded. You'll get a shock.
January 28, 1998
What is heat? What actually flows from a hot body to a cold body? -- AW, Pakistan
Heat is thermal energy that is flowing from one object to another. While several centuries ago, people thought
heat was a fluid, which they named "caloric," we now know that it is simply energy that is being transferred.
Heat moves via several mechanisms, including conduction, convection, and radiation. Conduction is the easiest
to visualize--the more rapidly jittering atoms and molecules in a hotter object will transfer some of their energy
to the more slowly jittering atoms in molecules in a colder object when you touch the two objects together. Even
though no atoms or molecules are exchanged, their energy is. In convection, moving fluid carries thermal energy
along with it from one object to another. In this case, there is material exchanged although usually only
temporarily. In radiation, the atoms and molecules exchange energy by sending thermal radiation back and forth.
Thermal radiation is electromagnetic waves and includes infrared light. A hotter object sends more infrared light
toward a colder object than vice versa, so the hotter object gives up thermal energy to the colder object.
Is it possible to create a magnet with more north poles than south poles? -- GS
No. All magnets that we know of have exactly equal amounts of north and south pole. That's because we have
never observed a pure north or a pure south pole in nature and you'd need such a pure north or south pole to
unbalance the poles of a magnet.
The absence of such "monopoles" is an interesting puzzle and scientists haven't given up hope of finding them.
Some theories predict that they should exist, but be very difficult to form artificially. There may be magnetic
monopoles left over from the big bang, but we haven't found any yet.
Is hydroplaning a form of sliding friction?
Yes. When you begin to skid on dry pavement, your car's wheels are experiencing sliding friction. The sliding
friction force on those wheels is pretty strong because rubber and asphalt grip one another quite well. But if there
is a layer of water in between the two surfaces, the water lubricates them and the force of sliding friction drops
precipitously. That's why you can skid so far when you are hydroplaning on a thin layer of water.
January 26, 1998
If you walk up 10 steps, one by one, do you exert the same amount of energy if you walk up the same set of 10 steps two
by two? How are energy and effort related, or are they?
Ideally, it doesn't matter how many steps you take with each step--the work you do in lifting yourself up a
staircase depends only on your starting height and your ending height (assuming that you don't accelerate or
decelerate in the overall process and thus change your kinetic energy, too). But there are inefficiencies in your
walking process that lead you to waste energy as heat in your own body. So the energy you convert from food
energy to gravitational potential energy in climbing the stairs is fixed, but the energy you use in carrying out this
procedure depends on how you do it. The extra energy you use mostly ends up as thermal energy, but some may
end up as sound or chemical changes in the staircase, etc.
If ball bearings create no friction, why do bearings have bearing grease as an essential ingredient?
Actually, some bearings are dry (no grease or oil) and still last a very long time. The problem is that the idea
touch-and-release behavior is hard to achieve in a bearing. The balls or rollers actually slip a tiny bit as they
rotate and they may rub against the sides or retainers in the bearing. This rubbing produces wear as well as
wasting energy. To reduce this wear and sliding friction, most bearings are lubricated.
How do anti-lock brake systems work?
If you brake your car too rapidly, the force of static friction between the wheels and the ground will become so
large that it will exceed its limit and the wheels will begin to skid across the ground. Once skidding occurs, the
stopping force becomes sliding friction instead of static friction. The sliding friction force is generally weaker
than the maximum static friction force, so the stopping rate drops. But more importantly, you lose steering when
the wheels skid. An anti-lock braking system senses when the wheels suddenly stop turning during braking and
briefly release the brakes. The wheel can then turn again and static friction can reappear between the wheel and
the ground.
January 23, 1998
How can a ball create thermal energy or "get hotter"?
When a ball bounces, some of its molecules slide across one another rather than simply stretching or bending.
This sliding leads to a form of internal sliding friction and sliding friction converts useful energy into thermal
energy. The more sliding friction that occurs within the ball, the less the ball stores energy for the rebound and
the worse the ball's bounce. The missing energy becomes thermal energy in the ball and the ball's temperature
increases.
January 21, 1998
You discussed how an egg doesn't bounce because it doesn't have time and instead it breaks. Why, then, does a mouse
ball (in a computer mouse) or a bowling ball not bounce? It doesn't break, so why doesn't the support force make it
bounce back upward. Does this relate to elasticity?
Actually, both a mouse ball and a bowling ball will bounce somewhat if you drop them on a suitably hard
surface. It does have to do with elasticity. During the impact, the ball's surface dents and the force that dents the
ball does work on the ball--the force on the ball's surface is inward and the ball's surface moves inward. Energy
is thus being invested in the ball's surface. What the ball does with this energy depends on the ball. If the ball is
an egg, the denting shatters the egg and the energy is wasted in the process of scrambling the egg's innards. But
in virtually any normal ball, some or most of the work done on the ball's surface is stored in the elastic forces
within the ball--this elastic potential energy, like all potential energies, is stored in forces. This stored energy
allows the surface to undent and do work on other things in the process. During the rebound, the ball's surface
undents. Although it's a little tricky to follow the exact flow of energy during the rebound, the elastic potential
energy in the dented ball becomes kinetic energy in the rebounding ball. But even the best balls waste some of
the energy involved in denting their surfaces. That's why balls never bounce perfectly and never return to their
original heights when dropped on a hard, stationary surface. Some balls are better than others at storing and
returning this energy, so they bounce better than others.
When an egg falls and hits the table, the table pushes up on it, doesn't it? The same with a bouncing ball?
Yes, when a falling object hits a table, the table pushes up on the falling object. What happens from then on
depends on the object's characteristics. The egg shatters as the table pushes on it and the ball bounces back
upward.
When a rubber ball bounces or rebounds, does the weight of the ball determine how many times it bounces?
Each time the ball bounces, it rises to a height that is a certain fraction of its height before that bounce. The ratio
of these two heights is the fraction of the ball's energy that is stored and returned during the bounce. A very
elastic ball will return about 90% of its energy after a bounce, returning to 90% of its original height after a
bounce. A relatively non-elastic ball may only return about 20% of its energy and bounce to only 20% of its
original height. It is this energy efficiency that determines how many times a ball bounces. The missing energy is
usually converted into thermal energy within the ball's internal structure.
What is thermal energy?
While we ordinarily associate energy with an object's overall movement or position or shape, the individual
atoms and molecules within the object can also have their own separate portions of energy. Thermal energy is the
energy associated with the motions and positions of the individual atoms within the object. While an object may
be sitting still, its atoms and molecules are always jittering about, so they have kinetic energies. When they push
against one another during a bounce, they also have potential energies. These internal energies, while hard to see,
are thermal energy.
I don't understand work done without any acceleration. Since F=ma and a=0, F=0 and thus W=0.
You are merging two equations out of context. The force you exert on an object can be non-zero without causing
that object to accelerate. For example, if someone else is pushing back on the object, the object may not
accelerate. If the object moves away from you as you push on it, then you'll be doing work on the object even
though it's not accelerating. The only context in which you can merge those two equations (Force=mass x
acceleration and Work=Force x distance) is when you are exerting the only force on the object. In that case, your
force is the one that determines the object's acceleration and your force is the one involved in doing work. In that
special case, if the object doesn't accelerate, then you do no work because you exert no force on the object! If
someone else is pushing the object, then the force causing it to accelerate is the net force and not just your force
on the object. As you can see, there are many forces around and you have to be careful tacking formulae together
without thinking carefully about the context in which they exist.
January 19, 1998
What effects do forces acting on an object which are not from the same pair have on one another? i.e. the force pulling
the egg downward and the potential force of the table? Are they equal upon impact and there a pair?
Different forces acting on a single object are not official pairs; not the pairs associated with Newton's third law of
action-reaction. While it is possible for an object to experience two different forces that happen to be exactly
equal in magnitude (amount) but opposite in direction, that doesn't have to be the case. When an egg falls and
hits a table, the egg's downward weight and the table's upward support force on the egg are equal in magnitude
only for a fleeting instant during the collision. That's because the table's support force starts at zero while the egg
is falling and then increases rapidly as the egg begins to push against the table's surface. For just an instant the
table pushes upward on the egg with a force equal in magnitude to the egg's weight. But the upward support
force continues to increase in strength and eventually pushes a hole in the egg's bottom.
If there is an upward force on the egg when it hits the table, why doesn't it bounce upward?
The enormous upward force on the egg when it hits the table does cause the egg to accelerate upward briefly.
The egg loses all of its downward velocity during this upward acceleration. But the egg breaks before it has a
chance to acquire any upward velocity and, having broken, it wastes all of its energy ripping itself apart into a
mess. If the egg had survived the impact and stored its energy, it probably would have bounced, at least a little.
But the upward force from the table diminished abruptly when the egg broke and the egg never began to head
upward for a real bounce.
How does the egg (sitting on a table) hold up the table? If the "weight vs. support force of table" is not always an equal
pair then how is the "support force of the egg vs. the table" an equal pair?
When an egg is sitting on a table, each object is exerting a support force on the other object. Those two support
forces are equal in magnitude (amount) but opposite in direction. To be specific, the table is pushing upward on
the egg with a support force and the egg is pushing downward on the table with a support force. Both forces have
the same magnitude--both are equal in magnitude to the egg's weight. The fact that the egg is pushing downward
on the table with a "support" force shows that not all support forces actually "support" the object they are exert
on. The egg isn't supporting the table at all. But a name is a name and on many occasions, support forces do
support the objects they're exerted on.
January 16, 1998
When people are able to bend spoons or move tables with their minds (if this is actually possible and not just a hoax),
what sort of force is being exerted on the object? Is it possible to create forces with the mind?
I'm afraid that spoon bending is simply a hoax. While there are electrochemical processes going on in the mind
that exert detectable forces on special probes located outside the head, these forces are so small that they are
incapable of doing anything as demanding as bending a spoon. Spoon bending and all other forms of telekinesis
are simply tricks played on gullible audiences.
Why is there more gravity acting on larger, more massive objects?
The fact that more massive objects also weigh more is just an observation of how the universe works. However,
any other behavior would lead to some weird consequences. Suppose, for example, that an object's weight didn't
depend on its mass, that all objects had the same weight. Then two separate balls would each weigh this standard
amount. But now suppose that you glued the two balls together. If you think of them as two separate balls that
are now attached, they should weigh twice the standard amount. But if you think of them as one oddly shaped
object, they should weigh just the standard amount. Something wouldn't be right. So the fact that weight is
proportional to mass is a sensible situation and also the way the universe actually works.
Why is it that when people jump, they don't bounce up?
A ball bounces because its surface is elastic and it stores energy during the brief period of collision when the ball
and floor are pushing very hard against one another. Much of this stored energy is released in a rebound that
tosses the ball back upward for another bounce. But people don't store energy well during a collision and they
don't rebound much. The energy that we should store is instead converted into thermal energy--we get hot rather
than bouncing back upward.
Why does the bigger ball have more gravity pulling on it? Because it weighs more? Which causes which?
The force that gravity exerts on an object is that object's weight. An object that has more gravity pulling on it
weighs more and vice versa.
When you throw a ball upward and claim that there is no upward force on it as it rises, why don't you count your hand?
The ball was thrown up, so there was an upward force on it! I'm confused.
While you are throwing the ball upward, you are pushing it upward and there is an upward force on the ball. But
as soon as the ball leaves your hand, that upward force vanishes and the ball travels upward due to its inertia
alone. In the discussion of that upward flight, I always said "after the ball leaves your hand," to exclude the time
when you are pushing upward on the ball. Starting and stopping demonstrations are often tricky and I meant you
to pay attention only to the period when the ball was in free fall.
When you drop a small rubber ball and a large rubber ball simultaneously, why do they both hit the floor at the same
time?
The fact that both balls fall together is the result of a remarkable balancing effect. Although the larger ball is
more massive than the smaller ball, making the larger ball harder to start or stop, the larger ball is also heavier
than the smaller ball, meaning that gravity pulls downward more on the larger ball. The larger ball's greater
weight exactly compensates for its greater mass, so that it is able to keep up with the smaller ball as the two
objects fall to the ground. In the absence of air resistance, the two balls will move exactly together-the larger ball
with its greater mass and greater weight will keep up with the smaller ball.
When you drop a baseball and a bowling ball, you say that its velocity acts faster and faster as it falls. How can you say
that the acceleration is constant at 9.8 m/s2? If it is falling faster and faster wouldn't the acceleration change also until the
object reaches terminal velocity and then it would be accelerating at 9.8 m/s2?
It's very important to distinguish velocity from acceleration. Acceleration is caused only by forces, so while a
ball is falling freely it is accelerating according to gravity alone. In that case it accelerates downward at 9.8 m/s2
throughout its fall (neglecting air resistance). But while the ball's acceleration is constant, its velocity isn't.
Instead, the ball's velocity gradually increases in the downward direction, which is to say that the ball accelerates
in the downward direction. Velocity doesn't "act"--only forces "act." Instead, a ball's velocity shifts more and
more toward the downward direction as it falls.
About terminal velocity: when an object descends very rapidly through the air, it experiences a large upward
force of air resistance. This new upward force becomes stronger as the downward speed of the object becomes
greater. Eventually this upward air resistance force balances the object's downward weight and the object stops
accelerating downward. It then descends at a constant velocity--obeying its inertia alone. This special downward
speed is known as "terminal velocity." An object's terminal velocity depends on the strength of gravity, the shape
and other characteristics of the object, and the density and other characteristics of the air.
How is there inertia on earth? I though that inertia was just in space.
Inertia is everywhere. Left to itself, an object will obey inertia and travel at constant velocity. In deep space, far
from any planet or star that exerts significant gravity, an object will exhibit this inertial motion. But on earth, the
earth's gravity introduces complications that make it harder to observe inertial motion. A ball that's thrown up in
the air still exhibits inertial effects, but its downward weight prevents the ball from following its inertia alone.
Instead, the ball gradually loses its upward speed and eventually begins to descend instead. So inertia is the basic
underlying principle of motion while gravity is a complicating factor.
How does the floor exert a force?
When you stand on the floor, the floor exerts two different kinds of forces on you--an upward support force that
balances your downward weight and horizontal frictional forces that prevent you from sliding across the floor.
Ultimately, both forces involve electromagnetic forces between the charged particles in the floor and the charged
particles in your feet. The support force develops as the atoms in the floor act to prevent the atoms in your feet
from overlapping with them. The frictional forces have a similar origin, although they involve microscopic
structure in the surfaces.
November 12, 1997
My daughter did a school project in which we placed a thermometer inside cloths of various colors. Black cloth showed
the highest temperature, blue next, then red, and finally white. Why is that?
Since light carries energy with it, a cloth that absorbs light also absorbs energy. In most cases, this absorbed
energy becomes thermal energy in the cloth. Because of this extra thermal energy, the cloth's temperature rises
and it begins to transfer the thermal energy to its surroundings as heat. Its temperature stops rising when the
thermal energy it receives from the light is exactly equal to the thermal energy it transfers to its surroundings as
heat. This final temperature depends on how much light it absorbs--if it absorbs lots of light, then it will reach a
high temperature before the balance of energy flow sets in.
A cloth's color is determined by how it absorbs and emits light. Black cloth absorbs essentially all light that hits
it, which is why its temperature rises so much. White cloth absorbs virtually no light, which is why it remains
cool. Colored cloths fall somewhere in between black and white. Blue cloth absorbs light in the green and red
portions of the spectrum while reflecting the blue portion. Red cloth absorbs light in the blue and green portions
of the spectrum while reflecting the red portion. Since most light sources put more energy in the red portion of
the spectrum than in the blue portion of the spectrum, the blue cloth absorbs more energy than the red cloth. So
the sequence of temperatures you observed is the one you should expect to observe.
One final note: most light sources also emit invisible infrared light, which also carries energy. Most of the light
from an incandescent lamp is infrared. You can't tell by looking at a piece of cloth how much infrared light it
absorbs and how much it reflects. Nonetheless, infrared light affects the cloth's temperature. A piece of white
cloth that absorbs infrared light may become surprisingly hot and a piece of black cloth that reflects infrared light
may not become as hot as you would expect.
November 11, 1997
Why does a roller coaster end on a lower level than where it starts? -- L, Staten Island, New York
A roller coaster is a gravity-powered train. Since it has no engine or other means of propulsion, it relies on
energy stored in the force of gravity to make it move. This energy, known as "gravitational potential energy,"
exists because separating the roller coaster from the earth requires work--they have to be pulled apart to separate
them. Since energy is a conserved quantity, meaning that it can't be created or destroyed, energy invested in the
roller coaster by pulling it away from the earth doesn't disappear. It becomes stored energy: gravitational
potential energy. The higher the roller coaster is above the earth's surface, the more gravitational potential energy
it has.
Since the top of the first hill is the highest point on the track, it's also the point at which the roller coaster's
gravitational potential energy is greatest. Moreover, as the roller coaster passes over the top of the first hill, its
total energy is greatest. Most of that total energy is gravitational potential energy but a small amount is kinetic
energy, the energy of motion.
From that point on, the roller coaster does two things with its energy. First, it begins to transform that energy
from one form to another--from gravitational potential energy to kinetic energy and from kinetic energy to
gravitational potential energy, back and forth. Second, it begins to transfer some of its energy to its environment,
mostly in the form of heat and sound. Each time the roller coaster goes downhill, its gravitational potential
energy decreases and its kinetic energy increases. Each time the roller coaster goes uphill, its kinetic energy
decreases and its gravitational potential energy increases. But each transfer of energy isn't complete because
some of the energy is lost to heat and sound. Because of this lost energy, the roller coaster can't return to its
original height after coasting down hill. That's why each successive hill must be lower than the previous hill.
Eventually the roller coaster has lost so much of its original total energy that the ride must end. With so little
total energy left, the roller coaster can't have much gravitational potential energy and must be much lower than
the top of the first hill.
It's then time for the riders to get off, new riders to board, and for a motor-driven chain to drag the roller coaster
back to the top of the hill to start the process again. The chain does work on the roller coaster, investing energy
into it so that it can carry its riders along the track at break-neck speed again. Overall, energy enters the roller
coaster by way of the chain and leaves the roller coaster as heat and sound. In the interim, it goes back and forth
between gravitational potential energy and kinetic energy as the roller coaster goes up and down the hills.
Is bouncing related to elasticity or hardness? Can a hard body rebound? -- DIY, Lyon, France
Bouncing is related to elasticity. Any object that stores energy when deformed will rebound when it collides with
a rigid surface. As long as the object is elastic, it doesn't matter whether it's hard or soft. It will still rebound from
a rigid surface. Thus both a rubber ball and a steel marble will rebound strongly when you drop them on a steel
anvil.
But hardness does have an important effect on bouncing from a non-rigid surface. When a hard object collides
with a non-rigid surface, the surface does some or all of the deforming so that the surface becomes involved in
the energy storage and bounce. If the surface is elastic, storing energy well when it deforms, then it will make the
object rebound strongly. That's what happens when a steel marble collides with a rubber block. However, if the
surface isn't very elastic, then the object will not rebound much. That's what happens when a steel marble
collides with a thick woolen carpet.
How does a dead ball work? -- DIY, Lyon, France
A dead ball, a ball that doesn't bounce, is one with enormous internal friction. A bouncy ball stores energy when
it collides with a surface and then returns this energy when it rebounds. But no ball is perfectly elastic, so some
of the collision energy extracted from the ball and surface when they collide is ultimately converted into heat
rather than being returned during the rebound. The deader the ball is, the less of the collision energy is returned
as rebound energy. A truly dead ball converts all of the collision energy into heat so that it doesn't rebound at all.
Most of the missing collision energy is lost because of sliding friction within the ball. Molecules move across
one another as the ball's surface dents inward and these molecules rub. This rubbing produces heat and
diminishes the elastic potential energy stored in the ball. When the ball subsequently undents, there just isn't as
much stored energy available for a strong rebound. The classic dead "ball" is a beanbag. When you throw a
beanbag at a wall, it doesn't rebound because all of its energy is lost through sliding friction between the beans as
the beanbag dents.
November 10, 1997
Is there any equipment that can track people in a large, dense forest? -- BRAR, India
To track someone in a forest, he must be emitting or reflecting something toward you and doing it in a way that
is different from his surroundings. For example, if he is talking in a quiet forest, you can track him by his sound
emissions. Or if he is exposed to sunlight in green surroundings, you can track him by his reflections of light.
But while both of these techniques work fine at short distances, they aren't so good at large distances in a dense
forest. A better scheme is to look for his thermal radiation. All objects emit thermal radiation to some extent and
the spectral character of this thermal radiation depends principally on the temperatures of the objects. If the
person is hotter than his surroundings, as is almost always the case, he will emit a different spectrum of thermal
radiation than his surrounds. Light sensors that operate in the deep infrared can detect a person's thermal
radiation and distinguish it from that of his cooler surroundings. Still, viewing that thermal radiation requires a
direct line-of-sight from the person to the infrared sensor, so if the forest is too dense, the person is untrackable.
Why does a badminton birdie have such a large tip? Does making it bigger protect the racket? -- J, California
The large, rounded head of a badminton birdie serves at least two purposes: it makes sure that the birdie bounces
predictably off the racket's string mesh and it protects the strings and birdie from damage. If the birdie's head
were smaller, it would strike at most a small area on one of the racket strings. If it hit that string squarely, the
birdie might bounce predictably. But if it hit at a glancing angle, the birdie would bounce off at a sharp angle. By
spreading out the contact between the birdie and the string mesh, the large head makes the birdie bounce as
though it had hit a solid surface rather than one with holes.
Spreading out the contact also prevents damage to the racket and birdie. If they collided over only a tiny area, the
forces they exerted on one another would be concentrated over that area and produce enormous local pressures.
These pressures could cut the birdie or break a string. But with the birdie's large head, the pressures involved are
mild and nothing breaks.
If you use a heavier racket, will you be able to hit a badminton birdie farther? -- J, California
Any time you hit an object with a racket or bat, there's a question about how heavy the racket or bat should be for
maximum distance. Actually, it isn't weight that's most important in a racket or bat, it's mass--the measure of the
racket or bat's inertia. The more massive a racket or bat is, the more inertia it has and the less it slows down when
it collides with something else. A more massive racket will slow less when it hits a birdie. From that observation,
you might think that larger mass is always better. But a more massive racket or bat is also harder to swing
because of its increased inertia.
So there are trade offs in racket or bat mass. For badminton, the birdie has so little mass that it barely slows the
racket when the two collide. Increasing the racket's mass would allow it to hit the birdie slightly farther, but only
if you continued to swing the racket as fast as before. Since increasing the racket mass will make it harder to
swing, it's probably not worthwhile. In all likelihood, people have experimented with racket masses and have
determined that the standard mass is just about optimal for the game.
What is an event horizon? -- KRH
An event horizon is the surface around a black hole from which not even light can escape. But to make it clearer
what that statement means, consider first what happens to the light from a flashlight that's resting on the surface
of a large planet. Light is affected by gravity--it falls just like everything else. The reason you never notice this
fact is that light travels so fast that it doesn't have time to fall very far. But suppose that the gravity on the planet
is extremely strong. If the flashlight is aimed horizontally, the light will fall and arc downward just enough that it
will hit the surface of the planet before escaping into space. To get the light to leave the planet, the flashlight
must be tipped a little above horizontal.
If the planet's gravity is even stronger, the flashlight will have to be tipped even more above horizontal. In fact, if
the gravity is sufficiently strong, light can only avoid hitting the planet if the flashlight is aimed almost straight
up. And beyond a certain strength of gravity, even pointing the flashlight straight up won't keep the light from
hitting the planet's surface.
When that situation occurs, an event horizon forms around the planet and forever separates the planet from the
universe around it. Actually, the planet ceases to exist as a complex object and is reduced to its most basic
characteristics: mass, electric charge, and angular momentum. The planet becomes a black hole. and light
emitted at or within this black hole's event horizon falls inward so strongly that it doesn't escape. Since nothing
can move faster than light, nothing else can escape from the black hole's event horizon either.
The nature of space and time at the event horizon are quite complicated and counter-intuitive. For example, an
object dropped into a black hole will appear to spread out on the event horizon without ever entering it. That's
because, to an outside observer, time slows down in the vicinity of the event horizon. By that, I mean that it takes
an infinite amount of our time for an object to fall through that event horizon. But the object itself doesn't
experience a change in the flow of time. For it, time passes normally and it zips right through the event horizon.
Finally, event horizons and the black holes that have them aren't truly black--quantum mechanical fluctuations at
the event horizon allow black holes to emit particles and radiation. This "Hawking radiation," discovered by
Stephen Hawking about 25 years ago, means that black holes aren't truly black. Nonetheless, objects that fall into
an event horizon never leave intact.
Is it possible to track a person based on the fact that they are listening to a radio receiver? -- BRAR, India
While tracking a radio transmitter is easy--you only need to follow the radio waves back to their source--you
might think that tracking a radio receiver is impossible. After all, a radio receiver appears to be a passive device
that collects radio waves rather than emitting them. But that's not entirely true. Sophisticated radio receivers
often use heterodyne techniques in which the signal from a local radio-frequency oscillator is mixed with the
signal coming from the antenna. The mixing process subtracts one frequency from the other so that antenna
signals from a particular radio station are shifted downward in frequency into the range the radio uses to create
sound. This mixing process allows the radio receiver to be very selective about which station it receives. The
receiver can easily distinguish the station that's nearest in frequency to its local oscillator from all the other
stations, just as its easy to tell which note on a piano is closest in pitch to a particular tuning fork.
But heterodyne techniques have a side effect: they cause the radio receiver to emit radio waves. These waves
originate with the local radio-frequency oscillator, and with other internal mixing frequencies such as the
intermediate frequency oscillator present in many sophisticated receivers. Because these oscillators don't use
very much power, the waves they emit aren't very strong. Nonetheless, they can be detected, particularly at short
range. For example, it's possible for police to detect a radar detector that contains its own local microwave
oscillator. Similarly, people who have tried to pirate microwave transmissions have been caught because of the
microwaves emitted from their receivers. In WWII, the Japanese were apparently very successful at locating US
forces by detecting the 455 kHz intermediate frequency oscillators in their radios--a problem that quickly led to a
redesign of the radios to prevent that 455 kHz signal from leaking onto the antennas (thanks to Tom Skinner for
pointing this out to me). As you can see, it is possible to track someone who is listening to the right type of radio
receiver. However, the radio waves from that receiver are going to be very weak and you won't be able to follow
them from a great distance.
Lunar gravity is partly what causes oceanic currents. If we had more than one moon orbiting Earth, what [if anything],
would happen to the oceans? -- MS, St. Charles, Missouri
While the moon's gravity is the major cause of tides (the sun plays a secondary role), the moon's gravity isn't
directly responsible for any true currents. Basically, water on the earth's surface swells up into two bulges: one
on the side of the earth nearest the moon and one on the side farthest from the moon. As the earth turns, these
bulges move across its surface and this movement is responsible for the tides.
If there were more than one moon, the tidal bulges would become misshapen. That is essentially what happens
because of the sun. As the moon and sun adopt different arrangements around the earth, the strengths of the tides
vary. The strongest tides (spring tides) occur when the moon and sun are on the same or opposite sides of the
earth. The weakest tides (neap tides) occur when the moon and sun are at 90° from one another. Extra moons
would probably just complicate this situation so that the strengths of the tides would vary erratically as the
moons shifted their positions around the earth. Since the timing of the tides is still basically determined by the
earth's rotation, there would still be approximately 2 highs and 2 lows a day.
October 30, 1997
I'm grateful for your work and the availability of your site. Though I think that your ignorant condemnation of the work
of other professionals about whose work you know absolutely nothing is contemptuous. Once again the arrogance of the
established order and refusal to open-minded investigation. I would not have this opinion if you had used the careful,
open-minded, systematic investigation that you espouse before you let your ego expose your ignorance. Carolyne Myss,
with a verifiable accuracy rate of 93% percent, should not be called a quack. I wonder if, in your answers on this site, you
could attain that rate of accuracy. I sincerely doubt it. In fact, with your over-blown ego, you could really benefit from her
work. So stick to the information about lightning and CDs and stay away from that which you obviously are quite
ignorant! -- Unsigned
This comment, which responds to a previous posting on this site, points out one of the most important
differences between physical science and pseudo-science: the fact that pseudo-science isn't troubled by its lack of
self-consistency.
Physical science, particularly physics itself, is completely self-consistent. By that I mean that the same set of
physical rules applies to every possible situation in the universe and that this set of rules never leads to
paradoxical results. Despite its complicated behavior, the universe is orderly and predictable. It's precisely this
order and predictability that is the basis for the whole field of physics.
In contrast, pseudo-science is eclectic--it draws from physics and magic as it sees fit. It uses the laws of physics
when it finds those laws useful and it ignores the laws of physics when they conflict with its interests. But the
laws of physics only make sense if they apply universally--if there were even one situation in which a law of
physics didn't apply, physics would lose its self-consistency and predictive power. That's just what happens with
pseudo-science when it begins to ignore the laws of physics on occasion. Moreover, the new rules that pseudoscience introduces to replace the ones it ignores make the trouble even worse. Overall, pseudo-science is
inconsistent and can't be counted on to predict anything.
Pseudo-science might argue that the laws of physics are correct as far as they go, but that they're incomplete. No
doubt the laws of physics are incomplete; physicists have frequently discovered improvements to the laws of
physics that have allowed them to make even more accurate predictions of the universe's behavior. But in the
years since the discoveries of relativity and quantum physics, the pace of such discoveries has slowed and what
remains to be understood is at a very deep and subtle level. It's extraordinarily unlikely that the laws of physics
as they're currently understood are wrong at a level that would allow a person to bend a spoon with their thoughts
alone or predict the order of a deck of cards without assistance. Just because I haven't dropped a particular book
doesn't prevent me from predicting that it will fall when I let go of it. I understand the laws that govern its motion
and I know that having it fly upward would violate those laws. Similarly, I don't have to watch someone try to
bend a spoon with their thoughts to know that it can't be done legitimately. Again, I understand the laws that
govern the spoon's condition and I know that having it bend without an identifiable force acting on it would
violate those laws. I also don't have to watch someone try to predict cards to know that it, too, can't be done
legitimately. Without a clear physical mechanism for transporting information from the cards to the person, a
mechanism that must involve forces or exchanges of particles, there is no way for the person to predict the cards.
I enjoy watching the pole-vaulters at the Olympics, especially Daly Thompson. Could you explain the physics of the pole
vault for me? -- ZG, Bullcreek, West Australia
The pole vault is all about energy and energy storage. Lifting a person upward takes energy because there is an
energy associated with altitude--gravitational potential energy. Lifting a person 5 or 6 meters upward takes a
considerable amount of energy and that energy has to come from somewhere. In the case of a pole-vaulter, most
of the lifting energy comes from the pole. But the pole also had to get the energy from somewhere and that
somewhere is the vaulter himself. Here is the story as it unfolds:
When the pole-vaulter stands ready to begin his jump, he is motionless on the ground and he has no kinetic
energy (energy of motion), minimal gravitational potential energy (energy of height), and no elastic energy in his
pole. All he has is chemical potential energy in his body, energy that he got by eating food. Now he begins to run
down the path toward the jump. As he does so, he converts chemical potential energy into kinetic energy. By the
time he plants his pole at the jump, his kinetic energy is quite large.
But once he plants the pole, the pole begins to bend. As it does, he slows down and his kinetic energy is partially
transferred to the pole, where it becomes elastic potential energy. The pole then begins to lift the vaulter upward,
returning its stored energy to him as gravitational potential energy. By the time the vaulter clears the bar, 5 or 6
meters above the ground, almost all of the energy in the situation is in the form of gravitational potential energy.
The vaulter has only just enough kinetic energy to carry him past the bar before he falls. On his way down, his
gravitational potential energy becomes kinetic energy and he hits the pit at high speed. The pit's padding extracts
his kinetic energy from him gently and converts that energy into thermal energy. This thermal energy then floats
off into the air as heat.
One interesting point about jumping technique involves body shape. The vaulter bends his body as he passes
over the bar so that his average height (his center of gravity) never actually gets above the bar. Since his
gravitational potential energy depends on his average height, rather than the height of his highest part, this
technique allows him to use less overall energy to clear the bar.
Why is it that when you stand in front of a flat mirror, your image is reversed horizontally (left-right) but remains the
same vertically (up-down)? -- CC, Martinsville, NJ
A mirror doesn't really flip your image horizontally or vertically. After all, the image of your head is still on top
and the image of your left hand is still on the left. What the mirror does flip is which way your image is facing.
For example, if you were facing north, then your image is facing south. This front-back reversal makes your
image fundamentally different from you in the same way a left shoe is fundamentally different from a right shoe.
No matter how you arrange those two shoes, they'll always be reversed in one direction. Similarly, no matter how
you arrange yourself and your image, they'll always be reversed in one direction.
While you're looking at your image, the reversed direction is the forward-backward direction. But it's natural to
imagine yourself in the place of your image. To do this you imagine turning around to face in the direction that
your image is facing. When you turn in this manner, you mentally eliminate the forward-backward reversal but
introduce a new reversal in its place: a left-right reversal. If you were to imagine standing on your head instead,
you would still eliminate the forward-backward reversal but would now introduce an up-down reversal. Since it's
hard to imagine standing on your head in order to face in the direction your image is facing, you tend to think
only about turning around. It's this imagined turning around that leads you to say that your image is reversed
horizontally.
October 22, 1997
What holds the atoms in a molecule together?
The atoms in a molecule are usually held together by the sharing or exchange of some of their electrons. When
two atoms share a pair of electrons, they form a covalent bond that lowers the overall energy of the atoms and
sticks the atoms together. About half of this energy reduction comes from an increase in the negatively charged
electron density between the atoms' positively charged nuclei and about half comes from a quantum mechanical
effect--giving the two electrons more room to move gives them longer wavelengths and lowers their kinetic
energies.
When two atoms exchange an electron, they form an ionic bond that again lowers the overall energy of the atoms
and sticks them together. Although moving the electron from one atom to the other requires some energy, the
two atomic ions that are formed by the transfer have opposite charges and attract one another strongly. The
reduction in energy that accompanies their attraction can easily exceed the energy needed to transfer the electron
so that the two atoms become permanently stuck to one another.
October 16, 1997
The earth's surface is moving at something like 950 mph as it rotates. Why don't we notice this when we are in an
airplane? -- DT, Nicosia, Cyprus
It's true that the earth's surface is moving eastward rapidly relative to the earth's center of mass. However, that
motion is very difficult to detect. When you are standing on the ground, you move with it and so does everything
around you, including the air. While you are actually traveling around in a huge circle once a day, for all
practical purposes we can imagine that you are traveling eastward in a straight line at a constant speed of 950
mph relative to the earth's center of mass. Ignoring the slight curvature of your motion, you are in what is known
as an inertial frame of reference, meaning a viewpoint that is not accelerating but is simply coasting steadily
through space.
You'll notice that I keep saying "relative to the earth's center of mass" when I discuss motion. I do that because
there is no special "absolute" frame of reference. Any inertial frame is as good as any other frame and your
current inertial frame is just as good as anyone else's. In fact, you are quite justified in declaring that your frame
of reference is stationary and that everyone else's frames of reference are moving. After all, you don't detect any
motion around you so why not declare that your frame is officially stationary. Since the air is also stationary in
that frame of reference, flying about in the air doesn't make things any more complicated. You are flying through
stationary air in your old stationary frame of reference. The only way in which the 950 mph speed appears now is
in comparing your frame of reference to the rest of the earth: in your frame of reference, the earth's center of
mass is moving westward at 950 mph.
I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn't that show that glass flows? -MJ
While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases
appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant
glass bubble on the end of a blowpipe or "punty" and this bubble was cut open at the end and spun into a huge
disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in
thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were
usually put in thick end down.
Modern studies of glass show that below the glass transition temperature, which is well above room temperature,
molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and
becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.
I understand that light waves cause electrically charged particles in matter to vibrate so that these particles can absorb and
reemit light, even in transparent materials. But doesn't that explanation contradict quantum theory, which states that only
specific photons corresponding to allowed electronic transitions can be absorbed? -- GS, Akron, OH
When a light wave passes through matter, the charged particles in that matter do respond--the light wave
contains an electric field that pushes on electrically charged particles. But how a particular charged particle
responds to the light wave depends on the frequency of the light wave and on the quantum states available to the
charged particle. While the charged particle will begin to vibrate back and forth at the light wave's frequency and
will begin to take energy from the light wave, the charged particle can only retain this energy permanently if
doing so will promote it to another permanent quantum state. Since light energy comes in discrete quanta known
as photons and the energy of a photon depends on the light's frequency, it's quite possible that the charged
particle will be unable to absorb the light permanently. In that case, the charged particle will soon reemit the
light.
In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon. As it
plays, the charged particle begins to shift into a new quantum state--a "virtual" state. This virtual state may or
may not be permanently allowed. If it is, it's called a real state and the charged particle may remain in it
indefinitely. In that case, the charged particle can truly absorb the photon and may never reemit it at all. But if
the virtual state turns out not to be a permanently allowed quantum state, the charged particle can't remain in it
long and must quickly return to its original state. In doing so, this charged particle reemits the photon it was
playing with. The closer the photon is to one that it can absorb permanently, meaning the closer the virtual
quantum state is to one of the real quantum states, the longer the charged particle can play with the photon before
recognizing that it must give the photon up.
A colored material is one in which the charged particles can permanently absorb certain photons of visible light.
Because this material only absorbs certain photons of light, it separates the components of white light and gives
that material a colored appearance.
A transparent material is one in which the charged particles can't permanently absorb any photons of visible
light. While these charged particles all try to absorb the visible light photons, they find that there are no
permanent quantum states available to them when they do. Instead, they play with the photons briefly and then
let them continue on their way. This playing process slows the light down. In general blue light slows down more
than red light in a transparent material because blue light photons contain more energy than red light photons.
The charged particles in the transparent material do have real permanent states available to them, but to reach
those states, the charged particles would have to absorb high-energy photons of ultraviolet light. While blue
photons don't have as much energy as ultraviolet photons, they have more energy than red photons do. As a
result, the charged particles in a transparent material can play with a blue photon longer than they can play with a
red photon--the virtual state produced by a blue photon is closer to the real states than is the virtual state
produced by a red photon. Because of this effect, the speed at which blue light passes through a transparent
material is significantly less than the speed at which red light passes through that material.
Finally, about quantum states: you can think of the real states of one of these charged particles the way you think
about the possible pitches of a guitar string. While you can jiggle the guitar string back and forth at any
frequency you like with your fingers, it will only vibrate naturally at certain specific frequencies. You can hear
these frequencies by plucking the string. If you whistle at the string and choose one of these specific frequencies
for your pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from your
whistle. But if you whistle at some other frequency, the string will only play briefly with your sound wave and
then send it on its way. The string playing with your sound waves is just like a charged particle in a transparent
material playing with a light wave. The physics of these two situations is remarkably similar.
What is the force produced when two cars crash? -- DT, Nicosia, Cyprus
There are two forces present when the cars collide: each car pushes on the other car so each car experiences a
separate force. As for the strength of these two forces, all I can say is that they are exactly equal in amount but
opposite in direction. That relationship between the forces is Newton's third law of motion, the law dealing with
action and reaction. In accordance with this law of motion, no matter how big or small the cars are, they will
always exert equal but oppositely directed forces on one another.
The amount of each force is determined by how fast the cars approach one another before they hit and by how
stiff their surfaces and frames are. If the cars are approaching rapidly and are extremely stiff and rigid, they will
exert enormous forces on one another when they collide and will do so for a very short period of time. During
that time, the cars will accelerate violently and their velocities will change radically. If you happened to be in one
of the cars, you would also accelerate violently in response to severe forces and would find the experience highly
unpleasant.
If, on the other hand, the cars are soft and squishy, they will exert much weaker forces on another and they will
accelerate much more gently for a long period of time. That will be true even if they were approaching one
another rapidly before impact. When the collision period is over, the cars will again have changed velocities
significantly but the weaker forces will have made those changes much more gradual. If you have to be in a
collision, chose the soft squishy cars over the stiff ones--the accelerations and forces are much weaker and less
injurious. That's why cars have crumple zones and airbags: they are trying to act squishy so that you don't get
hurt as much.
October 8, 1997
If the thermometer works on the concept of liquids expanding when heated, how can the glass not expand as well. I mean,
the glass expands, maybe the thermometer gets longer, or the hole in the middle where the liquid is, gets smaller or larger
or something but the glass must also expand, so why does the thermometer work or does it? -- RP, Hotchkiss, Colorado
You're right about the glass expanding along with the liquid inside it. But liquids normally expand more than
solids as their temperatures increase. That's because the atoms and molecules in a liquid have more freedom to
move around than those in a solid and they respond to increasing temperatures by forming less and less tightly
packed arrangements. Since the liquid in a thermometer expands more than the glass container around it, the
liquid level rises as the thermometer's temperature increases.
How does a halogen bulb work and is it really better than a regular bulb?
A halogen bulb uses a chemical trick to prolong the life of its filament. In a regular bulb, the filament slowly
thins as tungsten atoms evaporate from the white-hot surface. These lost atoms are carried upward by the inert
gases inside the bulb and gradually darken the bulb's upper surface. In a halogen bulb, the gases surrounding the
filament are chemically active and don't just deposit the lost atoms at the top of the bulb. Instead, they react with
those tungsten atoms to form volatile compounds. These compounds float around inside the bulb until they
collide with the filament again. The extreme heat of the filament then breaks the compounds apart and the
tungsten atoms stick to the filament.
This tungsten recycling process dramatically slows the filament's decay. Although the filament gradually
develops thin spots that eventually cause it to fail, the filament can operate at a higher temperature and still last
two or three times as long as the filament of a regular bulb. The hotter filament of a halogen bulb emits relatively
more blue light and relatively less infrared light than a regular bulb, giving it a whiter appearance and making it
more energy efficient.
Sometimes on television a high pitched noise breaks the windows in a house. I know that tubular objects such as wine
glasses will break when the frequency corresponds to the natural frequencies of the glass, but does flat sheet glass such as
windows experience this same effect? -- RF, Jackson, Michigan
In real life, only explosive sounds will break normal glass. That's because normal glass vibrates poorly and has
no strong natural frequencies. You can see this by tapping a glass window or cup--all you hear is a dull "thunk"
sound.
For an object to vibrate strongly in response to a tone, that object must exhibit a strong natural resonance and the
tone's pitch must be perfectly matched to the frequency of that resonance. A crystal wineglass vibrates well and
emits a clear tone when you tap it. If you listen to the pitch of that tone and then sing it loudly, you can make the
wineglass vibrate. A crystal windowpane would also have natural resonances and would vibrate in response to
the right tones. But it would take very loud sound at exactly the right pitch to break this windowpane. An
unamplified human voice can't break a crystal wineglass and it wouldn't be able to break the crystal windowpane
either.
Is glass in a gaseous state, a liquid state or a solid state? If I remember back to my freshman college year, it seems my
prof said it was in a highly viscous state; therefore a liquid. -- GC, Garland, Texas
The answer to that question is complicated--glass is neither a normal liquid nor a normal solid. While the atoms
in glass are essentially fixed in place like those in a normal solid, they are arranged in the disorderly fashion of a
liquid. For that reason, glass is often described as a frozen liquid--a liquid that has cooled and thickened to the
point where it has become rigid. But calling glass a liquid, even a frozen one, implies that glass can flow. Liquids
always respond to stresses by flowing. Since unheated glass can't flow in response to stress, it isn't a liquid at all.
It's really an amorphous or "glassy" solid--a solid that lacks crystalline order.
Can you get a suntan or sunburn through glass? -- SD, Farmington, Utah
Yes, but not as quickly as without the glass. While glass absorbs short wavelength ultraviolet light, it does pass
350 to 400 nanometer ultraviolet. While this longer wavelength ultraviolet is less harmful than the shorter
wavelength variety, you can still tan or burn if you get enough exposure. Glass is like sunscreen--it protects you
pretty well but it isn't perfect.
A center punch is a device used in extricating people from cars. You put tape over the glass window you want to pop,
place the center punch in the corner of that window, and simply press inward. This does something that causes the glass
to crack in a spider web pattern. The glass sticks to the tape, you push in enough glass to get your hand through, and
knock the rest outward. This technique works on any window except the front windshield, which is 3 layers(glass, plastic,
glass). Can you explain it? -- RSG, Boston, Massachusetts
A center punch is a common tool used to dent a surface prior to drilling. The drill bit follows the pointed dent
and the hole ends up passing right through it. But in the situation you describe, the center punch is being used to
damage the surface of a car window. When you push the handle of the center punch inward, you are compressing
a spring and storing energy. A mechanism inside the center punch eventually releases that spring and allows it to
push a small metal cylinder toward the tip of the punch. This cylinder strikes the tip of the punch and pushes it
violently into the glass. The glass chips.
In normal glass, this chipping would be barely noticeable. But the side and rear windows of a car are made of
tempered glass--glass that has been heat processed in such a way that its surfaces are under compression and its
body is under tension. Tempering strengthens the glass by making it more resistant to tearing. But once an injury
gets through the compressed surface of the tempered glass and enters the tense body, the glass rips itself apart.
The spider web pattern of tearing you observe is a feature of the tempered glass, not the center punch. Any deep
cut or chip in the tempered glass will cause this "dicing fracture" to occur.
In instructions for cleaning CDs, it always specifies cleaning the CD by wiping radially from the center out. Why does it
matter? -- AB, Toronto, Canada
Whenever you wipe a CD to clean it, there is a chance that you will scratch its surface. If that scratch is wide
enough, it may prevent the player's optical system from reading the data recorded beneath it and this loss of data
may make the CD unplayable. It turns out that tangential scratches are much more serious than radial scratches.
When the scratch is radial (extending outward from the center of the disc to its edge), the player should still be
able to reproduce the sound without a problem. That's because sound information is recorded in a spiral around
the disc and there is error-correcting information included in each arc shaped region of this spiral. Since a radial
scratch only destroys a small part of each arc it intersects, the player can use the error correcting information to
reproduce the sound perfectly.
But when the scratch is tangential (extending around the disc and along the spiral), it may prevent the player
from reading a large portion of an arc. If the player is unable to read enough of the arc to perform its error
correcting work, it can't reproduce the sound. That's why a tangential scratch can ruin a CD much more easily
than a radial scratch can. That's why you should never wipe a CD tangentially. Always clean them by wiping
from the center out.
How does a CD player pause a CD if the CD continues to turn? -- BC, Oxon, England
A CD player reads ahead of the sound it is playing so that it always has sound information from at least one full
turn of the disc in its memory. It has to read ahead as part of the error correcting process--the sound information
associated with one moment in time is actually distributed around the spiral rather than squeezed into one tiny
patch. This reading ahead is particularly important for a portable CD player, which usually saves several seconds
of sound information in its memory so that it will have time to recover if its optical system is shaken out of
alignment. When you pause the CD player, it reads ahead until its memory is full and then lets its optical system
hover while the disc continues to turn. When you unpause the player, it uses the sound information it has saved
in its memory to continue where it left off and its optical system resumes the reading ahead process.
October 7, 1997
Will ice cubes made out of heavy water (water that is rich in the heavy isotopes of hydrogen) sink to the bottom of a glass
of water? -- RN, Denmark
Yes. Heavy water ice is about 1% more dense than liquid water at its melting temperature of 3.82° C. I wouldn't
recommend drinking large amounts of heavy water, but you could make sinking ice cubes out of it.
Why does water sound loudest just before it reaches the boiling point, and then why does it get quieter once it actually
boils? -- KS
When you heat water on the stove, heat flows into the water from below and the water at the bottom of the pot
becomes a little hotter than the water above it. As a result, the water at the bottom of the pot boils first and its
steam bubbles begin to rise up through the cooler water above. As they rise, these steam bubbles cool and
collapse--they are crushed back into liquid water by the ambient air pressure. These collapsing steam bubbles are
noisy. When the water finally boils throughout, the steam bubbles no longer collapse as they rise and simply pop
softly at the surface of the liquid.
Why does cold water defrost things faster than hot water? -- BS, Chicago, IL
I can't think of any situation in which what you say would be true. Hot water should always defrost things faster
than cold water. That's because the rate of heat flow between two objects always increases as the temperature
difference between them increases. When you put frozen food in hot water, heat flows into that food faster than it
would from cold water because the temperature difference is larger.
Why can ice, water and steam co-exist at "triple point"? -- CL
Let's start with three simpler problems: the coexistences of ice and water, of water and steam, and of ice and
steam. Each pair of phases can coexist whenever the water molecules leaving one phase are replaced at an equal
rate by water molecules leaving the second phase. This isn't as hard as it sounds. In ice water, the water
molecules leaving the ice cubes for the liquid are replaced at an equal rate by water molecules leaving the liquid
for the ice cubes. In a sealed bottle of mineral water, the water molecules leaving the liquid for the water vapor
above it are replaced at an equal rate by water molecules leaving the water vapor for the liquid. And in an oldfashioned non-frostfree freezer with a tray of ice cubes, the water molecules leaving the ice cubes for the water
vapor around them are replaced at an equal rate by water molecules leaving the water vapor for the ice cubes.
In each case, there is some flexibility in temperature--these coexistence conditions can be reached over at least a
small range of temperature by varying the pressure on the system. In fact, at 0.03° C and a pressure of 6.11 torr;
pure water, pure ice, and pure steam can coexist as a threesome. At this triple point, water molecules will be
moving back and forth between all three phases but without producing any net change in the amount of ice,
water, or steam.
When I heat a cup of water in my microwave oven to 200 degrees, then put a spoonful of instant coffee in the hot water, it
foams up. Hot water from a coffee maker does not do this. Why does water heated in a microwave oven do this? -- WAH,
Library, Pennsylvania
The microwave oven is superheating the water to a temperature slightly above its boiling temperature. It can do
this because it doesn't help water boil the way a normal coffee maker does. For water to boil, two things must
occur. First, the water must reach or exceed its boiling temperature--the temperature at which a bubble of pure
steam inside the water becomes sturdy enough to avoid being crushed by atmospheric pressure. Second, bubbles
of pure steam must begin to nucleate inside the water. It's the latter requirement that's not being met in the water
you're heating with the microwave. Steam bubbles rarely form of their own accord unless the water is far above
its boiling temperature. That's because a pure nucleation event requires several water molecules to break free of
their neighbors simultaneously to form a tiny steam bubble and that's very unlikely at water's boiling
temperature. Instead, most steam bubbles form either at hot spots, or at impurities or imperfections--scratches in
a metal pot, the edge of a sugar crystal, a piece of floating debris. When you heat clean water in a glass container
using a microwave oven, there are no hot spots and almost no impurities or imperfections that would assist
boiling. As a result, the water has trouble boiling. But as soon as you add a powder to the superheated water, you
trigger the formation of steam bubbles and the liquid boils madly.
Is it possible to make ice with neutral buoyancy, so that if you placed it halfway down a glass of water and released it, it
would remain there and not float to the top or sink? B, Kent, England
Not without using something other than pure, normal water for the ice. The density of ice is always less than that
of water at the same pressure. While squeezing the ice will increase its density, it will also increase the density of
the water so the ice will always float. Of course, you could add dense materials to the ice to weight it down to
neutral buoyancy, but then it wouldn't be pure ice any more.
I know it's difficult to get drinking water from salt water, but why is it so expensive? -- MP, Chicago, IL
The simple answer is entropy--the ever-increasing disorder of the universe. Salt water is far more disordered than
the salt and water from which it's formed, so separating those components doesn't happen easily. The second law
of thermodynamics observes that the entropy of an isolated system cannot decrease--you can't reduce the
disorder of the salty water without paying for it elsewhere. In effect, you have to export the salty water's disorder
somewhere else as you separate it into pure water and pure salt.
In most cases, this exported disorder winds up in the energy used to desalinating sea water. You start with nicely
ordered energy--perhaps electricity or gasoline--and you end up with junk energy such as waste heat. While
some desalination techniques such as reverse osmosis can operate near the efficiency limits imposed by
thermodynamics, they can't avoid those limits. If you want to desalinate water, you must consume ordered
resources and those resources usually cost money (an exception is sunlight). The desalinating equipment is also
expensive. Until water becomes scarce enough or energy cheap enough, desalinated water will remain
uncommon in the United States.
I have found that turning on all the burners of my stove on a cold winter day makes the kitchen feel moderately warm but
putting a pot of water on to boil as well makes it feel much warmer, even if I use fewer burners. Why is that? -- PM,
Little Rock, Arkansas
When you simply heat the cold air, you lower its relative humidity--the heated air is holding a smaller fraction of
its maximum water molecule capacity and is effectively dry. Dry air always feels colder than humid air at the
same temperature. That's because water molecules are always evaporating from your skin. If the air is dry, these
evaporating molecules aren't replaced and they carry away significant amounts of heat. On a hot day, this
evaporation provides pleasant cooling but on a cold day it's much less welcome. If the air near your skin is
humid, water molecules will return to your skin almost as frequently as they leave and will bring back most of
the heat that you would have lost to evaporation. Thus humid air spoils evaporative cooling, making humid
weather unpleasant in the summer but quite nice in the winter.
Since cold water is drawn into a hot water heater at the same time that hot water is being drawn out, why doesn't the
water turn cold soon after you start taking a hot shower? -- NG, Golden, Colorado
A hot water heater is built so that hot water is drawn out of its top and cold water enters it at its bottom. Since hot
water is less dense than cold water, the hot water floats on the cold water and they don't mix significantly. As
you take your shower, you slowly deplete the hot water at the top of the tank and the level of cold water rises
upward. But the shower doesn't turn cold until almost all the hot water has left the tank and the cold water level
has risen to its top.
If I have two glass containers with equal amounts of water both at the same temperature (say 80° F), and put one in the
refrigerator and one in the freezer, which container will cool to 40° F first? Because the freezer is colder, I would guess
the freezer. -- JL, Eagan, MN
You're right. The greater the temperature difference between two objects, the faster heat flows between them.
This effect is useful whenever you forget to chill drinks for a party. Just don't leave a glass bottle in the freezer
too long; if the water inside freezes, it may expand enough to break the bottle.
I once read that if you were in a boat and dropped a cannonball into the water, the water level would actually go down. It
had to do with mass and displacement. Please explain in layman's terms. -- MJB, Lafayette, LA
While the cannonball is in your boat, its great weight pushes the boat deeper into the water. To support the
cannonball, the boat must displace the cannonball's weight in water--a result known as Archimedes principle.
Since the cannonball is very dense, the boat must displace perhaps 8 cannonball volumes of water in order to
obtain the buoyancy needed to support the cannonball. This displaced water appears on the surface of the lake so
that the lake's level rises.
Now suppose that you throw the cannonball overboard. The cannonball quickly sinks to the bottom. The boat
now floats higher than before because it no longer needs to displaces the extra 8 cannonball volumes of water.
Although the cannonball itself is displacing 1 cannonball volume of water, there are still 7 cannonball volumes
less water being displaced by objects in the water. As a result, the water level of the lake drops slightly when you
throw the cannonball overboard.
What features of the fuel rods used in reactors prevent them from becoming explosive? -- JG, Bateman, Australia
A nuclear reactor operates just below critical mass so that each radioactive decay in its fuel rods induces a large
but finite number of subsequent fissions. Since each chain reaction gradually weakens away to nothing, there is
no danger that the fuel will explode. But operating just below critical mass is a tricky business and it involves
careful control of the environment around the nuclear fuel rods. The operators use neutron absorbing control rods
to dampen the chain reactions and keep the fuel just below critical mass.
Fortunately, there are several effects that make controlled operation of a reactor relatively easy. Most
importantly, some of the neutrons involved in the chain reactions are delayed because they come from
radioactive decay processes. These delayed neutrons slow the reactor's response to changes--the chain reactions
take time to grow stronger and they take time to grow weaker. As a result, it's possible for a reactor to exceed
critical mass briefly without experiencing the exponentially growing chain reactions that we associate with
nuclear explosions. In fact, the only nuclear reactor that ever experienced these exponentially growing chain
reactions was Chernobyl. That flawed and mishandled reactor went so far into the super-critical regime that even
the neutron delaying effects couldn't prevent exponential chain reactions from occurring. The reactor superheated
and ripped itself apart.
What do the terms critical, sub-critical and super-critical mass really mean? -- JG, Bateman, Australia
Critical, sub-critical, and super-critical mass all refer to the chain reactions that occur in fissionable material--a
material in which nuclei can shatter or "fission" when struck by a passing neutron. When this nuclear fuel is at
critical mass, each nucleus that fissions directly induces an average of one subsequent fission. This situation
leads to a steady chain reaction in the fuel: the first fission causes a second fission, which causes a third fission,
and so on. Steady chain reactions of this sort are used in nuclear reactors.
When the fuel is below critical mass, there aren't quite enough nuclei around to keep the chain reactions
proceeding steadily and each chain gradually dies away. While such a sub-critical mass of fuel continues to
experience chain reactions, they aren't self-sustaining and depend on natural radioactive decay to restart them.
When the fuel is above critical mass, there are more than enough nuclei around to sustain the chain reactions. In
fact, each chain reaction grows exponentially in size with the passage of time. Since each fission directly induces
more than one subsequent fission, it takes only a few generations of fissions before there are astronomical
numbers of nuclei fissioning in the fuel. Explosive chain reactions of this sort occur in nuclear weapons.
What can cause a nuclear weapon to "fizzle"? -- WEM, Palo Alto, CA
Almost the instant the nuclear fuel reaches critical mass, it begins to release heat and explode. If this fuel
overheats and rips itself apart before most its nuclei have undergone fission, only a small fraction of the fuel's
nuclear energy will have been released in the explosion. There are at least two possible causes for such a "fizzle":
slow assembly of the super-critical mass needed for explosive chain reactions and poor containment of the
exploding fuel. A well designed fission bomb assembles its super-critical mass astonishingly quickly and it
shrouds that mass in an envelope that prevents it from exploding until most of the nuclei have had time to shatter.
Is critical mass the same for all fissionable materials? -- JG, Bateman, Australia
Critical mass is something of a misnomer because in addition to mass, it also depends on shape, density, and
even the objects surrounding the nuclear fuel. Anything that makes the nuclear fuel more efficient at using its
neutrons to induce fissions helps that fuel approach critical mass. The characteristics of the materials also play a
role. For example, fissioning plutonium 239 nuclei release more neutrons on average than fissioning uranium
235 nuclei. As a result, plutonium 239 is better at sustaining a chain reaction than uranium 235 and critical
masses of plutonium 239 are typically smaller than for uranium 235.
How is the super-critical mass achieved in nuclear weapons without it exploding prematurely? -- JG, Bateman, Australia
Apart from obtaining fissionable material, this is the biggest technical problem with building a nuclear weapon.
Although a fission bomb's nuclear fuel begins to heat up and explode almost from the instant it reaches critical
mass, just reaching critical mass isn't good enough. To use its fuel efficiently--to shatter most of its nuclei before
the fuel rips itself apart--the bomb must achieve a significantly super-critical mass. It needs the explosive chain
reactions that occur when each fission induces an average of far more than one subsequent fission.
There are two classic techniques for reaching super-critical mass. The technique used in the uranium bomb
dropped over Hiroshima in WWII involved a collision between two objects. A small cannon fired a piece of
uranium 235 into a nearly complete sphere of uranium 235. The uranium projectile entered the incomplete sphere
at enormous speed and made the overall structure a super-critical mass. But despite the rapid mechanical
assembly, the bomb still wasn't able to use its nuclei very efficiently. It wasn't sufficiently super-critical for an
efficient explosion.
The technique used in the two plutonium bombs, the Gadget tested in New Mexico and the Fat Man dropped
over Nagasaki, involved implosions. In each bomb, high explosives crushed a solid sphere of plutonium 239 so
that its density roughly doubled. With its nuclei packed more tightly together, this fuel surged through critical
mass and went well into the super-critical regime. It consumed a much larger fraction of its nuclei than the
uranium bomb and was thus a more efficient device. However, its design was so complicated and technically
demanding that its builders weren't sure it would work. That's why they tested it once on the sands of New
Mexico. The builders of the uranium bomb were confident enough of its design and too worried about wasting
precious uranium to test it.
How do they split the first atom in an atomic bomb? -- N, Houston, Texas
Once the bomb has assembled a super-critical mass of fissionable material, each chain reaction that occurs will
grow exponentially with time and lead to a catastrophic release of energy. But you're right in wondering just
what starts those chain reactions. The answer is natural radioactivity from a trigger material. While the nuclear
fuel's own radioactivity could provide those first few neutrons, it's generally not reliable enough. To make sure
that the chain reactions get started properly, most nuclear weapons introduce a highly radioactive neutronemitting trigger material into the nuclear fuel assembly.
How does current flow and return in a home electric hot water heater? I only see two black hot wires and no white return
wire. -- DT, Waianae, HI
Your hot water heater is powered by 240 volt electric power through the two black wires. Each black wire is hot,
meaning that its voltage fluctuates up and down significantly with respect to ground. In fact, each black wire is
effectively 120 volts away from ground on average, so that if you connected a normal light bulb between either
black wire and ground, it would light up normally. However, the two wires fluctuate in opposite directions
around ground potential and are said to be "180° out of phase" with one another. Thus when one wire is at +100
volts, the other wire is at -100 volts. As a result of their out of phase relationship, they are always twice as far
apart from one another as they are from ground. That's why the two wires are effectively 240 volts apart on
average.
Most homes in the United States receive 240 volt power in the form of two hot wires that are 180° out of phase,
in addition to a neutral wire. 120-volt lights and appliances are powered by one of the hot wires and the neutral
wire, with half the home depending on each of the two hot wires. 240-volt appliances use both hot wires.
October 6, 1997
How do airplanes work? What is the engineering behind how an airplane flies? -- ZJ, Bangalore, India
An airplane supports itself in flight by deflecting the passing airstream downward. The plane's wings push this
airstream downward and the airstream reacts by pushing the wings upward. This action/reaction effect is an
example of Newton's third law of motion, which observes that forces always come in equal but oppositely
directed pairs: if one object pushes on another, then the second object must push back on the first object with a
force of equal strength pointing in the opposite direction. Even air obeys this law so that when the plane's wings
push air downward, the air must push the wings upward in response. In level flight, the deflected air pushes
upward so hard that it supports the entire weight of the plane. Just how the airplane's wings deflect the airstream
downward to obtain this upward lift force is a marvel of fluid dynamics. We can view it from at least two
perspectives: a Newtonian perspective which concentrates on the accelerations of the passing airstream and a
Bernoullian perspective which concentrates on speeds and pressures in that airstream.
The Newtonian perspective is the most intuitive and where we will start. The airstream arriving at the forward or
"leading" edge of the airplane wing splits into two separate flows that travel over and under the wing,
respectively. The wing is shaped and tilted so that these two flows experience very different accelerations as they
travel around the wing. The flow that goes under the wing encounters a downward sloping surface that pushes it
downward and it accelerates downward. In response to this downward push, the air pushes upward on the bottom
of the wing and provides part of the force that supports the plane.
The air that flows over the wing follows a more complicated route. At first, this flow encounters an upward
sloping surface that pushes it upward and it accelerates upward. In response to this upward force, the air pushes
downward on the leading portion of the wing's top surface. But the wing's top surface is curved so that it soon
begins to slope downward rather than upward. When this happens, the airflow must accelerate downward to stay
in contact with it. A suction effect appears, in which the rear or "trailing" portion of the wing's top surface sucks
downward on the air and the air sucks upward on it in response. This upward suction force more than balances
the downward force at the leading edge of the wing so that the air flowing over the wing provides an overall
upward force on the wing.
Since both of these air flows produce upward forces on the wing, they act together to support the airplane's
weight. The air passing both under and over the wings is deflected downward and the plane remains suspended.
In the Bernoullian view, air flowing around a wing's sloping surfaces experiences changes in speed and pressure
that lead to an overall upward force on the wing. The fact that each speed change is accompanied by a pressure
change is the result of a conservation of energy in air passing a stationary surface--when the air's speed and
motional energy increase, the air's pressure and pressure energy must decrease to compensate. In short, when air
flowing around the wing speeds up, its pressure drops and when it slows down, its pressure rises.
When air going under the wing encounters the downward sloping bottom surface, it slows down. As a result, the
air's pressure rises and it exerts a strong upward force on the wing. But when air going over the wing encounters
the up and down sloping top surface, it slows down and then speeds up. As a result, the air's pressure first rises
and then drops dramatically, and it exerts a very weak overall downward force on the wing. Because the upward
force on the bottom of the wing is much stronger than the downward force on the top of the wing, there is an
upward overall pressure force on the wing. This upward force can be strong enough to support the weight of the
airplane.
But despite the apparent differences between these two descriptions of airplane flight, they are completely
equivalent. The upward pressure force of the Bernoullian perspective is exactly the same as the upward reaction
force of the Newtonian perspective. They are simply two ways of looking at the force produced by deflecting an
airstream, a force known as lift.
Can lightning strike a flying airplane? -- DC, Denver, CO
An object doesn't have to be on the ground to be a target for lightning. In fact, most lightning strikes don't reach
the ground at all--they occur between different clouds. All that's needed for a lightning strike between two
objects is for them to have very different voltages, because that difference in voltages means that energy will be
released when electricity flows between the objects.
If an airplane's voltage begins to differ significantly from that of its surroundings, it's going to have trouble.
Sooner or later, it will encounter something that will exchange electric charge with it and the results may be
disastrous. To avoid a lightning strike, the airplane must keep its voltage near that of its surroundings. That's why
it has static dissipaters on the tips of its wings. These sharp metal spikes use a phenomenon known as a corona
discharge to spray unwanted electric charges into the air behind the plane. Any stray charges that the plane picks
up by rubbing against the air or by passing through electrically charged clouds are quickly released to the air so
that the plane's voltage never differs significantly from that of its surroundings and it never sticks out as a target
for lightning. While an unlucky plane may still get caught in an exchange of lightning between two other objects,
the use of static dissipaters significantly reduces its chances of being hit directly.
Suppose I were to fall from an airplane that is cruising at about 30,000 feet. What would kill me, the fall itself or the
sudden deceleration as I intersect with the planet? -- ZE, Woodinville, WA
In effect, you would be a skydiver without a parachute and would survive up until the moment of impact with the
ground. Like any skydiver who has just left a forward-moving airplane, you would initially accelerate downward
(due to gravity) and backward (due to air resistance). In those first few seconds, you would lose your forward
velocity and would begin traveling downward rapidly. But soon you would be traveling downward so rapidly
through the air that air resistance would keep you from picking up any more speed. You would then coast
downward at a constant speed and would feel your normal weight. If you closed your eyes at this point, you
would feel as though you were suspended on a strong upward stream of air. Unfortunately, this situation wouldn't
last forever--you would eventually reach the ground. At that point, the ground would exert a tremendous upward
force on you in order to stop you from penetrating into its surface. This upward force would cause you to
decelerate very rapidly and it would also do you in.
October 3, 1997
What is the difference between an elastic collision and an inelastic one? How does an inelastic collision work and why?
When two objects collide with one another, they usually bounce. What distinguishes an elastic collision from an
inelastic collision is the extent to which that bounce retains the objects' total kinetic energy--the sum of their
energies of motion. In an elastic collision, all of the kinetic energy that the two objects had before the collision is
returned to them after the bounce, although it may be distributed differently between them. In an inelastic
collision, at least some of their overall kinetic energy is transformed into another form during the bounce and the
two objects have less total kinetic energy after the bounce than they had before it.
Just where the missing energy goes during an inelastic collision depends on the objects. When large objects
collide, most of this missing energy usually becomes heat and sound. In fact, the only objects that ever
experience perfectly elastic collisions are atoms and molecules--the air molecules in front of you collide
countless times each second and often do so in perfectly elastic collisions. When the collisions aren't elastic, the
missing energy often becomes rotational energy or occasionally vibrational energy in the molecules. Actually,
some of the collisions between air molecules are superelastic, meaning that the air molecules leave the collision
with more total kinetic energy than they had before it. This extra energy came from stored energy in the
molecules--typically from their rotational or vibrational energies. Such superelastic collisions can also occur in
large objects, such as when a pin collides with a toy balloon.
Returning to inelastic collisions, one of the best examples is a head-on automobile accident. In that case, the
collision is often highly inelastic--most of the two cars' total kinetic energy is transformed into another form and
they barely bounce at all. Much of this missing kinetic energy goes into deforming and heating the metal in the
front of the car. That's why well-designed cars have so called "crumple zones" that are meant to absorb energy
during a collision. The last place you want this energy to go is into the occupants of the car. In fact, the
occupants will do best if they transfer most of their kinetic energies into their airbags.
I am a huge figure skating fan and was wondering if you could explain to me the physics of a triple axle jump? My
friends and I are always asking ourselves how it's done. -- AF
While I don't know the details of the jump, there are some basic physics issues that must be present. At a
fundamental level, the skater approaches the jump in a non-spinning state, leaps into the air while acquiring a
spin, spins three times in the air, lands on the ice while giving up the spin, and then leaves the jump in a nonspinning state. Most of the physics is in spin, so that's what I'll discuss.
To start herself spinning, something must exert a twist on the skater and that something is the ice. She uses her
skates to twist the ice in one direction and, as a result, the ice twists her in the opposite direction. This effect is an
example of the action/reaction principle known as Newton's third law of motion. Because of the ice's twist on
her, she acquires angular momentum during her takeoff. Angular momentum is a form of momentum that's
associated with rotation and, like normal momentum, angular momentum is important for one special reason: it's
a conserved physical quantity, meaning that it cannot be created or destroyed; it can only be transferred between
objects. The ice transfers angular momentum to the skater during her takeoff and she retains that angular
momentum throughout her flight. She only gives up the angular momentum when she lands and the ice can twist
her again.
During her flight, her angular momentum causes her to spin but the rate at which she spins depends on her shape.
The narrower she is, the faster she spins. This effect is familiar to anyone who has watched a skater spin on the
tip of one skate. If she starts spinning with her arms spread widely and then pulls them in so that she becomes
very narrow, her rate of rotation increases dramatically. That's because while she is on the tip of one skate, the
ice can't twist her and she spins with a fixed amount of angular momentum. By changing her shape to become as
narrow as possible, she allows this angular momentum to make her spin very quickly. And this same rapid
rotation occurs in the triple axle jump. The jumper starts the jump with arms and legs widely spread and then
pulls into a narrow shape so that she spins rapidly in the air.
Finally, in landing the skater must stop herself from spinning and she does this by twisting the ice in reverse. The
ice again reacts by twisting her in reverse, slowing her spin and removing her angular momentum. She skates
away smoothly without much spin.
October 1, 1997
What is reverse osmosis and how it is used in the process of purifying seawater for drinking water? -- CS
In the form used for water desalination, reverse osmosis involves a special membrane that allows water
molecules to pass through it while blocking the movement of salt ions. When water molecules are free to move
between two volumes of water, they move in whichever direction reduces their chemical potential energy. The
concept of a chemical potential is part of statistical physics--the area of physics that deals with vast collections of
particles--and it depends partly on energy and partly on probability. Factors that contribute to a water molecule's
chemical potential are the purity of the water and the water's pressure. Increasing the salt content of the water
lowers a water molecule's chemical potential while increasing the water's pressure raises its chemical potential.
Because salty water has a lower chemical potential for water molecules than pure water, water molecules tend to
move from purer water to saltier water. This type of flow is known as osmosis. To slow or stop osmosis, you
must raise the chemical potential on the saltier side by applying pressure. The more you squeeze the saltier side,
the higher the chemical potential there gets and the slower water molecules move from the purer side to the
saltier side. If you squeeze hard enough, you can actually make the water molecules move backwards--toward
the purer side! This flow of water molecules from the saltier water toward the purer water with the application of
extreme pressure is known as reverse osmosis.
In commercial desalination, high-pressure seawater is pushed into jellyroll structures containing the semipermeable membranes. The pressure of the salty water is so high that the water molecules flow through the
membrane from the salty water side to the pure water side. This pure water is collected for drinking.
How come if I stand on the balcony of my third story apartment and drop a hose to the swimming pool down below, I
can't suck any water up through the hose into my mouth?
While it may seem that you are somehow attracting the water to your mouth when you suck, you are really just
making it possible for air pressure to push the water up toward you. By removing much of the air from within the
hose, you are lowering the air pressure in the hose. There is then a pressure imbalance at the bottom end of the
hose: the pressure outside the hose is higher than the pressure inside it. It's this pressure imbalance that pushes
water into the hose and upward toward your mouth.
But air pressure can't push the water upward forever. As the column of water in the hose rises, its weight
increases. Atmospheric pressure can only lift the column of water so high before the upward force on the water is
balanced by the water's downward weight. Even if you remove all of the air inside the hose, atmospheric
pressure can only support a column of water about 30 feet tall inside the hose. If you're higher than that on your
balcony, the water won't reach you no matter how hard you try. The only way to send the water higher is to put a
pump at the bottom end of the hose. This pump can push upward harder than atmospheric pressure can and it can
support a taller column of water. That's why deep home wells have submersible pumps at their bottoms--they
must pump the water upward because it's impossible to suck it upward more than 30 feet from above.
Does gravity have a speed at which it acts upon another body? -- CP, Billings, Montana
Yes, the speed of light. The gravitational interaction between two objects can be viewed as the exchange of
particles called "gravitons," just as the electromagnetic interaction between two objects can be viewed as the
exchange of particles called "photons." Gravitons and photons are both massless particles and therefore travel at
a special speed: the "speed of light." Since light is easier to work with than gravity, people discovered this
special speed in the context of light first. If gravity had been easier to work with, they might have named it "the
speed of gravity" instead. Sometime in the not too distant future, gravity-wave detectors such as the LIGO
project will begin to observe gravity waves traveling through space from nearby cosmic events, particularly star
collapses. These gravity waves will reach us at essentially the same time as light waves from those events since
the gravity and light travel at the same speed.
How does a cassette tape recorder work? -- TW, Ottawa, Ontario
Like any tape recorder, a cassette recorder uses the magnetization of the tape's surface to represent sound. The
tape is actually a thin plastic film that's coated with microscopic cigar-shaped permanent magnets. These
particles are aligned with the tape's length and can be magnetized in either of two directions--they can have their
north magnetic poles pointing in the direction of tape motion or away from that direction. In a blank tape, the
particles are magnetized randomly so that there are as many of them magnetized in one direction as the other. In
this balanced arrangement, the tape is effectively non-magnetic. But in a recorded tape, the balance is upset and
the tape has patches of strong magnetization. These magnetized patches represent sound.
When you are recording sound on the tape, the microphone measures the air pressure changes associated with the
sound and produces a fluctuating electric current that represents those changes. This current is amplified and
used to operate an electromagnet in the recording head. The electromagnet magnetizes the tape--it flips the
magnetization of some of those tiny magnetic particles so that the tape becomes effectively magnetized in one
direction or the other. The larger the pressure change at the microphone, the more current flows through the
electromagnet and the deeper the magnetization penetrates into the tape's surface. After recording, the tape is
covered with tiny patches of magnetization, of various depths and directions. These magnetized patches retain
the sound information indefinitely.
During playback, the tape moves past the playback head. As the magnetic fields from magnetized regions of the
tape sweep past the playback head, they cause a fluctuating electric current to flow in that head. The process
involved is called electromagnetic induction; a moving or changing magnetic field produces an electric field,
which in turn pushes an electric current through a wire. The current from the playback head is amplified and used
to operate speakers, which reproduce the original sound.
The rest of the cassette recorder is just transport mechanism--wheels and motors that move the tape smoothly and
steadily past the recording or playback heads (which are often the same object). There is also an erase head that
demagnetizes the tape prior to recording. It's an electromagnet that flips its magnetic field back and forth very
rapidly so that it leaves the tiny magnetic particles that pass near it with randomly oriented magnetizations.
I understand that the speed of electricity varies with the conductor, but is supposedly 2/3 the speed of light. I had thought
the speed would equal the speed of light. Why isn't it? -- AP
Although electricity involves the movement of electrically charged particles through conducting materials, it can
also be viewed in terms of electromagnetic waves. For example, programs that reach your home through a cable
TV line are actually being carried by electromagnetic waves that travel in the cylindrical space between coaxial
cable's central wire and the tubular metal shield around it. These waves would travel at the speed of light, except
that whenever charged particles in the wires interact with the passing waves, they introduce delays. The charged
particles in the wires don't respond as quickly as empty space does to changes in electric or magnetic fields, so
they delay these changes and therefore slow down the waves. The materials that insulate the wires also influence
the speed of the electricity by responding slowly to the changing fields. The fastest wires are ones with carefully
chosen shapes and almost empty space for insulation. In general, the less the charges in the wire respond to the
passing electromagnetic waves, the faster those waves can move.
September 16, 1997
Why are swept wings preferred for transonic/supersonic flight, but not for lower speeds? -- CL
While the designers of low speed planes focus primarily on lift and drag, designers of high speed planes must
also consider shock waves--pressure disturbances that fan out in cones from regions where the plane's surface
encounters supersonic airflow. The faster a plane goes, the easier it is for the plane's wings to generate enough
lift to support it, but the more likelihood there is that some portions of the airflow around the plane will exceed
the speed of sound and produce shock waves. Since a transonic or supersonic plane needs only relatively small
wings to support itself, the designers concentrate on shock wave control. Sweeping the wings back allows them
to avoid some of their own shock waves, increasing their energy efficiencies and avoiding shock wave-induced
surface damage to the wings. Slower planes can't use swept wings easily because they don't generate enough lift
at low speeds.
How does one find out the speed of a quark? Is it 7000 times the speed of light? -- D
It seems that quarks are forever trapped inside the particles they comprise--no one has ever seen an isolated
quark. But inside one of those particles, the quarks move at tremendous speeds. Their high speeds are a
consequence of quantum mechanics and the uncertainty principle--whenever a particle (such as a quark) is
confined to a small region of space (i.e. its location is relatively well defined), then its momentum must be
extremely uncertain and its speed can be enormous. In fact, a substantial portion of the mass/energy of quarkbased particles such as protons and neutrons comes from the kinetic energy of the fast-moving quarks inside
them.
But despite these high speeds, the quarks never exceed the speed of light. As a massive particle such as a quark
approaches the speed of light, its momentum and kinetic energy grow without bounds. For that reason, even if
you gave all the energy in the world to a single quark, its speed would still remain just a hair less than the speed
of light.
What happens to a permanent magnet's magnetic field if its temperature is lowered? What happens to a magnetic field at
absolute zero?
Thermal energy is actually bad for permanent magnets, reducing or even destroying their magnetizations. That's
because thermal energy is related to randomness and permanent magnetization is related to order. Not
surprisingly, cooling a permanent magnet improves its ordering and makes its magnetization stronger (or at least
less likely to become weaker with time). At absolute zero, a permanent magnet's magnetic field will be in great
shape--assuming that the magnet itself doesn't suffer any mechanical damage during the cooling process.
September 15, 1997
What are the effects of water pressure on fish, submarines and divers?
All three of these objects contain solids, liquids, and gases, so I'll begin by describing how pressure affects those
three states of matter. Solids and liquids are essentially incompressible, meaning that as the pressure on a solid or
a liquid increases, its volume doesn't change very much. Without extraordinary tools, you simply can't squeeze a
liter of water or liter-sized block of copper into a half-liter container. Gases, on the other hand, are relatively
compressible. With increasing pressure on it, a certain quantity of gas (as measured by weight) will occupy less
and less volume. For example, you can squeeze a closet full of air into a scuba tank.
Applying these observations to the three objects, it's clear that the solid and liquid portions of these objects aren't
affected very much by the pressure, but the gaseous portions are. In a fish or diver, the gas-filled parts (the swim
bladder in a fish and the lungs in a diver) become smaller as the fish or diver go deeper in the water and are
exposed to more pressure. In a submarine, the hull of the submarine must support the pressure outside so that the
pressure of the air inside the submarine doesn't increase. If the pressure did reach the air inside the submarine,
that air would occupy less and less volume and the submarine would crush. That's why the hull of a submarine
must be so strong--it must hide the tremendous water pressure outside the hull from the air inside the hull.
Apart from these mechanical effects on the three objects, there is one other interesting effect to consider.
Increasing pressure makes gases more soluble in liquids. Thus at greater depths and pressures, the fish and diver
can have more gases dissolved in their blood and tissues. Decompression illness, commonly called "the bends",
occurs when the pressure on a diver is suddenly reduced by a rapid ascent from great depth. Gases that were
soluble in that diver's tissue at the initial high pressure suddenly become less soluble in that diver's tissue at the
final low pressure. If the gas comes out of solution inside the diver's tissue, it causes damage and pain.
I recently read in a sales brochure for a major international energy services company that the speed of light had been
exceeded in 1995. Is this true? If so, could you explain how this was accomplished? -- TS
For very fundamental reasons, the speed of light in vacuum cannot be exceeded. Calling it the "speed of light" is
something of a misnomer--it is the fundamental speed at which all massless particles travel. Since light was the
first massless particle to be studied in detail, it was the first particle seen to travel at this special speed.
While nothing can travel faster than this special speed, it's easy to go slower. In fact, light itself travels more
slowly than this when it passes through a material. Whenever light encounters matter, its interactions with the
charged particles in that matter delay its movement. For example, light travels only about 2/3 of its vacuum
speed while traveling in glass. Because of this slowing of light, it is possible for massive objects to exceed the
speed at which light travels through a material. For example, if you send very, very energetic charged particles
(such as those from a research accelerator) into matter, those particles may move faster than light can move in
that matter. When this happens, the charged particles emit electromagnetic shock waves known as Cherenkov
radiation--there is light emitted from each particle as it moves.
I suppose that the brochure could have been talking about this light/matter interaction. But since that effect has
been observed for decades, there is nothing special about 1995. More likely, the brochure is talking about
nonsense.
How do bipolar transistors work? -- BR
A bipolar transistor is a sandwich consisting of three layers of doped semiconductor. A pure semiconductor such
as silicon or germanium has no mobile electric charges and is effectively an insulator (at least at low
temperatures). Dope semiconductor has impurities in it that give the semiconductor some mobile electric
charges, either positive or negative. Because it contains mobile charges, doped semiconductor conducts
electricity. Doped semiconductor containing mobile negative charges is called "n-type" and that with mobile
positive charges is called "p-type." In a bipolar transistor, the two outer layers of the sandwich are of the same
type and the middle layer is of the opposite type. Thus a typical bipolar transistor is an npn sandwich--the two
end layers are n-type and the middle layer is p-type.
When an npn sandwich is constructed, the two junctions between layers experience a natural charge migration-mobile negative charges spill out of the n-type material on either end and into the p-type material in the middle.
This flow of charge creates special "depletion regions" around the physical p-n junctions. In this depletion
regions, there are no mobile electric charges any more--the mobile negative and positive charges have cancelled
one another out!
Because of the two depletion regions, current cannot flow from one end of the sandwich to the other. But if you
wire up the npn sandwich--actually an npn bipolar transistor--so that negative charges are injected into one end
layer (the "emitter") and positive charges are injected into the middle layer (the "base"), the depletion region
between those two layers shrinks and effectively goes away. Current begins to flow through that end of the
sandwich, from the base to the emitter. But because the middle layer of the sandwich is very thin, the depletion
region between the base and the second end of the sandwich (the "collector") also shrinks. If you wire the
collector so that positive charges are injected into it, current will begin to flow through the entire sandwich, from
the collector to the emitter. The amount of current flowing from the collector to the emitter is proportional to the
amount of current flowing from the base to the emitter. Since a small amount of current flowing from the base to
the emitter controls a much larger current flowing from the collector to the emitter, the transistor allows a small
current to control a large current. This effect is the basis of electronic amplification--the synthesis of a larger
copy of an electrical signal.
I cannot understand a step-up transformer. Why is the voltage doubled when we double the secondary turns? What isn't it
possible to have a dc transformer; since the law of induction says that when a current passes through a conductor it
provides a magnetic field, isn't it the same as ac? -- C
A transformer only works with ac current because it relies on changes in a magnetic field. It is the changing
magnetic field around the transformer's primary coil of wire that produces the electric field that actually propels
current through the transformer's secondary coil of wire.
When dc current passes through the primary coil of wire, the coil does have a magnetic field around it, but it
doesn't have an electric field around it. The electric field is what pushes electric charges through the secondary
coil to transfer power from the primary coil to the secondary coil. In contrast, when ac current passes through
that primary coil of wire, the magnetic field around the coil flips back and forth in direction and this changing
magnetic field gives rise to an electric field around the coil. It is this electric field that pushes on electrically
charged particles--typically electrons--in the secondary coil of wire. These electrons pick up speed and energy as
they move around the secondary coil's turns. The more turns these charged particles go through, the more energy
they pick up. That's why doubling the turns in a transformer's secondary coil doubles the voltage of the current
leaving the secondary coil.
How do slot machines work? -- DD, Thunder Bay, Ontario, Canada
A slot machine is a classic demonstration of rotational inertia. When you pull on the lever, you are exerting a
torque (a twist) on the three disks contained inside the machine. These disks undergo angular acceleration--they
begin turning toward you faster and faster as you complete the pull. When you stop pulling on the lever, the lever
decouples itself from the disks and they continue to spin because of their rotational inertia alone--they are
coasting. However, their bearings aren't very good and they experience frictional torques that gradually slow
them down. They eventually stop turning altogether and then an electromechanical system determines whether
you have won. Each disk is actually part of a complicated rotary switch and the positions of the three disks
determine whether current can flow to various places on an electromechanical counter. That counter controls the
release of coins--coins that are dropped one by one into a tray if you win. Sadly, computerized gambling
machines are slowly replacing the beautifully engineered electromechanical ones. These new machines are just
video games that handle money--they have little of the elegant mechanical and electromechanical physics that
makes the real slot machines so interesting.
August 12, 1997
Is it true that you can determine the distance of a lightning bolt by counting the second that are between the sound of the
thunder and the flash of the light? I heard that every second represents one mile in distance. -- LS, Los Gatos, CA
You can tell how far away a lightning flash is by counting the time separating the flash from the thunderclap.
Every five seconds is about a mile. The reason that this technique works is that light and sound travel at very
different speeds. The light and sound are created simultaneously, but the light travels much faster than the sound.
You see the flash almost immediately after it actually occurs, but the thunderclap takes time to reach your ears.
You can determine how long it takes sound to travel from the lightning bolt to your ears by counting the seconds
between the flash and the thunderclap. Since it takes sound about 5 seconds to travel a mile, you can determine
the distance to the lightning bolt in miles by dividing the seconds of sound delay by 5.
Why is there always snow on mountaintops, even if the weather in the valley is not cold? -- GV, El Paso, Texas
The atmosphere maintains a natural temperature gradient of about 10° C (which is equivalent to 18° F) per
kilometer in dry air and about 6 or 7° C (which is equivalent to about 12° F) per kilometer in moist air. The
higher you look in the lower atmosphere, the colder the air is. Because of this gradient, it may be 20° C (68° F)
in the valley and 0° C (32° F) at the top of a 2,000 meter high mountain.
This temperature gradient has its origin in the physics of gases--when a gas expands and does work on its
surroundings, its temperature decreases. To see why this effect is important, imagine that you have a plastic bag
that's partially filled with valley air. If you carry this bag up the side of the mountain, you will find that the bag's
volume will gradually increase. That's because there will be less and less air overhead as you climb and the
pressure that this air exerts on the bag will diminish. With less pressure keeping it small, the air in the bag will
expand and the bag will fill up more and more. But for the bag's size to increase, it must push the air around it
out of the way. Pushing this air away takes work and energy, and this energy comes from the valley air inside the
bag. Since the valley air has only one form of energy it can give up--thermal energy--its temperature decreases as
it expands. By the time you reach the top of the mountain, your bag of valley air will have cooled dramatically. If
it started at 20° C, its temperature may have dropped to 0° C, cold enough for snow.
If you now turn around and walk back down the mountain, the increasing air pressure will gradually squeeze
your bag of valley air back down to its original size. In doing do, the surrounding air will do work on your valley
air, giving it energy, and will increase that air's thermal energy--the valley air will warm up! When you reach the
valley, the air in your bag will have returned to its original temperature.
Air often rises and falls in the atmosphere and, as it does, it experiences these same changes in temperature. Air
cools as it blows up into the mountains (often causing rain to form) and warms as it flows down out of the
mountains (producing dry mountain winds). These effects maintain a temperature gradient in the atmosphere that
allows snow to remain on mountaintops even when it's relatively warm in the valleys.
Recently, my doctor attached a small clip to my index finger that allowed a machine to not only measure my pulse rate
but my blood gasses too. No needles were involved. How does this work? -- CM, New York, New York
The red blood cells in your blood contain large amounts of a complicated and brightly colored molecule known
as hemoglobin. This molecule's ability to bind and later release oxygen molecules is what allows blood to carry
oxygen efficiently throughout your body.
Each hemoglobin molecule contains four heme groups, the iron-containing structures that actually form the
reversible bond with oxygen molecules and that also give the hemoglobin its color. However, this color depends
on the oxidization state of the heme group--red when the heme group is binding oxygen and blue-purple when
the heme group is alone. That color difference explains why someone who is holding their breath may "turn
blue"--their hemoglobin is lacking in oxygen. The clip you wore was analyzing the color of your blood to
determine the extent of oxygenation in its hemoglobin. It measured your pulse rate by looking for periodic
fluctuations in the opacity of your finger, brought on by changes in your finger's blood content with each
heartbeat.
Is wearing rubber-soled shoes more dangerous or less dangerous if you are hit by lightning? -- JH, Santa Fe, New Mexico
Once lightning strikes you, whether or not you are wearing rubber-soled shoes will make little difference. The
voltages involved in lightning are so enormous (hundreds of millions of volts) that the insulating character of
rubber soles will be completely overwhelmed. If the electric current can't pass through your rubber soles, it will
simply form an electric arc around them or through them.
However, I would guess that rubber-soled shoes provide some slight protection against being hit by lightning in
the first place. Lightning tends to strike objects that have acquired an electric charge that is opposite that of the
cloud overhead. This opposite charge naturally appears on grounded conducting objects because the cloud's
charge pulls opposite charges up from the ground and onto the objects. Once this charging has taken place, the
object is a prime target for a lightning strike.
If you are standing alone and barefoot on the top of a mountain during a thunderstorm, the cloud will draw
opposite charge up from the ground through your feet and you will become very highly charged. There are even
photographs of people on mountaintops with their hair standing up because of this charging effect.
Unfortunately, some of these people were struck by lightning shortly after experiencing this effect. If you ever
experience it, run for your life down the mountain! It's possible that wearing rubber soles shoes will prevent or
delay this charging effect, and it might keep you from being struck by lightning. But I sure wouldn't count on it.
August 11, 1997
Why are there no bubbles in carbonated water until you open the sealed cap? Why are the bubbles inside the opened
bottle then larger than the ones in a glass? -- EP
When the bottle is sealed, its contents are in equilibrium. In this context, equilibrium means that while carbon
dioxide gas molecules are continuously shifting from solution in the water to independence in the gas underneath
the cap, there is no net movement of gas molecules between the two places. Since the company that bottled the
water put a great many gas molecules in the bottle, the concentration of dissolved molecules in the water is high
and so is the density of molecules in the gas under the cap. This high density of gaseous carbon dioxide
molecules under the cap makes the pressure inside the bottle quite high, which is why the bottle's surface is taut
and hard.
While you can't see it in this unopened bottle, there is activity both at the surface of the water and within the
water. At the water's surface, carbon dioxide molecules are constantly leaving the water for the gas under the cap
and returning from the gas under the cap to the water. The rates of departure and return are equal, so that nothing
happens overall. Within the water, tiny bubbles are also forming occasionally. But these tiny bubbles, which
nucleate through random fluctuations within the liquid or more often at defects in the bottle's walls, can't grow.
Even though these bubbles contain gaseous carbon dioxide molecules, the molecules aren't dense enough to keep
the bubbles from being crushed by the pressurized water. So these tiny bubbles form and collapse without ever
becoming noticeable.
However, once you remove the top from the bottle, everything changes. The bottle's contents are no longer in
equilibrium. To begin with, carbon dioxide molecules that leave the surface of the water are no longer replaced
by molecules returning to the liquid. That's one reason why an opened bottle of carbonated water begins to lose
its dissolved carbon dioxide and become "flat." Secondly, without its trapped portion of dense carbon dioxide
gas, the bottle is no longer pressurized and it stops being taut and hard (assuming that it's made of plastic rather
than gas). Thirdly, with the loss of pressure, the water in the bottle stops crushing the tiny gas bubbles that form
within it. In fact, once one of those bubbles forms, carbon dioxide molecules can enter it from the liquid just as
they enter the gas at the top of the bottle. As a result, each bubble that forms grows larger and larger. Since the
gas in a bubble is less dense than water, the bubble begins to float upward until it reaches the top of the bottle.
Because the bottle is taller than a typical water glass, a bubble has more time to grow before reaching the top in
the bottle than it would have in the glass. That's one reason why the bubbles in a bottle are taller than in a glass.
Another reason is that the concentration of dissolved carbon dioxide molecules is higher while the water is in the
bottle than it is by the time the water reaches the glass, so that bubbles grow faster in the bottle than in the glass.
How does an acetylene miner's lamp work? How does a propane gas lamp work? Why do gas lamps need a mantle and
what is the mantle made of? -- DK, Washington, DC
An acetylene miner's lamp produces acetylene gas through the reaction of solid calcium carbide with water. An
ingenious system allows the production of gas to self-regulate--the gas pressure normally keeps the water away
from the calcium carbide so that gas is only generated when the lamp runs short on gas. In contrast, a propane
lamp obtains its gas from pressurized liquid propane. Whenever the propane lamp runs short on gas, the falling
gas pressure allows more liquid propane to evaporate.
Only the propane lamp needs a mantle to produce bright light. That's because the hot gas molecules that are
produced by propane combustion aren't very good at radiating their thermal energy as visible light. The mantle
extracts thermal energy from the passing gas molecules and becomes incandescent--it converts much of its
thermal energy into thermal radiation, including visible light. Mantles are actually delicate ceramic structures
consisting of metal oxides, including thorium oxide. Thorium is a naturally occurring radioactive element,
similar to uranium, and lamp mantles are one of the few unregulated uses of thorium.
In contrast, the acetylene miner's lamp works pretty well without a mantle. I think that's because the flame
contains lots of tiny carbon particles that act as the mantle and emit an adequate spectrum of yellow thermal
radiation. Many of these particles then go on to become soot. A candle flame emits yellow light in the same
manner.
One last feature of a properly constructed miner's lamp, a safety lamp, is that it can't ignite gases around it even
if those gases are present in explosive concentrations. That's because the lamp's flame is surrounded by a fine
metal mesh. This mesh draws heat out of any gas within its holes and thus prevents the flame inside the mesh
from igniting any gas outside the mesh.
August 7, 1997
Is there rain above the clouds? -- JM, Arlington Heights, Illinois
No. If you are above the clouds, then the sky above you is free from droplets of condensed moisture. While that
doesn't mean that there is no water overhead, that water must be entirely in the form of gaseous water molecules.
Since rain forms when droplets of condensed moisture grow large enough to descend rapidly through the air, the
absence of any condensed droplets makes it impossible for full raindrops to form. In short, no clouds overhead,
no rain.
How does an automatic transmission in a car work? -- ORL, Trondheim, Norway
An automatic transmission contains two major components: a fluid coupling that controls the transfer of torque
from the engine to the rest of the transmission and a gearbox that controls the mechanical advantage between the
engine and the wheels. The fluid coupling resembles two fans with a liquid circulating between them. The engine
turns one fan, technically known as an "impeller," and this impeller pushes transmission fluid toward the second
impeller. As the liquid flows through the second impeller, it exerts a twist (a "torque") on the impeller. If the car
is moving or is allowed to move, this torque will cause the impeller to turn and, with it, the wheels of the car. If,
however, the car is stopped and the brake is on, the transmission fluid will flow through the second impeller
without effect. Overall, the fluid coupling allows the efficient transfer of power from the engine to the wheels
without any direct mechanical linkage that would cause trouble when the car comes to a stop.
Between the second impeller and the wheels is a gearbox. The second impeller of the fluid coupling causes
several of the gears in this box to turn and they, in turn, cause other gears to turn. Eventually, this system of
gears causes the wheels of the car to turn. Along with these gears are several friction plates that can be brought
into contact with one another by the transmission to change the relative rotation rates between the second
impeller and the car's wheels. These changes in relative rotation rate give the car the variable mechanical
advantage it needs to be able to both climb steep hills and drive fast on flat roadways.
Finally, some cars combine parts of the gear box with the fluid coupling in what is called a "torque converter."
Here the two impellers in the fluid coupling have different shapes so that they naturally turn at different rates.
This asymmetric arrangement eliminates the need for some gears in the gearbox itself.
Is there a formula or equation for figuring out the pressure of air at a certain altitude? -- DLH, Conifer CO
Unfortunately, the answer is no. The atmosphere is too complicated to be described by a simple formula or
equation, although you can always fit a formulaic curve to measured pressure values if you make that formula
flexible enough. The complications arise largely because of thermodynamic issues: air expands as it moves
upward in the atmosphere and this expansion causes the air to cool. As a result of this cooling, the air in the
atmosphere doesn't have a uniform temperature and, without a uniform temperature, the air's pressure is difficult
to predict. Radiative heating of the greenhouse gases and phase changes in the air moisture content further
complicate the atmosphere's temperature profile and consequently its pressure profile. If you want to know the
air pressure at specific altitude, you do best to look it up in a table.
In science, we learned that a color's energy depends on its wavelength--that violet light with its short wavelength has
more energy than red light with its long wavelength. But in art, we learned that red, orange, and yellow are warm and
blue and violet are cool. Is that because of how the people feel about the colors, like fire is red and water is blue? -- ON,
Istanbul, Turkey
Both of your observations are correct: short wavelength light, such as violet, carries more energy per particle (per
"photon") than long wavelength light, such as red, and red light does appear "warmer" than blue light. But the
latter observation is one of feelings and psychology, rather than of physics. It is ironic that colors we associate
with cold and low thermal energies are actually associated with higher energy light particles than are colors we
associate with heat and high thermal energies.
I know that the medium of electromagnetic waves is a photon. What is a photon? What is it made of? -- ON, Istanbul,
Turkey
First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as
they change with time and they travel together through empty space. An electromagnetic wave of this sort carries
energy with it because electric and magnetic fields both contain energy. That much was well understood by the
end of the 19th century, but something new was discovered at the beginning of the 20th century: an
electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of
energy, units that are commonly called "quanta." An electromagnetic wave that carries only one quanta of energy
is called a "photon."
The amount of energy that a photon carries depends on the frequency of that photon--the higher the frequency,
the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules,
which is why they provide our eyes with such useful information about the objects around us--we see how this
visible light is interacting with the world around us.
I work finding sites for cellular & PCS wireless telephone antennae. I would like to know how radio waves work and how
they are able to carry voice and data information. What are these waves and do they exist naturally or do we set them up
using electric charges? -- PAB, Madison, WI
Radio waves are a class of electromagnetic waves, specifically the lowest frequency, longest wavelength
electromagnetic waves. Actually, the electromagnetic waves used in cellular & PCS transmissions are technically
known as microwaves because they have wavelengths of less than 1 meter, but there are no important differences
between radio waves and microwaves.
Like all electromagnetic waves, radio waves and microwaves consist of coupled electric and magnetic fields that
sustain one another in stable structures that move rapidly through empty space. Because an electromagnetic
wave's electric field changes with time, it is able to create the wave's magnetic field and, because its magnetic
field changes with time, that magnetic field is able to create the wave's electric field. Since they consist only of
electric and magnetic fields, these waves cannot stay still--they must move (although you can trap them between
mirrors so that they appear to stand in one place as they bounce back and forth). While they contain no true mass,
they do contain energy and an electromagnetic wave carries energy from one place to another.
Electromagnetic waves are created whenever electrically charged particles change speed or direction; whenever
they accelerate. Since there are accelerating electric charges everywhere--thermal energy keeps them moving
about--there are also electromagnetic waves everywhere. But the radio waves used in communications systems
are generated deliberately by moving electric charges back and forth. When charges are sent up and down a radio
antenna, these charges are accelerating and they form complicated electric and magnetic fields that include
electromagnetic waves. Once launched, those electromagnetic waves propagate through space at approximately
the speed of light.
To send information with radio waves, a transmitter makes modifications in one or more the wave's
characteristics. In an amplitude modulation scheme (AM), the transmitter changes the strength or "amplitude" of
the wave to convey information--like sending radio smoke signals. In the frequency modulation scheme (FM),
the transmitter changes the frequency of the wave to convey information--like whistling a tune with a
complicated melody.
How does a VCR Plus system work? Are codes built in for every possibility of channel and time or does it calculate
somehow? I know that if you enter a random number (including single digits) that some program is scheduled. -- LK,
Huntington, West Virginia
The VCR Plus codes contain just enough information to tell the VCR what time and day a program starts, what
channel that program is on, and how long it will last. What is remarkable about these codes is not that they exist,
but that many of them are so short. A long number that contained the complete date, the entire channel number,
and the length of the program in minutes would obvious fulfill the requirements, but the actually numbers are
never that long. While I don't know the precise encoding scheme, the date is clearly compressed--a daily or
weekly program is represented by a very small code--and so is the record time for programs with a common
duration. The VCR Plus codes get significantly longer when they must represent one-time only shows and shows
with complicated durations. Even then, the date is truncated so that there are no current codes to represent a show
five years in the future.
August 5, 1997
How does a rice cooker know when to turn off? -- JS, Tokyo, Japan
The rice cooker turns off when there is no longer enough liquid water on its heating element to keep that
element's temperature at the boiling temperature of water (212° F or 100° C). As long as the element is covered
with liquid water, it is hard for that element's temperature to rise above water's boiling temperature. That's
because as the water boils, all of the thermal energy produced in the heating element is converted very efficiently
into chemical potential energy in the resulting steam. In short, boiling water remains at 212° F even as you add
lots of thermal energy into it.
But as soon as the liquid water is gone (and, fortuitously, the rice is fully cooked), there is nothing left to keep
the heating element's temperature from rising. As more electric energy enters the element and becomes thermal
energy, the element gets hotter and hotter. A thermostat, probably a bimetallic strip like that used in most
toasters, senses the sudden temperature rise. It releases a switch that turns off the electric power to the rice
cooker.
You stated (elsewhere) that thermodynamics overwhelms just about everything sooner or later. Could you explain why? - MT, San Antonio, TX
One of the principal observations of thermodynamics (and statistical mechanics, a related field) is that vast,
complicated systems naturally evolve from relatively unlikely arrangements to relatively likely arrangements.
This trend is driven by the laws of probability and the fact that improbable things don't happen often. Here's an
example: consider your sock drawer, which contains 100 each of red and blue socks (it's a large drawer and you
really like socks). Suppose you arrange the drawer so that all the red socks are on one side and all the blue socks
are on the other. This arrangement is highly improbable--it didn't happen by chance; you caused it to be ordered.
If you now turn out the light and randomly exchange socks within the drawer, you're awfully likely to destroy
this orderly situation. When you turn the light back on, you will almost certainly have a mixture of red and blue
socks on each side of the drawer. You could turn the light back out and try to use chance to return the socks to
their original state, but your chances of succeeding are very small. Even though the system you are playing with
has only 200 objects in it, the laws of probability are already making it nearly impossible to order it by chance
alone. By the time you deal with bulk matter, which contains vast numbers of individual atoms or electrons or
bits of energy, chance and the laws of probability dominate everything. Even when you try to impose order on a
system, the laws of probability limit your success: there are no perfect crystals, perfectly clean rooms, flawless
structures. These objects aren't forbidden by the laws of motion, they are simply too unlikely to ever occur.
How does a fan compare to a propeller? Why does a fan blow air while a propeller has "lift"? -- DB, Austin, TX
A fan and a propeller are actually the same thing. Both are rotating wings that push the air in one direction and
experience a reaction force in the opposite direction as a result. Each experiences a "lift" force, typically called
"thrust," in the direction opposite the airflow. If you put a strong fan on a low-friction cart or a good skateboard,
it will accelerate forward as it pushes the air backward. Similarly, if you prevent a propeller plane from moving,
its spinning blades will act as powerful fans.
How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? -- AM,
Mexico
Most CD's are made from polycarbonate plastic (though other plastics with the same index of refraction are
occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum
removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come
occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped
within the tangle of plastic molecules, you're probably out of luck. Removing such a stain will require wearing
away some of the plastic. Since the disc's surface finish must remain smooth and the thickness of the disc
shouldn't change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the
printed side of the disc--it has only a thin layer of varnish protecting the delicate aluminum layer from injury.
Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to
render the disc clear again by filling the tiny air gaps that make it white with another plastic. I'll bet that a clear
furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.
I once saw a green sunrise. Can you explain this?
Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible
first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from
behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual
yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed)
in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html.
According to the author of that site, Andrew Young, given a low enough horizon, which is the primary
consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly
see green flashes at most sunsets.
How does the carbon in an organic material affect the flow of light through it? -- TM
When light passes into a material, it interacts primarily with the negatively charged electrons in that material.
Since light consists in part of electric fields and electric fields push on charged particles, light pushes on
electrons. If the electrons in a material can't move long distances and can't shift from one quantum state to
another as the result of the light forces, then all that will happen to the light as it passes through the material is
that it will be delayed and possibly redirected. But if the electrons in the material can move long distance or shift
between states, then there is the chance that the light will be absorbed by the material and that the light energy
will become some other type of energy inside the material.
Which of these possibilities occurs in a particular organic material depends on the precise structure of that
material. Carbon atoms can be part of transparent organic materials, such as sugar, or of opaque organic
materials, such as asphalt. The carbon atoms and their neighbors determine the behaviors of their electrons and
these electrons in turn determine the optical properties of the materials.
The frequency at which microwave ovens operate is about 2.45 GHz, which is about the resonant frequency of the free
water molecule. Can you calculate this resonant frequency or was it determined experimentally? -- GW
While most microwave ovens operate at 2.45 GHz, that frequency is not a resonant frequency for the water
molecule. In fact, using a frequency that water molecules responded to strongly (as in a resonance) would be a
serious mistake--the microwaves would all be absorbed by water molecules at the surface of the food and the
center of the food would remain raw. Instead, the 2.45 GHz frequency was chosen because it is absorbed weakly
enough in liquid water (not free water molecules) that the waves maintain good strength even deep inside a
typical piece of food. Higher frequencies would penetrate less well and cook less evenly. Lower frequencies
would penetrate better, but would be absorbed so weakly that they wouldn't cook well. The 2.45 GHz frequency
is a reasonable compromise between the two extremes.
Why does a single phase 220 volt motor run off two legs of a three-phase circuit?
In three-phase power, the voltages of the three power wires fluctuate up and down cyclically so that they are
"120 degrees" apart. By "120 degrees" apart, I mean that each wire reaches its peak voltage at a separate time-first the X wire, then the Y wire, and then the Z wire--with the Y wire reaching its peak 1/3 of the 360 degree
cycle (or 120 degrees) after the X wire and the Z wire reaching its peak 1/3 of the 360 degree cycle (or 120
degrees) after the Y wire.
The specific voltages and their relationships with ground or a possible fourth "neutral" wire depend on the exact
type of transformer arrangement that supplies your home or business. In the standard "Delta" arrangement
(which you can find discussed at sites dealing with power distribution), the voltage differences between any pair
of the three phases is typically 240 VAC. In the standard "Wye" arrangement, the typical voltage difference
between any pair of phases is 208 VAC and the voltage difference between any single phase and ground is 120
VAC. And in the "Center-Tapped Grounded Delta" arrangement, the voltage difference between any pair of
phases is 240 VAC and the voltage difference between a single phase and neutral is 120, 120, and 208 VAC
respectively (yes, the three phases behave differently in this third arrangement).
If you run a single-phase 220 VAC motor from two wires of a Delta arrangement power outlet, that motor will
receive a little more voltage (240 VAC) than it was designed for and if you run it from two wires of a Wye
arrangement outlet, it will receive a little less voltage (208 VAC) than appropriate. Still, the motor will probably
run adequately and it's unlikely that you'll ever notice the difference.
In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in
the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive?
-- G
The magnetic interaction between the stator and the rotor is repulsive--the rotor is pushed around in a circle by
the stator's magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that
instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field)
that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in
that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet-the two
will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is
recognized formally in "Lenz's Law". For reasons having to do with resistive energy loss and heating, the
repulsive forces in front of and behind the moving magnet don't cancel perfectly, leading to a magnetic drag
force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the
moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the
rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive-pushes not pulls.
If you wrap a three-phase power cord into a coil and allow it to deliver power to equipment, will the coil develop
magnetic fields and, as a consequence exhibit both an inductive reactance and a voltage drop? -- JH
If any current reaching the equipment through the three-phase power cord returns through that same power cord,
then the net current in the cord is always exactly zero. Despite the complicated voltage and current relationships
between the three power wires, one simple fact remains: the equipment can't store electric charge. As a result,
any current that flows toward the equipment must be balanced by a current flowing away from the equipment,
and if both flows are in the same power cord, they'll cancel perfectly. Since there is no net current flowing
through the power cord, it develops no magnetic field and exhibits no inductive reactance or voltage drop.
Does a moving magnet use up its energy when it generates electricity? Does this mean that the term "permanent magnet"
is a misnomer because its magnetism can be used up? -- MT, San Antonio, TX
When a moving magnet generates electricity, it does transfer energy to the electric current. However, that energy
comes from either the magnet's kinetic energy (its energy of motion) or from whatever is pushing the magnet
forward. The magnet's magnetism is basically unchanged by this process.
Nonetheless, a large permanent magnet isn't really permanent. The random fluctuations of thermal energy and
the influences of passing magnetic fields gradually demagnetize large permanent magnets. However, good
permanent magnets demagnetize so slowly that the changes are completely undetectable. You might have to wait
a billion years to detect any significant weakening in the magnetic field around such a magnet.
I am doing a science fair project on conductors and insulators. What are some of the best and worst conductors of
electricity? -- LM
The best conventional conductors are silver, copper, gold, and aluminum. What makes them good conductors is
that electrons move through them for relatively long distances without colliding with anything that wastes their
energy. These materials become better conductors as their purities increase and as their temperatures decrease. A
cold, near-perfect crystal is ideal, because all of the atoms are then neatly arranged and nearly motionless, and
the electrons can move through them with minimal disruption. However, there is a class of even better
conductors: the so-called "superconductors." These materials allow electric current to travel through them will
absolutely no loss of energy. The carriers of electric current are no longer simply independent electrons; they are
typically pairs of electrons. Still, superconductivity appears because the moving charged particles can no longer
suffer collisions that waste their energy-they move with perfect ease. We would be using superconductors
everywhere in place of copper or aluminum wires if it weren't for the fact that superconductors only behave that
way at low temperatures.
As for the best insulators, I'd vote for good crystals of salts like lithium fluoride and sodium chloride (table salt),
and covalently-bound substances like aluminum oxide (sapphire) or diamond. All of these materials are pretty
nearly perfect insulators.
I've used metal detectors that only pick up gold signals. How does that work? -- MB
While metal detectors can easily distinguish between ferromagnetic metals such as steel and non-ferromagnetic
metals such as aluminum, gold, silver, and copper, it is difficult for them to distinguish between the particular
members of those two classes. Ferromagnetic metals are ones that have intrinsic magnetic structure and respond
very strongly to outside magnetic fields. The non-ferromagnetic metals have no intrinsic magnetic structure but
can be made magnetic when electric currents are driven through them.
Good metal detectors produce electromagnetic fields that cause currents to flow through nearby metal objects
and then detect the magnetism that results. Unfortunately, identifying what type of non-ferromagnetic metal is
responding to a metal detector is hard. Mark Rowan, Chief Engineer at White's Electronics of Sweet Home,
Oregon, a manufacturer of consumer metal detecting equipment, notes that their detectors are able to classify
non-ferromagnetic metal objects based on the ratio of an object's inductance to its resistivity. They can reliably
distinguish between all denominations of U.S. coins--for example, nickels are relatively more resistive than
copper and clad coins, and quarters are more inductive than smaller dimes. The primary mechanism they use in
these measurements is to look at the phase shift between transmitted and received signals (signals typically at, or
slightly above, audio frequencies). However, they are unable to identify objects like gold nuggets where the size,
shape, and alloy composition are unknown.
I have read articles about research into anti-gravity. Do you think it is really possible? -- JG
No, I don't think that anti-gravity is possible. The interpretation of gravity found in Einstein's General Theory of
Relativity is as a curvature of space-time around a concentration of mass/energy. That curvature has a specific
sign, leading to what can be viewed as an attractive force. There is no mechanism for reversing the sign of the
curvature and creating a repulsive force--anti-gravity. I know of only one case, involving a collision between two
rapidly spinning black holes, in which two objects repel one another through gravitational effects. But that
bizarre case is hardly the anti-gravity that people would hope to find.
August 4, 1997
Why do carbonated beverages "burn" your throat? -- TS
When carbon dioxide gas (CO2) dissolves in water (H2O), its molecules often cling to water molecules in such a
way that they form carbonic acid molecules (H2CO3). Carbonic acid is a weak acid, an acid in which most
molecules are completely intact at any given moment. But some of those molecules are dissociated and exist as
two dissolved fragments: a negatively charged HCO3- ion and a positively charged H+ ion. The H+ ions are
responsible for acidity--the higher their concentration in a solution, the more acidic that solution is. The presence
of carbonic acid in carbonated water makes that water acidic--the more carbonated, the more acidic. What you're
feeling when you drink a carbonated beverage is the moderate acidity of that beverage "irritating" your throat.
Why don't batteries work as well in cold environments? -- KS
A battery uses electrochemical processes to provide power to a current passing it. This statement means that if
you send an electric charge through the battery in the normal direction, that charge will emerge from the battery
with more energy than it had when it entered the battery. But while it might seem that the number of electric
charges passing through the battery each second doesn't matter--that each charge will pick up the usual amount
of extra energy during its passage--that's not always the case. To understand this fact, let's look at how charges
"pass through" the battery and how they pick up energy.
What's really happening is that electrochemical processes are spontaneously separating charges from one another
inside the battery and placing those separated charges on the battery's terminals--the battery's negative terminal
becomes negatively charged and its positive terminal becomes positively charged. This charge separating process
proceeds in a random, statistical manner until enough charges accumulate on the terminals to prevent any further
charge separation. Because like charges repel one another, sufficiently large accumulations of positive charges
on the positive terminal and negative charges on the negative terminal stop further arrivals of those charges.
But when you send a positive charge through a wire and onto the battery's negative terminal, you reduce the
amount of negative charge there and weaken the repulsive forces. As a result, the chemicals in the battery
separate another pair of charges. The battery's negative terminal returns to normal, but now there is an extra
positive charge on the battery's positive terminal. This extra charge flows away through a wire. Overall, it
appears that your positive charge "passed through" the battery--entering the battery's negative terminal and
emerging from the positive terminal with more energy than it had when it arrived at the negative terminal. But
what really happened was that the battery's chemicals separated another pair of charges.
In a warm environment, the battery's chemicals can separate charges rapidly and can keep up with reasonably
large currents of arriving charges. But in a cold battery, the electrochemical processes slow down and it becomes
hard for the battery to keep up. If you try to send too much current through the battery while it's cold, it is unable
to replace the charges on its terminals quickly enough and it voltage sags--it doesn't have enough separated
charges on its terminals to give the charges "passing through" it their full increase in energy. If you use a battery
while it's very cold, you should be careful not to send too much current through it because it will become
inefficient and will provide less than its usual voltage.
I would like to get your opinion of the general subject of "healing science." This has come up as a topic of conversation
in our family. I've seen many articles on this subject which often contain references to physics terms, such as vibrational
healing" and "energy medicine." They sometimes claim the existence of a human energy field, or aura, which "penetrates
and surrounds the physical body, and contains the template for the body, the thoughts, the emotions and the spirituality."
Imbalances, blockages and distortions in the flow of the energy field have a direct correlation to physical, emotional,
mental and spiritual "dis-ease" and problems, it is claimed. Furthermore, it is claimed that one can learn how to sense and
correct an energy imbalance before it expresses itself as physical illness, as well as recover emotionally and physically
from an illness you may already have. Some even claim that long-distance healing works. "Based on Einstein's theory
that time and space are relative," they say, "not only can the energy field be worked on by directly placing hands on the
body or a few inches above the body, but also from across the room, or across the continent. Long-distance clientele
experience the healing work as if they were in the office." While these claims would seem to have no foundation in
scientific fact, I pause when I see endorsements by supposedly educated people such as Richard Gerber, M.D., author of
Vibrational Medicine, and Caroline Myss, author of Anatomy of the Spirit (who has her B.A. in Journalism, her M.A. in
Theology, and her Ph.D. in Energy Medicine). Reportedly, "Harvard-trained neurosurgeon C. Norman Shealy estimates
Myss' 'medical intuitive readings' to be 93% accurate." This reminds me of something I recently saw about Albert
Abrams, M.D.--a reputedly brilliant and well-respected American diagnostician. In the early 1930s, in an apparent effort
to clone his talents so he could handle his patient overload, he invented two machines based on his theory of radionic
diagnosis. One was the "Dynamizer" that could diagnose any illness and the other was the "Oscilloclast" which could
cure any illness by restoring the person's harmony. Through a series of double-blind tests conducted by Scientific
American, these devices were conclusively shown to be sheer quackery. Amazingly, Abrams is still held as a "true
genius" in some circles, e.g., http://www.healing.org/only-contents.html (See chapter 1 -- Albert Abrams and Radionics
Diagnosis) What do you tell your college students -- and other people who may be naive to science -- about this stuff
(without being disrespectful)? -- JB
You have every reason to be skeptical about this sort of activity. Despite its length, I have included your entire
question here because it gives me an opportunity to point out some of the differences between science and
pseudo-science. You have written a wonderful survey of some of the quackery that exists in our society and have
illustrated beautifully the widespread view that science is fundamentally nothing more than gibberish. I cringe as
I read your review of "healing science" because in that description I see science, a field that has been developed
with care by people I respect and admire, tossed cavalierly into the gutter by self-important know-nothings who
aren't worth a moments notice. That these miserable individuals draw such attention, often at the expense of far
more deserving real scientists--or worse, by "standing on the shoulders" of those real scientists--is a tragedy of
modern society. It's just dreadful.
Let me begin to pick up the pieces by pointing out that terms like "human energy field", "vibrational medicine",
and "energy imbalance" are simply meaningless and that the use of "Einstein's Theory" to justify healing-at-adistance is typical of people who don't have a clue about what science actually is. The meaningless misuse of
scientific terms and the uninformed and careless misapplication of scientific techniques is an activity called
pseudo-science. Pseudo-science may sound and look like science, but the two have almost nothing else in
common. Among the benefits of a good college education is learning how vast is the world of human knowledge,
recognizing how little you know of that world, discovering how much others have already thought about
everything you can imagine, and finding out how dangerous it is to venture unprepared into any area you do not
know well. Most of these pseudo-scientific quacks are either oblivious of their own ignorance or so arrogant that
they dismiss the work of others as not worthy of their attention. Either way, they make terrible students and,
consequently, useless teachers. You'll do best to leave their books on the shelves.
Because real science is not buzzwords, simply stringing together the words of science does not make one a
scientist. Science is an intense, self-reflective, skeptical, objective investigative process in which we try to form
conceptual models for the universe and its contents, and try to test those models against the universe itself. We
do this modeling and testing over and over again, improving and perfecting the models and discarding or
modifying models that do not appear consistent with actual observations. Accurate models are valuable because
they have predictive power--you can tell in advance how something will behave if you have modeled it correctly.
In the course of these scientific investigations, concepts arise which deserve names and so we assign names to
them. In that manner, words such as "energy" and "vibration" have entered our language. Each such word has a
very specific meaning and applies only in a specific context. Thus the word "force" was assigned to the concept
we commonly refer to as a "push" or a "pull" and applies in the context of interactions between objects. The
expression "the force be with you" has nothing to do with physics--the word "force" in that phrase doesn't mean a
push or a pull and has nothing to do with the interactions between objects. As you can see, taken out of its
applicable context and used carelessly in another usually renders a scientific word completely meaningless.
Alas, the average person doesn't understand science, doesn't speak its language, and cannot distinguish the
correct use of the language of science from the meaningless gibberish of pseudo-science. As anyone who has
spent time exploring the web ought to have discovered, highly polished prose and graphics is no guarantee of
intelligent content. That's certainly true of what appears to be scientific material. I am further saddened to see
that even the titles of academia are deemed fair game by the quacks. While the physics term "energy" and the
biological word "medicine" can appear together in a sentence about cancer treatment or medical imaging, that's
not what the person claiming to have a Ph.D. in "Energy Medicine" has in mind. That degree was probably
granted by a group that understands neither physics nor medicine. There may be a place for non-traditional
medicine because medicine is not an exact science--there is often more than one correct answer in medicine and
there are poorly understood issues in medicine even at fairly basic levels.
However, physics is an exact science, with mechanical predictability (within the limitations of quantum
mechanics) and only one truly correct answer to each question. Its self-consistent and quantitative nature leaves
physics with no room for conflicting explanations. Like most academic physicists, I occasionally receive selfpublished books and manuscripts from people claiming to have discovered an entirely new physics that is far
superior to the current one. And like most academic physicists, I flip briefly through these unreviewed
documents and then, with a moment's sadness that the authors have wasted so much time, effort, and money, I
toss them into the recycling bin. It's not that we scientists are close minded medieval keepers of the dogma, it's
that these "new physics" offerings are the works of ignorant people who don't know what they don't know.
Unlike real scientific revolutionaries like Galileo and Einstein, these people don't understand the strengths and
weaknesses of the current scientific models. Their new offerings are usually inconsistent, fail to correctly model
the real universe, add unnecessary complexity to simple phenomena, or all three. It's extraordinarily unlikely that
anyone will ever successfully overthrow the basic laws of physics, not because no one will accept a new physics
if it's actually correct but because the current physics already explains things with such incredible accuracy and
predictive power. Developments in physics come almost exclusively at its frontier, where the current
understanding of physics is known to be imperfect or incomplete, and that is probably where those developments
will probably always occur.
So to return to your question, I would tell my students that I think that the "healing sciences" as you have
identified them are neither.
What do some permanent-magnet generators have stainless-steel axles? -- RC, Port-au-Prince, Haiti
Many forms of stainless steel, including those designated as "18-8 stainless," are completely non-magnetic. In
contrast to normal steel, which has a microscopic magnetic structure and is easily magnetized by a strong
magnetic field, these non-magnetic stainless steels are entirely free of magnetic structure. They cannot be
magnetized, even temporarily. In machinery that contains strong permanent magnets, using non-magnetic
stainless steel for the mechanical parts avoids undesirable attractions between parts and distortions of the
required magnetic fields. While copper, aluminum, or brass could also be used--they are non-magnetic as well-stainless steels are generally much tougher metals.
How does a projector work?
A projector is essentially a camera that's operating backward. When you take a picture of a tree, all of the light
striking the camera lens from a particular leaf is bent together to one small spot on the film. Overall, light from
each leaf is bent together to a corresponding spot on the film and a pattern of light that looks just like the tree--a
real image of the tree--forms on the surface of the film. The film records this pattern of light through
photochemical processes, and subsequent development causes the film to display this captured light pattern
forever. Because of the nature of the bending process, the real image that forms on the film is upside-down and
backward. Because it forms so near the camera lens, it's also much smaller than the tree itself.
A projector just reverses this process. Now light starts out from an illuminated piece of developed film--such as a
slide containing an image of a tree. Now the projector lens bends all of the light striking it from a particular leaf
spot on the slide together to one small spot on a distant projection screen. Again, light from each leaf on the slide
is bent together to a corresponding spot on the screen and a pattern of light that looks just like the slide--a real
image of the slide--forms on the surface of the projection screen. As before, this image is upside-down and
backwards, which is why you must be careful how you orient a slide in a projector, lest you produce an inverted
image on the screen.
Why are metal-halide lamps so efficient?
Metal-halide lamps are actually high-pressure mercury lamps with small amounts of metal-halides added to
improve the color balance. Light in such a lamp is created by an electric arc--electricity is passing through a gas
in the lamp and causing violent collisions within the gas. These collisions transfer energy to the mercury and
other gaseous atoms in the lamp and these atoms usually emit that energy as light. Overall, an electric current
passes through the lamp and gives up most of its energy as light and heat in the gas. As you've noted, the lamp is
relatively efficient, meaning that it produces more light and less heat than ordinary incandescent or halogen
lamps. However, metal-halide lamps aren't quite as energy efficient as fluorescent lamps.
What makes a metal-halide lamp so efficient is that there are relatively few ways for the lamp to waste energy as
heat. While collisionally excited mercury atoms normally emit most of their stored energy as ultraviolet light-the basis for fluorescent lamps--they can't do this in a high-pressure environment. A phenomenon called
"radiation trapping" makes it almost impossible for this ultraviolet light to escape from a dense vapor of mercury,
so a high-pressure mercury lamp emits mostly visible light. Even without the metal-halides, a high-pressure
mercury lamp emits a brilliant blue-white glow. The metal-halides boost the reds and other colors in the lamp to
make its light "warmer" and more like sunlight.
Next time you watch one of these lamps warm up, observe how its colors change. When it first starts up, its
pressure is low and it emits mostly invisible ultraviolet light (which is absorbed by the lamp's glass envelope).
But as the lamp heats up and its pressure increases, the rich, white light gradually develops. Incidentally, if the
power to a hot lamp is interrupted, the lamp has to cool down before it can restart because it only starts well at
low pressures.
Given a certain chemical structure, can it be determined which spectrum of light that molecule will absorb? Are there any
known compounds that charge their color or intensity when exposed to electric fields? - GS
While it is possible in principle to calculate the exact spectrum of light that a molecule will absorb, in practice it
is normally extremely difficult. It's a matter of complexity--the quantum mechanical equations describing a
molecule's electromagnetic structure are easy to write down but extraordinarily difficult to solve, even in
approximation. One of the great challenges of atomic and molecular physics and physical chemistry is
determining the full quantum mechanical structure of atoms and molecules through calculation alone. Except
with small atoms and molecules, it's awfully hard but not impossible. As computers get faster and approximation
schemes get better, the calculated spectra of molecules get closer to their experimental values.
As for compounds that change their optical properties while in electric fields, the answer is yes--all compounds
exhibit such changes, although they may be undetectably small. However, I can't think of any isolated molecules
that change dramatically in normal fields. Still, electric fields can alter the "selection rules"--the symmetry-based
laws that often control which optical transitions can or cannot occur. It's possible that a modest electric field will
turn on or off import optical transitions in some molecules so that they exhibit large color changes in small
fields. Still, I can't think of any useful examples.
How much current can a power generator produce and how does that current vary as you introduce more load onto the
generator?
There is no fundamental limit to how much current a generator can handle, however, the characteristics of the
generator's wiring, its magnetic fields, and the machinery turning it all tend to limit its current capacity. A
generator's wires aren't perfect and, as the current passing through the generator increases, its wires waste more
and more power. Like any wiring, a generator's wires convert electric power into thermal power in proportion to
the square of the current. Thus if you double the current in the generator, you quadruple the power loss. While
this power loss and the resulting heat are trivial at low currents, they become serious problems at high currents.
Increasing the current in the generator also affects its magnetic fields because currents are magnetic. At a low
current, the current's magnetism can be ignored. But when a generator is handling a very large current, the
magnetic fields associated with that current are no longer small perturbations on the generator's normal magnetic
fields and the generator may not perform properly any more.
Finally, a generator's job is to transfer energy from a mechanical system to the electric current passing through it.
As the amount of current in the generator increases, the amount of work that the mechanical system provides
must also increase--the generator becomes harder to turn. There will always be a limit to how much torque an
engine or crank can exert on the generator to keep it spinning and thus there will be a limit to how much current
the generator can handle.
As for how the current varies with load: the more current the load permits to pass through it, the more current
will pass through the generator. Assuming that the generator is well built and has very little electric resistance,
the load will serve to limit the current. The generator will then deliver just as much current as the load will
permit. If the load permits more current, the generator will deliver more. As a result, the wires in the generator
will waste more power as heat, the magnetic fields in the generator will become more complicated, and the
device powering the generator will have to work harder to keep the generator turning.
August 1, 1997
What is the relative insulating value of various levels of vacuum? For example, how insulating is 1/2 atmosphere as
compared to full atmosphere?
Amazingly enough, air's ability to carry heat doesn't change much as you reduce its pressure and density as long
as you stay above about a thousandth of atmospheric pressure and density. That's because reducing the density of
air molecules may leave fewer particles to carry heat, but it also allows them to travel farther before they collide
with other molecules. The reduction in molecular density is almost perfectly cancelled by an increase in the mean
free path those molecules travel between collisions--there are fewer heat carriers, but they can move more easily.
It isn't until you reach very low pressures and densities--so that the mean free path begins to approach the size of
the enclosed gas--that reducing the air pressure and density begins to decrease the air's ability to carry heat.
That's why even a small leakage of gas into a vacuum flask spoils that flask's insulating characteristics. However,
you can decrease the "air's" ability to carry heat by increasing the mass of its molecules--heavier particles such as
carbon dioxide or krypton travel more slowly than normal air molecules and don't carry heat as well.
July 19, 1997
When water boils the "air bubbles" rising from the bottom of the pan seem to be created spontaneously out of nothing. I
told my son that they are not air bubbles but rather water vapor. Is that correct? -- JG, Austin, TX
Yes, these bubbles contain water vapor, not air. The reason that you don't see them until the water reaches its
boiling temperature is that a bubble containing only water vapor isn't stable at lower temperatures--the
surrounding air pressure will crush it. But once the water temperature is high enough, the water vapor bubbles
are stable and they grow while rising to the top of the water.
Suppose that I fill a rigid container with water and that this container will not expand or contract as its temperature
changes. Will the water turn to ice when I cool it below 0° C? -- PL, Taikoo Shing, Hong Kong
Since water normally expands as it forms ice, the rigid container will prevent it from freezing at 0° C. If the
container was completely filled with water at room temperature, then an "empty" region will appear inside the
container when you first begin to cool it toward freezing. That's because water contracts as you cool it toward 4°
C. The "empty" region isn't really empty, it contains gaseous water vapor. But once the water's temperature drops
below 4° C, the water begins to expand as it cools. It will first expand into the "empty" region, but when that
region becomes full the water will no longer be able to expand. Instead, its pressure will begin to rise
dramatically. This elevated pressure is what will ultimately prevent the water from freezing at 0° C--high
pressure depresses water's freezing temperature. Although the water will eventually freeze, you'll have to cool it
far below 0° C for that to occur.
Does it make sense to raise the thermostat setting on your air conditioner when you leave your house, since when you
come back, you have to lower it again and the unit has to work more? Are there any energy savings? -- AN, Herndon, VA
You will save energy and money by raising the thermostat setting when you leave your home and then lower it
again when you return. That's because the rate at which heat flows into your home from outside is roughly
proportional to the difference between the indoor and outdoor temperatures. By letting the indoor temperature
rise, you slow the heat flow into your home. With less heat flowing into your home, the air conditioner doesn't
have to pump as much heat outside and that saves energy. Moreover, an air conditioner is more energy efficient
when the indoor temperature is closer to the outdoor temperature, so letting the indoor air warm up saves even
more energy. While the air conditioner does have to work steadily for a while when you return to your home, its
efficiency is still good during that time and the energy saved while you were away more than makes up for the
energy consumed when you return.
When I buy a role of undeveloped film, it has a particular weight. After I have taken a picture but before I develop the
film, does it weigh more or less? Does it matter what I take a picture of? -- CV, Warrenville, IL
I think that a small number of atoms leave the film when it's exposed to light, so your exposed film probably
weighs less than it did when you bought it. That's because light causes charge transfers within the grains of silver
salts, changing silver-halide molecules into silver atoms and halogen atoms, and the halogen atoms probably
leave the film or allow other atoms to leave instead. The silver atoms remain in the film, where clusters of three
or four of them form the latent image--a cluster triggers the complete conversion of a silver-halide grain into
silver during the development process. But the halogen atoms don't remain in the silver-halide grains. While it's
possible that these halogen atoms are stabilized in the emulsion, so that the emulsion's weight remains constant,
my guess is that they either diffuse out of the film or displace other atoms in the emulsion. Those displaced
atoms would then leave the emulsion. Overall, I suspect that atoms leave the film when it's exposed and that the
film becomes ever-so-slightly lighter.
I should point out, however, that the energy absorbed by the film does have a weight and that if the only effect of
exposing film to light were that the film absorbed this additional energy, then the film's weight would increase by
a fantastically small amount. But the chemistry that results from this energy absorption certainly swamps the
weight of the light energy.
I think that the speed of light could be broken by turning a very long lever. If the lever is long enough and you have
enough power to turn it, the end of the lever will travel faster than the speed of light. Is this so? -- NL, Hong Kong
I'm afraid that this technique won't work--the torque you would have to exert on the lever to make its end
approach the speed of light would become infinite and the energy you would have to transfer to the lever would
also become infinite. The Newtonian laws of motion aren't accurate at such high speeds and the full relativistic
laws are required. With this shift to relativistic motion come changes in the relationship between force and
acceleration, and between torque and angular acceleration. The faster the end of the lever moves, the harder it is
to increase its speed any further. As the lever tip approaches the speed of light, it becomes essentially impossible
to make it move faster.
As if this problem weren't enough, there is another problem: if you aren't extremely patient, the lever will bend
as you turn it, forming a spiral rather than a long arm that sweeps through space. That's because the lever is kept
straight by internal forces. While you are twisting the lever to make it turn faster, you are unbalancing these
internal forces and causing the lever to bend. The long lever you describe will actually curl into a spiral and its
end speed will never come close to the speed of light.
What changes occur to wood when it is permanently bent with the aid of steam? -- MH, Perth, West Australia
The main structural component of wood is cellulose, a polymer (plastic) consisting of long molecular chains of
sugars. While cellulose is extremely useful and is by far the most common polymer/plastic in the world, it can't
be melted because the temperature at which its molecular chains begin to move relative to one another is above
the temperature at which those molecular chains begin to fall apart. In short, cellulose decomposes before it
melts. Shaping or reshaping cellulose is very difficult, though chemical processes have made it possible to
reform cellulose into such materials as cellophane and rayon.
The process you describe, bending wood while heating the wood with steam, takes advantage of the fact that
cellulose molecules bind strongly to water molecules and that the water molecules then lubricate the chains so
that they can move relative to one another. Water is said to be a "plasticizer" for cellulose. Heat, water, and stress
allow the cellulose chains to slide slowly across one another. With enough patience, the wood's internal structure
can be changed forever. When the heat, water, and stress are then removed, the wood keeps its new shape.
I can understand that the strings of bubbles from the side of a glass of champagne are due to nucleating dirt or other
imperfections in the glass surface, but what causes those strings of bubbles in the center of the fluid? They are quite
persistent. Are they just dust? -- BM, Tehachapi, CA
If there were no impurities or imperfections in a glass of champagne, bubbles would only form through statistical
fluctuations--random effects would occasionally bring enough gas molecules together to form (nucleate) a
bubble and that bubble would grow and rise to the surface. But such spontaneously nucleated bubbles are
extremely rare and form randomly throughout the fluid, rather than in chains of steady bubbles. In fact, bubbles
would be so rare in this impurity-free liquid that you would probably not even notice them--the champagne
would slowly go flat by losing gas molecules from its surface alone.
In real champagne, chains of bubbles do rise upward from the center of the fluid. These bubbles are clearly
forming at suspended impurities. All it takes is a tiny piece of dust to trigger bubble formation. If you swirl the
champagne slightly, you should be able to see these suspended chains of bubbles move, indicating that the
impurities that are triggering them are also moving with the fluid.
Why is a rainbow in an arch? Does it have something to do with an equal distance from me to the raindrops and if so, is
the arc really a parabola? -- MM, Seattle, WA
A rainbow is truly circular, not parabolic. Passing through the exact center of that circle is the line that runs
between the sun and your head. Each colored arc of the rainbow is located at a particular angle away from this
line--the red arc is farther from the line than the violet arc is.
It is difficult for me to understand current flowing from a battery through a circuit. A battery has both a positive end and a
negative end. Which direction does the current flow? -- SK
When you connect a battery in a circuit, negatively charged electrons flow away from the battery's negative end
and they return toward the battery's positive end. The battery then pumps the electrons back to its negatively
charged end and they begin the journey all over again (hence the name "circuit"). But because the electrons have
a negative charge, current does not flow in their direction. Instead, current is defined as flowing in the direction
of positive charge flow. In the present case, current flows from the battery's positive end, through the circuit, and
back to the battery's negative end. Current is thus flowing in the direction opposite to the direction of electron
movement! If you want to know which way current is flowing, you can normally find the direction in which
electrons are flowing and then reverse it. Life for physicists and electrical engineers would be so much simpler if
Benjamin Franklin hadn't made an unfortunate choice that gave electrons--the principal carriers of electricity--a
negative electric charge. We have been living with the consequences of that choice ever since.
I understand how computer information travels as bits from source to destination, but how does each bit travel through
the wire at a molecular level? -- JCC, Atlanta, Georgia
At the simplest level, a bit travels as a packet of positive or negative charge through a wire. To start this
movement, the source injects a small amount of charge onto the end of the wire. Since like charges repel and
opposite charges attract, this new charge pushes on charges further down the wire, and those charges push on
charges still farther down the wire, and so on. Overall, a wave of forces and responses rushes along the wire until
it reaches the destination end of the wire. There charges flow off the wire and into the destination device. While
these charges aren't really the same ones that were put on the wire by the source, they have the same charge and
one can imagine that charge has simply moved from the source device to the destination device by way of the
wire. The destination device can examine this charge to determine whether the source was sending a 0 or a 1.
Why can we see through glass and some minerals? -- KH, Newport Beach, CA
Light consists of electromagnetic waves--fluctuating electric and magnetic fields that travel through space at
enormous speeds. As light passes through an insulator such as glass, diamond, quartz, or salt, the light's
fluctuating electric and magnetic fields cause electric charges in the insulator to vibrate back and forth. This
interaction between light and the charged particles in a material is the first step in absorption--the material is
"trying" to absorb the light. But light carries energy with it and any material that absorbs light must be prepared
to accept the light's energy. The charged particles in insulators generally have no quantum states that allow them
to accept that light energy. As a result, the insulator's charged particles respond to the light as it passes, but they
can't actually absorb the light. The light simply passes through the insulator. However, the light is delayed by its
interaction with the charged particles in the insulator (the speed of light in a material is less than the speed of
light in vacuum) and the light may be redirected (reflected or scattered) by encounters with inhomogeneities. So
glass and the other insulators don't absorb light and are often transparent. Those that aren't transparent are usually
white--they scatter light in all directions.
June 30, 1997
How does a catalytic converter help emissions in a car? -- JAM, Littleton, Colorado
While the burned gases that emerge from an ideal car engine would consist only of water vapor, carbon dioxide,
and nitrogen gas, a real car engine is far from ideal. In addition to these gases, a real engine emits nitrogen
oxides, carbon monoxide, and various unburned hydrocarbons left over from the gasoline. Because these gases
are major contributors to urban smog, car manufacturers have been forced to reduce them in various ways.
One of the most effective tools for eliminating the unburned hydrocarbons and carbon monoxide is a catalytic
converter. It is essentially a pipe containing a ceramic honeycomb on which there are countless tiny particles of
platinum and palladium. As the unwanted molecules pass through the honeycomb, they land on the metal
particles briefly and are combined with oxygen atoms to form water vapor and carbon dioxide. The catalytic
converter is burning these molecules in a controlled way, with the precious metal particles acting as catalysts to
assist the burning process.
Like all catalysts, these particles are not consumed in the process of burning the gases, but they can easily be
contaminated. That's why it's so important not to put leaded gasoline in a car with a catalytic converter--one tank
of leaded gas is all it takes to lead-coat the tiny platinum and palladium particles and to render them useless.
Another interesting note is that the catalytic converter is usually located on the underside of the car, protected
only by a thin metal shield. The converter becomes very hot in operation, both because hot exhaust gas is passing
through it and because the controlled combustion taking place inside it heats it up. Don't park a car with a
catalytic converter over a pile of leaves! Many an autumn car fire has started when a hot catalytic converter
ignited the pile of leaves beneath it.
We know that high speeds cause time to distort. We also have found wormholes in space that connect two distant points.
Therefore, by entering a wormhole we can travel through time. How can we create a wormhole and control its destination
point? -- JB, Union, New Hampshire
Near some large concentration of mass/energy, the equations of general relativity do admit solutions that have
two open ends and that could be interpreted as being wormholes. However, there is no widely accepted
interpretation of these solutions and no evidence that such solutions are actually realized in our universe. While
there are some physicists and astrophysicists who remain hopeful that wormholes will ultimately be found, the
only ones I've ever heard about are in science fiction stories.
Even if such exotic structures do exist, there is also no evidence that people could traverse the severely distorted
space-time between the two open ends without being destroyed and without having an infinite amount of time
pass in the rest of the universe while they were en route. If all of these issues aren't enough to discourage you, let
me add that the possibility of engineering wormholes to connect specific regions of space-time is extraordinarily
remote. Working with a wormhole would be at least as difficult as working with a black hole and I, for one, hope
never to encounter such a destructive and dangerous object.
How does gravity bend visible light? -- AHM, Pasadena, California
According to the concept of inertia, established by Galileo and Newton several hundred years ago, an object
that's not experiencing any pushes or pulls will continue to move in a straight line at a steady pace--in short, it
travels at a constant velocity. This observation can also be stated simply as an object in motion continues in
motion and an object at rest remains at rest.
When Newton formulated his theory of gravity, he viewed gravity as exerting forces on objects--it pulled them
toward one another so that they no longer followed their straight inertial paths. That's why a ball arcs through the
air, gradually turning toward the ground as the earth's gravity pulls it downward. This interpretation of gravity
was very successful and remains extremely useful to this day.
However, there is a second interpretation of gravity: the one offered by Einstein in the general theory of
relativity. According to this interpretation, concentrations of mass/energy warp space-time so that objects that are
following inertial paths--called geodesics--no longer travel in simple straight lines. In effect, a ball arcs through
the air because it is following a curved geodesic path and not because it is experiencing a force. While this exotic
interpretation for gravity isn't all that useful for slow moving objects like balls--Newtonian gravity is much more
practical in that case--it's important when dealing with fast moving objects like light. Light also follows
geodesics, but because it travels so quickly its geodesics tend to be rather straight. Even light passing just above
the surface of the sun bends only just enough to measure. Still, one of the most important confirmations of
general relativity came during a total solar eclipse when light from a star was found to bend slightly as it passed
by the sun's obscured surface.
Finally, I should say that you can also interpret the bending of light in terms of Newtonian gravity--that because
light contains energy, it acquires a weight when gravity is present and this weight causes its path to bend.
However, this Newtonian observation omits so much of the intrigue and beauty that comes with the bending of
space-time that I prefer the more modern interpretation.
When I warm more than one cup of coffee or milk together in a microwave oven, some of them warm more than others.
Why does this happen? Is there something wrong with our microwave oven? -- ON, Istanbul, Turkey
When the microwaves bounce around inside the oven's cooking chamber, they experience an effect called
interference. Interference occurs when similar waves, or portions of the same wave, follow different paths to the
same region in space. As they pass through that region, their crests and troughs ride up on top of one another and
they interfere. Sometimes the crests of one wave ride on the crests of the other wave, creating enormous crests-an effect called constructive interference. However, it is also possible for the crests of one wave to ride on the
troughs of the other wave, so that they cancel one another out--an effect called destructive interference.
These interference effects are quite visible in wave waves, but they also make themselves apparent in
microwaves. In your oven, they lead to regions of the cooking chamber that heat quickly (regions where the
microwaves experience constructive interference) and regions that don't heat well at all (regions where they
experience destructive interference). Because these fast and slow cooking regions can't be avoided, many
microwave ovens incorporate turntables to keep the food moving through the various regions inside the oven.
Some ovens use rotating metal paddles to stir that microwaves around inside the cooking chamber, so that the
fast and slow cooking regions move about.
Your experience with uneven heating of coffee or milk is an example of this interference problem. The solution
is to move the cups occasionally while they are being heated.
What was the difficulty in developing the blue LED? -- JM, Hoboken, NJ
A light emitting diode (an LED) produces light when a current of electrons passes through the junction between
its two pieces of semiconductor--from a n type semiconductor cathode to an p type semiconductor anode. The
LED's light is actually produced in the anode when an electron that has just crossed the p-n junction and is
orbiting a positively charged region (called a "hole") drops into the hole to fill it. In filling the hole, the electron
releases energy and that energy becomes light through a process called fluorescence.
The energy in a particle of light (a photon) is related the color of that light--with blue photons having more
energy than red photons. Here is where the difficulty in making blue LED's comes in: to produce a blue photon,
the electron in an LED must give up lots of energy as it fills the hole in the anode. This need for a large energy
release places a severe demand on the semiconductors from which the blue LED is made. These semiconductors
need an unusually large band gap--the energy spacing between two types of paths that electrons can follow in the
semiconductor. It wasn't until recently that good quality semiconductors with the appropriate electrical
characteristics were available for this task.
When a rear-wheel drive truck goes up a hill, do its rear wheels gain traction because of a transfer of weight to its rear
wheels? I think it depends on the center of gravity, right? -- DA, Issaquah, Washington
The traction a wheel experience depends largely on how hard it's being pushed into the roadway. When the truck
is on level pavement, the roadway prevents the wheel from sinking into it by pushing upward on the wheel with a
force called a support force. Because a wheel's traction is roughly proportional to the support force it's
experiencing, the harder the wheel is pushed into the roadway, the more traction that wheel has.
Since a truck has its heavy engine in front, the front wheels bear more of its weight than the rear wheels and they
experience more traction than the rear wheels. But as the truck tilts upward on the hill, the weight of its engine is
born more and more by the rear wheels. In physics terms, the truck's center of gravity, which is almost over the
front wheels while the truck is level, shifts to be more and more over the rear wheels as the truck tilts upward.
However, the extra weight that the rear wheels are supporting as the truck tilts doesn't improve their traction.
That's because this extra weight isn't being supported entirely by support forces--much of it is being supported
instead by friction between the rear wheels and the roadway. In fact, the support forces exerted by the roadway
on the rear wheels to keep them from sinking into the pavement actually become weaker as the truck tilts uphill,
so the truck loses traction as the tilt increases. Since traction is responsible for the friction that is also supporting
the truck, the truck is in danger of slipping down the road. There is clearly a limit to how steep the roadway can
get before the truck begins to slide.
June 26, 1997
If a given volume of water is placed in a container and frozen, will it weigh more, less, or remain the same relative to
when it was in a liquid state? -- RL, Denver, Colorado
Freezing water has virtually no effect on its weight--as long as the same number of water molecules remain in
the container, the overall weight of the container and water/ice won't change significantly. But water does
expand as it freezes, so the container will become more full as the ice forms. Water's expansion upon freezing
makes ice less dense--less mass per volume--than liquid water. This decrease in density explains why ice floats
on water and why pipes often break as the water inside them freezes.
However, you'll notice that I said "freezing the water has virtually no effect on its weight." In reality, the water
does lose a tiny fraction of its weight. That's because to freeze the water, you must remove some of the water's
energy. As Einstein pointed out with his famous formula E=mc2, energy and mass are related to one another and
since mass acquires weight when it's near the earth, so does energy. Because the thermal energy in liquid water
has a tiny weight, when you remove some of this thermal energy from the water, the water loses some of its
weight. But don't expect to measure this weight loss with a common scale--the weight change is on the order of
one part in a trillion, a factor that's presently beyond the precision of even the most advanced research measuring
devices.
If you take a compressed metal spring and place it in a container of metal dissolving acid, what happens to the energy in
the spring assuming the entire spring dissolves at one time? -- BR, Mount Pleasant, SC
That energy becomes thermal energy in the metal/acid solution. Before the spring dissolves, the energy it stores
is actually found in the forces between adjacent metal atoms. The crystals in the metal are slightly distorted,
bringing the atoms in these crystals a little too close or a little too far from one another. Since each of these
displaced atoms has a little extra potential energy, it is a little more chemically reactive than normal. When the
acid attacks one of these atoms and pulls it away from the crystal, the atom comes away a little more easily than
normal because it brings with it a little extra energy. This extra energy enters the solution, making the solution a
little warmer than it would have become had the spring not been compressed.
Could the traffic flow on freeways be modeled as a one-dimensional gas? You can see waves of motion among the cars
and these waves travel faster as the cars pack more tightly. -- RH, Escondido, California
There are many similarities between the cars traveling on a freeway and the molecules in a gas. As you point
out, disturbances at one point in the traffic cause ripples of motion to spread backward through the cars--similar
to what happens in a gas. However, normal gas molecules only interact with one another when they actually
touch, while cars interact at much larger distances--unlike gas molecules, cars don't do so well when they collide
with one another. To avoid collisions, the drivers watch what's happening far ahead of them and react
accordingly. In that sense, traffic's behavior resembles that of a non-neutral plasma--a gas of charged particles
that all have the same electric charge and therefore repel one another even at large distances. If you were to send
such a plasma through a narrow pipe, its particles would jostle back and forth as they tried to stay as far as
possible from one another. Ripples of motion would pass through the plasma and this motion would be very
similar to that of cars on a freeway.
You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can
they protect us from microwaves when they aren't even able to keep sound contained? You can hear popcorn popping
through the walls. -- RB, Beltsville, MD
The fact that sound waves can pass through the cooking chamber's metal walls doesn't mean that microwaves
can. These two types of waves are very different and the chamber's walls handle them very differently.
Any type of wave will partially reflect from a surface if passing through that surface causes the wave's speed to
change or, more generally, introduces a change in the "impedance" the wave experiences. Impedance is a
quantity that relates various parts of a wave to one another--it relates pressure to velocity in sound and it relates
the electric field to the magnetic field in a microwave. Since both sound waves and microwaves change speeds
and impedances when they encounter the cooking chamber's metal walls, they both partially reflect. The sound
that you hear when popcorn pops inside the oven is slightly muffled because the sound is having some trouble
escaping from the cooking chamber. However, the impedance change for the microwaves is so enormous that the
reflection is complete. No microwaves at all escape from the cooking chamber! The same effect occurs when you
hold a large mirror up in front of your face. You can hear what's happening on the other side of the mirror
because some sound can pass through the mirror. But light is completely reflected and you can't see through the
mirror at all.
How does lightning damage electrical appliances that are properly grounded and have their power switches in the off
position? Doesn't that eliminate a path for the electricity? -- RDU, Atlanta, Georgia
When lightning strikes a power line, it pours enormous amounts of electric charge onto that wire. These like
charges repel one another and they quickly spread out all over the wire. If this wire enters your home, the charges
traveling along it will flow into any appliance that's plugged in, whether it's turned on or not. But if the appliance
is turned off, this charge will reach the open switch and it will come to a stop, at least temporarily.
What matters then is just how much charge enters the appliance. The open switch would normally block the
passage of electricity, which is why the appliance doesn't operate while it's turned off. But as charge accumulates
on one side of the switch, the voltage at that point rises higher and higher. When the voltage becomes high
enough, as it easily does after a lightning strike, the charges can leap into the air and travel to the other side of
the switch even though the two sides don't touch one another. Another view of this disaster is that the like
charges on one side of the switch repel one another so vigorously that some of them are pushed through the air to
the other side of the switch. As a result of this movement of charges through the air--an electric arc--current
passes through the appliance as though it were turned on. If this current exceeds what the appliance can tolerate,
the appliance will be destroyed. Even grounding the appliance may not help--charges can flow uncontrollably
through the appliance and, while some charges take paths to ground, others flow through sensitive components
and destroy them.
If there was a hole drilled directly through the center of the earth and a ball was dropped into it, what would happen to the
ball? Would it oscillate up and down in the hole until it remained suspended in the center? -- JC, Dallas, TX
Yes, if the hole were drilled from the north pole to the south pole, the ball would behave just as you say.
Assuming that there were no air resistance, the ball would drop through the center of the earth and rise to the
surface on the other side. It would then return via the same path and travel all the way back to your hand. This
motion would repeat over and over again, with the ball taking 84 minutes to go from your hand to your hand.
That time is the same as it would take a satellite to orbit the earth once at sea level. In effect, the ball is orbiting
through the earth rather than around it!
However, because there would be air resistance unless you maintained a vacuum inside the hole, the ball
wouldn't rise to its original height after each passage through the earth. It would gradually loss energy and speed,
and would eventually settle down at the very center of the earth.
Finally, the reason for drilling the hole from the north pole to south pole is to avoid complications due to the
earth's rotation. If you were to drill the hole anywhere but through the earth's rotational axis, the ball would hit
the sides of the hole as it fell and its behavior would be altered.
June 25, 1997
What makes an airplane fly? -- SDH, Vicksburg, MS
While there are several ways to understand how air supports a plane's weight, I will look at it first in terms of the
deflection of the air flowing past the plane's wings. As the plane moves forward, air flows both over and under
the plane's wings. It flows across the wing from its leading edge to its trailing edge. The air that strikes the
inclined lower surface of the wing is deflected downward and leaves the wing's trailing edge with a slight
downward component to its motion. The air that flows over the arced and inclined upper surface of the wing
travels a more complicated route, curving up, over, and down before leaving the wing's trailing edge with a slight
downward component to its motion. In both cases, the wing has made the air accelerate downward by pushing
the air downward and it is the nature of our universe that the air must push upward on the plane in response. It's a
case of action and reaction: if one object pushes on another, the second object must push back on the first object
with an equal but oppositely directed force. So the plane's wing pushes down on the air and the air pushes up on
the plane. When the plane is moving fast enough and the wings are properly shaped and/or tilted, the upward
force that the air exerts on the wings can support the weight of the plane and suspend it in the air.
Another important view of flight involves air pressure in the streams of air flowing over and under the plane.
When the air passing under the wing curves downward, it actually does so because the pressure just under the
wing is higher than the pressure far from the wing--the air stream is experiencing an overall downward force due
to this pressure imbalance and this downward force is deflecting the air stream downward. When the air passing
over the wing arcs up, over, and down, it is also doing so because the pressure just above the wing is different
from that far from the wing. In this case, the pressure just over the wing's leading edge is quite high--enough to
deflect the air stream upward initially. But the pressure over the rest of the wing's upper surface is very low and
the air stream curves inward toward the wing; arcing downward so that it leaves the wing's trailing edge with a
small downward component to its motion. Overall, there is a low average pressure above the wing and a high
average pressure below it. This pressure imbalance produces an overall upward force on the wing and supports
the plane's weight.
These two views of flight--one involving deflection of the air stream and the other involving pressure
imbalances--are intimately related to one another and really only two descriptions of the same process.
Incidentally, the low pressure just over most the wing causes the air flowing over that wing to speed up. That's
Bernoulli's equation in action--when air following a streamline experiences a drop in pressure, it accelerates in
the forward direction.
How does the tachometer in a new car work? It looks like a magnet wrapped with wire that's located very near a sawtoothed wheel that spins as the engine turns. -- TR, Provo, UT
The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its
teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes
magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent
magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing
magnetic fields produce electric fields, the tooth's movement causes an electric field to appear around the
magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and
generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when
that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic
field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by
how frequently the current in the coil reverses directions or by how much voltage the coil generates.
June 24, 1997
How does an ultrasonic bath work? -- PT
An ultrasonic cleaner exposes a bath of liquid to very intense, very high frequency sound. Sound itself consists
of regions of high and low pressure that move through a material as waves. As these waves pass through the
liquid in the bath, each tiny portion of liquid vibrates back and forth in response to these pressure fluctuations.
Near the surface of an object immersed in the bath, the liquid is pushed first toward the object and then away
from it. The pressures involved are large and the changes in velocity within the liquid are so intense that
occasionally the liquid will actually pull away completely from the object so that a tiny empty cavity forms. In
effect, the liquid is jumping up and down on the object's surface and it occasionally jumps so hard that it leaves
the surface altogether. Cavities of this sort are unstable and the liquid soon returns to the object. When it does
return, the liquid collides violently with the surface and the liquid's pressure skyrockets as it transfers all of its
momentum to the object in millionths of a second. This "cavitation" process is what cleans objects immersed in
the ultrasonic bath--the dirt and grime are pounded free by the liquid when it returns to fill cavities that have
formed during the vibrations.
How would you figure out how much pressure a 100 lb. woman's high heel would produce as she walks? -- JB, Boulder,
Colorado
If the woman were standing still, with about half her weight on the heel of her right shoe, she would be exerting
a force of 50 pounds on the floor under that heel. Since a spiked heel is about 0.33 inches on a side, its surface
area is about 0.1 square inches (0.33 inches times 0.33 inches). Since a force of 50 pounds is applied to an area
of 0.1 square inches, the pressure on the floor is 50 pounds divided by 0.1 square inches or 500 pounds per
square inch. That's about 30 times as much pressure as the atmosphere exerts on objects at sea level.
But when the woman is walking, she often lands hard on that heel, so that it supports her entire weight and then
some. The extra force comes about because she is accelerating--when she lands, she is heading downward and
the floor must push upward extra hard on her to stop her downward motion. If we suppose that the total
downward force she exerts on the heel reaches a peak of 200 pounds--not at all unreasonable--the pressure the
shoe exerts on the floor reaches 2000 pounds per square inch. No wonder spiked heels damage floors and present
a serious hazard to nearby toes!
is a photon a specific unit of measurement of light? Has it been decided if light is a particle or a wave? Why? -- J,
Australia
There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave--for
example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle--for example,
it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light,
the smallest packet of light that can exist. You can't have half a photon of light--it's all or nothing. The amount of
energy in a particular photon of light depends on the frequency (or wavelength) of that light.
I have heard that there is a substantial cost to starting a fluorescent light fixture. When entering and exiting a room
frequently, is it better to leave a fluorescent light turned on, or to turn it off when leaving each time? -- GEW
Whenever you turn on a fluorescent lamp, a small amount of metal is sputtered away from the electrodes at each
end of the tube. These electrodes are what provide electric power to the gas discharge inside the lamp and
sputtering is a process in which fast moving ions (electrically charged atoms) crash into a surface and knock
atoms out of that surface. Because sputtering is most severe during start up, a typical fluorescent tube can only
start a few thousand times before its electrodes begin to fail. To avoid the expense and hassle of having to
replace the tube frequently, you shouldn't cycle the lamp more than once every ten minutes. If you will only be
away for a minute or two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off.
Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is
nonsense. It's just a myth.
How does the "night vision" mode of the car rear view mirror work? -- P
The glass in the rear view mirror is cut so that it forms a thin wedge--it's thicker at the top than it is at the bottom.
Its back surface is fully mirrored by a layer of aluminum. For daytime use, the mirror is oriented so that light
from behind the car enters the glass, reflects from the layer of aluminum on the back surface, and returns through
the glass to your eyes.
But when you tip the mirror upward for night use, the mirrored back surface presents you only with a view of the
car's darkened ceiling. However, there is a weak second reflection from the clear front surface of the mirror-whenever light changes speeds, as it does upon entering the glass, some of that light reflects. About 4% of the
light striking the front surface of the mirror from behind the car reflects without entering the glass and is directed
toward your eyes. Since the image you see is about 25 times dimmer than normal, it doesn't blind you the way a
reflection from the mirrored surface would.
Could you explain the meaning of polarization in optics? Please try to associate it with water waves if possible, to help
me visualize it, and avoid the use of electric and magnetic fields. -- AM, Yavne, Israel
I can't completely avoid electric and magnetic fields because polarization in optics is associated with a wave's
electric field. I also can't depend entirely on water waves because they only have one (transverse) polarization.
Still, I will try.
First, consider a wave traveling toward us on the surface of a lake. Suppose that this wave passes under a small
boat and I ask you which way the wave is making the boat move. You would tell me that the boat is moving up
and down. I would then tell you that the wave is vertically polarized because it causes objects that it encounters
to move up and down rhythmically.
Unfortunately, pure water won't do for the next step because it won't support horizontally polarized waves. So
let's imagine that some ecological disaster has turned the entire lake into gelatin. An explosion at the side of the
lake now causes a wave to begin heading toward us on the gelatin lake, but this strange wave involves a side-toside motion of the lake's surface. Now when the wave passes under the boat, the boat moves side-to-side
rhythmically. In this case the wave is horizontally polarized because it causes objects that it encounters to move
left and right rhythmically.
Now let's return to optics. When an electromagnetic wave heads toward us, its electric fields will push any
electrically charged particles it encounters back and forth rhythmically. If we watch one of these charged
particles as the wave passes it and observe that this particle moves up and down, then the wave is vertically
polarized. If instead the charged particle moves left and right, then the wave is horizontally polarized.
During a recent ice storm, I was standing in my front doorway before dawn and the entire southern sky turned brilliant
blue-green for about five seconds or more. What caused this effect? People who missed it tell me it was just a transformer
"blowing up" but I've seen one blow up on our street and there is no comparison. The light I saw virtually filled the entire
horizon.
You probably saw a sustained high-voltage arc between high-tension wires and/or the ground. I would guess that
the ice pulled down one of the wires or caused a tree to fall across them. While transformer explosions often
involve hundreds of kilowatts of electric power being turned into light and heat, most of that light is hidden from
view inside the transformer. Such an explosion can be dramatic, with some nice sparks and flashes, but it's
usually not very bright. However, when a high-tension wire arcs, a significant fraction of the many megawatts of
power flowing through the arc is converted directly into light. In effect, a high-pressure arc lamp forms right in
the air and it looks like a camera flash that just keeps going until something stops the arc or the power is shut off.
The blue-green color you saw comes from characteristics of the air and metal wires involved in the arc. As you
saw, a couple of million watts of light are enough to light up the predawn sky quite effectively!
There is, however, an alternative explanation: you may have seen the "green flash" that occasionally appears just
as the sun reaches the horizon at sunrise or sunset. This flash is a refraction effect in the atmosphere in which
only blue-green light from the sun reaches the viewer's eyes for a second or two while the sun is just below the
horizon. However, this green flash should appear in the eastern sky just before dawn, not the southern sky.
What is the speed with which electric power is transmitted through the power grid? Believe it or not, the education center
at an important nuclear power plant claims that "electrons travel at the speed of light," an obvious impossibility for
current in a copper wire. What is the maximum speed of an electron in a commercial electric power grid? in a
superconductor? -- AW, Alexandria, VA
Amazingly enough, the speed at which electric power travels through a wire is very different from the speed at
which electrons move through that wire. In most wires, electric power travels at very nearly the speed of light
while the electrons themselves travel only millimeters per second! This statement is true whether the electricity
is traveling in a copper wire or a superconductor!
To understand how this difference in speeds is possible, think about what happens when you turn on the water to
a long hose. If that hose is already filled with water, water will immediately begin pouring out of the hose's end
even though the water is flowing quite slowly through the hose. While the water itself moves slowly, the water's
effects travel through the hose at the speed of sound in water--several miles per second! Water at the end of the
hose "knows" that you have opened the faucet long before new water from the faucet arrives.
Similarly, when you turn on a flashlight, electrons begin to flow out of the battery's negative terminal at speeds
of only a few millimeters per second. But these electrons don't have to travel all the way to the light bulb for the
bulb to light up. When these electrons leave the battery, they push on the electrons in front of them, which push
on the electrons in front of them, and so on. They produce an electromagnetic wave that rushes through the wire
at an incredible speed. As a result, electrons begin flowing through the light bulb only a few billionths of a
second after the first electron left the battery. So while the electrons that carry electricity through the power grid
flow rather slowly, the power they deliver moves remarkably fast.
June 23, 1997
I heard some time ago about a car that uses microwaves to heat the air in front of it so that it creates a vacuum. The
relatively higher pressure behind then pushes it forward. Is this possible? -- RM, Toronto, Ontario
Even if microwaves were effective at heating air, which they are not, this heating would not propel the car
forward. The air in front of the car would become hot, but its pressure would remain almost unchanged. Instead,
the air would expand to occupy a larger volume and would then be lifted upward by the cooler air around it ("hot
air rises"). Cooler air would flow in to replace the escaping hot air and the car would simply sit there with a
steady stream of hot air rising in front of it.
Could you suspend a car on hot air produced below it? -- RM, Toronto, Ontario
For the buoyancy of hot air to suspend a car, you would need a lot of it--in effect you would have to turn the car
into a hot air balloon. That's because the lifting force experienced by hot air is really supplied by the cooler air
around it and this upward buoyant force is proportional to the volume of hot air being lifted. Since a car is pretty
heavy, the volume of hot air required will be enormous.
However, if you trap the air underneath the car, so that its volume can't increase, and then heat that air, its
pressure will rise. This increased pressure below the car would produce an overall upward pressure force on the
car and could support the car's weight. In effect, you would be creating a ground-effect hovercraft in which the
elevated pressure of trapped hot air supports the weight of the vehicle. But it would be easier and less energyintensive to pump air underneath your hovercraft with a big fan. That's what most ground-effect vehicles do.
They pack extra air molecules underneath themselves and then allow those molecules to support their weight.
Furthermore, because air molecules are always leaking out from beneath the vehicle, you'll need a fan to replace
them anyway.
How does the automatic cutoff valve on a gasoline pump work? How is it able to shut off the gas before the nozzle has
become immersed in the liquid? I don't see how the pump could be so sensitive to back pressure in the gasoline. -- NG,
Bloomsburg, PA
As you suspect, the pump isn't able to detect the change in gasoline pressure that occurs when the fill level
reaches the nozzle. Instead, the nozzle uses several hidden components to shut itself off when the tank is full.
There is a small hole near the end of the nozzle that becomes blocked by the liquid gasoline as soon as the fill
level reaches that hole. Blocking this hole with gasoline is what shuts off the valve. There is actually a thin tube
inside the main gasoline delivery hose that operates this valve system. That tube runs from the hole in the nozzle
to a vacuum pump inside the gasoline-pumping unit. While the pump is dispensing gasoline into a partially filled
tank, air flows easily into the nozzle's hole and the pressure inside the thin tube remains close to atmospheric
pressure. But when the level of gasoline rises high enough, it essentially blocks the hole and the pressure inside
the thin tube drops. This pressure drop is what triggers the valve and stops the gasoline flow. Look for the hole
near the end of the metal nozzle next time you fill your car with gasoline. In most cases, it's easy to see.
Could microwave heating be used to treat sewage to wipe out disease organisms in it? -- KO
While microwave heating could be used to sterilize sewage, it's not the most energy efficient or inexpensive
technique. Microwave heating is really only worthwhile in cases where you can't reach the inside of an object
directly--as is the case in most solid foods. Since sewage is essentially liquid, it can be heated quickly and
efficiently by passing it close to a hot surface. Just about anything can be used to heat that surface--electricity,
natural gas, coal, you name it.
But to be even more energy efficient, the sewage that was just sterilized a minute ago and is still hot can be used
to heat the sewage that is about to be treated! A well designed thermal treatment facility could employ "countercurrent exchange"--that is it could pass the hot, treated material through a heat exchanger to allow it to transfer
most of its excess heat to the cooler, untreated material that is about to be sterilized. By recycling the heat in this
manner, the facility could avoid having to burn so much fuel. The only drawback with this technique is that the
heat exchanger must be leak-proof--it must keep the sterilized material from touching and being contaminated by
the unsterilized material.
With the amount of wind that's produced by high-speed traffic on expressways, why don't electric companies put windpowered generators in the center lanes? Using this (wasted) wind to generate electricity would be cheaper, safer, and
environmentally friendlier than the power plants that they are running now. -- DJA
While wind generators are being used experimentally to charge batteries in roadway equipment that can't be
reached with power lines, there are at least three reasons why such generators aren't in large scale use. First, wind
generators that connect to the AC power grid work most efficiently when they turn at a steady rate--the generator
itself must remain in synch with the cyclic alternating current in the electric power lines. The intermittent and
sporadic winds produced by passing cars and trucks aren't really suitable for such wind generators.
Second, to make efficient use of the wind created by traffic, hundreds of wind generators would have to be
installed on each mile of expressway. Since wind generators are expensive, it's much more cost effective to put
them on windy ridges out in the country or by the seashore.
Third, the wind generators you propose would actually extract energy from the cars and trucks and reduce their
gas mileages! That fact might surprise you, since it would seem that extracting energy from the wind wouldn't
have any effect on the cars and trucks that created that wind. But the wind and the vehicles continue to interact as
they move along the expressway--each vehicle drags a pocket of air with it and interfering with this air pocket
has the effect of interfering with the vehicle! The vehicle uses energy to maintain this moving air pocket and it
burns additional fuel. An aerodynamically well-designed vehicle has a relatively small air pocket, but there is a
limit to what can be done. To reduce the energy cost of maintaining the air pocket, the vehicle's driver can steer it
into the air pocket behind another vehicle so that the two vehicles share a single air pocket. The lead vehicle then
provides most of the energy needed to keep the air pocket moving. This technique of sharing an air pocket is
called "drafting" and is frequently used by bicycle racers. But while drafting makes it easier for many vehicles to
keep their air pockets moving, the wind generators that you propose would make it harder--they would steal
energy from the air pockets of every passing vehicle and make those vehicles fight harder to keep their air
pockets moving.
A better way to save energy would be to encourage large-scale drafting in some safe way. Having chains of
independent cars tailgate one another would be energy efficient, but would cause horrific accidents. However,
assembling those cars into a tightly coupled "train" may someday become possible with advances in technology
and computer controls.
June 20, 1997
I fight a constant battle with mildew in the Pacific Northwest. I can buy solid chemicals to put in my closets, which take
water out of the air, eventually creating a bucket full of water. Do these devices actually lower the moisture content of the
air or do they just make me feel like I'm doing something? -- MD
How much effect these drying agents have depends on how much air they're exposed to. Water molecules are
continuously going back and forth between the air and everything exposed to that air--your clothing, your hair,
the walls of your home, the contents of a saltshaker, and the drawers in a wooden bureau. The water molecules
land on and take off from every surface, like busy miniature airports. The rate at which water molecules land on
an object depends on how humid the air is. The rate at which water molecules leave that object depends on how
hot the object is and on how tightly water molecules cling to it.
The landing and leaving processes are in perpetual competition and the fastest one wins. If the air is humid and
the object is cold or attractive to water molecules, the landing process dominates and water condenses out of the
air and onto the object. If the air is dry and the object is hot or doesn't bind water molecules well, taking off
dominates and water evaporates from the object into the air.
Your problem is that the air in your closets is very humid and landing is winning--too much water is condensing
on your walls. To stop this condensation, you either have to heat the walls, so that water molecules leave them
faster, or reduce the humidity of the air, so that water molecules land less often. Putting a material that binds
water molecules into your closets changes the balance of landing and taking off--water molecules that land on
this material don't return to the air often so the humidity of the air diminishes. With less humidity in the air, the
rate at which water molecules land on the walls also diminishes.
But this drying effect only works if the air in the closet is trapped there. If your closet exchanges air quickly with
outdoor air, the water molecules removed by the drying agent will be quickly replaced with new water molecules
from outside. In effect, you will be trying to dry the great outdoors, a hopeless task. To make the most of this
drying agent, you should let it work on as little air as possible by sealing the closet and slowing the exchange of
air with outside. Better yet, replace the drying agent with a dehumidifier. A dehumidifier accumulates water
molecules from the air by presenting the air with a chilled surface. Water molecules land on the cold surface and
then don't have enough energy to return to the air. They are trapped by the cold rather than by chemical binding.
June 18, 1997
We heated a cup of water in a microwave oven for 2-1/2 minutes and then added a spoonful of sugar to it. A rush of tiny
bubbles ensued. Did the sugar crystals nucleate boiling water molecules that were trapped by surrounding cooler
molecules or did they nucleate the release of dissolved air? -- VC
When you heated the water in the microwave oven, you raised its temperature above its boiling temperature, yet
it did not boil. While the water was hot enough to boil--that is, any steam bubble that formed in this hot water
would have a pressure at least equal to atmospheric pressure and would not be crushed by the surrounding air-the water was having a difficult time forming steam bubbles. For a bubble to appear, several water molecules
must simultaneously break free of their neighbors to form a bubble nucleus. Once this nucleation has occurred,
additional water molecules can evaporate into the bubble, making it grow. This nucleation is rare in pure water
near its boiling temperature; in most cases it is assisted by hot spots at the bottom of a pot on the stove or by
imperfections in the container holding the water. But when you heat water in a glass or glazed ceramic container
in a microwave oven, there are no hot spots or surface imperfections to nucleate the bubbles. The water
superheats above its boiling temperature. When you add sugar crystals to this superheated water, the crystal's
sharp edges and points assist the nucleation of steam bubbles and the water boils violently.
Your suggestions for why the bubbles appear raise two interesting points. First, in a thermal system such as hot
water, you can't identify some molecules as being boiling hot and others as being cooler--temperature is a
property of the entire system and not of individual molecules. However, at a given instant, there are molecules
with more energy than their neighbors and it is these energetic molecules that may break free of their neighbors
to form a bubble nucleus.
Second, water often contains dissolved gases and these gases come out of solution when the water is heated.
While many of the gas molecules leave through the water's surface, some of them may leave as bubbles from
within the water. This gas bubble formation requires nucleation as well, which is why these bubbles often appear
on the inner surfaces of a metal pot on the stove--flaws in the pot's surface assist bubble nucleation. But these gas
bubbles aren't what you observed; there just isn't that much dissolve gas. You can prove that the bubbles you
observe are steam: repeat the experiment several times with the same water. Each time you heat the water and
add sugar, it bubbles wildly--something that wouldn't be possible if you were simply releasing dissolved gases
from the water.
June 10, 1997
How is the skin better hydrated by vapor as opposed to liquid water? Wrapping yourself in a damp sheet is more effective
at treating the dryness of eczema than taking a bath. -- CW
When your skin is immersed in pure water, the only molecules that ever collide with its surface are water
molecules. That might seem to be the ideal situation for keeping skin moist, however such immersion can have
other unintended consequences. First, any water soluble atoms, molecules, and ions that can move to the surface
of your skin will dissolve away in the surrounding water and you'll never see them again. Second, any water
soluble atoms, molecules, and ions that can't move to the surface of your skin will draw water into your skin by
way of osmosis--the pure water will flow into your skin cells in an attempt to dilute the dissolved particles inside
those cells. After a relatively short time, the cells of your skin will contain many more water molecules than
before and your skin will look all wrinkly. This flow of water soluble materials out of your skin and water into
your skin may not be so wonderful for your eczema.
When you wrap yourself in a wet cloth, you are ensuring that the relative humidity near the surface of your skin
will be close to 100%. Air molecules will still be present around your skin but now there will be essentially no
net transfer of water between your skin and the surrounding air--water molecules will leave your skin for the air
at roughly the same rate as water molecules return to your skin from the air. In effect, you are stopping
evaporation from your skin and very little else. Stopping evaporation from your skin will also cause it to
accumulate moisture, but this time the new moisture will come from within your body. Water molecules that
would have left your skin had it been surrounded by dry air are now staying in your skin, where they add to the
moisture in your skin. Overall, you skin will contain more water but it will not have lost as many water-soluble
chemicals and it will not have water driven into it by osmotic pressure. It may be this more gentle moisturizing
effect that makes wrapping yourself in a damp sheet more pleasant for your eczema than immersing yourself in
water.
Is light a particle or a ray? -- CG
Light is both a particle and a ray (a wave). Its wave character was known and understood for many years before
its particle character was discovered. That a film of clear soap exhibits colors is one of many demonstrations that
light travels as waves, and such demonstrations were well understood in the 19th century. But it wasn't until the
early 20th century that people discovered the particle character of light. They found that light is absorbed in
discrete packets of energy or quanta, and these quanta of light energy were called photons. As a simple rule of
thumb, you can think of light as exhibiting wave-like properties while it's traveling, but particle-like properties
when it's being emitted or absorbed. This dual nature of light is complicated but unavoidable; it's a consequence
of the quantum mechanical nature of our universe.
If I measure current from a photocell, am I indirectly measuring power as well? -- MR
As long as current is free to flow from one end of the photocell to the other, the amount of current flowing
through that circuit is almost exactly proportional to the number of light particles (photons) striking the photocell
each second. Since the rate at which photons strike a photocell is generally proportional to the light power
striking that photocell, you can use a measurement of current to make a measurement of light power. While there
are a few subtle details that you must be careful about, particularly changes in the light spectrum and
unanticipated impediments to the free flow of current through the circuit, this relationship between the current
and the light power is very useful. For example, most camera light meters use photocells to determine exposures.
How does a photocell absorb light and turn it into power? -- MR
A photocell is actually a large diode--a one-way device for electric current. Like most diodes, the photocell
consists of two different layers of chemically altered or "doped" semiconductors, the anode layer and the cathode
layer, and the junction between these two layers has the peculiar property that it normally allows electrons to
cross it in only one direction. There is what's called a "depletion region" at the junction, a very thin insulating
layer with two electrically charged surfaces--the surface on the cathode side is positively charged and the surface
on the anode side is negatively charged.
When an electron, which is negatively charged, approaches the depletion region from the anode side, it first
encounters the depletion region's negatively charged surface and is repelled. But when the electron approaches
from the cathode side, it first encounters the depletion region's positively charged surface and is attracted. If it
has enough energy when it approaches the depletion region from the cathode side, the electron can cross the
depletion region to reach the anode layer. Thus electrons can move relatively easily from the photocell's cathode
layer to its anode layer but they can't go back.
When a photocell is exposed to light, some of the light particles (photons) are absorbed in the diode's cathode
layer. When such an absorption occurs, the photon's energy may be transferred to an electron in the cathode,
giving that electron the energy it needs to cross the depletion region and reach the anode. But once the electron
has arrived at the anode it can't return to the cathode directly across the depletion region. Instead, it must flow
through an external circuit in order to return to the cathode. As that electron flows through the external circuit, it
can give up some of its energy, obtained from the light photon, to devices in that circuit. In that manner, light
energy has provided energy to an electrically powered device.
Can a photocell ever absorb too much light? If it can, does it explode or simply stop absorbing light? -- MR
Since not all of the light power absorbed by a photocell is converted into electric power, a photocell that's
exposed to too much light will overheat. High temperatures are disastrous for all semiconductor devices,
including computer chips and photocells. If a semiconductor device overheats slightly, the excessive thermal
energy will change the electronic properties of the semiconductor layers so that these layers won't behave as they
were chemically prepared to do. In an overheated photocell, charge will be allowed to flow backward so that the
photocell will become less energy efficient. But if a semiconductor device overheats seriously, the
semiconductor layers will change permanently--atoms, molecules, and entire structures will migrate and
rearrange, and the device will never work properly again.
By itself, an overheated photocell won't fail dramatically; it will just stop working. If you've overheated it
severely, it will remain broken from then on. But if the photocell is part of a larger collection of power
generating elements that continues to produce power, that photocell may suddenly consume all of the power
from the other elements. In that case, the photocell may explode as its temperature skyrockets.
Why are sparks generated when iron is brought in contact with a spinning grinding wheel? -- JF, Rochester, NY
When the iron touches the spinning wheel, the two experience sliding or "dynamic" friction--the iron acts to slow
the wheel while the wheel acts to move the iron. Because you hold the iron in place, it doesn't move but its
surface begins to experience severe wear--the iron is skidding across the surface of the wheel. Sharp projections
from the wheel are tearing particles away from the iron and throwing them in the direction of the wheel surface's
motion. Because the two surfaces, iron and wheel, are pushing on one another and they are moving relative to
one another in the directions of their forces, they are doing physical work on one another--meaning that they are
exchanging energy. This energy is actually being converted from the wheel's rotational energy into thermal
energy in the iron and in the wheel, both of which become hot. You can feel similar heating by rubbing you
hands against one another vigorously. The wheel's surface begins to glow red-hot and the particles that fly off the
iron emerge so hot that they burn in the air. The sparks you see are the iron particles burning up. Depending on
what type of iron or steel you use, you'll see different spark patterns. An expert can actually identify an alloy by
this pattern.
June 6, 1997
How are incandescent light bulbs made? -- SU
The glass enclosures are made from a ribbon of hot glass that's first thickened and then blown into molds to form
the bulb shapes. These enclosures are then cooled, cut from the ribbon, and their insides are coated with the
diffusing material that gives the finished bulb its soft white appearance.
The filament is formed by drawing tungsten metal into a very fine wire. This wire, typically only 42 microns
(0.0017 inches) in diameter is first wound into a coil and then this coil is itself wound into a coil. The mandrels
used in these two coiling processes are trapped in the coils and must be dissolved away with acids after the
filament has been annealed.
The finished filament is clamped or welded to the power leads, which have already been embedded in a glass
supporting structure. This glass support is inserted into a bulb and the two glass parts are fused together. A tube
in the glass support allows the manufacturer to pump the air out of the bulb and then reintroduce various inert
gases. When virtually all of the oxygen has been eliminated from the bulb, the tube is cut off and the opening is
sealed. Once the base of the bulb has been attached, the bulb is ready for use.
What are positive and negative g's?
Let me start with the concept of inertia. Like all objects in this universe, we naturally tend to keep doing what
we're doing--if we are stationary, we tend to remain stationary, and if we are moving, we tend to keep moving in
a straight line at a steady pace. In fact, the only way that your speed and/or direction of travel (in short, your
velocity) can change is if something pushes on you. When that happens, you accelerate (which is to say your
velocity changes).
Whenever you accelerate, the various parts of your body can no longer follow their inertia; they must accelerate,
too. This acceleration requires forces within your body and you can feel these forces. In fact, they make it feel as
though a new type of gravity were acting on the parts of your body. You can't distinguish true gravity from the
experience of acceleration because they feel exactly the same. The strength of this gravity-like experience
depends on how fast you accelerate and it points in the direction opposite your acceleration. If you accelerate
upward, as you do when an elevator first starts moving upward, this gravity-like sensation points downward and
you feel extra heavy (the experience of "positive g's") If you accelerate downward, as you do when a rising
elevator comes to a stop, this gravity-like sensation points upward and you feel unusually light (the experience of
"negative g's") Since there is no fundamental limit to how rapidly one can accelerate, these positive and negative
g's can become extremely strong and can easily feel stronger than the true force of gravity. However, when these
gravity-like sensations become a few times stronger than gravity itself, they become difficult to tolerate. That's
why elevators start and stop gradually and why the turns on roller coasters aren't too sharp.
How does a single lens reflex camera work?
When rays of light from a distant object reach the camera's lens, those rays are spreading apart or "diverging."
You can understand this by following the rays of light from one spot on the object, say the tip of a person's nose.
The rays of light reflected from the nose spread outward in all directions and only a small portion of them passes
into the camera's lens. These light rays are diverging from one another as they travel.
The camera's lens is a converging lens, meaning that it bends the paths of these light rays so that they diverge
less after passing through it. In fact, the lens bends the rays so much that they begin to come together or
"converge" after the lens and all the rays of light from the person's nose merge to a single point in space
somewhere beyond the lens. Exactly how far from the lens the rays come together depends on the structure of the
lens and on the distance between it and the person's nose. When you focus the lens, you're moving the lens so
that the rays come together at just the right place to illuminate a single spot on a piece of photographic film.
When the distance between the lens and film is just right, all the light from each point on the person comes
together at a corresponding point on the film. The lens is then forming a real image of the person on the film and
the film records this pattern of light to make a photograph.
In a single lens reflex camera, light passing through the lens doesn't always fall on the film. Most of the time, this
light is redirected by a mirror that follows the lens so that the real image forms on a special glass sheet near the
top of the camera. When you look through the viewfinder of the camera, you are actually using a magnifying
glass to inspecting this real image, making the camera effectively a telescope. You (or the camera, if it is
automatic) then focus the lens to form a sharp real image on the glass sheet before taking the picture. Since this
glass sheet is the same optical distance from the lens as the film is, focusing on the glass is equivalent to focusing
on the film. When you take the picture, the redirecting mirror quickly flips out of the way and a shutter opens to
allow light from the lens to fall directly onto the camera's photographic film. For a brief moment, light from the
person passes through the lens and onto the film, forming a real image that is permanently recorded on the film.
Then the shutter closes and the mirror swings back to its normal position.
June 3, 1997
What happens to ice when it is left in the freezer--does it evaporate? I have noticed that over time the ice cubes shrink? -J&K
When you leave ice in a frostless refrigerator, it gradually sublimes and shrinks away to nothing. Sublimation is
equivalent to evaporation, except it involves a solid converting directly into a gas. The surface of an ice cube is a
busy place, with water molecules landing and taking off all the time. If more water molecules land than leave,
the ice cube will grow in size. If more water molecules leave than land, the ice cube will shrink. The water
molecule landing rate is determined by how much moisture there is in the air. In a frostless refrigerator, the air is
extremely dry, meaning that it contains very few water molecules. Thus the landing rate in a frostless refrigerator
is very low and the ice cubes shrink. If you watch the ice cubes in an older style refrigerator, you will find that
they grow over time because the air in that refrigerator is moist and the landing rate is high. Incidentally, this
sublimation of water molecules from ice is why snow disappears gradually even when the weather remains cold
and is also how freeze drying of food is done.
If living organisms maintain their order by exporting disorder to their environment, do they create more disorder than the
order they maintain? -- CC
Living organisms create more disorder in their surroundings than they create order in themselves. Overall the
disorder of the combined system--organisms and environment--increases. This result is an unavoidable
consequence of the second law of thermodynamics, which notes that the entropy (disorder) of an isolated system
can never decrease. While it is possible in principle for a living organism to export disorder so efficiently that the
overall disorder remains unchanged, that perfection is never achieved. Instead, living organisms export far more
disorder than is required for them to maintain order in themselves. As a result, living organisms are net producers
of disorder.
In that respect, people are much more vigorous producers of disorder than most other living organisms. People
seek order not only in their bodies, but also in the objects around them and they achieve this ordering by
consuming order in their environment--fossil fuels, minerals, pure water--at a furious pace and producing
disorder in its place--burned gases, garbage, polluted water. Fortunately, sunlight is a tremendous source of order
for our earth and it undoes some of the disordering caused by living organisms. However, we are consuming
much of the order that sunlight stored on earth over millions of years in only a few generations. At this pace,
we're destined to have troubles with the disorder we're creating. Many of the environmental issues that face us
today can be viewed from this order/disorder perspective: we have to learn how to create less disorder.
May 25, 1997
How is the high explosive used in a fission bomb detonated so precisely together? - F, United Kingdom
Most modern nuclear weapons produce a super-critical mass of fissionable nuclear fuel by crushing a sphere of
that material with high explosives. As the material's size shrinks, its density increases and it passes rapidly
through critical mass to achieve a highly super-critical mass. Nuclear chain reactions then grow exponentially in
the material and huge amounts of energy are released. However, the process of crushing a solid sphere of metal
to several times its normal density requires sophisticated high explosives triggered at precisely the right
moments. The triggering is done with very high-speed electronic devices and explosive detonators that respond
almost instantly to high voltage pulses. Perhaps the most critical components in this system are high speed, high
voltage switches known as krytron tubes. Because these devices have limited uses outside of nuclear weapons,
their export is tightly controlled and it's a big news story whenever someone is caught trying to smuggle them
outside the United States.
How do you determine the critical mass of a particular radioactive element or isotope? - F, United Kingdom
This questions asks how you can predict the amount of a fissionable nuclear fuel you must assemble in order for
that fuel to experience self-sustaining nuclear fission chain reactions. A self-sustaining nuclear chain reaction can
only occur when each fission within that material causes an average of one subsequent fission. The size, shape,
and density of the nuclear fuel are important to the chain reaction because they determine how much opportunity
fragments from one fission event will have at inducing subsequent events elsewhere within the fuel. A properly
shaped piece of fuel that is just large enough and dense enough to experience a self-sustaining nuclear chain
reaction is said to be at critical mass. Below the critical mass, the chain reaction won't be able to sustain itself
and will gradually dwindle away. Above the critical mass, the chain reaction will grow stronger exponentially.
Since crossing the threshold from below critical mass to above critical mass has dramatic consequences, it can be
quite important to know the point at which it occurs.
The basic calculation of critical mass is straightforward in principle, but it requires a thorough understanding of
the nuclear fuel. Because you need to know how likely one nuclear fission is to cause a subsequent nuclear
fission, you must know both the types of fragments you can expect from the first nuclear fission and the
likelihood that each fragment will induce a subsequent fission in another atomic nucleus before that it escapes
from the nuclear fuel. Because the range of possible fragments, their kinetic energies, and their paths through the
nuclear fuel are so vast, an accurate calculation of critical mass is extremely complicated. As an indication of the
difficulty, note that fission fragments may bounce off nuclei without inducing fission, so that you must consider
bent paths as well as straight ones. Not surprisingly, the calculation of critical mass is too difficult to do exactly,
even with the help of computers. In fact, one of the reasons that Germany didn't develop nuclear weapons during
World War II was that its scientists miscalculated the critical mass of a fission bomb based on enriched uranium
and thought that they would need many tons of enriched uranium rather than the true critical mass of about 52
kilograms. Certain that a critical mass of enriched uranium was unattainable, they didn't pursue the project.
What about the effects of microwaves on the cellular structure of the item in the oven? I've heard that cells are ruptured
violently by microwave radiation and that the ingestion of such materials affects the immune system. - AB
Just about any cooking damages the cells of the food being cooked, so microwave cooking is nothing unusual.
Since our digestive systems destroy cells in the food we eat, cellular damage in cooking is inconsequential. As
for the rumors about the unhealthiness of food cooked in a microwave oven, these are simply myths promulgated
by people who don't understand what microwaves are and fear them irrationally. The world was awash in
microwaves from natural sources long before the developments of electricity and microwave ovens.
Is there a standard time that one should wait before eating food that has been heated in a microwave oven? - M
Apart from the usual precautions with hot food, there is nothing unsafe about food cooked in a microwave oven.
You can eat it the instant the microwave oven turns off. The microwaves in the oven are absorbed so quickly that
they vanish almost immediately after the oven stops producing them. By the time you get the oven door open,
there is nothing hazardous left inside the cooking chamber or in the food. However, a microwave oven tends to
heat foods unevenly, particularly if they were initially frozen. Thus you should be careful to stir the food or test
its temperature at various places so that you don't burn yourself. You should be particularly wary of solid foods,
such as raisin biscuits, that are generally dry but have moist, microwave-absorbing objects inside them. Those
moist objects can become dangerously hot and have been known to cause life-threatening burns in people who
tried to swallow them without letting them cool off.
That said, a reader notes that the uneven cooking in a microwave oven can lead to bacterial safety problems--if
parts of the food aren't heated sufficiently to kill dangerous bacteria, then you could be exposing yourself to
those bacteria. He suggests using the microwave oven for reheating only. He also notes that the lack of surface
heating leaves the food relatively tasteless, as compared to more conventional cooking.
Does microwave cooking affect the nutritional value of food?
No more so than conventional heating does. Overheating some nutrients can damage them, so that microwave
cooking does affect food's nutritional value. But microwave cooking is far less likely to cause serious molecular
damage to food than flame broiling or frying.
May 19, 1997
You stated that thermodynamics overwhelms just about everything sooner or later. Could you explain why? -- MT, San
Antonio, Texas
Thermodynamics is a statistical science that deals with systems that are so complicated or vast that they can't be
followed in complete detail. It makes predictions of behavior based on probability theory and while some of its
laws predict probable outcomes rather than certain outcomes, a sufficiently probably event is effectively a certain
event. For example, I can say with near certainty that if you play the lottery 50 times, you won't win the jackpot
50 times. I can't be truly certain of that fact, but the likelihood of my prediction being correct is pretty good.
In a sense, probability is destiny. Thermodynamics observes that vast systems tend to evolve toward the mostly
likely configurations. To understand this process, consider what happens when you mix hot and cold water. The
most likely final configuration for the mixed water is for it to reach a uniform temperature about half way in
between the two original temperatures. While it's possible for the water to end up extremely hot in one place and
extremely cold in another, that outcome is extremely unlikely. It's so unlikely that it never happens.
So in what sense does thermodynamics overwhelm things? The world is filled with relatively ordered
arrangements and these ordered arrangements are unlikely by themselves (how they came to be ordered in the
first place is another matter for another questions). If you take a crystal vase and drop it on the floor, it's going to
evolve toward a more likely arrangement of atoms and dropping it a second time isn't going to return it toward its
original unlikely state. In short, ordered systems naturally drift toward disorder when given a chance. How
quickly they drift depends on their situation. A coffee cup will remain a nicely ordered object for thousands or
millions of years if you don't disturb it. But in a hot environment, or one that is chemically aggressive, it may not
last very long.
One last thought: how do living organisms maintain their order in the face of this tendency to disorder? They do
it by consuming order and exporting disorder--they eat ordered foods and release disordered wastes to their
surroundings.
I understand that an ear thermometer measures a person's temperature by studying the thermal radiation emitted by their
ear. What is the farthest range that a person can emit thermal radiation that can still be received? Does this range depend
on how hot the inner person is? -- M
The thermal radiation that a person emits is mostly infrared light and, like all light, it can travel forever if nothing
gets in its way. In principle, if you can observe something through a telescope, you can also measure its
temperature. For example, astronomers can measure the temperature of a distant star by studying the star's
spectrum of thermal radiation.
However, there are several complications when using this technique to measure a person's temperature. First,
anything that lies between the person and you, and that absorbs or emit thermal radiation, will affect your
measurement. That's because some of the thermal radiation that appears to be coming from the person may be
coming from those in between things. Fortunately, air is moderately transparent to thermal radiation but many
other things aren't. In fact, to get an accurate reading of person's temperature, you'd have to cool the telescope
and the light detector so that they don't add their own thermal radiation to what you observe. You'd also have to
use a mirror telescope because glass optics absorb infrared light.
Second, the temperature that you observe will be that of the person's skin and not their inner core temperature.
That's because the person's skin absorbs any infrared light from inside the person and it emits its own infrared
light to the world around the person. You can't observe infrared light from inside the person because the person's
skin blocks your view. All you see is their skin temperature.
May 16, 1997
How does an ear thermometer work so quickly? -- SN, West Covina, California
An ear thermometer examines the spectrum of thermal radiation emitted by the inner surfaces of a person's ear.
All objects emit thermal electromagnetic radiation and that radiation is characteristic of their temperatures--the
hotter an object is, the brighter its thermal radiation and the more that radiation shifts toward shorter
wavelengths. The thermal radiation from a person's ear is in the invisible infrared portion of the light spectrum,
which is why you can't see people glowing. But the ear thermometer can see this infrared light and it uses the
light to determine the ear's temperature. The thermometer's thermal radiation sensor is very fast, which accounts
for the speed of the measurement.
What are the relative efficiencies of the fission and fusion reactions in thermonuclear weapons? Is every last grain of
fissile and fusible matter converted to energy or is there a loss somewhere?
While both fission and fusion convert substantial fractions of the mass in a thermonuclear weapon into energy,
most of the bomb's initial matter remains matter, not energy. When a uranium nucleus fissions to become smaller
nuclei, about 0.1% of the uranium nucleus's mass becomes energy. When two deuterium nuclei--the heavy
isotope of hydrogen--fuse together to become helium, about 0.3% of the deuterium nuclei's masses become
energy. Despite these seemingly small percentages, this scale of matter to energy conversion dwarfs that of
chemical explosives, which convert only parts per billion of their masses into energy.
While fusion is somewhat more energy efficient than fission, that's not the whole reason why hydrogen bombs
(thermonuclear bombs) are more powerful than uranium bombs (fission bombs). The main reason is that
thermonuclear bombs can be much larger than fission bombs because there is no upper limit to the amount of
hydrogen you can assemble in a small region of space. In contrast, if you assemble too much fissile uranium in a
small region of space, a chain reaction will begin and the material will overheat and explode. At the height of the
cold war, the Soviet Union built gigantic thermonuclear weapons with explosive yields as large as 100 megatons
of TNT.
Is there any easy way to mold plastics?
The easiest way to mold plastics is to form them directly inside a mold. Most plastics are made by attaching
small molecules to one another in a process called polymerization. You begin with one or more small molecules
or "monomers" and cause them to link together into in a "polymer." You can initiate this polymerization with
chemical catalysts, light, or even heat. There are many plastic-forming systems that you can buy commercially.
You simply mix a few chemicals together, pour the mixture into a mold and wait. Once the polymerization has
finished, you have a molded piece of plastic.
If you don't want to do the polymerization yourself, you can start with a finished plastic and melt it. Most plastics
that haven't been vulcanized into one giant molecule (as is done in rubber tires) will melt at high enough
temperatures (although some burn or decompose before they melt). These molten plastics can be stretched,
squeezed, or poured into molds to make just about any shape you like.
What function does the Degauss button actually perform on computer monitors and why is it not available for
televisions? -- JF, San Francisco, California
Both color monitors and color televisions create their color images by combining the three primary colors of
light--red, green, and blue. Each display has an intricate pattern of red, green, and blue phosphor dots or stripes
on the inside surface of its picture tube and it produces full color images by adjusting the brightness balance of
these tiny glowing spots. Beams of electrons are directed at these phosphors from the back of the picture tube
and their impacts with the phosphors cause the phosphors to fluoresce--emit light.
Because the picture tube can't direct its electron beams accurately enough to hit specific red, green, or blue
phosphor regions, it needs help from a shadow mask that's located a short distance before the phosphor layer.
This thin metal grillwork shades the light-producing phosphors from the wrong electrons. The picture tube has
three separate beams of electrons, one for each primary color, and the grillwork ensures that electrons in the red
beam are only able to strike phosphors that produce red light. The same goes for the blue beam and the green
beam.
The grillwork must stay in perfect registry with the pattern of phosphors on the inside of the picture tube, even as
their temperatures change. That's why this grillwork is made of Invar, a special steel alloy that doesn't change
size when its temperature changes. Unfortunately, Invar can be magnetized and its magnetic fields can then steer
the electrons so that they strike the wrong phosphors. If you were to hold a strong magnet near the face of a
computer monitor, you would probably magnetize the Invar shadow mask and spoil the color balance of the
images on the monitor.
To demagnetize the Invar, you must expose it to a magnetic field that fluctuates back and forth and gradually
diminishes to zero. The Invar's magnetization would also fluctuate back and forth and would dwindle to nothing
by the time the demagnetizing field had vanished. Traditionally, this demagnetizing was done with a large wire
coil that was powered by alternating current so that its magnetic field fluctuated back and forth. This coil was
gradually moved away from the picture tube so that the influence of its magnetic field slowly diminished to zero,
leaving the Invar completely demagnetized. In good computer monitors, this coil and an automatic power source
for it are built in. When you push the degauss button, you see a burst of colors as the demagnetizing coil's
fluctuating magnetic field erases the magnetization of the shadow mask and also steers the electrons wildly.
Apparently, degaussing circuitry has been built into all color televisions sets for the past 20 or 30 years. When
you turn on your television, a demagnetizing coil activates briefly and removes minor magnetization from the
television's invar mask.
What do engineers have to consider about waves when they are building bridges? -- K
There are two answers to this question because there are two possible interpretations of the word "waves." If you
mean waves in the water beneath the bridge, then naturally the engineers must plan for the forces exerted on the
bridge by the moving water that flows around its surfaces. But a more interesting wave issue is waves in the
bridge itself. The bridge's surface can experience waves, just as a taut rope or a long beam can have waves
running through it. For example, when a heavy object drops on the surface of the bridge, a ripple heads outward
along the bridge surface and doesn't stop completely until it reaches the ends of the bridge. In fact, the wave will
reflect from various portions of the bridge and its effects may not disappear for many seconds after the incident
that started the waves.
Most of the time, these waves aren't important and can be ignored. But occasionally some special event will
cause enormous waves to begin traveling through a bridge. The classic example was the Tacoma Narrows Bridge
in Washington State that collapsed in 1940 when wind-driven waves in its surface ripped it apart. The entire
collapse was captured on film and is a fascinating to watch. When a large group of soldiers crosses a footbridge,
they are often instructed to break step so that their rhythmic cadence doesn't excite intense waves that might
damage the bridge. In general, modern bridges are engineered to dampen these waves--wasting their energy
through friction or friction-like effects so that they die away quickly. While it might be fun to watch waves
traveling along the surface of a bridge from a safe vantage point, you probably wouldn't want to be on a bridge
when it was experiencing strong ones.
May 15, 1997
If you were out in space and could see every individual person clearly, would it look like they were walking at a slant? -KD, McMinnville, OR
To the astronauts orbiting the earth, up and down have very little meaning. Because they are falling all the time,
these astronauts have no feeling of weight and can't tell up from down without looking. If an astronaut were to
look at a person walking on the ground below, that person might easily appear at a strange angle, depending on
the astronaut's orientation and point of view.
Why do you hear different music coming from a compact disc when the laser of the CD player is just going around the
same part of the CD over and over again? -- KD, McMinnville, OR
The CD player's laser doesn't really go over the same part of the CD over and over again. As the disc turns, the
laser slowly moves outward from the middle of the disc toward its edge. The laser beam is focused to an
extremely small spot inside the disc and it is carefully following a tight spiral ridge in the aluminum layer inside.
This ridge runs continuously from the center of the disc to its edge. With each revolution of the disc, the laser
works its way outward by one more turn of the spiral. The ridge has interruptions in it every so often and it is this
pattern of interruptions that contains the information needed to reproduce sound.
How does a parabolic microphone work? -- KL, Regina, Saskatchewan
A parabolic microphone is effectively a mirror telescope for sound. When sound waves strike the dense, rigid
surface of the parabolic dish, they partially reflect. This reflection occurs because sound travels much faster in a
rigid solid than in the air and changes in the speed of a wave cause part of it to reflect. In this case, the reflection
redirects the sound waves inward because the reflecting surface is curved and the sound waves form a real image
of the distant source that produced them. While you can't see this real image with your eyes, you can hear it with
your ears. If you were to mount a large parabolic dish so that it faced horizontally and then moved your ear
around in the focal plane of the dish, you would hear sounds coming from various objects far away from the
dish. The same effect occurs for light when it bounces off a curved mirror--a real mirror telescope. A TV satellite
dish is the same thing, but this time for microwaves! In all three cases, the real images that form are upside
down. To make a parabolic microphone, you normally put a conventional microphone in the central focus of a
parabolic surface so that the microphone receives all the sound coming from objects directly in front of the
parabola. To listen to different objects, you simply steer the parabola from one to the other. This is exactly what
a TV satellite dish does when it wants to "listen" to a different satellite--it steers from one to the other.
I heard of a laser induced fluorescence instrument that is used in aiding cancer diagnosis. Could you tell me how this
instrument works?
You are probably referring to a device developed at the BC Cancer Research Center in Vancouver, British
Columbia and now available commercially from Xillix Technologies. A scientist from that research center gave
me the following description of their technique.
The instrument is based on the discovery that most tissues when illuminated by blue or UV light emit a natural
fluorescence spectral signature known as autofluorescence. This fluorescence signature is the sum of the
emission of the various biochemical fluorphores present in the tissue. If the tissue chemical or physical structure
changes, then the spectral signature changes. By exploiting differences in the spectral signature between
cancerous and healthy tissue one can create an imaging device that can "see" the difference in the color of the
autofluorescence of the tissue and detect changes that may indicate the presence of cancer. The sensors used to
see the low levels of fluorescence light employ similar technology to military night vision devices. Once areas of
change are located and confirmed by analysis of a biopsy sample treatment can begin. This technique is
primarily useful for early stage cancers that are not visually apparent to a physician.
How can I build an AM radio?
That's a very open ended question so I'll describe the simplest AM radio I can think of--a crystal radio. A crystal
radio already addresses most of the issues of AM radio and more sophisticated AM radios just improve on its
performance.
You need only four basic components for a crystal radio: an antenna, a tank circuit, a diode, and a highimpedance earphone.
The antenna is a long wire that projects upward into the electromagnetic fields of the passing radio wave so that
electric charges begin to move up and down its length. The ideal length for this wire is a quarter of the
wavelength of the wave you're trying to receive, but since that's hundreds of meters for a typical AM station,
you'll have to settle for a shorter than ideal antenna.
The tank circuit is a coil of wire that's connected at each end to the two ends of a capacitor. In a typical crystal
radio, one of these items--either the coil or the capacitor--is adjustable and forms the tuning element that allows
you to select a particular AM station. The tank circuit is a resonant device--electric charges and current flow
back and forth through it rhythmically at a specific frequency. If that resonant frequency is adjusted so that it
coincides with the transmission frequency of an AM radio station, the small currents flowing in the antenna that's
connected to the tank circuit will excite large movements of charge and current in the tank circuit.
The diode is also connected to the tank circuit. Its job is to extract some of the charge that oscillates back and
forth in the tank circuit and to send that charge to the earphone. By allowing current to flow only in one
direction, the diode samples the overall amount of charge moving in the tank circuit. What it passes to the
earphone is a measure of how strong the radio wave is, which is actually the form in which the AM radio station
is transmitting sound information.
The high-impedance earphone uses the diode's tiny charge deliveries to reproduce sound. The diaphragm inside
the earphone moves back and forth as the amount of charge passing through the diode fluctuates up and down.
Each time the radio wave increases in strength, the diaphragm moves in one direction. Each time the radio wave
decreases in strength, the diaphragm moves in the other direction. Thus as the radio station varies the strength of
its radio wave, the earphone's diaphragm moves back and forth and it reproduces the sound.
I heard on a news report that there is a paint that will generate heat from a 12-volt battery. What can you tell me about
this subject? -- JF
Generating heat from a battery is relatively easy. All you need is a material that conducts electricity only
moderately well and you're in business. If you allow current to flow through that material from the battery's
positive terminal to its negative terminal, the current will lose energy as it struggles to get through the material
and the current's lost energy will become thermal energy in the material. The only difficult part of this task is in
choosing the right material so that it doesn't produce too much or too little heat. In short, the electric resistance of
the finished material has to be in the right range. For a solid system that you can cut and tailor, that's not much of
a problem. But for a paint, it could be tricky. To make an inexpensive paint, it would probably need to use
carbon powder as the electric conductor. A thin layer of carbon granules held in place by a plastic of some sort
would probably provide a suitable conducting surface that would become warm when you allowed current to
flow through it from a battery. There are copper and silver conducting paints that might also work, but these are
rather expensive and I'm not sure how they behave at elevated temperatures.
May 14, 1997
For home canning it is necessary to thoroughly sterilize the containers. In the past, I have had to boil the jars in a large
container. This is dangerous. If I were to moisten the jars and place them in the microwave, would there be enough heat
to sterilize them? -- CM
While you could sterilize jars in a microwave oven, doing so would be extremely dangerous. Your chances of
successfully sterilizing the jars without blowing one of them up is very small. Here is an explanation.
When you place a canning jar in boiling water, what you are really doing is exposing that jar to a water bath at a
temperature of 212° F (100° C). Boiling water self-regulates its temperature very accurately, making it a
wonderful reference for cooking. Below water's boiling temperature, water molecules evaporate relatively slowly
from the surface of water so that when you add heat to the water, it tends to get hotter and hotter. But once the
water begins to boil--meaning that evaporation begins to occur within the body of the water--water molecules
evaporate so rapidly that when you add heat to the water, more of it converts into steam and its temperature
doesn't change much. When you boil canning jars for 5 minutes, you are simply making sure that the canning jars
sit at about 212° F for about 5 minutes; long enough to kill bacteria in the jars. Since the boiling temperature of
water diminishes at high altitudes and lower atmospheric pressures, you must wait longer for your jars to be
adequately sterilized if you live in the mountains.
Microwave cooking wouldn't heat the jars to any specific temperature. As you cooked the jars in a microwave
oven, their contents would become hotter and hotter. Even if we ignore the fact that microwave cooking is
uneven, so that the temperature inside each jar won't be uniform, there will be nothing special about the
temperature 212° F. If you cook the food long enough, its temperature will reach 212° F, but will then keep
rising. As it does, the water vapor in the jars will become more and more dense and its pressure will rise higher
and higher. If the canning jar had been properly capped, the metal lid ought to be loose enough to allow this
steam to escape. However, the canning system wasn't designed to handle large amounts of escaping steam and an
over-tightened jar might not permit the steam to escape at all. With the steam trapped inside, the pressure inside
the jar may become large enough to cause it to explode. Since too little time in the microwave oven will leave
the jars unsterilized and too much time in the microwave oven may cause them to explode, I suggest sticking to
the tried and true method of sterilizing your jars in boiling water.
I was told by an electrician to use 130-volt bulbs, which he said were outlawed by the electric bulb makers because they
last so long. He said that electricians can buy them and not the public. I found them and have used them for 5 years and
he is right! They last forever. Why is that? How do they compare to more energy efficient lights? -- J
When you use a bulb designed for 130 volts in a fixture that operates at 120 volts, the bulb's filament runs at less
than its rated temperature. This temperature change has two consequences--one good and one bad. The good
news is that operating the filament at less than its normal temperature slows the evaporation of tungsten atoms
and prolongs the filament's life. That's why your bulbs are lasting so long. The bad news is that incandescent
bulbs become much less energy efficient as you lower their filament temperatures. The light emitted by the
filament is thermal radiation and its color spectrum and brightness depend almost exclusively on its temperature.
These 130-volt bulbs emit redder and dimmer light than a normal bulb and they are significantly less energy
efficient as a result. Incandescent bulbs already emit far more invisible infrared light than visible light and
operating them at reduced temperatures only makes this problem worse. I recently read the statement "this bulb
burns cooler than a normal bulb" on a package of super-long-life bulbs--as though burning cooler was a good
thing rather than a serious shortcoming.
As energy becomes more and more precious, making the most of it becomes more and more important. I would
suggest saving these 130-volt bulbs for fixtures that are so difficult to reach that you want to avoid changing
bulbs at all costs. In more easily accessible fixtures, replacing bulbs is only a minor inconvenience associated
with improved energy efficiency. Better still, switch to fluorescent lamps--which are much more energy efficient
than even the best incandescent lamps.
What is the chemistry involved with natural dyes adhering to surfaces? -- AG, Aloha, OR
Unless a chemical reaction binds them permanently in place, dye molecules that are soluble enough to wash into
fabrics are equally likely to wash back out of the fabrics later on. To remain in place, the dyes must undergo
chemical reactions that attach them to the fibers of the fabric. Some dyes react spontaneously to the fabric
molecules but many others need help. The traditional scheme for binding dyes to fabrics involves mordents-relatively colorless chemicals that bind to both fabric and dye, and that hold the two together. Tannic acid and
various metal salts have been used as mordents for centuries. They form insoluble compounds that wedge
themselves into hollow spaces in the fibers and then bind chemically to the dye molecules. These mordents hold
the dye molecules in place in much the same way that technical climbing gear holds rock climbers to the face of
a cliff.
How does cathodic protection work? -- MM, Dominican Republic
The rusting of damp steel is an electrochemical reaction in which iron atoms in the steel are converted into
positively charged iron ions (Fe2+) in the water. However, each iron atom that becomes an ion releases two
negatively charged electrons and rusting can only continue if there is a suitable destination for these electrons.
Normally, the electrons pass through the steel metal and are used together with oxygen molecules to form
negatively charged hydroxide ions (OH-) in the water. Overall, the rate at which the steel rusts is limited by how
quickly hydroxide ions can be formed to use up the electrons.
Cathodic protection is a scheme in which a piece of reactive metal, typically magnesium, is connected to the
steel to form an electrochemical cell. Magnesium ions (Mg2+) form more easily than iron ions and enough
electrons are given up by the magnesium atoms as they become positive ions to completely dominate the
hydroxide ion formation process. With nowhere for their electrons to go, the iron atoms can't become iron ions
and rusting can't proceed. As long as the magnesium metal, often called the "sacrificial anode", remains intact
and connected to the steel, the steel won't rust significantly.
As an alternative to this approach, some companies use a power supply to pump negative charges onto the steel
to prevent it from rusting. Pipeline companies often do this and that action has led to some interesting
complications: metal objects that are brought into contact with such a pipeline can be protected against rusting as
well. For example, when people chained their bicycles to protected pipelines, the bicycles became part of the
protected materials. This may have been good for the bicycles, but it confused the pipeline companies who found
that they needed to pump extra charge onto the pipelines to handle the increased load. It was particularly bad
when the bicycles accidentally grounded the pipelines and allowed the negative charges to escape.
Can light be bent by electric fields, magnetic fields, and gravity fields? If so, can these fields be made to make light travel
in a circle? -- RS
Light consists of electromagnetic waves, meaning that it is composed of electric and magnetic fields. While light
isn't affected by other electric or magnetic fields, it is affected by gravitational fields. Like everything else in our
universe, light falls when exposed to gravity. However, because light travels so fast, it's very hard to detect that it
falls. The first observation of light falling in a gravitational field was made during a total eclipse in 1919 and
served as dramatic confirmation of the predictions of Einstein's general theory of relativity. As for light traveling
in a circle, this can occur near the surface of a black hole. When light traveling tangent to the surface of the black
hole falls at just the right rate, it will orbit the black hole indefinitely.
I recently place a green tomato in the microwave oven. I forgot to turn on the microwave and in the morning the tomato
was ripe. Can you explain this? -- KH
No. When a microwave oven is off, the cooking chamber contains nothing special at all--just some trapped air
and perhaps a little light that enters through the window. Even when it is operating, a microwave oven never
produces any ionizing (high energy) radiation so there are no long-term effects such as radioactivity present in
the cooking chamber when the oven is off. The tomato was simply sitting in a sealed metal box overnight. Since
some fruits ripen faster in sealed environments, perhaps that accounts for your observation.
Does a device that has radio waves and uses ozone and negative ions have the ability to clean the air in my home? -KTR, Halifax, Nova Scotia
There are many simple electronic devices that claim to clean the air in your home by making negative ions and
ozone (if they involve any radio waves, it's a minor side effect of their internal electronics). The claim is
accurate--they do make both ozone and negative ions, and they do clean the air in your home. However, that's
not the whole story. First, ozone may have the "fresh" smell that occurs after a thunderstorm (a potent producer
of ozone), but ozone is a powerful oxidizing agent and chemical irritant that's considered an environmental
pollutant rather than a charming scent. The manufacturers are taking a nuisance effect and touting it as a
"valuable feature." Second, the negative charges emitted by these electronic devices attach themselves to dust,
ash, pollen, and smoke particles and cause those particles to bind themselves to your walls and furniture. The air
really does become cleaner, but every surface in your home becomes dirtier as a result.
If you're seriously interested in cleaning the air in your home, you are probably better off with a full electrostatic
air cleaner. Small home versions of this common industrial workhorse are easy to obtain at a local heating and
air conditioning store. Properly designed machines use positive ions to avoid producing ozone and provide a
negatively charged surface for the positively charged dirt to stick to so that it doesn't deposit itself on your walls.
April 27, 1997
Why do we have time? -- KD, McMinnville, Oregon
Time is the fourth dimension, similar to but not equivalent to the three spatial dimensions. With four dimensions
in our universe, we need four values to specify the exact location of each event--three values that specify that
event's location in space and one value that specifies its location in time. Space and time are intimately related so
that we perceive time in terms of space and space in terms of time. For example, you sense the distance of a
remote city by how long it would take you to get there. Similarly, you sense the large separation between two
moments in time by how far you could travel between those two moments. But as to "why we have time," I can
only answer that it's part of the nature of our universe.
Is it possible that time is not just an abstraction but also a sort of resonant force that can be contained and manipulated
within a controlled environment? -- SK, Cape Town, South Africa
Time is a dimension, much like the three spatial dimensions. Objects and events are located in time, just as they
are located in space. Because time is part of the framework in which objects and events exist, and not an object
or an event, time can't be manipulated easily. So the short answer to your question is no, time can't be contained
or manipulated. However, time and space are related and how we perceive the two depends on our velocity--the
special theory of relativity. Moreover, time and space can be warped by the presence of mass/energy--the general
theory of relativity. Still, the dream of playing with space-time like it was taffy that could be stretch, bent, and
folded at will is just that, a dream. It takes an enormous concentration of mass/energy to cause even the most
barely perceptible deformations of space-time and even the effects of celestial objects on space-time are limited.
Finally, about the expression "resonant force": a resonance is a motion or action that spontaneously follows a
repetitive cycle while a force is a push or a pull, an influence that causes something to accelerate. Thus, the
expression "resonant force" is interesting sounding jargon but it doesn't have any meaning.
How do neon lights work? -- MT, Cement City, MI
A neon light uses a very high voltage to propel an electric current through a low-density gas of neon atoms.
These neon atoms are trapped inside a glass tube and the current passes between two metal electrodes at opposite
ends of that tube. A high voltage power supply--typically a neon sign transformer--pumps a large number of
negative charges onto one electrode and a large number of positive charges onto the other electrode. Because like
charges repel while opposite charges attract, there are strong forces pushing the charges from one electrode
toward those on the other electrode. Eventually, charges at the two ends of the tube begin to leap off the
electrodes and into the neon gas so that they can flow toward one another. Current begins to flow through the
tube. As the charges move through the gas, they frequently collide with neon atoms and occasionally transfer
some of their energies to those neon atoms. During such an energy transfer, an electron in the neon atom shifts
from its normal orbital to a higher energy orbital in which the electron doesn't normally travel. The electron soon
returns to its normal orbital and releases a particle of light--a photon--in the process. Since the most common
orbital shift in an excited neon atom releases a particle of red light, a neon light emits a bright, reddish glow.
Do the resonant frequencies of the elements change as the magnetic fields they reside in change? Can an element such as
iron be made to resonate at the magnetic field strength of the earth? -- JP, Blakeslee, PA
The terms "resonant" and "resonate" are general expressions that refer to repetitive motions or actions that occur
spontaneously within a system. Elements exhibit many different resonant behaviors in different situations, so I
must pick an appropriate resonant behavior in order to answer your question.
The best choice I can think of is nuclear magnetic resonance (NMR)--an effect that involves the flipping of an
atomic nucleus's magnetic poles. Most atomic nuclei--the massive positively charged nuggets at the centers of
atoms--are magnetic. When you put an atom with a magnetic nucleus in a magnetic field, the atom acquires a
certain amount of potential energy that depends on whether that magnetic nucleus is aligned with the magnetic
field or not. The extent to which the atom's nucleus is aligned with the field can be changed by exposing it to an
electromagnetic wave of the right frequency. This electromagnetic wave provides or absorbs the required energy
to allow the nucleus's magnetization to flip. The nucleus exhibits a resonance in response to the correct
electromagnetic wave--a phenomenon called "nuclear magnetic resonance." This frequency at which this
resonance occurs depends on the nucleus, on the magnetic field, and on the magnetic environment of the nucleus.
The resonance occurs for any magnetic nucleus, in any field, but how interesting or useful the resonance is
depends on the situation. So the answers to both questions are yes, but that doesn't mean the effects are
important.
When two identical items are cooked, one with a microwave oven and the other on the stove, which will cool faster? -CR
If the distributions of temperatures inside the items were the same after cooking, they would cool at the same
rate. However, a microwave oven tends to cook relatively evenly throughout the food while the stove tends to
cook from the outside of the food inward. That means that food cooked in a microwave oven tends to have more
thermal energy near its center than food cooked on a stove, even when those foods contain the same total amount
of thermal energy. Since foods lose heat through their surfaces, the extra thermal energy in the food cook by
microwave will take longer to flow out to the surface of the food and from there to its surroundings. All else
being equal, I would expect the food cooked in the microwave oven to cool slightly slower than the food cooked
on the stovetop.
How does an integrated circuit perform computations? I know that it has transistors embedded into it, but how can a
circuit of semiconductors be used for multiplication? -- DF, Marina Del Rey, California
The transistors used in digital integrated circuits, including microprocessors, act primarily as electronically
controlled switches. These transistor switches permit the electric charge on or electric current in one wire to
control the electric charge on or current in another wire. In digital electronics, a wire's charge or current state is
used to represent a single binary digit--either a 1 or a 0. By combining transistors in modestly complicated
arrangements, the states of several wires together can control the states of several other wires. This increased
complexity allows for simple functions such as binary addition to be performed--for example, the charges on two
wires can be used to control the charges on two other wires so that the charges on the second pair of wires
represent the single binary sum of the two individual numbers represented by charges on the first pair of wires.
More complicated adders can be assembled from more transistors and finally multipliers can be assembled from
a collection of adders. Overall, it only takes a few arrangements of electrically controlled switches to form the
primitive elements from which incredibly complicated digital processors can be built.
How does a fan motor work? -- JM, Toronto, Ontario
A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary
electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows
through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of
these electromagnets carries an alternating current that it receives from the power line and its magnetic poles
fluctuate back and forth as the direction of current through it fluctuates back and forth. These electromagnets are
arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These
moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the
rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning
rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to
different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different
rates.
What are the frequency characteristics of transformers? Are they related to the circuit components and the ratio of
primary to secondary turns around the iron core? -- JM, Lakewood, Colorado
The frequency characteristics of a transformer are determined principally by the materials in the transformer's
core. Power flows from the primary circuit to the secondary circuit by way of the magnetization of the
transformer's core. With each half-cycle of the alternating current in the primary circuit, the transformer's core
must magnetize and demagnetize. A transformer core's ability to magnetize and demagnetize properly depends
on the frequency of the alternating current in the transformer's coils. If that frequency is too low, the core may
saturate--reach its maximum possible magnetization--during the half-cycle. In that case, the core will not be able
to transfer the requisite amount of energy to the secondary coil and the power transferred between the two coils
will be inadequate. That's why low frequency transformers often contain huge iron cores--cores that avoid
saturation by spreading out the magnetization and stored energy over large volumes of iron.
On the other hand, if the frequency of current in the primary is too high, the core may be unable to magnetize
and demagnetize fast enough to keep up with it and the power transfer will again be inadequate. The core may
also become hot due to friction-like losses in the core material. That's why high frequency transformers use
special core materials such as ferrite powders or even air. Although air (or really empty space) can't store large
amounts of energy in small volumes when it magnetizes, it can respond extremely quickly. Air-core transformers
operate well at extremely high frequencies.
What causes the phases of the moon? -- CH, Denver, Colorado
Except during an eclipse, one half of the moon's surface is bathed in sunlight while the other half is in shadow.
The phases of moon occur because we can only see half the moon at any moment and the fractions of lighted and
shadowed moon that we see vary with about a four-week cycle--the lunar month. For example, when the moon is
almost on the opposite side of the earth from the sun, we see only the lighted side of the moon and the moon
appears full. When the moon is on the same side of the earth as the sun, we see only the shadowed side of the
moon and it appears almost non-existent--a new moon. Each lunar month, our vantage point gradually evolves so
that we see the new moon become a growing crescent moon, a half moon, a gibbous moon, and a full moon, a
gibbous moon, a half moon, a shrinking crescent moon, and finally a new moon again. You can see this effect by
illuminating a soccer ball with a bright flashlight and then walking around the soccer ball. You'll see the phases
of the soccer ball.
April 15, 1997
How can you demonstrate that sounds are waves produced by the vibration of material objects? -- TP, Huntington Park,
California
I can't think of an easy way to make sound waves visible while they travel through air, but it's relatively easy to
make sound waves visible as they travel through materials. If you choose a system in which the sound waves
bounce back and forth many times through a material, you can sometimes see the sound waves as they move. For
example, partially fill a crystal wine glass with water and then rub your wet finger gently around the rim of the
glass. With some practice, you'll be able to get the wine glass to emit a pure tone as your finger alternately sticks
and slips its way around the glass rim. As this tone appears--the vibration of the crystal glass itself--the water
will begin to exhibit beautiful ripple patterns. You should be able to see these ripples by looking at a bright light
reflected from the water's surface. The ripples are sound waves that are travel through the water, back and forth,
as the glass vibrates.
Another system that makes the movement of waves visible is a stiff, thin aluminum plate that's supported rigidly
and horizontally at only one point. If you sprinkle fine sand lightly over the surface of this plate and then bow its
edge with a violin bow, it will begin vibrating with a clear tone. As it vibrates, the sand will drift into places
where there is very little surface motion--the nodes of the vibrating surface. Once again, sound waves are
traveling back and forth across this surface and the up-down motions squeeze the sand into certain parts of the
plate. In this case, the surface's vibrations and the sound waves in that surface are the same thing--in example of
the fact that vibrations and sound waves are intimately related and are in many respects exactly the same thing.
How does the telephone work? -- JB, Sydney, Nova Scotia
A telephone uses an electric current to convey sound information from your home to that of a friend. When the
two of you are talking on the telephone, the telephone company is sending a steady electric current through your
telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into
your telephone's microphone, the current that your telephone draws from the telephone company fluctuates up
and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice
at the microphone.
Because the telephones are sharing the total current, any change in the current through your telephone causes a
change in the current through your friend's telephone. Thus as you talk, the current through your friend's
telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and
rarefying the air. The resulting air pressure fluctuations reproduce the sound of your voice. Although the nature
of telephones and the circuits connecting them have changed radically in the past few decades, the telephone
system still functions in a manner that at least simulates this behavior.
How does a relay work? -- CS, Fairfax, Virginia
A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since
electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes
through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of
wire is a moveable piece of iron. While iron normally appears nonmagnetic when it's by itself, it becomes highly
magnetic whenever it's exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows
through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves
various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil
remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When
the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the
two apart and the electric contacts return to their original positions.
Why are there two tides per day? -- JF
The tide is caused primarily by the moon's gravity. Gravity is what keeps the moon and earth together as a pair-the moon and earth orbit one another because each is exerting an attractive force on the other. While they are
effectively falling toward one another as the result of this gravitational attraction, their sideways motion keeps
them from smashing together and they instead travel in elliptical paths around a common center of mass. But the
moon's gravity is slightly stronger on the near side of the earth than it is on the far side of the earth. As a result,
the water on the near side of the earth bulges outward toward the moon. The water on the far side of the earth
also bulges outward because the earth itself is falling toward the moon slightly faster than that more distant water
is. The distant water is being left behind as a bulge.
There are thus two separate tidal bulges in the earth's oceans: one on the side nearest the moon and one on the
side farthest from the moon. But the earth rotates once a day, so these bulges move across the earth's surface.
Since there are two bulges, a typical seashore passes through two bulges a day. At those times, the tide is high.
During the times when the seashore is between bulges, the tide is low. Because the moon moves as the earth
turns, high tides occur about 12 hours and 26 minutes apart, rather than every 12 hours. Since local water must
flow to form the bulges as the earth rotates, there are cases where the tides are delayed as the water struggles to
move through a channel. However, even in those cases, the high tides occur every 12 hours and 26 minutes. The
sun's gravity also contributes to the tides, but its effects are smaller and serve mostly to vary the heights of high
and low tide.
April 8, 1997
I'm helping on a lesson plan for grades 3-12 where students make ice cream. Adding salt to the ice makes the ice colder.
I'm having trouble explaining why we put salt on the roads to melt ice, but in making ice cream the salt actually lowers
the temperature of the ice. -- N
These two observations--that salt melts ice and that salt makes ice colder--are actually consistent with one
another. When you add salt to ice, you make a relatively ordered mixture--pure crystalline ice and pure
crystalline salt. This orderly arrangement is looked on unfavorably by nature; given a chance, nature tends to
maximize randomness. There is a much more disorderly arrangement available--salt water--and nature tends
toward disorderly arrangements. When you put the salt and ice together, nature's tendency toward randomness
begins to drive the system to rearrange. The ice begins to melt so that the salt can dissolve in it. Although the
melting of ice requires energy, the randomness this melting and dissolving produces makes this process take
place. The energy needed to melt the ice is extracted from the remaining ice and that ice gets colder. When
you're making ice cream, some of the energy needed to melt the ice also comes from the ice cream mix, so that it
gets colder, too. If there is enough salt around, the ice will melt completely to form very cold salt water--the
desired result with salt on a slippery sidewalk. The salt water remains liquid well below the normal freezing
temperature of water because forming ice crystals would require the salt and water to separate from one another-an orderly and therefore unlikely event. In short, nature's trend toward disorder causes salt to melt ice, even
though that melting lowers the temperatures of everything involved well below the freezing temperature of pure
water.
What is the relationship between gravitational force and electromagnetic force? -- TPC, Foster, OK
As yet, there is no direct relationship between those two forces. Our best current understanding of gravitational
forces is as disturbances in the structure of space itself while our best current understanding of electromagnetic
forces involves the exchanges of particles known as virtual photons. However, physicists are trying to develop a
quantum theory of gravity that would identify gravitational forces with the exchange of particles known as
gravitons. How closely such a quantum theory of gravity would resemble the current quantum theory of
electromagnetic forces (a theory called quantum electrodynamics) is uncertain. It's also uncertain whether those
two quantum theories will be able to merge together into a single more complete theory. Only time will tell.
How would I go about making a camera that's more than just a pinhole camera? -- JL, Longview, WA
While a pinhole will project the image of a scene on a piece of film, it doesn't collect very much light. That's why
a pinhole camera requires very long exposures. A better camera makes use of a converging lens. If you hold a
magnifying glass several inches away from a white sheet of paper, you will see that it forms a real image of
anything on the other side of it--particularly bright things such as light bulbs or well-lighted windows. A typical
camera uses a converging lens that's not unlike a magnifying glass to form an image of this sort. You could use a
magnifying glass to build a camera, but I'd suggest that you start with a camera and rebuild it yourself. Go to a
company that processes film and see if they will give you any used disposable cameras. These cameras are of
essentially no value to them and they either discard them or recycle them. If you ask around, you should find a
photo shop that will give you a couple. You can then disassemble them. You'll find a very nice lens, a shutter
system, a film advance mechanism, and so on. You can use a toothpick or small screwdriver to turn the exposure
dial backward so that the camera behaves as though it still has film left. You can then "advance the (nonexistent) film" by turning the film sensing gears in the back of the camera with your fingers until the shutter
cocks. Finally, you can press the shutter release and watch the shutter open the lens to light. Disposable cameras
are great because if you break something in your experimenting, you can just throw away your mistake.
How does an overhead projector work? -- SR, Hartford, CT
An overhead projector uses a converging lens and a mirror to project a real image of your transparency onto a
screen. A lamp brightly illuminates the transparency and a special surface under the transparency (actually a
Fresnel lens) directs the light from the transparency through the projector's main lens. This lens bends the light
rays in such a way that all of the rays spreading outward from one point on the transparency bend back together
and merge to one point on the screen. For example, if you make a green dot on the transparency, light rays
spread outward from that green dot and some of them pass through the main lens. The lens bends these rays back
together so that they form a single green dot on the screen. There is a single point on the screen for the light rays
from each point on the transparency.
The pattern of light that forms on the screen is called a real image because it looks just like the original object--in
this case the transparency--and it's real, meaning that you can touch it with your hand. Real images are usually
upside-down and backward, but the overhead projector uses its mirror to flip the image over so that it appears
right side up. Because of this vertical flip, the side-to-side reversal is a good thing--the right side of the
transparency becomes the left side of the screen image (as viewed by the same person) and the screen image is
readable.
What path does sunlight follow for you to see a mirage? -- XF
The first step in explaining a mirage is to understand why the sky is blue, or why it has any color at all. If it
weren't for the earth's atmosphere, the sky would be black and dotted with stars. That's how the moon's sky
appears. But the earth's atmosphere deflects some of the sunlight that passes through it, particularly shortwavelength light such as blue and violet, and this scattered light (Rayleigh scattering) gives the sky its bluish
cast. When you look at the blue sky, you're seeing particles of light that have been scattered away from their
original paths into new paths so that they reach your eyes from all directions.
The blue light from the sky normally travels directly toward your eyes so that you see it coming from the sky.
But when there is a layer of very hot air near the ground in the distance, some of the blue light from the sky in
front of you bends upward toward your eyes. This light was traveling toward the ground in front of you at a very
shallow angle but it didn't hit the ground. Instead, its entry into the hot air layer bent it upward so that it arced
away from the ground and toward your eyes. When you look at the ground far in front of you, you see this
deflected light from the blue sky turned up at you by the air and it looks as though it has reflected from a layer of
water in front of you. This bending of light that occurs when light goes from higher-density cold air to lowerdensity hot air is called refraction, the same effect that bends light as light enters a camera lens or a raindrop or a
glass of water. Whenever light changes speeds, it can experience refraction and light speeds up in going from
cold air to hot air. In this case, the light bends upward, missing the ground and eventually reaching your eyes.
April 7, 1997
I read a recent article about the FCC requiring all TV stations to switch to digital signals instead of analog ones by 2006.
How are digital signals different from analog signals, and will they work with our current TV's? -- JP
Current video signals use continuous physical quantities to represent the brightness and color of the spots on a
television screen. For example, the current in a video cable can take any value and that value is used to represent
the brightness and color of the spots. This use of a continuous physical quantity (such as current) to represent a
continuous physical quantity (such as brightness) is called analog representation.
In a digital video signal, a physical quantity first represents numbers and then these numbers represent the
brightness and color of the spots. The physical quantity representing the numbers doesn't have to be continuous.
For example, a current that's on could represent the number 1 while a current that's off could represent the
number 0. A certain pattern of on and off currents could represent larger numbers and these numbers could then
represent brightness and color. This use of a continuous or non-continuous physical quantity (such as
magnetization, charge, or current) to represent numbers and then these numbers to represent a continuous
physical quantity (such as brightness) is called digital representation.
One advantage of digital representation is that it's relatively immune to noise. In analog representation, any
disturbance in the continuous physical quantity representing the information leads directly to a disturbance in the
recovered information. For example, if the strength of a radio wave is representing brightness and color on your
television (the current technique), then any disturbance of the radio wave leads directly to a damaged image on
your television. But in digital representation, small changes in the physical quantity that's carrying the
information won't change the numbers that are obtained from that physical quantity and will thus have absolutely
no effect on the recovered information. For example, if the strength of a radio wave is representing numbers in
digital format, using binary (base two) encoding, then a small disturbance of the radio wave will not affect the
binary numbers that are recovered from the radio wave. To see why that's true, imagine representing the number
1 as a powerful radio wave and a 0 as no radio wave at all. It's pretty easy to tell a powerful radio wave from an
absent one so that, even if there is some radio interference around, it's unlikely to confuse the receiver.
Moreover, even if noise does occasionally confuse the receiver about a number or two, the digital scheme can
include redundant information that allows the receiver to identify errors and to fix them! That's why a compact
disk is so immune to noise--even if there is a flaw or dirty spot on the disk, there is enough redundant digital
information to reproduce the music flawlessly.
The other advantage to digital representation is that digital compression techniques become possible. A typical
video signal contains lots of unnecessary and duplicated information. For example, when two people are standing
in a room and the only things that are changing with time are the images of those two people, there is really no
reason to keep sending an image of the room itself from the broadcast station to your home. Digital compression
can identify redundant information and remove it from the transmission. In doing so, it can use the
communication channel more efficiently.
By adopting a digital transmission scheme, the FCC has recognized that broadcasters will be able to send much
clearer, more detailed images using digital representations than with the current analog representations, while
still occupying the same portions of the electromagnetic spectrum. However, there is a cost--current televisions
will not work directly with these new digital signals. To fix that shortcoming, there will be inexpensive
converters that receive the new digital signals and recreate the analog signals needed for current televisions. This
conversion will allow older televisions to keep working, but the new digital televisions will be designed to make
better use of the enhanced details in the transmissions. The new transmissions will contain about 4 times the
detail of current transmissions so that the images will be sharper as well as more immune to noise than the
current transmissions.
April 3, 1997
Why does water freeze at very low pressure? I saw an experiment in which a small amount of water first boiled and then
froze solid when exposed to a vacuum. -- BLG, Old Bridge, NJ
Water molecules are always leaving the surface of liquid water and when they do, they carry away more than
their fair share of the water's thermal energy. Placing the water in a vacuum speeds this process because (1) it
prevents those gaseous water molecules from returning to the liquid water, in which case they would return the
thermal energy, and (2) it makes it possible for bubbles of water vapor to remain stable inside the liquid water
even at low temperature, so that the water can boil. Overall, the main effect of putting the water in a vacuum is
that its molecules leave rapidly and don't return. Since each leaving water molecule takes away more than its fair
share of thermal energy, the water molecules that remain behind become cooler and cooler. You experience this
effect when evaporating water from your skin makes you feel cold. In the present case, this cooling is so
effective that the remaining water cools all the way to water's freezing point and the water begins to crystallize
into ice. Water molecules continue to leave the surface of ice, a process called sublimation, so that even the ice
gradually gets colder in the vacuum.
How much steam is required to produce a unit of power? -- DKB, Dubai
There is no easy answer to this question, but for an interesting reason. First, "power" is a measure of energy per
time (e.g. joules per second or BTUs per hour) so any answer would have to involve the amount of steam per
time (e.g. kilograms per second or cubic meters per hour). But even recognizing that requirement, I can't answer
the question. First, I'd need to know the temperature of the steam. The hotter the steam, the more thermal energy
it contains and the more energy it could provide. For more complicated reasons, I'd also have to know the
pressure of the steam. But there is a fourth issue: even knowing the amount of steam involved and the
temperature and pressure of that steam, the amount of useful energy that can be extracted from that steam
depends on the existence of a colder object. You can't turn thermal energy--the type of energy that steam
contains--directly into useful work or into electric energy in a continuous manner. You must use the steam in a
"heat engine", converting a fraction of its thermal energy into work as that thermal energy flows as heat from the
hot steam to a colder object. This requirement is established by the laws of thermodynamics and there is no way
to get around it. The hotter the steam and the colder the object, the larger the fraction of the steam's thermal
energy you can convert to work. However, there is no way to convert all of the steam's thermal energy into work
continuously.
How does a standard water pump work? -- ML, Wilmington, NC
The water pumps in most cars are centrifugal pumps. These pumps work by spinning water around in a circle
inside a cylindrical pump housing. The pump makes the water spin by pushing it with an impeller. The blades of
this impeller project outward from an axle like the arms of turnstile and, as the impeller spins, the water spins
with it. As the water spins, the pressure near the outer edge of the pump housing becomes much higher than near
the center of the impeller. There are many ways to understand this rise in pressure, and here are two:
First, you can view the water between the impeller blades as an object traveling in a circle. Objects don't
naturally travel in a circle--they need an inward force to cause them to accelerate inward as they spin. Without
such an inward force, an object will travel in a straight line and won't complete the circle. In a centrifugal pump,
that inward force is provided by high-pressure water near the outer edge of the pump housing. The water at the
edge of the pump pushes inward on the water between the impeller blades and makes it possible for that water to
travel in a circle. The water pressure at the edge of the turning impeller rises until it's able to keep water circling
with the impeller blades.
You can also view the water as an incompressible fluid, one that obeys Bernoulli's equation in the appropriate
contexts. As water drifts outward between the impeller blades of the pump, it must move faster and faster
because its circular path is getting larger and larger. The impeller blades do work on the water so it moves faster
and faster. By the time the water has reached the outer edge of the impeller, it's moving quite fast. But when the
water leaves the impeller and arrives at the outer edge of the cylindrical pump housing, it slows down. Here is
where Bernoulli's equation figures in. As the water slows down and its kinetic energy decreases, that water's
pressure potential energy increases (to conserve energy). Thus the slowing is accompanied by a pressure rise.
That's why the water pressure at the outer edge of the pump housing is higher than the water pressure near the
center of the impeller.
When water is actively flowing through the pump, arriving through a hole near the center of the impeller and
leaving through a hole near the outer edge of the pump housing, the pressure rise between center and edge of the
pump isn't as large. However, this pressure rise never completely disappears and it's what propels the water
through the car's cooling system.
If one accepts the existence of black holes, would it be plausible to assume that a "white hole" exists on the opposite end
due to captured light by the black hole?
I think not. Depending on your frame of reference, the passage of material into a simple black hole--one that isn't
spinning very fast and that doesn't have a great deal of electric charge in it--has one of two results. If you are
traveling with the material, things proceed more or less normally as you pass the point of no return--the so-called
"event horizon" from which even light can't escape. You accompany the material all the way to the center of the
black hole--its "singularity"--and are crushed to infinite density. If instead of traveling with the material, you
remain outside the black hole looking in toward it, you see the material approach the event horizon but without
ever quite entering its surface. In fact, all of the material that went into forming the black hole in the first place,
plus all the material that has fallen into the black hole since its formation, appear to reside forever on the event
horizon surface. In effect, the material never quite gets to the black hole. Since the material never quite gets to
the black hole, there is no need for it to reemerge elsewhere from a "white hole."
However, there are more complicated black holes--ones involving angular momentum and electric charge--that
have more complicated structures. In falling into one of these black holes, it is apparently possible to miss the
singularity. There is some discussion of such material reemerging from the "other end" of one of this black holes
but I believe that there are serious problems with such two-ended interpretations of the equations governing such
black holes.
What type of laser is in a laser printer? -- DFC, Asheville, NC
A laser printer uses a single diode laser that's scanned across the surface of the photoconductor drum by a rapidly
turning, multifaceted mirror. These diode lasers are very similar the ones used in laser pointers or supermarket
barcode readers. The multifaceted mirrors are typically octagonal prisms that are aluminized to make them
highly reflecting and spun by a motor. The laser beam bounces off the spinning mirror and its reflection sweeps
across the photoconductor. Modulating the current supplying power to the diode laser causes its brightness to
fluctuate so that it writes information on the surface of the photoconductor.
What are atoms made of? -- Fifth Grade Class, Knifley, KY
My answer to that question depends on the level of detail you're interested in. As an example of what I mean by
that statement, imagine describing what a simple house is made of. At the coarsest level, you might say that it
consists of a floor, a ceiling, four walls, and a roof. At a greater level of detail, you might say that it consists of
many boards, some tarpaper, and lots of nails. At a still finer level of detail, you might say that it consists of
atoms and molecules, and... you get the point. So it is with atoms. I'll answer the question at a fairly coarse level
of detail, one that's familiar to many people, and then say a word or two about the next level of detail.
The principal constituents of an atom are protons, neutrons, and electrons. These are three most important
subatomic particles; the main building blocks of matter in the same way that wood, bricks, and steel are the
major building blocks of houses. Each of these particles has a mass--the measure of their inertia--and two of
them, electrons and protons, are electrically charged. Each electron has one unit of negative charge while each
proton has one unit of positive charge. Because an atom is normally electrically neutral--its positive and negative
charges must balance--it has an equal number of electrons and protons. The number of neutrons in an atom is
somewhat flexible.
These particles, electrons, protons, and neutrons, are held together by several types of forces. The protons and
neutrons, which are relatively massive, stick to one another at the center of the atom and form a dense object
called the atomic nucleus. The particles in the nucleus are held together by the "nuclear" force, which binds
together protons and neutrons that are touching one another. This nuclear force is quite strong and is able to
overcome the strongly repulsive electromagnetic forces that the protons in the nucleus exert on one another--like
electric charges repel one another and the protons are all positively charged. The electrons circulate around the
atom's nucleus, held in place by the strongly attractive electromagnetic forces that protons exert on electrons-opposite electric charges attract one another and the electrons are negatively charged while the protons are
positively charged.
The electrons do most of the circulating around the nucleus, rather than the other way around, because they are
much less massive than the nucleus. As with the planets around the sun, the less massive objects tend to orbit the
more massive objects. At a basic level, you can view an atom as a tiny solar system with its neutrons and protons
at the center and its electrons orbiting around this central nucleus. Quantum physics dramatically complicates
this picture, but it's a helpful picture nonetheless.
At the next level of detail, the protons and neutrons themselves have structure--they are built out of yet smaller
particles known as quarks. The particles also stick to one another by tossing particles back and forth--particles
including photons and gluons. But that is a whole new story.
What is sonar? -- BK, Australia
Sonar stands for "sound navigation ranging" and involves the bouncing of sound waves from objects to
determine where those objects are. It's based on the reflection of sound waves from objects. Whenever a wave of
any sort moves from one medium to another and experiences a change in speed (or more generally, a change in
impedance), part of that wave reflects. Because sound travels much faster in solids than it does in air, some
sound reflects when it moves from air to rock--which is why you hear echoes when you yell at a mountain! But
even more subtle changes in the speed of sound will cause modest reflections. Thus a sophisticated sound
generator and receiver can detect objects immersed in water or buried in the ground. Another form of sonar is
used in medical imaging--ultrasonic imaging.
April 2, 1997
Why and how does water conduct electricity? -- SM, Murrysville, PA
Water molecules are electrically neutral and do not accelerate in response to electric fields. For that reason, a
liquid consisting only of water molecules wouldn't conduct electricity. However, real water contains things other
than water molecules. Even in completely pure water, about 1 in every 10,000,000 water molecules is found to
have dissociated into a hydrogen ion (H+) and a hydroxide ion (OH-). These electrically charged ions do
accelerate in response to electric fields and they make it possible for even the purest water to conduct electricity
weakly. Adding impurities, particularly ionic impurities such as salts, makes water an even better conductor of
electricity.
I understand that for a steam engine to produce useful work, you need a difference in temperatures. My question is
whether the difference in temperatures between cold glacier ice and the warmer air could be used to drive a steam engine
and generate electricity. -- LNH & AJH, Juneau, Alaska
As you clearly recognize, any heat engine--a machine that converts thermal energy into work--can only do its job
while heat is flowing from a hotter object to a colder object. That limitation is imposed by the second law of
thermodynamics--a statistical law that observes that the disorder of an isolated system can never decrease. A heat
engine's theoretical efficiency at turning thermal energy into work improves as the temperature difference
between its hotter and colder objects increases. Since the air temperature is hotter than the glacier temperature,
there is the possibility to convert some of the air's thermal energy into work as heat flows from the air to the
glacier. In short, what you suggest could be done.
Unfortunately, most practical heat engines work best when the hotter object is really hot. For example, a steam
engine works best when the hotter object is hot enough to produce very high temperature, high pressure steam.
To operate a steam engine with outside air as the hotter object and cold ice as the colder object, the steam engine
would have to operate at very low pressure. In fact, it would operate well below atmospheric pressure in a
carefully sealed environment. Steam might not even be the best choice for a working fluid--you might do better
with a refrigerant such as the various Freon replacements. In effect, your heat engine would be an air conditioner
run backward--providing electric power rather than consuming it. Although this could be done, it would probably
not be cost effective. The heat exchangers needed to obtain heat from the air and to deliver most of that heat to
the glacier, as well as all the machinery of the heat engine itself, would probably make the electricity you
generated too expensive. Just because something can be done doesn't mean that it's worth doing. Until other
sources of energy become more expensive, this one won't pay for itself.
How might an ion engine work? -- DAA, San Diego, CA
One possible ion engine uses mercury as a propellant. The mercury starts as a liquid in a small tank, but its atoms
slowly evaporate to form a low-density gas. An electric discharge through this gas, such as occurs inside a
fluorescent lamp, knocks electrons off some of the mercury atoms. When a mercury atom loses an electron, it
becomes a positively charged mercury ion and can be accelerated from the discharge by electric fields. In the ion
propulsion engine, an electric field extracts and accelerates the mercury ions toward a hole in the side of a
spaceship. The mercury ions are ejected into space at enormous speeds. As they accelerate, the mercury ions
exert reaction forces on the engine and these forces are what propel the spaceship forward. Overall, the mercury
ions accelerate in one direction while the spaceship accelerates in the other direction. To keep the spaceship
electrically neutral, the engine also ejects electrons into space. However, mercury ions provide most of the
engine's thrust.
What is the general theory of operation of a hydraulic turbine? -- GS, Fort Worth, Texas
A hydraulic turbine is essentially a fan run backward--while a fan adds energy to a passing fluid, a turbine
extracts energy from a passing fluid. You can think of the fluid's effects on the turbine blades in two different but
equivalent ways. In one view, the fluid is deflected by its encounter with the canted turbine blades and as the
blades push the fluid in one direction, the fluid pushes the blades in the opposite direction. This reaction force
that the fluid exerts on the blades causes those blades to spin and does work on them--energy is transferred from
the fluid to the blades.
In the other view, the blades "fly" through the fluid like the wings of an airplane. The fluid flow around each
blade is such that the pressure is higher on one side of the blade than the other and the blade experiences a net
force toward the lower pressure side. The blades move in the direction of this force, so the passing fluid does
work on them--energy is transferred from the fluid to the blades.
These two views are completely equivalent. The fluid leaves the turbine blades traveling more slowly or at lower
pressure, and it acquires a rotation in the direction opposite the turbine's rotation.
What are some general uses of X-rays other than medical? -- SD, Raleigh, NC
There are so many non-medical uses for X-rays that I'll limit myself to two: industrial imaging and X-ray
crystallography. Industrial X-ray imaging is used frequently in manufacturing to inspect finished materials. An
important example of this imaging is in weld inspection. After a sheet of steel has been rolled into a pipe and the
seam of that pipe has been welded closed, it's often important to inspect the weld to be sure that it's solid and
leak free. Sometimes a weld that looks perfect to the eye has hollow spots or other flaws that can only be seen by
looking through the material of the weld. This inspection is done with high energy X-rays--X-rays that are able
to penetrate a thick steel plate to look for bubbles or unwanted inclusions.
X-ray crystallography is an important tool for materials science and molecular biology. Just as the colored
interference patterns that appear on a soap bubble when sunlight reflects from that bubble tell you something
about the structure of that soap bubble, so the X-rays that reflect from a crystal tell you something about the
structure of that crystal. X-rays experience interference after they reflect from a crystal and the interference
patterns can tell you where individual atoms are located within a crystal or within the molecules from which the
crystal is made. Materials scientists use this information to understand the crystals they have produced while
molecular biologists use it to understand the molecular structures of complicated biological molecules.
Why is it that when you put two electric lamps into a circuit in parallel with one another, the current through the circuit
increases, while when you put those two lamps in series with one another, the current through the circuit decreases?
When the two lamps are in parallel with one another, they share the current passing through the rest of the
circuit. Current arriving at the two lamps can pass through either lamp before continuing its trip around the
circuit. The two lamps operate independently and each one draws the current that it normally does when it
experiences the voltage drop provided by the rest of the circuit. With both lamps providing a path for current, the
current through the rest of the circuit is the sum of the currents through the two lamps.
But when the two lamps are in series with one another, each lamp carries the entire current passing through the
circuit. Current arriving at the two lamps must pass first through one lamp and then through the other lamp
before continuing its trip around the circuit. There is no need to add the currents passing through the lamps
because it is the same current in each lamp. Moreover, the voltage drop provided by the rest of the circuit is
being shared by the two lamps so that each lamp experiences roughly half the overall voltage drop. Since lamps
draw less current as the voltage drop they experience decreases, these lamps draw less current when they must
share the voltage drop. Thus the current passing through the circuit is much less when the two lamps are inserted
into the circuit in series than in parallel.
How do long range metal detectors work? -- AS
In general, metal detectors find metal objects by looking for their electromagnetic responses. For example, you
can tell when an iron or steel object is nearby by waving a magnet around. If you feel something attracting the
magnet, you can be pretty sure that there is a piece of iron or steel nearby. Similarly, if you wave a strong magnet
rapidly across an aluminum or copper surface, you'll feel a drag effect as the moving magnet causes electric
currents to flow in the metal surface--electric currents are themselves magnetic.
Of course, a real metal detector is much more sensitive than your hands are, but it's using similar principles to
detect nearby metal. Most often, a metal detector uses a coil of wire with an alternating current in it to create a
rapidly changing magnetic field around the coil. If that changing magnetic field enters a piece of nearby metal,
the metal responds. If the metal is ferromagnetic--meaning that it has intrinsic magnetic order like iron or steel-it will respond strongly with its own magnetic field. If the metal is non-ferromagnetic--meaning that it doesn't
have the appropriate intrinsic magnetic order--it will respond more weakly with magnetic fields that are caused
by electric currents that begin to flow through it.
In a short range metal detector, the detector looks for the direct interaction of its magnetic field and a nearby
piece of metal. That nearby metal changes the characteristics of the detector's wire coil in a way that's relatively
easy to detect. But in a longer-range metal detector, the electromagnetic coil must actually radiate an
electromagnetic wave and then look for the reflection of this electromagnetic wave from a more distant piece of
metal. That's because the magnetic field of the coil doesn't extend outward forever--it dies away a few diameters
of the coil away from the coil itself. For the metal detector to look for metal farther away, it needs help carrying
the magnetic field through space. By combining an electric field with the magnetic field, the long-range metal
detector creates an electromagnetic wave--a radio wave--that travels independently through space.
Electromagnetic waves reflect from many things, particularly objects that conduct electricity. So the long-range
metal detector launches an electromagnetic wave and then looks for the reflection of that wave. This wave
reflection technique is the basis for sonar (sound waves) and radar (radio waves), and it can be used to find
metals deep in the ground. Unfortunately, the ground itself conducts electricity to some extent, so it becomes
harder and harder to distinguish the reflections from metal from the reflections from other things in the ground.
What is torque? -- JPT, Calgary, Alberta
A torque is a physicist's word for a twist or a spin. When you twist the top off a jar, you are exerting a torque on
the jar and causing it to undergo an angular acceleration--it begins to rotate faster and faster in the direction of
your torque. Similarly, when you spin a toy top, you do this by exerting a torque on the top and it again
undergoes an angular acceleration.
April 1, 1997
How does a siren work? -- MM, Waterloo, Iowa
A siren uses a perforated disk or drum to alternately block and unblock a stream of air. The classic siren has a
spinning disk with a pattern of holes around its periphery. This disk is spun in front of a jet of air, producing
pressure pulses that we hear as sound. A more modern siren has a spinning centrifugal fan that propels air
radially outward through a pattern of holes in a drum around the fan. This centrifugal siren is much louder than
the disc siren because the centrifugal system pushes large pulses of air through many openings at once, whereas
the disc siren only has one pulsed source of air.
How does an operational amplifier work? -- BR
An operational amplifier is an extremely high gain differential voltage amplifier--a device that compares the
voltages of two inputs and produces an output voltage that's many times the difference between their voltages.
How the operational amplifier performs this subtraction and multiplication process depends on the type of
operational amplifier, but in most cases two input voltages control how current is shared between two paths of a
parallel circuit. Even a tiny difference between the input voltages produces a large current difference in the two
paths--the path that's controlled by the higher voltage input carries a much larger current than the other path. The
imbalance in currents between the two paths produces significant voltage differences in their components and
these voltage differences are again compared in a second stage of differential voltage amplification. Eventually
the differences in currents and voltage become quite large and a final amplifier stage is used to produce either a
large positive output voltage or a large negative output voltage, depending on which input has the higher voltage.
In a typical application, feedback is used to keep the two input voltages very close to one another, so that the
output voltage actually falls in between its two extremes. At that operating point, the operational amplifier is
exquisitely sensitive to even the tiniest changes in its input voltages and makes a wonderful amplifier for small
electric signals.
How does a video recorder work? -- SH, Sault Ste. Marie, Ontario
A video recorder is much like a normal tape recorder, except that it records far more information each second.
When you play an audiotape in a normal tape recorder, small magnetized regions of tape move past a playback
head. This playback head consists of an iron ring with a narrow gap in it and there is a coil of wire wrapped
around the ring. As the magnetized regions of the tape pass near the ring's gap, they magnetize the ring. The
ring's magnetization changes as the tape moves and these changing magnetizations cause currents to flow in the
coil of wire. These currents are amplified and used to reproduce sound. When you record the tape, the recorder
sends currents through the wire coil, magnetizing the iron ring and causing it to magnetize the region of tape
that's near the gap in the ring.
In a video recorder, the tape moves too slowly to produce the millions of the magnetization changes needed each
second to represent a video signal. So instead of moving the tape past the playback head, the video recorder
moves the playback head past the tape. As the tape travels slowly through the recorder, the playback head spins
past it on a smooth cylindrical support. The tape is wrapped part way around this support and two or more
playback heads take turns detecting the patches of magnetization on the tape's surface. The tape is tilted slightly
with respect to the spinning heads so that the heads sweep both along the tape and across its width. That way, the
entire surface of the tape is used to record the immense amount of information needed to reproduce images on a
television screen. During recording, currents are sent through the heads so that they magnetize the tape rather
than reading its magnetization.
How are magnets made and what are they made of? -- S, San Francisco, CA
The strongest modern magnets are made by assembling lots of tiny magnetic particles into a solid object. These
magnetic particles are "intrinsically" magnetic, meaning that the atoms from which the particles are formed
retain their magnetism in coming together as a solid. Electrons are naturally magnetic and most atoms exhibit the
magnetism of their electrons. But as these atoms come together to form a solid, most of them lose their
magnetism. For example, copper, aluminum, gold, and silver are all nonmagnetic solids built from magnetic
atoms. There are only a few materials that don't lose their atomic magnetism and might be suitable for making
permanent magnets. However, most of these magnetic materials only exhibit their magnetism when exposed to
other magnets--when they're alone, their magnetism is mostly hidden. For example, iron and steel are magnetic
materials but they only appear strongly magnetic when you bring a permanent magnet near them.
To make a strong permanent magnet, you must find a material that is both intrinsically magnetic and that is able
to stay magnetic when it's by itself. Materials that hide their magnetism when alone do this by allowing their
magnetic structure to break up into tiny pieces that all point in different directions. Each of these tiny magnetic
pieces is called a magnetic domain, and iron and steel are normally composed of many magnetic domains. A
good permanent magnet material is one that is intrinsically magnetic and that resists the formation of randomly
oriented magnetic domains. A very effective way to make such permanent magnet materials is to assemble lots
of tiny magnetic particles. Each of these particles is shaped in a way that makes one of its ends a north pole and
its other end a south pole, and that makes it extremely hard for these two poles to exchange places. The particles
are then aligned with one another and bonded together to form a permanent magnet. To make sure that the
particles all have their north poles at one end and their south poles at the other end, the finished magnet is
exposed to an extremely strong magnetic field--one so strong that it flips any misaligned magnetic particles into
alignment with the others. After being magnetized in this manner, the permanent magnet is very hard to
demagnetize, which is just what you want in a permanent magnet.
The most common magnet materials are Ferrite and Alnico. Ferrite magnets are made from a mixture of iron
oxide and barium, strontium, or lead oxide. Alnico magnets are made from aluminum, nickel, iron, and cobalt,
and consist of tiny particles of an iron-nickel-aluminum alloy inside an iron-cobalt alloy. But the strongest
modern magnets are made from an iron-neodymium-boron alloy. The latter magnets are very resistant to
demagnetization and the forces they exert on one another are amazingly strong.
How does an electromagnetic doorbell work? -- SH, Sault Ste. Marie, Ontario
When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of
electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete,
current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the
coil of wire is an iron rod. When this the coil of wire--also called a solenoid--becomes magnetic, so does the iron
rod. The iron rod becomes magnetic in such a way that it's attracted toward and into the solenoid, and it
accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid,
but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of
the solenoid that it strikes a bell on the far side--the doorbell! The rod rebounds from the bell and reverses is
motion. It has traveled so far out the other side of the solenoid that it's attracted back in the opposite direction.
The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different
pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the
solenoid and doesn't move again until you stop pushing the button. Once you release the button, the current in the
circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its
original position at one end of the solenoid.
How does a rail gun work?
A rail gun is a device that uses an electromagnetic force to accelerate a projectile to very high speeds. This
acceleration technique is based on the fact that whenever an electrically charged particle moves in the presence
of a magnetic field, it experiences a force that pushes it perpendicular to both its direction of travel and the
magnetic field. In a rail gun, this perpendicular magnetic force--known as the Lorentz force--pushes the
projectile along two metal rails and can accelerate it to almost limitless speeds.
The rail gun's projectile must conduct electricity and it completes the electric circuit formed by two parallel
metal rails and a high current power source. During the rail gun's operation, current flows out of the power
source through one rail, passes through the projectile, and returns to the power source through the other rail. As it
passes through the two rails, the electric current produces an intense magnetic field between the rails. The
projectile is exposed to this magnetic field and as charged particles pass through the projectile, they experience a
Lorentz force that pushes them and the projectile in one direction along the rails. The projectile picks up speed as
it travels along the rails and doesn't stop accelerating until the current ceases or it leaves the rails. In practice, the
power sources used in most rail guns is a large bank of capacitors. These devices store separated electric charge
and supply enormous currents to the rails for a brief period of time.
March 28, 1997
How do the display lasers used in sporting events work? I think it has something to do with mirrors.
They do use mirrors. When you bounce a laser beam from a mirror, any small change in the mirror's orientation
can cause a large change in the beam's final destination. Simple laser light shows bounce lasers from low-mass
mirrors that are mounted on elastic membranes. As those membranes are driven into motion by sound waves, the
mirrors tip and turn and the laser beams move around in beautiful patterns on a distant screen or wall. In laser
light shows that produce specific shapes and images, the mirrors that steer the laser beams are driven by highspeed electromagnetic mechanisms that can change a mirror's angle dramatically in thousandths of a second.
With several of this electromagnetically controlled mirrors working together and guided by a computer, the beam
can be steered to draw complicated shapes on a screen or other surface.
March 27, 1997
A company claims that if you place their sealed liquid-filled plastic ball into your washing machine, you can eliminate
the need for caustic detergents, improving the ecology and saving the planet. The claim is that this ball changes the ionic
charge of the water and "magically releases" the dirt from your clothing. Is it possible to use ions to clean as well or better
than detergent? -- RO, Garden City, MI
I'm afraid that this claim is nonsense and, like the stone in "stone soup," the ball does nothing at all. The old-time
medicine show didn't really disappear, it just evolved into a more modern form. Since the ball doesn't add or
remove chemicals from the water, it can't alter the numbers of neutral and ionic particles in the water. But ions
have very little to do with how water cleans clothes anyway. Water is already a wonderful solvent for salts and
sugars, so you can clean many soils from your clothes with just water alone. But water is a poor solvent for oils
and fats because oil and fat molecules don't bind well to water molecules. That's where detergents come into
play--they form shells called micelles around the oil and fat molecules and render those molecules soluble in
water. Without detergents, you'll have trouble cleaning oils and fats from your clothes. Since oils and fats aren't
affected one way or the other by ions, even the ball's claimed activity won't help them to dissolve in the water.
How does an air conditioner work? -- RL
An air conditioner uses a condensable working fluid--a chemical that easily converts from a gas to a liquid and
vice versa--to transfer heat from the air inside of a home to the outside air. This process involves three major
components and at least one fan. The three major components are a compressor, a condenser, and an evaporator.
The compressor and condenser are usually located on the outside air portion of the air conditioner while the
evaporator is located on the inside air portion. The working fluid passes through the insides of these three
components in order, over and over again, so I'll start examining what happens to the working fluid as it enters
the compressor.
The working fluid arrives at the compressor as a cool, low pressure gas. The compressor squeezes this working
fluid, packing its molecules more tightly together so that their density and pressure increase. The squeezing
process also does work on the working fluid, increasing its energy and therefore its temperature. The working
fluid leaves the compressor as a hot, high-pressure gas and flows into the condenser. The condenser has metal
fins all around it that assist the working fluid in transferring heat to the surrounding outdoor air. As this transfer
takes place, the closely spaced molecules of the working fluid begin to stick to one another, releasing additional
thermal energy into the surrounding air and causing the working fluid to transform into a liquid. By the time the
working fluid leaves the condenser, its temperature has almost dropped back down to the outdoor temperature
but it is now a liquid rather than a gas.
This high pressure liquid then flows into the evaporator through a narrow orifice. This orifice allows the liquid's
pressure to drop so that it begins to evaporate into a gas. As it evaporates, it extracts heat from the air around the
evaporator because that heat is needed to separate the molecules of the working fluid. Like the condenser, the
evaporator has metal fins to assist it in exchanging thermal energy with the surrounding air. By the time the
working fluid leaves the evaporator, it is a cool, low-pressure gas. It then returns to the compressor to begin its
trip all over again.
Overall, the working fluid releases heat into the outside air and absorbs heat from the inside air. The direction of
heat transfer, from a cooler region to a hotter region, is the reverse of normal and requires an input of ordered
energy so that it doesn't violate the second law of thermodynamics (the disorder of an isolated system can never
decrease). This ordered energy is used to operate the compressor and is converted into thermal energy in the
process. This additional disordered thermal energy enters the outside air and makes up for the additional order
that's given to the indoor air as that air is cooled.
What makes an airplane fly? -- BO, Pemberton, MN
As an airplane's wing moves through the air, the airstream approaching the wing separates into a flow over the
top of the wing and a flow under the bottom of the wing. The wing is shaped and tilted so that the flow over the
wing follows a longer path to arrive at the sharp trailing edge of the wing than the flow under the wing must
follow. Because it has a shorter distance to travel, the flow under the wing initially arrives at the trailing edge of
the wing first and flows up and around that trailing edge to meet the flow over the wing. This type of flow has a
kink in it at the wing's trail edge and is unstable. A few moments after the wing begins moving through the air,
the kink at the trailing edge blows away from the wing altogether. This kink leaves as a vortex--a whirling
cyclone of air--and as it does, it causes the flow over the wing to speed up so that the two airflows join together
cleanly at the wing's trailing edge. To increase its speed, the flow over the wing converts some of its pressure
energy into kinetic energy. Because the flow over the wing has used up some of its pressure energy, and thus
experienced a drop in pressure, there is an unbalanced pressure across the wing: the pressure beneath the wing is
greater than the pressure above the wing. This imbalance in pressure leads to an overall upward force on the
wing and this upward force is what supports the plane's weight so that it remains suspended in the air. Overall,
the airstream is deflected downward as the result of this complicated flow pattern around the wing and the air
pushes the wing upward in response. A nice image of the airstream leaving a plane's wings can be seen at the
Canon website, http://www.usa.canon.com/explorers/flight.html.
What is the difference between crystal and glass?
The "crystal" that's used in fine glassware is actually a glass, but it is chemically different from the glass that's
used in more common glassware. Both materials are formed by melting together a mixture of silicon dioxide
(also called quartz or silica) and other chemicals and both are glasses, meaning that their atoms are arranged
haphazardly and not in the crystalline lattices of such materials as salt or sugar. But the chemicals that are added
to silicon dioxide to make normal glassware--sodium oxide and calcium oxide--make the glass easier to melt and
work with at the expense of strength. That's why normal glassware is relatively soft, emitting a dull sound when
you rap it because it experiences lots of internal friction. In contrast, the chemicals added to silicon dioxide to
make "crystal" glassware include lead oxide, which makes the glass easier to melt but doesn't weaken the glass
nearly so much. Lead "crystal" glassware is relatively hard and emits a ringing tone when you rap it because it
experiences very little internal friction.
Why is incandescent lighting better in residential construction than metal halide, high-pressure sodium, or mercury vapor
lighting systems? -- JC, Halifax, Nova Scotia
While incandescent lighting isn't nearly as energy efficient as those other light systems, it produces a more eye
pleasing light than some of the alternatives. Our eyes are optimized for sunlight, so that we find the spectrum of
light from hot objects particularly pleasant. The heart of an incandescent bulb is a hot tungsten filament. Highpressure arc lamps such as sodium vapor or mercury vapor lamps (metal halide lamps are just somewhat colorcorrected high pressure mercury vapor lamps) produce a much less even spectrum of light. High-pressure sodium
vapor lamps are wonderfully energy efficient, but their light is orange or pink. High-pressure mercury vapor
lamps are also quite energy efficient, but their light is somewhat bluish. Even metal halide lamps aren't quite
white. The other problem with high-pressure arc lamps is that they take time to warm up and then can't be
restarted until they cool off. They're best in applications that don't require them to be turned on or off frequently.
A much better choice, both in terms of energy efficiency and light color, is a fluorescent or compact fluorescent
lamp. Such lamps typically use less than 25% of the energy required for comparable incandescent lighting,
provide excellent color rendering that can be chosen to match that of incandescent lighting, and they last much
longer than incandescent bulbs. Even though compact fluorescent lamps are more expensive than incandescent
bulbs up front, they last so much longer and save so much energy that each one typically saves you about $45
over its working life.
How do neon lights work? -- MT, Cement City, MI
A neon light uses a high voltage transformer to place electric charges on the wires at each end of a neon-filled
glass tube. One end of the tube receives positive charges and the other end receives negative charges. Since like
charges repel one another, the vast numbers of like charges at each end push apart strongly and some of them
leave the wire and enter the neon gas. Once they're in the gas, these charges are draw quickly toward the opposite
charge at the far end of the tube. As they travel through the tube, these moving charges pick up speed and kinetic
energy but they occasionally collide with neon atoms as they travel and can transfer some of their kinetic
energies to the neon atoms. The neon atoms retain this extra energy only briefly before getting rid of it in the
form of visible light--the familiar red glow of a neon lamp. Overall, electric charges stream from one end of the
tube to the other, frequently colliding with the neon atoms and causing those atoms to emit red light. If you look
closely at a neon lamp, you'll see that it is the gas itself that's emitting the red light.
I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both
heated by microwaves even though they have no dipole moments? -- EB
Aluminum foil and graphitic carbon are both conductors of electricity. When they're exposed to microwaves, the
electric fields in those microwaves causes currents to flow through them. If the aluminum were thick enough, it
would be able to handle the currents without trouble. But aluminum is very thin and the current that flows
through it may be more than it can tolerate, particularly if it's only a narrow strip. It then becomes very hot. The
effect is the same as would happen if you plugged the aluminum foil into an electric outlet and sent current
through it that way. The same heating occurs in the carbon--the current that flows in it heats it up. In short,
relatively poor conductors of electricity become hot in a microwave because they permit currents to flow in
response to the microwave electric fields but then can't tolerate those currents without becoming hot.
Assuming microwave ovens cook on the principle of "moist" heat cookery, what are the general effects of microwave
cooking on various foods, including effects on chemical structure? -- EJ, Sydney, Australia
Microwave ovens cook by depositing thermal energy in the water molecules, which isn't the same as cooking
food in moist hot air. Microwave cooking tends to heat food uniformly throughout where as more conventional
"moist" heat cooking still heats food from the outside in. Nonetheless, the chemical effects on food are very
similar for both types of cooking. Virtually all of these effects are caused by elevating the temperatures of the
food. I'm not an expert on the chemistry of cooking, but elevated temperatures certainly denature proteins and
caramelize sugars.
How do radios work?
A radio station launches a radio wave by moving electric charges rhythmically up and down their antenna. As
this electric charge accelerates back and forth, it produces a changing electric field--a structure in space that
pushes on electric charges--and a changing magnetic field--a structure in space that pushes on magnetic poles.
Because the electric field changes with time, it creates the magnetic field and because the magnetic field changes
with time, it creates the electric field. The two travel off across space as a pair, endlessly recreating one another
in an electromagnetic wave that will continue to the ends of the universe. However, when this wave encounters
the antenna of your radio, its electric field begins to push electric charges up and down on that antenna. Your
radio senses this motion of electric charges and thus detects the passing radio wave.
To convey audio information (sound) to you radio, the radio station makes one of several changes to the radio
wave it transmits. In the AM or Amplitude Modulation technique, it adjusts the amount of charge it moves up
and down its antenna, and hence the strength of its radio wave, in order to signal which way to move the speaker
of your radio. These movements of the speaker are what cause your radio to emit sound. In the FM or Frequency
Modulation technique, the radio station adjusts the precise frequency at which it moves charge up and down its
antenna. Your radio senses these slight changes in frequency and moves its speaker accordingly.
March 26, 1997
When you were saying that even humans travel as waves (which I can picture), is this the theory behind how the people
in the show Startrek are "beamed" to certain planets and back to the ship?
The fact that all objects, including people, travel as waves in our universe is probably not what the writers of
Startrek had in mind when they "invented" the transporter. In Startrek, the transporter seems to disassemble the
people involved at one location and then reconstruct them at another. That disassembly/reassembly process is
purely science fiction while the wave propagation of matter is quite real. We never notice this wave propagation
for large objects because their wave effects are too small to detect and because watching an object propagate
prevents its wave properties from having any significant consequences. Each observation of an object tends to
localize it and minimize its wave properties, so that watching an object moves makes the effects of its wave
properties minimal.
I know that photons are particles of light--but how are photons related to the "excited" electrons in the atoms of a gas
discharge?
An atom in a gas discharge emits light when one of its electrons shifts from an orbital with extra energy into an
empty orbital in which it will have less energy. Since an electron can only travel around the atom's nucleus in an
allowed orbit--an orbital--and the energy it has while in that orbital is very specifically defined, such a shift from
one orbital to another results in the emission of a photon of light with a very specific energy. Because a photon's
energy is directly proportional to the frequency of the light, and light's frequency and wavelength are related by
the speed of light, the amount of energy the electron gives up in shifting from one orbital to another determines
the photon's energy, frequency, and wavelength.
March 25, 1997
When you walk on snow when it is cold (-20° C), the snow squeaks; but when it is relatively warm (-5° C) the snow
doesn't squeak. Why? -- PW, Alberta, CA
Near ice's melting temperature, the surfaces within warm snow become more and more liquid-like. These liquidlike surfaces not only allow the warm snow to stick together as firm snowballs, but they act as lubricants so that
the snow is particularly slippery. At much lower temperatures, the snow's surfaces are much more solid and they
slide uneasily and noisily across one another. The cold snow squeaks because it hasn't "been oiled."
Is it true that water that has been previously boiled will boil faster than water that hasn't been boiled? -- HE, Haddonfield,
NJ
I don't think so. The only effect that bringing water to a boil has on the water is to drive dissolved gases out of
solution. Once the water returns to room temperature, it's essentially the same as it was before it was heated to
boiling, except that it contains very little dissolved air. It may be that this absence of dissolved air will allow the
water to boil slightly faster the next time around, but I doubt that you'd be able to detect a difference.
In his Lectures on the Elements of Chemistry, Joseph Black discussed his difficulty in understanding latent heat. He
performed an experiment where water in a tube was brought below freezing without a phase change. The water remained
in this equilibrium as long as the tube of water was not disturbed. When it was disturbed, the water instantly turned to ice,
releasing enough heat to raise the temperature of the ice to 0° C. Please explain why the system remained in equilibrium
until it was acted upon by some external motion. -- EDH, Annapolis, MD
The water in Black's tube was in an unstable equilibrium state known as supercooled water. Supercooled water
tends to spontaneously convert into ice. When part of this supercooled water does convert to ice, it releases
enough latent heat energy to raise its temperature and that of the remaining water to 0° C, thereby terminating the
phase transition before all of the water has become ice.
But in the experiment you describe, the supercooled water was having trouble nucleating the initial seed ice
crystal on which the remaining water could crystallize. Given enough time, that water would have spontaneously
formed a seed crystal and the growth of the ice crystal would have proceeded rapidly after that. However, Black
accelerated the formation of the seed crystal by shaking the tube. A defect at the surface of the tube or a piece of
dust then acted as the trigger and helped the seed ice crystal form. The water then crystallized rapidly around this
seed crystal. After the ice had formed, the water was truly in equilibrium.
What is the difference between internal and external combustion engines?
External combustion engines burn a fuel outside of the engine and produce a hot working fluid that then powers
the engine. The classic example of an external combustion engine is a steam engine. Internal combustion engines
burn fuel directly in the engine and use the fuel and the gases resulting from its combustion as the working fluid
that powers the engine. An automobile engine is a fine example of an internal combustion engine.
How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? -- SF, Lake
Almanor, CA
The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room
appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle's yellow glow is
about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be
hotter still, except it's usually colored artificially by the presence of excited atoms. Atomic emissions are colored
because atoms can't emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they
preferentially emit only specific colors. That's why when you burn copper, you see blue-green light, even when
the copper isn't very hot. The copper atoms just can't emit red or yellow light, even though those would be the
more appropriate colors at the temperature of the burning copper.
If you have four carts of equal weights, one with small wheels, one with large wheels, one with small wheels in front and
large wheels in back, and one with large wheels in front and small wheels in back, which cart will be easiest to move? -PK
The cart with the small wheels will be easiest to move. That's because, as the cart starts moving, each kilogram
of mass in the wheels acquires twice as much energy as each kilogram of mass in the cart itself. Keeping the
mass of the wheels low by making the wheels small reduces the energy in the overall cart and makes it easier to
start or stop.
When an object is free falling, I understand that the earth's gravity causes its velocity to increase at 10 meters/second2 in
the downward direction. Is there a point at which this object would reach a "terminal velocity" in the earth's atmosphere
and cease to accelerate? -- CS, Sykesville, MD
Yes, most objects will reach a terminal velocity and stop accelerating downward. The faster an object drops, the
more air resistance it experiences. This air resistance pushes the object upward and at least partially cancels the
downward force of gravity--the object's weight. When the object's downward speed becomes high enough, the
upward air resistance force exactly cancels the object's downward weight. At that point, the object experiences
zero net force and it no longer accelerates. Instead, it descends at a constant downward velocity--its terminal
velocity. This terminal velocity is determined partly by the object's density and size and partly by its
aerodynamics. Large, dense, and aerodynamic objects tend to have very large terminal velocities while small,
low-density, non-aerodynamic objects tend to have very small terminal velocities.
When raisins are added to a solution containing water, baking soda, and vinegar, why do the raisins dance? -- RE, Troy,
IL
Baking soda and vinegar react in water to release carbon dioxide molecules. If the chemicals are sufficiently
dilute in the water, the carbon dioxide molecules may remain dissolved in the water almost indefinitely. But
when the water has impurities in it, the carbon dioxide molecules tend to come out of solution as gas bubbles at
those impurities. The impurities allow the molecules to form tiny gas bubbles--a process called nucleation. In the
present case, the raisins serve as the impurities that nucleate gas bubbles. As the gas bubbles grow on the
surfaces of the raisins, the raisins experience upward buoyant forces from the surrounding water. The bubbles
float upward, carrying the raisins with them and causing the raisins "to dance."
I've heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?
Since both gases and liquids are fluids, the earth's atmosphere is certainly a fluid. Any material that flows in
response to sheer stress (tearing) is considered a fluid. The earth's atmosphere flows in responses to sheer stress-for example when you drive your car past another car, the air in between experiences this tearing and it flows in a
complicated fashion. Winds are another important example of fluid flow in the earth's atmosphere.
Can you suggest an experiment to prove that a helium balloon floats because helium is lighter than oxygen? -- CR
If you have a balance scale, you can do a series of comparisons. First compare a cup of water to a cup of salad
oil, using the balance, to show that the salad oil is less dense than the water. Then show that the salad oil floats
on water. Then compare an air-filled balloon to an identical helium balloon, using the balance, to show that the
helium is less dense than air. Then show that the helium floats on air. It's just like the salad oil on water, but now
it's the helium on air. You can't simply pour the helium on the air to show that it floats, because they'll mix. So
you leave the helium wrapped up in a rubber balloon and then let it float on air. It floats just fine!
What is ink made of? -- JD, Langley, British Columbia
Ink is made of light absorbing pigment particles or dye molecules that are suspended in a fluid that contains a
dissolved binder chemical. When the ink is deposited on a sheet of paper, the binder's solvent diffuses into the
paper or evaporates into the air, leaving the pigment particles or dye molecules bound to the paper by the binder.
Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible
light is not absorbed and yet it slows down. What's going on? -- FH, Waltham, MA
When a light wave enters matter, the light wave's electric field causes charged particles in the matter to
accelerate back and forth. That's because an electric field exerts forces on charged particles. The light wave gives
up some of its energy to these charged particles and is partially absorbed in the process. However, the charged
particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit
electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier.
Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it
encounters, so that the light continues on its way as though nothing had happened.
However, something has happened--the light wave has been delayed ever so slightly. This absorption and
reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles
in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as
glass or diamond, the light wave can be slowed substantially.
Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more
effectively absorbed and reemitted by the atoms in most transparent materials. That's because when a high
frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons'
motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are
able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave
down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower
and the electrons' motions are so large that they quickly reach the boundaries of the atom. As a result, those
electrons aren't able to jiggle back and forth as far as they should and they don't slow the light wave down as
well.
What is infrared light? -- AC, Teaneck, NJ
Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their
wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the
number of electric field maximums that pass by a specific point in space each second). Infrared light has longer
wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher
frequencies than visible light. We can't see infrared or ultraviolet lights because the cells of retinas aren't
sensitive to these lights. Nonetheless, we can often tell when those lights are present--we may feel infrared light
as heat on our skins and we may find ourselves sunburned by ultraviolet light.
I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are
significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the
hole size now? -- KBH, Logan, Utah
If the electric field isn't changing with time, then it can't enter a metal cage no matter how large the cage's holes
are. In effect, the constant electric field has an infinite wavelength and can't propagate through holes of any finite
size. However, the holes don't stop an electromagnetic wave instantly--the wave does penetrate a short distance
into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of
about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that
are 1 centimeter in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If
the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an
effective shield if it is extremely large. To avoid having to use a very large cage, it's better to use small holes.
How do microwave ovens affect people fitted with pacemakers? -- W
If a microwave oven doesn't leak microwaves, then it won't affect such people at all. However, if microwaves do
leak from a particular microwave oven, they will cause undesirable currents to flow in the electric leads of the
pacemaker. That's because a microwave consists of electric and magnetic fields, and an electric field exerts
forces on charged particles. The mobile charged particles in the pacemaker's electric wiring will experience these
forces as the microwave encounters them and they will move back and forth with the microwave's fluctuating
electric field. The pacemaker's wiring isn't meant to carry these unexpected current flows, and the pacemaker
and/or the person attached to it may experience unpleasant effects. While such problems are very unlikely, it
makes sense to warn pacemaker users whenever a microwave oven is in use.
Do hand carried microwave heaters exist or must the microwaves always be enclosed, as they are in a microwave oven? - AL, Umea, Sweden
My understanding is that there are microwave heating systems that are not enclosed and that are used in medical
therapies to provide deep warming to injured tissues in medical patients. But apart from such devices, I've never
heard of unenclosed microwave heaters. That's because such heaters would be dangerous, since a user would be
exposed to the heating effects of the microwaves. To keep the microwave heating under control, microwave
ovens always carefully enclose the microwaves in a metal cooking chamber from which they can't escape.
I've heard the reason an antenna, such as the one on your car, is so long is because it needs to be large enough for the long
radio waves to pass into it. Is this true? Why are antennas for radio stations so tall and slender? -- LW, Blacksburg, VA
A vertical pole radio antenna receives a radio wave by allowing that wave to push electric charges up and down
the antenna. The radio senses this moving charge and is thus aware of the passing radio wave. The ideal length of
a vertical receiving antenna is a quarter of the wavelength of the radio wave it's trying to receive--in which case,
charge that the radio wave's electric field pushes up and down the antenna has just enough time to reach the end
of the antenna before it has to reverse directions.
The waves used for standard AM radio transmissions have very long wavelengths--typically 300 meters--so that
they require vertical pole antennas that are about 75 meters long for optimal reception. An antenna of that length
is also optimal for radio transmission, which is why the antennas of AM radio stations are so long and slender.
However, because such long antennas are inconvenient for most AM receivers, most AM receivers use small
magnetic antennas. A magnetic antenna is a device containing an iron-like material called ferrite that draws in
magnetic flux lines like a sponge. A coil of wire is wound around this ferrite so that as the magnetic flux lines of
a passing radio wave enter the ferrite, they induces electric currents into the coil of wire. This coil then acts as
the antenna.
But the waves used in FM radio transmission have much shorter wavelengths--typically 3 meters--so that
antennas of about 75 centimeters are all that's needed. The vertical pole radio antenna on your car is designed to
receive these FM waves. The antennas of FM radio stations are also rather short, but they are usually mounted
high up on a pole so that the whole structure looks like an AM radio antenna. However, if you look near the top
of an FM radio tower, you'll see the actual FM antenna as a much smaller structure.
If an 8-ohm speaker was connected in parallel to an 8-ohm resistor, would the new impedance be 4 ohms?
Yes. When you connect two 8-ohm devices in parallel, so that they share a current between them, they act as a
single 4-ohm device.
What kind of tape recorders are the best: cassette recorders or the ones with bigger spools? -- HB, Stde, Sweden
The audio quality of analog tape recording improves as the tape moves faster past the recording and playback
heads. That's because the faster tape motion spreads out the magnetized regions of tape over greater distances on
the tape's surface. A cassette tape moves so slowly that oppositely magnetized regions are often bunched tightly
together and they demagnetize one another. This demagnetization produces high-pitched noise in the recording.
In contrast, a reel-to-reel tape that moves rapidly past the heads has magnetized regions that are widely spaced on
the tape's surface and that are much less susceptible to demagnetization and noise.
What is the formula for finding the power in an AC circuit?
If an appliance receiving power from an AC power source behaves as an electric resistor--meaning that the
current passing through it is proportional to the voltage drop across it--then it's easy to calculate the power being
consumed by this appliance. You simply multiply the voltage drop across the appliance (measured in volts) by
the current passing through the appliance (measured in amperes) to obtain the power (measured in watts). The
voltage drop across the appliance indicates how much energy the appliance extracts from each unit of charge
pass through it and the current passing through the appliance is the measure of how many units of charge are
passing through the appliance each second. Thus the product of voltage drop times current gives the energy that
the appliance extracts from the current each second, which is the power extracted by the appliance. On the other
hand, if the appliance behaves like an inductor or capacitor--meaning that the current passing through it isn't
proportional to the voltage drop across it--it's much harder to calculate the power that the appliance is
consuming.
How does an electric welder work? -- JE
An electric welder sends an electric current through an ionized gas, forming a pattern of current flow through the
gas that is known as an arc. The ionized gases in this arc consist of electrons that are negatively charged and
atoms or molecules that have lost electrons to become positively charged. The electrons flow toward the
positively charged metal at one end of the arc while the positively charged ion flow toward the negatively
charged metal at the other end of the arc. As these charged particles move, they collide frequently with one
another and with gas atoms or molecules along their paths, and they convert some of their electric energies into
thermal energy. These collisions also produce additional ions. The enormous amounts of thermal energy
produced by collisions as the charged particles flow through the arc melts the metals at the ends of the arc so that
these metals can be fused together.
What is the most effective way to electronically measure the level of charge of a lead acid battery? -- RS
The voltage of any battery--the amount of energy it gives to each positive charge that it transfers from its
negative terminal to its positive terminal--increases slightly when the battery is fully charged. That's because
when the battery is fully charged and its chemicals are highly ordered, the laws of thermodynamics that
encourage the development of disorder act to increase the battery's disorder through effects that also increase the
battery's voltage. But as the battery discharges, these thermodynamic effects fade and the battery's voltage
diminishes slightly. So the easiest way to determine the battery's charging status electronically is to look at the
voltage rise across the battery when little or no current is flowing through it. The higher the voltage, the more
fully charged the battery is.
How fast can maglev trains travel? -- AP
As long as the track is straight enough that the train doesn't experience severe accelerations up, down, left, or
right, there is no limit to how fast it can go. In fact, the levitation process becomes more and more energy
efficient as the speed increases. However, the moving train does experience a pressure drag force (a type of air
resistance) that increases roughly as the square of the train's speed. The power needed to overcome this drag
force increases as the cube of the train's speed, making it impractical to propel the train forward above a certain
speed.
Why does a body at rest remain at rest and a body in motion remain in motion, in the absence of unbalanced force? -AW, Karachi, Pakistan
That observation, known as Newton's first law of motion, is one of the fundamental characteristics of the
universe. I could answer simply that that's the way the universe works. But a more specific answer is that the
universe exhibits translational symmetry--meaning that the laws of physics are the same from your current
vantage point as they would be if you shifted a meter to your left. Shifting your vantage point along some linear
path--a process called translation--doesn't affect the laws of physics. The laws of physics are said to be
symmetric with respect to translations and, because translations of any size are possible, this symmetry is
considered to be continuous in character (as opposed to mirror reflection, which is a discrete symmetry).
Whenever the laws of physics exhibit a continuous symmetry of this sort, there is a related conserved quantity.
The conserved quantity that accompanies translational symmetry is known as momentum. An isolated object's
momentum can't change because momentum is a conserved quantity--it can't be created or destroyed. Since
momentum is related to motion, an isolated object that's at rest and has no momentum must remain at rest with
no momentum. And an isolated object that's moving and has a certain momentum must remain in motion with
that same momentum.
Incidentally, the laws of physics also exhibit rotational symmetry--meaning that turning your head doesn't
change the laws of physics--and this symmetry leads to the existence of a conserved quantity known as angular
momentum. The laws of physics also don't change with the passage of time, a temporal symmetry that leads to
the existence of a conserved quantity known as energy.
March 21, 1997
Why does food become soggy after heating in the microwave oven, particularly pastry?
A normal oven heats foods by exposing them to hot air and thermal radiation. It cooks the foods from the outside
in. As a result, a normal oven tends to make the surfaces of food dry and crispy because it heats those outer
surfaces first and drives the water out of them. A microwave oven heats the food by heating the water in that
food. It cooks foods from the inside out. As a result, a microwave oven tends to drive water out of the middle of
the food and into the outer layers of that food. The outer layers are essentially "steamed" and steaming makes
everything soggy.
How did the sniperscopes used in World War II work? They seem to have used an invisible light to illuminate the target
and the sniper then looked through the scope and was able to see the target. -- ND, Florence, Montana
These sniperscopes used infrared light to illuminate their targets and then detected this infrared light with the
help of an infrared-sensitive photocathode. Producing infrared light is easy; any incandescent bulb produces
large amounts of it. The sniperscope simply filtered out the visible light from an incandescent bulb, leaving only
the invisible infrared light to illuminate the target.
Understanding the photocathode system requires an examination of the interactions of light and metal. Whenever
a particle of light--a photon--strikes a metal surface, there is the possibility that the photon will eject an electron
from that metal surface. However, each type of metal requires a certain minimum photon energy before it will
release an electron. Because infrared light photons carry very little energy, they can only eject electrons from
very special metals. The sniperscope contained a very thin layer of one such infrared-sensitive metal.
Actually, this metal layer was deposited on a transparent glass window that formed the front end of a vacuum
tube. Light from the scene in front of the sniper passed through a converging lens that formed a real image of the
scene on the metal layer. The metal layer was so thin that light striking its front surface through the glass window
caused electrons to emerge from its back surface. Electrons ejected from the back of the metal layer were
accelerated by a high voltage that was applied between this metal photocathode layer and a phosphor-coated
anode layer located very nearby. Each electron acquired so much energy during its brief flight that it caused the
phosphors on the anode to glow brightly when it hit them. The electron flight path was short so that electrons
emitted by a certain spot on the photocathode would hit a corresponding spot on the phosphor anode and the
sniper would see a clear image of the scene in front of the sniperscope.
Because one infrared photon striking the photocathode could lead to the release of dozens of photons from the
phosphors on the anode, this sniperscope provided a modest amount of "image intensification." But modern
starlight scopes go far beyond this level of amplification. Like the old sniperscope, these modern devices also use
a photocathode to turn a pattern of light from the real image of a lens into a pattern of free electrons. But the
starlight scope then amplifies these electrons by sending them through narrow channels that have highly charged
walls. As the electrons bounce their ways through the channels, they knock out hundreds, then thousands, then
even millions of other electrons so that each original photon can release more than a million electrons from the
amplifying system. When these electrons strike the phosphor-coated anode, the image they produce is bright and
visible, so that the person looking at the anode can effectively see when each photon of light strikes the
photocathode and initiates one of these electron cascades. With such incredible light sensitivity, there is no
longer any need to actively illuminate the target with infrared light--even starlight is enough illumination to
make the target visible through the starlight scope's image intensification system.
March 18, 1997
How does a steam whistle work? -- DR
As far as I know, a steam whistle is just a whistle that's blown by steam rather than air. The principle behind a
whistle is straightforward: the air inside the whistle is driven into intense vibration by the stream of gas blown
across a slot-shaped opening. This stream of gas is directed at the sharp edge on the far side of the opening and
might or might not actually enter the whistle. If air happens to be flowing out of the slot-shaped opening as the
stream flows across the slot, the outgoing air will deflect the stream outward and that stream won't enter the
whistle. But if air happens to be flowing into the slot as the stream crosses the slot, the stream will be deflected
into the whistle. This situation leads to an amplifying effect: if any air is flowing into the slot, the whole stream
of gas will flow into the slot. If any air is flowing out of the slot, the whole stream of gas will flow out of the slot.
Now air inside the whistle is never perfectly still--it's always sloshing back and forth at least a tiny bit, much like
water sloshes in a basin. As a result, there is always a little motion of air in or out of the slot. When the stream of
gas begins to blow across the slot, it amplifies any tiny motions of air inside the whistle so that they become
more and more vigorous. Soon the air inside the whistle is vibrating intensely and the resulting pressure
fluctuations radiate outward from the whistle as sound.
This same principle is active in many other musical devices, including pipe organs and flutes. In a steam whistle,
the stream of gas that drives this vibration is steam rather than air. Water is heated in a boiler until it forms
moderately high-pressure steam and then the steam is released through a valve to a large whistle, which sounds
loudly.
How is infrared light produced?
There are many ways of producing infrared light. First, any warm surface emits infrared light. For example, a
heat lamp or an electric space heater emits enormous amounts of it. That's because the thermal radiation of a
warm object lies mostly in the invisible infrared portion of the electromagnetic spectrum.
Second, many light-emitting electronic devices emit infrared light. For example, the light emitting diodes in a
television remote control unit emit infrared light. In this case, the infrared light is emitted by electrons that are
shifting from one group of quantum levels in a semiconductor to another group--from conduction levels to
valence levels. This emission isn't thermal radiation; it doesn't involve heat.
Lastly, some infrared light is produced by lasers. In this case, excited atoms or atomic-like systems amplify
passing infrared light to produce enormous numbers of identical light particles--identical photons. Infrared
industrial lasers are commonly used to machine everything from greeting cards to steel plates.
How does wing shape affect flight?
During flight, an airplane wing obtains an upward lift force by making the air flowing over its top surface travel
faster than air flowing under its bottom surface. When the air over its top speeds up, that air's pressure drops.
Since the pressure of the slower moving air under the wing is larger than the pressure of the faster moving air
over the wing, there is a net upward force on the wing due to this pressure imbalance and the wing is lifted
upward. A wing also experiences drag forces--or air resistance--that tend to slow the plane down. But as long as
an airplane wing doesn't cause the airstreams flowing around it to separate from its surface, it will experience
relatively little pressure drag force; the most important drag force for a large, fast-moving object.
The details of the airplane wing's surfaces have relatively subtle affects on the wing's performance. While most
wings are asymmetric, with broadly curved top surfaces and relatively flat bottom surfaces, that isn't essential.
It's quite possible to use wings that are symmetric, with the same curvature on their tops as on their bottoms. But
a symmetric wing won't obtain an upward lift force unless it's tilted upward, while an asymmetric wing can
obtain lift even when it's horizontal. A broader, more highly curved wing can also obtain more lift at a lower
speed, as required for slow moving propeller planes. So wing shapes are often dictated by the desired flight angle
and speed of a particular airplane and its wings.
How does a toilet work? -- JJ, Stafford VA
A toilet is actually a very clever device that makes use of a siphon to extract the water from its bowl. A siphon is
an inverted U-shaped pipe that can transfers water from a higher reservoir to a lower reservoir by lifting that
water upward from the higher reservoir and then lowering it into the lower reservoir. In fact, the water is simply
seeking its level, just as it would if you connected the two reservoirs with a pipe at their bottoms. In that case, the
water in the higher reservoir would flow out of it and into the lower reservoir, propelled by the higher water
pressure at the bottom of the higher reservoir. In the case of a siphon, it's still the higher water pressure in the
higher reservoir that causes the water to flow toward the lower reservoir, but in the siphon the water must
temporarily flow above the water levels in either reservoir on its way to the lower reservoir. The water is able to
rise upward a short distance with the help of air pressure, which provides the temporary push needed to lift the
water up and over to the lower reservoir. At the top of the siphon, there is a partial vacuum--a region of space
with a pressure that's less than atmospheric pressure. The same kind of partial vacuum exists in a drinking straw
when you suck on it and is what allows atmospheric pressure to push the beverage up toward your mouth.
In the toilet, the bowl is the higher reservoir and the sewer is the lower reservoir. The pipe that connects the bowl
to the sewer rises once it leaves your view and then descends toward the sewer. Normally, that rising portion of
the pipe isn't filled water--water only fills enough of the pipe to prevent sewer gases from flowing out into the
room. As a result of this incomplete filling, the siphon doesn't transfer any water. But when you flush the toilet, a
deluge of water from a storage tank rapidly fills the bowl and floods the siphon tube. The siphon then begins to
function. It transfers water from the higher reservoir (the toilet bowl) to the lower reservoir (the sewer) and it
doesn't stop until the bowl is basically empty. At that point, the siphon stops working because air enters the Ushaped tube with a familiar sound and water again accumulates in the bowl. When the storage tank has refilled
with water, the toilet is ready for action again.
How can you make a hologram? -- JM, Kettering, OH
The classic technique for making a hologram begins with splitting the light from a laser into two parts. Part of
the laser light is used to illuminate a scene while the other part is used to illuminate a piece of film placed in
front of the scene. Actually, the film is exposed to light from two sources: (1) the second part of the laser beam
and (2) a portion of the first part of the laser beam that the objects reflect toward the film. Lights from these two
sources don't simply add when they reach the film; they interfere with one another. Laser light is unusual in that
it is coherent light--a giant wave consisting of numerous identical particles of light. When the wave from the
laser and the wave reflected from the objects meet at the film, they interfere. When the crest of one wave joins
the crest of the other wave, the two waves form an extra large crest--constructive interference. But when the crest
of one wave joins the trough of the other wave, the two waves cancel and produce essentially nothing-destructive interference. Because of this interference, the film ends up recording not only the intensity
information that we associate with normal photography; it also records phase information that is an important
aspect of waves. This phase information indicates where crests and troughs in the wave occurred. Because the
hologram contains both kinds of information, it allows a viewer to see things that they would not see in a simple
photograph.
To make a hologram, you should take a laser and split its light into two unequal portions with the help of a laser
beam-splitter (or even a glass slide). The laser should operate at only a single wavelength, so that its light is
highly coherent, and it should have a coherence length much longer than any distance in the scene--two
requirements that are met by most common continuous-wave lasers, including laser pointers and basic heliumneon lasers. Send the stronger portion of the laser beam through a diverging lens and allow it to illuminate a
scene that is otherwise in complete darkness. Light reflected from this scene should reach the film holder in
which the hologram will be made. Send the weaker portion of the laser beam through another diverging lens and
allow it to illuminate the film holder from the scene side. For best results, the light reflected from the scene on
the film holder should be about as bright as light from this second beam.
Now place fine-grained black and white film in the film holder. Be sure that the film is sensitive to the laser
light--some black and white films aren't sensitive to red light. Allow light to strike the film for long enough to
expose it. Finally, develop the film and observe the developed film while it's illuminated from behind with laser
light that has been spread out by a diverging lens. You should see the original scene as a three-dimensional
image.
Unfortunately, there is one detail I've omitted until now. To make sure that the phase information is properly
recorded, you must be sure that nothing moves by even a fraction of a wavelength of laser light during the entire
exposure period. That's a very demanding requirement. Vibrations are everywhere and they will spoil the
hologram. If you want this technique to work, you'll have to isolate everything--the laser, the optics, the scene,
and the film--from vibrations. In a laboratory, this vibration isolation is done by floating a massive optics table
on a cushion of air. All of the objects involved in making the hologram are rigidly attached to this table so that
they can't move. As an alternative, you can put all the objects for the hologram on as rigid and massive a surface
as you can find and support that surface on a thick layer of foam rubber. Make the holograms at night when there
is little traffic of any sort around and be sure that nothing is jiggling about nearby that might shake the floor even
a little bit. If you're careful, you ought to be able to create a hologram with such an arrangement.
What is a VU meter on tape deck? How does it differ from a dB meter? I know that the best recording is achieved when
the needle hovers around the zero and that the sound distorts above zero and is barely audible the lower into the negative
numbers you go, but what are the meanings of the plus and minus readings? -- GF, California
VU and dB meters both measure the audio power involved in recording and they both use logarithmic scales to
report that power. Because of these logarithmic scales, a factor of 10 increase in power produces an increase of
10 in both the VU reading and the dB reading. For example, -20 dB is 10 times the power of -30 dB. In both
measures, the zero is chosen as the highest acceptable power--the highest power for which distortion is
acceptable.
Where VU and dB differ is in how they measure audio power. VU is short for "volume units" and it is a measure
of average audio power. A VU meter responds relatively slowly and considers the sound volume over a period of
time. Its zero is set to the level at which there is 1% total harmonic distortion in the recorded signal. dB is short
for "decibels" and it is a measure of instantaneous audio power. A dB meter responds very rapidly and considers
the audio power at each instant. Its zero is set to the level at which there is 3% total harmonic distortion. Because
of these differences in zero definitions, the dB meter's zero is roughly at the VU meter's +8. Nonetheless, both
meters are important and both should be kept at or below zero to avoid significant distortion in a recording. In
certain situations, such as when there are sudden loud sounds or with instruments that are very rich in harmonics,
it's possible to have the dB meter read above zero even though the VU meter remains below zero.
Please explain ideal mechanical advantage and actual mechanical advantage. How can I demonstrate these two ideas? -- S
Mechanical advantage is any process that allows you to exchange force for distance (or torque for angle) while
performing a particular task. The amount of mechanical work you must do (i.e., the amount of energy you must
supply) to perform that task won't change, but the relationship of force and distance (or torque and angle) will.
For example, you can increase the altitude of a wooden block by 1 meter either by lifting it straight upward 1
meter or by pushing it several meters uphill along a ramp. In the first case, you'll have to exert a large upward
force on the block but you won't have to move it very far to complete the task. In the second case, you'll have to
exert a much smaller uphill force on the block but you'll have to move it a long way along the ramp. If you
multiply the force you exert on the block times the distance that block travels while rising 1 meter, you'll find
that it's exactly the same in either case. You've simply calculated the work required to raise the block 1 meter and
that work won't change, regardless of how you perform the task! That's the crucial issue with mechanical
advantage--it doesn't let you avoid doing the work, it just lets you do that work with a small (or larger) force
exerted over a longer (or shorter) distance. In a situation involving rotation, mechanical advantage lets you do the
same work with a smaller (or larger) torque exerted over a larger (or smaller) angle. In all of these cases, you're
doing the same amount of work but you're making it more palatable by adjusting the balance between force and
distance or between torque and angle.
As for actual mechanical advantage, it's simply a recognition that any mechanical system involves imperfections.
The work that you do with the help of a machine doesn't all go toward your goal. Instead, you end up doing some
work against sliding friction or air resistance and that work is lost to thermal energy. For example, when you
slide a block up a ramp, friction with the ramp wastes some of your energy. If you multiply the uphill force you
exert on the block while pushing it up the hill times the distance it travels along the ramp, you'll find that you
must do somewhat more work while raising the block 1 meter than you would have done by simply lifting the
block directly upward that 1 meter. So ideal mechanical advantage assumes no change in the work you do while
actual mechanical advantage recognizes that you're going to end up doing extra work whenever you employ a
machine to obtain mechanical advantage.
What makes a three-way touch lamp work? What makes a three-way light bulb work? - CY
A three-way touch lamp is much like a simple touch lamp--it detects your touch by applying a high frequency
alternating charge to the lamp's surfaces and uses this fluctuating charge to measure the lamp's electric
capacitance--the ease with which charge can moved on or off the lamp's surfaces. When you touch the lamp, the
lamp's capacitance changes and the lamp's electronics detect this change.
In a three-way touch lamp, the lamp's electronics control 4 different light levels alternately: dim, medium, bright,
and off. How these light levels are obtained depends on the lamp. If the lamp uses a three-way light bulb, which
contains two separate filaments, then it can obtain the 3 brightness levels by turning on one or both of the
filaments. It uses just the small filament for dim, just the large filament for medium, and both filaments for
bright. That's exactly what a normal three-way lamp does.
But if the lamp uses a normal bulb and obtains three light levels from it, then it uses the same technique as a
dimmer switch. In this technique, an electronic switching device called a triac is used to limit the times during
which electric current can flow through the bulb and deliver power to it. In the bright setting, the triac permits
current to flow through the bulb at all times and the bulb appears as bright as possible. But in the dim or medium
settings, the triac prevents current from flowing at certain times. The triac takes advantage of the fact that the
power flowing through a household lamp is alternating current--current that reverses directions 120 times a
second (in the United States) for a total of 60 full cycles of reversal, over and back, each second (60 Hz). At the
beginning of each current reversal, the electronic devices that control the triac start a timer. This timer allows
those devices to wait a certain amount of time before they trigger the triac and allow it to begin carrying current
to the light bulb. Once triggered, the triac will allow current to flow through the bulb until the next reversal of
current in the power line. Thus the amount of energy that reaches the bulb during each half-cycle of the power
line depends on how long the electronic devices wait before triggering the triac. The longer they wait, the less
energy will reach the bulb and the dimmer it will glow. In the bright setting, the triac is triggered immediately
after each current reversal so that power always flows to the bulb and it glows brightly. But in the medium and
dim settings, the triac is triggered well into the half-cycle that follows the reversal. A normal dimmer gives you
complete control over this delay, but a three-way touch switch only provides three preset delays. The medium
setting has a medium delay while the dim setting has a long delay.
How does a "touch lamp" work? -- LAM, Enosburg Falls, VT
A touch lamp detects your touch by looking for changes in the electric properties of the lamp's surfaces. It
monitors these properties by putting a fluctuating electric charge on them. As electric current flows toward the
bulb through the lamp's wires, it passes through an electronic device that places a high frequency (about 60 kHz)
alternating current onto those wires. This added current causes the lamp's surfaces to take on a small fluctuating
electric charge--first positive, then negative, then positive, over and over again. This surface charging involves
electrostatic forces, which extend long distances between charged objects, and occurs even though the lamp's
surfaces aren't directly connected to the lamp's wires. The more surface the lamp has, the more easily it can hold
that electric charge--the greater the lamp's electric capacitance.
When you plug the lamp in, the electronic device uses its fluctuating charge to determine how easy it is to add or
subtract charge from the lamp's surfaces. In other words, it measures the lamp's capacitance. It then begins to
look for changes in that capacitance. When you touch the lamp, or even come close to its surfaces, your body
effectively adds to the lamp's surface and its capacitance increases significantly. The electronic device detects
this increase in capacitance and switches the lamp's state from on to off or from off to on. The fact that you don't
have to touch the lamp to affect its capacitance means that a touch lamp can have insulating paint on its metal
surfaces yet still detect your touch. You can also buy touch lamp modules that plug into the wall and turn the
lamp that's connected to them into a touch lamp. These modules are so sensitive to capacitance changes in the
lamp that you can trigger them just by touching the lamp cord.
March 17, 1997
Why can you put a can of frozen concentrate juice in the microwave? The metal doesn't spark or burn.
The microwaves in a microwave oven consist of electric and magnetic fields. Since electric fields push on
electric charges, microwaves cause electric currents to flow through any metal objects they encounter. These
movements of current don't necessarily cause any problems in a microwave oven. In fact, metal objects only
cause trouble in the microwave oven when they are so thin or narrow that they can't tolerate the electric currents
that flow through them or when they have such sharp ends that electric charges leap off them as sparks. A thin
object like a twist-tie can't tolerate the currents and becomes very hot. Its sharp ends also allow charges to leap
out into the air as sparks. But the thick, rounded end of a juice concentrate can easily tolerates the currents sent
through it by the microwaves and doesn't have the sharp ends needed to send charges into the air as sparks. It
doesn't present any problem for the microwave oven.
If you stand near a microwave oven, looking at your food, is it dangerous--tissue damage or make you blind?
Properly built and undamaged microwave ovens leak so few microwaves that they aren't dangerous at all. Even if
they did leak enough to be in violation of the safety limits, those safety limits are very conservative. While there
is no reason to court disaster by holding your face right up to the microwave for hours and hours, it shouldn't hurt
you at all.
March 13, 1997
How do sound waves travel in space? -- PS
When sound travels in air, it takes the form of compressions and rarefactions of that air. Similar compressions
and rarefactions occur when sound travels in a liquid or in a solid. But sound can't travel through space because
space is entirely empty. Sound requires a medium in which to travel and space doesn't contain any such medium.
Astronauts talk to each other by radio during space walks. With nothing at all between them, they simply can't
hear one another directly.
How does a snow making machine work? -- IB, Blue Ash, OH
A snow-making machine simply sprays a fine mist of water high into the cold air overhead, so that that mist can
freeze into tiny particles of ice before falling back to the ground. If the air is cold enough, the mist will solidify
before it hits the ground and before it has time to evaporate into water vapor. This freezing process isn't as simple
as it sounds because water can't turn into an ice crystal without a seed on which that crystal can grow. Forming a
seed crystal is a random process in which a couple of water molecules accidentally arrange themselves in a
crystalline lattice. In snow making, each water droplet has only a few seconds in which to freeze and it can easily
take that long for a seed crystal to form. However, people have found that adding certain chemicals or other
materials to the water before spraying it into the air can speed the formation of seed crystals and dramatically
increase the fraction of water that becomes artificial snow.
What is convection? -- DB, Corona, CA
Convection is the transfer of heat by a circulating fluid, such as air or water. This heat is carried from a hotter
object to a colder object. The fluid first passes near the hotter object and receives heat. The fluid becomes
warmer and more buoyant, and it's lifted upward by the colder fluid around it--just as a hot air balloon is lifted
upward by the colder air around it. The rising fluid carries the heat with it. Eventually the rising fluid spreads
outward and it pass near colder objects, giving up its heat. The fluid becomes cooler and less buoyant, and soon
it begins to descend back toward the ground. Eventually it's drawn back past the hotter object and this cycle
begins again.
What is a vortex? -- M
A vortex is a region of fluid that's circulating in one direction around a line passing through that region. If you
imagine yourself looking along that line, you would see the fluid flowing either clockwise or counter-clockwise
around the line itself. Tornadoes and whirlpools are both vortices since they involve fluids circulating in one
direction around a central line.
What is an H-Bomb made of?
A hydrogen bomb or thermonuclear bomb is a nuclear weapon that obtains most of its energy from the fusion of
hydrogen nuclei into helium nuclei. This fusion typically involves deuterium and tritium nuclei, the heavy
isotopes of hydrogen. Deuterium is a stable, naturally occurring isotope with one proton and one neutron in its
nucleus, and can be extracted from normal water. Tritium is an artificial, radioactive isotope with one proton and
two neutrons in its nucleus, and can be formed in nuclear reactors or, during a nuclear explosion, by the exposure
of lithium nuclei to the neutrons formed in that explosion.
Since hydrogen nuclei are positively charged, they repel one another. To get these heavy hydrogen nuclei close
enough together to fuse into helium nuclei, the hydrogen nuclei must be heated to fantastic temperatures. This
heating is done with a fission bomb--a uranium or plutonium bomb. When the fission bomb explodes, its heat is
enough to trigger the hydrogen bomb.
How does a strobe light work? -- JM, Kettering, OH
A strobe light passes a brief, intense pulse of electric current through a gas, which then emits a brilliant burst of
light. The gas is usually one of two inert gases, xenon or krypton, that emit relatively white light when they're
struck by the fast moving electrons in the electric current. When it hits a xenon or krypton atom, an electron may
give up some of its kinetic energy--its energy of motion--to the electrons in the atom. Those atomic electrons
shift from their usual orbitals (quantum mechanically allowed orbits) to higher-energy orbitals that they usually
don't travel in. The atomic electrons remain only briefly in these higher-energy orbitals before dropping back to
their original orbitals. As they drop back down, these electrons give up their extra energy as light. Because
krypton and xenon atoms have a great many electrons and their electronic structures are very complicated, they
emit light over a broad range of wavelengths. Moreover, the gases are at relatively high pressures and collisions
between the atoms while they are emitting light further smooth out the spectrum of light they produce. Thus the
strobe emits a rich, white light during the moments while current is passing through the gas.
Supplying the enormous current needed to maintain the brief arc in the strobe's gas is done with the help of a
capacitor, a device that stores separated electric charge. A high voltage power supply pumps positive charge
from the capacitor's negative plate to its positive plate, until there is a huge charge imbalance between those two
plates. You can often hear a whistling sound as this power supply does its work. The capacitor plates are
connected to one another through the gas-filled flashlamp that will eventually produce the light. However,
current can't pass through the gas in the flashlamp until some electric charges are injected into the gas. These
initial charges are usually produced by a high voltage pulse applied to a wire that wraps around the middle of the
flashlamp. When a few charges are inserted into the gas, they accelerate rapidly toward the positive or negative
wires that extend from the charged capacitor. As these charges pick up speed, they begin to collide with the gas
atoms and they deposit energy in those atoms. Electrons are occasionally knocked out of atoms or out of the
wires at the end of the flashlamp and these new charges that enter the gas also begin to accelerate toward the
wires. A cascade of collisions quickly leads to a violent arc of charged particles flowing through the flashlamp
and colliding with the gas atoms. The flashlamp emits its brilliant burst of light that terminates only when the
capacitor's separated electric charges and stored energy are exhausted.
How does a radio receive transmissions from one station and not another, and how does it turn them into audible waves? - T, Chester, VT
A radio wave contains an electric field that pushes on any electric charge it encounters. That's why, when a radio
wave passes the antenna of your radio, it causes electric charges in that antenna to accelerate up and down. There
is also a resonant circuit connected to the antenna--a circuit that oscillates strongly only when charge is pushed
up and down the antenna at exactly the circuit's resonant frequency. If the circuit's resonant frequency is the same
as that of the radio wave, the small pushes exerted on charges in the antenna add up so that charge moves more
and more vigorously through the resonant circuit. But if your radio isn't tuned to the frequency of the radio wave,
the overall motion of charge on the antenna and this resonant circuit is small. That's why your radio only
responds to the radio transmission of one station and not others. To understand this effect, imagine pushing a
child on a swing. If you push rhythmically at just the right frequency, the child will swing higher and higher. But
if you push rhythmically at the wrong frequency, the child will just jitter about a bit.
Once charge is moving strongly through the resonant circuit in your radio, the radio can monitor various features
of that moving charge. If the station is using the AM or amplitude modulation technique to represent sound, your
radio studies the amount of charge moving back and forth through the resonant circuit. When that flow of
charge--that current--is strong, it moves the speaker cone toward you and produces a compression of the air.
When that current is weak, it moves the speaker cone away from you and produces a rarefaction of the air. These
changes in air density and pressure reproduce the sound that the station is transmitting.
If the station is using the FM or frequency modulation technique to represent sound, your radio studies the
frequency at which charge moves back and forth in the resonant circuit. Very small changes in this frequency,
caused by frequency changes in the radio wave itself, are used to control the speaker cone in your radio. When
the frequency is raised slightly above normal, your radio moves the speaker cone toward you and produces a
compression of the air. When the frequency is lowered slightly below normal, your radio moves the speaker cone
away from you and produces a rarefaction of the air. Again, these changes in air density and pressure produce
sound.
What are some unusual conductors of electricity?
How about graphite and cadmium sulfide? Graphite, such as that in the lead of a pencil, conducts electricity even
though it's not formally a metal. If you draw a dark line on a sheet of paper, that line can act as a wire for
sensitive electric circuits. Cadmium sulfide is a photoconductor--a material that is electrically insulating in the
dark but that conducts electricity when exposed to light. Photoconductors of this sort are used in some light
sensors, as well as in xerographic copiers and laser printers.
Is it possible to charge batteries using static electricity? Can lightning or atmospheric charges be stored in a capacitor and
then released into a cell for charging? -- JM, Lafayette, NT
Yes, static electricity has energy associated with it and that energy can be used to charge batteries, at least in
principle. Static electricity is literally stationary separated electric charges--essentially separated charges stored
on capacitor-like surfaces. As you suggest, it may be easiest to transfer these separated charges into a real
capacitor and then to use this charged capacitor to recharge an electrochemical cell. Whether such a procedure
can be carried out efficiently and in a cost-effective manner isn't clear to me. The charges involved in lightning
have so much energy per charge--so much voltage--that they're hard to use for anything. Even the charges that
you accumulate when you rub your feet on a wool carpet on a cold, dry winter day acquire an enormous amount
of energy per charge. To charge most batteries, you need lots of low energy charges, not the small numbers of
high-energy charges that are typical of static electricity. Using this tiny current of high-energy charges to charge
a battery is equivalent to trying to fill a swimming pool with water from a high-pressure car-washing nozzle--too
little water under too much pressure. You can do it, but there are better ways.
What is a magnet?
A magnet is an object that has magnetic poles and therefore exerts forces or torques (twists) on other magnets.
There are two types of these magnetic poles--called, for historical reasons, north and south. Like poles repel
(north repels north and south repels south) while opposite poles attract (north attracts south). Since isolated north
and south magnetic poles have never been found in nature, magnets always have equal amounts of north and
south magnetic poles, making them magnetically neutral overall. In a permanent magnet, the magnetism
originates in the electrons from which the magnet is formed. Electrons are intrinsically magnetic, each with its
own north and south magnetic poles, and they give the permanent magnet its overall north and south poles.
March 8, 1997
What is pH and why is it so important to my garden pond and spa? -- NW, California
pH is a measure of the concentration of dissolved hydrogen ions in water. When a hydrogen atom loses an
electron and becomes a hydrogen ion--a proton--it can dissolve nicely in water. Actually, this proton sticks itself
to the oxygen atom of a water molecule, producing a hydronium ion (H3O+) that is then carried around by shells
of water molecules. The higher the concentration of hydrogen (or hydronium) ions in water, the lower the water's
pH. More specifically, pH is negative the log (base 10) of the molar hydrogen ion concentration. That means that
water with a pH of 6 has ten times as many hydrogen ions per liter as water with a pH of 7.
Pure water naturally contains some hydrogen ions, formed by water molecules that have spontaneously
dissociated into hydrogen ions (H+) and hydroxide ions (OH-). Pure water has enough of these hydrogen ions in
it to give it a pH of 7. But if you dissolve acidic materials in the water, materials that tend to produce hydrogen
ions, the pH of the water will drop. If you dissolve basic materials in the water, materials that tend to bind with
hydrogen ions and reduce their concentration, the pH of the water will rise. Water with too many or too few
hydrogen ions tends to be chemically aggressive and we do best in water that has a pH near 7.
Our problem concerns temperature. At different temperatures, solubility of compounds varies. If we extract water from a
pond at two degrees Celsius and then test it at room temperature, our reading isn't going to be accurate. On the other
hand, it isn't practical for us to perform out tests outside. The substances we are testing are nitrites, nitrates, ammonia, pH,
hardness, oxygen level, phosphates, temperature, and ORP. -- J&E, Missouri
If you collect pond water at 2° C and then bring it into a room at 20° C, there will be a few subtle changes in the
water's contents. While the amounts of various dissolved materials can't change unless atoms move in or out of
the water, how they interact with one does change somewhat with temperature. I would be very surprised if
anything that's dissolved in that pond water comes out of solution when you warm it to room temperature, so if
all you want to do is to determine the concentrations of various dissolved materials, go ahead and do it at room
temperature. You might have to be careful with dissolved gases, because it's relatively easy for gas molecules to
enter or leave the pond water without your noticing that it's happening, but the nitrites, nitrates, hardness, and
phosphates aren't going anywhere. Ammonia can leave as a gas, so you should be a little careful with it. I don't
know enough about ORP (oxidization reduction potential) to say anything about it. But you'll have to be very
careful with oxygen concentration because you can modify this just by pouring the water through air and making
bubbles.
However, to be sure that the contents of the pond water are interacting with one another just as they were in the
pond, you should cool the water back down to 2° C before making any measurements. This is particularly
important for pH measurements, since water's pH decreases slightly with increasing temperature.
How does fog form? -- KB
The interface between a droplet of water and the air around it is a busy place. Water molecules are constantly
leaving the droplet to become water vapor in the air and water molecules in the air are constantly returning to the
droplet as liquid water. What determines whether the droplet grows or shrinks is the difference between these
two rates. If more water molecules return to the droplet than leave, the droplet will grow. If more water
molecules leave the droplet than return, the droplet will shrink. How often water molecules leave the droplet
depends on the droplet's temperature. How often water molecules return to the droplet depends on the moisture
content of the air.
This dynamic balance of growth and shrinkage occurs right in the middle of the air all the time. Tiny water
droplets form by accident, even in reasonably dry air, but in most cases they quickly shrink back to nothing
because the leaving rate is higher than the returning rate. However, when air that contains lots of moisture
experiences a decrease in temperature, the returning rate can exceed the leaving rate. When that happens, the tiny
droplets that appear by accident don't immediately disappear. Instead, they grow larger and larger. Depending on
the altitude, we call the white mist that results clouds or fog.
How does dry ice work to freeze things? -- JH
Solid carbon dioxide or "dry ice" sublimes into gaseous carbon dioxide at a temperature well below 0° C. Since it
takes energy to separate the molecules of carbon dioxide from one another, the dry ice absorbs heat as it
sublimes and takes that heat out of any warmer objects nearby. Those nearby objects become colder and colder
as the heat leaves them and eventually they begin to freeze.
How do you make an energy converter to convert water into energy? -- SB
I'm afraid that there is no simple way to convert water into energy. People have been trying to use fusion to
extract the nuclear energy stored in the hydrogen nuclei in water. But while billions of dollars have been spent on
research, there is no viable scheme for this process for controlled fusion in sight. The stars are powered by
hydrogen fusion, but people on the earth aren't likely to be using it as a source for peaceful energy any time soon.
How does a halogen lamp get so hot?
Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot
tungsten wire. In fact, the wire in a halogen lamp is allowed to get even hotter than the one in a normal bulb. But
while the glass envelope of a normal bulb gets only moderately hot during use, the glass envelope of a halogen
bulb gets extremely hot. That's because the halogen bulb is using a chemical trick to keep tungsten atoms from
getting away from the filament. Each time one of those tungsten atoms tries to leave, it's picked up by halogen
molecules inside the glass envelope and returned to the filament. These halogen molecules can even pick the
tungsten atoms up off the glass envelope and return them to the filament, but only if the glass envelope is
allowed to get extremely hot. That's why the glass envelope of the halogen bulb is allowed to run so hot--if it
weren't, it would accumulate the tungsten atoms permanently and it would darken. And since the tungsten atoms
wouldn't be returned the filament, the filament wouldn't last as long.
Is it possible to construct "home-made" thermal windows (double pan) so condensation can be avoided? I work in stained
glass and want to make an energy efficient window. -- JAA, York, PA
Yes, you should be able to make your own thermal windows. The value of having two vertical panes of glass that
are separated by a narrow gap is that heat has trouble flowing across gap. While air is a poor conductor of heat, it
carries heat reasonably well via convection. But with only a narrow gap of air between two vertical glass panes,
convection doesn't work well. Air heated by its contact with the warmer pane tends to flow directly upward,
rather than toward the cooler pane. Similarly, air cooled by its contact with the cooler pane tends to flow directly
downward, rather than toward the warmer pane.
But as you've anticipated, you may have trouble with condensation on the inside surface of the cooler pan. Your
best bet at avoiding this problem is to completely seal the space between the two panes and to fill it with very dry
air or even bottled nitrogen gas--which can be obtained cheaply from a local gas supply company. You'd have to
blow the dry air or nitrogen in through one hole and allow the trapped air to flow out through another hole. After
the trapped air has been replaced several times with dry gas and you're sure there is little moisture left between
the panes, you can stop replacing the air and seal both holes. But with stained glass, you have many potential
gaps through which moisture can enter the trapped air, so achieving a seal could be very difficult. In that case,
you might just put a desiccant at one edge of the window. Drierite is an inexpensive material that resembles little
white pebbles and that can absorb quite a bit of moisture. If you put some Drierite between the two panes before
you did your best to seal the space between them, I would expect the Drierite to remove enough moisture from
the trapped air to avoid condensation problems. After a few years, enough moisture may have leaked in through
cracks to cause trouble, in which case you would simply replace the Drierite. One useful type of Drierite is blue
when fresh and turns pink when it has absorbed its fill of moisture.
Can an object be heated no hotter than the temperature of the flame beneath it? For example, if the temperature of a
candle flame is 1770° C and the melting point of the solid being heated above it is 1800° C, would the solid ever melt if
the flame were held under it long enough? -- MR, Ohio
The answer is a qualified no. Heat always flows from hotter objects to colder objects, so the solid can't get any
hotter than the flame that's heating it. But this observation is stems from the laws of thermodynamics,
particularly the second law of thermodynamics. Unlike Newton's laws of motion, which are rigid, inviolable laws
that are never, even violated in our universe, the second law of thermodynamics is a statistical laws--it says that
certain events are extremely unlikely but doesn't say that they are truly impossible. The flow of heat from hotter
to colder is a statistical law, not a rigid mechanical law. So it is possible, although extraordinarily unlikely, that
heat can flow from the 1770° C flame to the 1799° C solid and warm that solid all the way to 1800° C. However,
for any reasonable sized solid (say, more than 10 atoms), the possibility of this occurring is going to be so
unbelievably small as to be ridiculous. It's as unlikely as taken a crystal wineglass that has been crushed into dust
and then dropping it on the floor and having the impact reassemble the wineglass into its original pristine form.
The laws of motion don't forbid such as fantastic result, but it sure would be unlikely. I've tried it several times
myself, without success. But then, you're not going to be able to melt your solid with a not-hot-enough flame,
either. You'd have to wait a few ages of the universe just to have that solid climb a tiny fraction of a degree
above the temperature of the flame. For 20 degrees... forget it.
Can a rocket, starting back toward the earth from 30,000 feet, reach the speed of sound before reaching the earth? -- WJT,
Crystal, MN
Some rockets probably reach the speed of sound in a few hundred feet heading upward, so that reaching the
speed of sound in 30,000 feet heading downward would be a simple task. In fact, if you dropped a highly
aerodynamic object such as a rocket from 30,000 feet, it could reach the speed of sound even without any
propulsion! Gravity alone will accelerate it to about 130% of the speed of sound.
Do airplanes travel faster from east to west or west to east in the United States? -- SU, Lawrence, KS
Airplanes travel faster from west to east in the United States. That's because the prevailing winds at out latitudes
are eastward and they blow the airplane toward the east. When the airplane flies toward the east, it has a tail wind
and travels faster with respect to the ground. When the airplane flies toward the west, it has a headwind and
travels slower with respect to the ground.
Our area has been flooded recently (Kentucky, Indiana) by about 15 inches of rain. How is it that the Ohio River has risen
so many feet and not just 15 inches? -- RK
The Ohio River is carrying water collected by vast areas surrounding the river and this accumulated volume of
water is enough to raise the river's level by many feet. Similarly, if you collected all the rain water that
accumulated on your yard and poured that water into a bathtub, the level of water in the bathtub would rise far
more than 15 inches.
My husband and I watch Star Trek often. He says that travel at warp speeds (faster than the speed of light) is impossible
and that Einstein's theories prove it. Is this true? -- JL, Las Cruces, NM
I'm afraid that travel at or above light speed is simply impossible and that "warp speed" travel is just a
Hollywood fantasy. Einstein's special relativity forbids objects with mass from reaching or exceeding the speed
of light and even if there were some way to travel vast distances in less time than it would take light to cover
those distances, but without actually traveling at light speed, such travel would violate some important principles
of causality--you would be able to meet your own grandparents as children and that sort of thing.
One of the reasons that Hollywood ignores real physics so often is that real physics is almost wilder than fiction.
Suppose that you decided to travel to a star 5 light-years away from the earth and that you have a starship that
can almost reach the speed of light (another nearly impossible thing, but let's ignore that problem). If you travel
to the star at almost the speed of light, make one loop around it, and head right back to earth, I will have aged 10
years while waiting for you to return. However, you will only have aged days or weeks, depending on just how
close you came to the speed of light. During the trip, we will have disagreed on many physical quantities,
particularly the times at which various events occurred and the distances between objects. The mixing of time
and space that occur when two people move rapidly relative to one another would be so disorienting to movie or
television viewers that Hollywood ignores or simplifies these effects.
How fast does the earth wobble and why does it wobble? -- MF, Tokyo, Japan
The earth's rotational axis wobbles around in a circle once every 25,800 years because of torques (twists) exerted
on it by the moon's gravity. The moon's gravity is able to twist the earth slightly because the earth isn't quite
spherical. The earth's rotation causes it to bulge outward a little around its equator and it is this bulging that
allows the moon to exert a torque on the earth.
How far away is the moon?
It's about 235,000 miles (375,000 kilometers) away from the earth's surface. However, it's drifting about 1.3
inches (3.5 centimeters) farther away every year. That's because tides on the rotating earth gently pull the moon
forward in its orbit as they slowly extract energy from the earth's rotation. Because of this transfer of energy
from the earth's rotation to the moon's orbit, the moon is gradually slipping farther away from the earth.
How do conductors and insulators work? -- SN, Beverly, MA
Because of the quantum physic that dominates the behaviors of tiny objects in our universe, electrons can't travel
in every path you can imagine; they can only travel in one of the paths that are allowed by quantum physics-paths that are called orbitals in atoms and levels in solids. When a material is assembled out of its constituent
atoms, those atoms bring with them both their electrons and their quantum orbitals. These orbitals merge and
blend as the atoms touch and they shift to form bands of levels in the resulting solid. The electrons in this solid
end up traveling in the levels with the lowest energies. Because of the Pauli exclusion principle, only one
indistinguishable electron can travel in each level. Since there are effectively two types of electrons, spin-up and
spin-down, only two electrons can travel in each level of the solid.
In a conductor, there are many unused levels available within easy reach of the electrons. If the electrons have to
begin moving toward the left, in order to carry an electric current, some of the electrons that are in right-heading
levels can shift into empty left-heading levels in order to let that current flow. But in an insulator, all of the easily
accessible levels are filled and the electrons can't shift to other levels in order to carry current in a particular
direction. While there are empty levels around, an electron would need a large increase in its energy to begin
traveling in one of these empty levels. As a result, the electrons in an insulator can't carry an electric current.
What role do gravity and inertia play in making a roller coaster work? -- B
Gravity provides the energy source for a roller coaster and inertia is what keeps the roller coaster moving when
the track is level or uphill. Once the roller coaster is at the top of the first hill and detaches from the lifting chain,
the only energy it has is gravitational potential energy (and a little kinetic energy--the energy of motion). But
once it begins to roll down the hill, its gravitational potential energy diminishes and its kinetic energy increases.
Since kinetic energy is related to speed, they both increase together.
At the bottom of the first hill, the roller coaster has very little gravitational potential energy left, but it does have
lots of kinetic energy. The roller coaster also keeps moving, despite the absence of gravitational potential energy.
You can view its continued forward motion as either the result of having lots of kinetic energy or a consequence
of having inertia. Inertia is a feature of everything in our universe--a tendency of all objects to keep doing what
they're doing. If an object is stationary, it tends to remain station. If an object was moving forward at a certain
speed, it tends to keep moving forward at a certain speed. Inertia tends to keep the roller coaster moving forward
along the track at a certain speed, even when nothing is pushing on the roller coaster. While the roller coaster
will slow down as it rises up the next hill, its inertia keeps it moving forward.
Being born in the early 60's, I grew up knowing that you could send a nuclear sub around the world on a chunk of
uranium the size of a golf ball and that the half-life of plutonium was 38,000 years. So why does the world now have so
much nuclear waste to get rid of? Why, if something has a half-life of many thousands of years, is it waste after only a
few? -- SG, Sydney, Australia
First, nuclear waste isn't 100% radioactive atoms. Much of it is radioactively contaminated material--normal
materials that contain enough radioactive atoms to be considered hazardous. Second, nuclear reactors don't wait
for radioactive materials to decay via spontaneous processes, the ones that are responsible for half-lives. Instead,
they induce the radioactive decays using chain reactions. In a nuclear fission reactor, the spontaneous decay of
one uranium or plutonium nucleus is used to induce decays in other uranium or plutonium nuclei. In this manner,
huge fractions of the uranium or plutonium nuclei can be "used up" in only a few years. In fact, in a nuclear
fission bomb, many or most of the uranium or plutonium nuclei are consumed in less than a millionth of a second
because of these induced fissions. Half-life has almost nothing to do with a fission bomb. It becomes nuclear
waste so fast you can't imagine it.
How is glass made?
Common window glass is made by melting a mixture of quartz sand (silicon dioxide), soda (sodium oxide), and
lime (calcium oxide). The quartz is the network forming material that forms the basic structure of the glass. The
soda makes it much easier to melt and work with--along with making the glass weaker and more temperature
sensitive. The lime prevents the soda-rich glass from dissolving in water.
How do analog to digital converters change the analog input signal into a stream of numbers? -- RME, Santa Monica, CA
A typical analog-to-digital converter (ADC) uses a process called "successive approximation" to find a binary
number that accurately represents the voltage on an input wire. It samples the voltage on the input wire at one
moment in time and then gradually constructs a binary number representing that voltage. The ADC tries various
binary numbers and uses a digital-to-analog converter to form a voltage from each number. It compares the two
voltages, the original and its approximation, to determine how close its current guess is to the correct value. With
each successive approximation, it adds a bit a precision to its measurement so that after 16 approximations, it has
a 16 bit number that accurately represents the voltage on the input wire.
For applications requiring even faster measurements, there are flash ADCs. These devices synthesize the entire
range of possible voltages and then compare the input voltage directly with the complete collection of possible
voltages. Since 8 binary bits can represent 256 possible numbers, an 8 bit flash ADC synthesizes 255 different
voltages and makes 255 voltage comparisons simultaneously. It instantly determines where among the various
voltages the input voltage falls and it reports this value in billionths of a second.
Why is the Hubble telescope in space rather than on earth? -- L
The earth's atmosphere has poor optical properties that seriously diminish the resolving powers of even the finest
earth-based telescopes. You can see these optical problems by watching the warm air rise above a radiator or hot
pavement on a summer day. The little swirls and eddies of heated air distort the scenery beyond them. Earthbased telescopes have to look at the stars through several miles of swirling, inhomogeneous atmosphere and they
struggle to compensate for the imaging problems this air causes. Most world-class telescopes are located on
mountaintops, far from lighted urban centers and away from humidity and clouds. But even the sky above these
mountaintop observatories causes problems. By putting Hubble in space, they got rid of all atmospheric
problems--air turbulence, clouds, and nearby lighting. They also made it possible for Hubble to operate around
the clock by eliminating the blue sky that blinds telescopes during the day.
How does a picture camera work? -- HW, Ypsilanti, MI
A picture camera uses a lens to form a real image of a distant scene on the surface of a sheet of film. The lens
bends rays of light so that all the light from a certain spot on the scene that passes through the lens comes
together to a single point on the film. You can see this real image formation process with a magnifying glass.
Just go into a darkened room with one window on a sunny day and hold the magnifying glass a few inches away
from the wall opposite the window. You should see an inverted image of the window and the scene outside it
projected on the wall. If you don't move the lens toward or away from the wall until that image forms.
Everything else about a camera is just helping that lens form its image on the film in a controlled fashion. The
camera's shutter limits the amount of time that light has to form this image. The focus controls make sure that
light from the object you are interested in forms a sharp image on the film and doesn't appear blurry.
What is zero point energy? -- AWG, Karachi, Pakistan
All objects in our universe have wave-like characteristics that manifest themselves in certain circumstances.
These wave-like characteristics become more significant as objects become smaller. Their wave-like
characteristics allow small particles to have ill-defined locations. To understand what I mean by "ill-defined
locations", consider a wave on the surface of a lake. There is no one point at which this wave is located--it is
located over a region of the water's surface. Waves don't have well defined locations. Similarly, if you observe an
electron, which is really a wave, there is no one point at which that electron is located--it is located over a region
of space. Because of the detailed relationships between wavelength, frequency, and energy, the smaller the
region of space in which the electron-wave can be found, the higher its energy must be. Thus an electron that is
localized at all--that is known to be within a certain region of space--must have a certain minimum energy, even
if it is stationary. This minimum energy is called zero point energy and it is a consequence of trying to localize
the particle within a certain region of space. Since the zero point energy is a base level and can't be reduced, you
can't use zero point energy to do anything useful. It's just there.
How does a black light work? -- JLM, Kettering, OH
I think that most black lights are gas discharge lamps that resemble normal fluorescent lamps. However, while a
normal fluorescent light uses fluorescent phosphors to convert the ultraviolet light produced by its mercury
discharge into visible light, a black light allows that ultraviolet light to emerge from the lamp unchanged. The
ultraviolet light from a mercury discharge has too short a wavelength to be useful or safe as artistic black light,
so other gases are likely to be used. The lamps are probably filtered so that they emit relatively little visible light
or short wavelength ultraviolet light.
How do the spectrums of different light sources differ? For example, when you look at an incandescent bulb through a
spectroscope, do you see colors other than what you see when you look at a fluorescent bulb? -- EC, Tokyo, Japan
The spectrum of light from an incandescent bulb is what is known as a blackbody thermal spectrum--the light
produced by a hot object. A blackbody spectrum is relatively featureless--you can't even tell what material is
producing the light; only what temperature it has. All the wavelengths of light are present in thermal radiation
and their intensities vary smoothly with wavelength. For the filament temperature of a normal incandescent bulb,
the reds are brighter than the greens and the blues are rather weak.
A fluorescent bulb pieces together white light out of several separate colored lights. The spectrum of light from a
fluorescent lamp is not simple or featureless--many wavelengths are essentially missing and the intensities of the
remaining wavelengths don't vary smoothly with wavelength. Viewed through a spectroscope, the light from a
fluorescent light has many bright bands of color interspersed with relatively dark bands.
When you spray water from a garden hose into the air, with the sun behind you, you see a rainbow which appears to
stretch right across the sky, in the same way that rainbows form by normal rain appear. In the garden hose case, the water
droplets are only a few feet in front of the observer. Is the image of a normal rainbow also only a few feet away or is it
formed by droplets within the total volume of the rain shower? If this latter case is true, does the rainbow in fact form a
complete circle that is cut off by the horizon? -- RP, Solihull, England
A rainbow isn't an image that originates at a specific distance away from your eyes. It consists of rays of colored
light that travel at particular angles away from the water droplets that produce them. You see red light coming
toward you from a certain angle because at that angle, the water droplets are all sending red light toward you. In
the garden hose case, the water droplets are so densely arranged that they are able to create a brilliant rainbow in
only a few meters of thickness. In a typical rainstorm, sunlight must travel through hundreds or thousands of
meters of raindrops to produce an intense rainbow. When you look up toward the red arc of the normal rainbow,
you are seeing light directed toward your eyes by millions of water droplets, some close and others distant, that
are all sending a part of the red portion of the sunlight striking them toward you and the other wavelengths of
sunlight elsewhere.
You are correct that a normal rainbow is cut off abruptly by the horizon and that it would continue down below
to form a full circle if the ground weren't in the way. People in airplanes sometimes see full 360° rainbows.
When you hold a flashlight to your hand, some of the light comes through. What light frequencies shine through people?
Is it possible to see inside people? -- PC
Biological tissues themselves are relatively transparent. They're not good conductors of electricity and electric
insulators are typically transparent (quartz, diamond, sapphire, salt, sugar). But we also contain some pigment
molecules that are highly absorbing of certain wavelengths of light. For example, the hemoglobin molecules in
blood absorb green and blue light quite strongly, so that they appear red. When you look at a flashlight through
your hand, the light appears red because of this absorption of green and blue light by hemoglobin. If you use a
bright enough red light source and are willing to look very carefully, probably with sophisticated light sensing
devices, you can probably see a little light coming through a person's body. But that light will probably have
bounced several times during its passage, so that you won't be able to learn anything about what the person's
internal organs look like. To get a better view of what a person's insides look like, you need light that penetrates
more effectively and that doesn't bounce very often. Moreover, you must employ techniques to that block this
bouncing light as much as possible so that you only see light that travels straight through the person. The light
that does this isn't visible light--it's X-rays. X-rays are very high frequency, very short wavelength "light" (or
rather electromagnetic waves). Tissue doesn't absorb these X-rays much at all and they can go through people to
form images.
What would you hypothesize the effects of black light bulbs to be on the tanning of human skin?
I would expect that certain black light sources would cause tanning with only modest burning while other black
light sources would cause burning with only modest tanning. Black light--also known as ultraviolet light-consists of very energetic light particles. The particles or photons of ultraviolet light contain enough energy to
break chemical bonds and rearrange molecules. When you're exposed to such energetic light, it causes damage to
molecules in your skin cells and your skin may respond by darkening in the process we call "tanning." But
ultraviolet light is a general term that covers a broad range of wavelengths and photon energies. Long
wavelength/low energy ultraviolet light tends to cause tanning while short wavelength/high energy ultraviolet
light tends to cause burning--it directly kills cells. But these differences aren't sharp and any ultraviolet light will
cause some amount of skin damage.
Is it possible isolate a room or part of it totally from microwaves? -- DMJ
Because conducting surfaces reflect electromagnetic waves, you can shield a room from electromagnetic waves
by enclosing it in conducting surfaces. For example, a room surrounded by metal mirrors will be completely
black inside because light won't be able to enter it. Furthermore, if the electromagnetic waves that you're trying
to exclude have reasonably long wavelengths, you can put holes in the conducting surfaces because
electromagnetic waves can't pass through holes in a conducting surface if those holes are substantially smaller
than their wavelengths. So, to shield your room from microwaves, I'd suggest enclosing it in copper screening
with holes that are no more than a few millimeters in diameter. Many scientific experiments are performed in
such screen rooms, which are generally called Faraday cages.
Have you made RF leakage measurements on a sample of microwave ovens? I understand that the FDA requires that if
measured 5 cm away from any of the oven's surfaces, the RF leakage must be less than 1 mW/cm2 for new ovens and less
than 5 mW/cm2 over the oven's life time. I'm just curious what actual measurements reveal about a "typically used" oven.
-- S
I've measured several ovens and have only found one that leaks a measurable amount of microwave power. That
leaker is an oven that I've used in countless demonstrations and have taken apart several times (it appears on
page 514 of my book). Considering the abuse that poor oven has had, it's doing pretty well. At a talk I gave
yesterday, I couldn't get it to leak more than about 1 mW/cm2 even though I was measuring microwave power
directly on the edge of the oven door--the most vulnerable point in the oven. Given that this oven's door sags
several millimeters as the result of its rough treatment, that's not bad. In short, I doubt that there are many leaky
microwave ovens around that haven't been dropped, crushed in shipping, or otherwise suffered serious
mechanical injury.
What does the inside of a radio look like and what is the difference between AM and FM?
These days, radios just look like electronic circuit boards inside. You'd have some trouble telling the difference
between a radio and a computer. AM and FM are both techniques whereby the radio station tells your radio
which way to move the diaphragm of its speaker and by how much, in order to make sound. In the AM or
Amplitude Modulation technique, the station raises or lowers the power of its radio wave to tell your radio to
move its speaker diaphragm toward you or away from you, respectively. The higher the power of the radio wave,
the more your radio pushes its diaphragm toward you. In the FM or Frequency Modulation technique, the station
raises or lowers the frequency of its radio wave slightly to tell your radio to move its speaker diaphragm toward
you or away from you, respectively. The more it raises the frequency of its radio wave, the more your radio
pushes its diaphragm toward you.
I'm a poor student and can't afford the deposit for a telephone line. Is there any kind of telephone or radio that I can use to
communicate with other people? -- AG, Tulsa, OK
Yes, you can use a radio to communicate with your friends, but they will also have to have radios. Amateur radio
has been popular almost since the invention of radio and the most accessible version of this hobby, citizen band
or CB radio, was extremely popular in the 60's and 70's. You can still buy CB radios and communicate with
friends directly through the air, but the general interest in CB radio has waned in recent years. Unfortunately, you
can't make your friend's radio ring to alert them to begin listening. You'll have to anticipate your "call." Also,
there is no privacy on conventional radio--any nearby person with a similar radio can listen in.
How did wire recorders work? -- MW, San Diego, CA
The original recording scheme invented by Poulson used a wire as the recording medium, rather than a tape. It
recorded audio information as the magnetization of a steel wire in much the same way that a modern tape
recorder records audio information as the magnetization of iron particles on the surface of a plastic tape. Both
devices record the air pressure changes associated with sound as magnetization changes in a magnetizable
surface--the higher the air pressure, the deeper the magnetization in a particular direction; the lower the air
pressure, the deeper the magnetization in the opposite direction.
How does a transformer lessen voltage? -- C
When you send an alternating current through the primary coil of wire in a transformer, that current produces a
magnetic field in the transformer. Because the current in the primary coil is changing with time--it's an
alternating current--this magnetic field is changing and changing magnetic fields are accompanied by electric
fields. In the transformer, this electric field pushes electric charges around the secondary coil of wire in the
transformer. Since these electric charges are pushed in the direction they are traveling, work is being done on
them and their energies are increasing. However, in the transformer you mention, the secondary coil of wire has
fewer turns in it that the primary coil of wire. As a result, the charges don't receive as much energy per charge (as
much voltage) as the charges in the primary coil are giving up. This type of transformer, in which the secondary
coil has fewer turns of wire than the primary coil, is called a step-down transformer and reduces the voltage of an
alternating current.
How do I make a battery that will charge using wind power? -- K
Any rechargeable battery will do for this job, although I'd recommend using a lead-acid battery. To charge it, you
need a wind-powered DC generator. You can make such a generator by attaching a DC motor to the blades of a
fan and providing some weather-vane mechanism to ensure that the fan always points into the wind. The wind
will then cause the fan to spin, and with it the motor. Wind energy will become mechanical energy and that will
in turn become electric energy. The DC motor will act as a generator and will produce electric power.
To make this generator recharge the battery, you first need to ensure that the motor can generate a voltage that's
at least 20% higher than the voltage of the battery while the wind is blowing at its usual rate. If it can't, you need
a higher voltage motor or a lower voltage battery. Now you should connect the negative output wire of the
generator to the negative terminal of the battery and use a power rectifier (a power diode) to connect the positive
output wire of the generator to the positive terminal of the battery. You need this diode to prevent the battery
from sending its power into the motor and making the fan turn when the wind isn't blowing hard. If the fan starts
turning when you've inserted the diode, you have it installed backward. When correctly inserted, the diode will
prevent the battery from operating the fan so that the fan can only charge the battery. When the wind starts
blowing and the fan starts turning, it will charge the battery.
What would be a legitimate form of propulsion for magnetic trains? -- DS, Kenton, OH and MB, Willows, CA
The most sensible propulsion system for a magnetically levitated train would be a linear electric motor. This
motor would consist of electromagnets on the train and electromagnets on the track. By turning these
electromagnets on and off at carefully chosen moments, they can be used to pull or push the train forward for
propulsion or backward for breaking. The timing is important because, for propulsion, the magnet on the train
must always be attracted toward the track magnet in front of it and repelled by the track magnet behind it. For
breaking, this relationship must be reversed.
What is a superconducting magnet? -- JS, Montreal, Quebec
Electric currents are magnetic. That's the basis for electromagnets--if you run an electric current around a coil of
wire, that coil of wire will develop a north magnetic pole at one end and a south magnetic pole at the other end.
But an electromagnet made with normal copper wires consumes electric power all the time. The current passing
through those wires wastes energy because of friction-like effects in the copper and the wires become hot. The
electromagnet also needs a power source to keep its current flowing.
However, a superconducting electromagnet is one in which the wires are superconducting--the current passing
through them doesn't waste any power. Once a current has been started in a coil of superconducting wire, it flows
forever. Since it doesn't waste any power, that current needs no source of power and produces no thermal energy.
In fact, you can buy superconducting magnets with the current already started at the factory. As long as the wires
are kept cold (as they must be to remain superconducting), the current will continue to flow and the coil will
remain magnetic forever.
What causes different types of lightning? -- CR, Tokyo, Japan
Cloud to ground lightning is caused by difference in electric charges between the cloud and the ground. Cloud to
cloud lightning is caused by difference in electric charges among the clouds themselves. While I'm not at all
expert on the subject, I would guess that the various different types of lightning discharges are caused by
differences in the distances between charged objects, by variations in the local electric conductivity of the air and
clouds during a discharge, and by the sizes and shapes of the clouds and ground.
Why is it that the times at which the moon sets and rises change so much from day to day?
The moon orbits the earth about every 27.3 days, so its position relative to the sun changes from day to day.
Because of the moon's movement around the earth, the moon rises and sets about 1 hour later every day. When
the moon is on the sun side of the earth, it rises at sunrise and sets at sunset. Fourteen days later, when the moon
is on the side opposite the sun, it rises at sunset and sets at sunrise.
March 6, 1997
What makes a soap bubble round? -- MZ, Massachusetts
The molecules in a liquid are touching one another and this touching reduces the molecules' potential energies.
Separating the molecules and reducing the extent of their touching requires energy and is something that the
liquid won't normally do on its own. The molecules at the surface of a liquid have fewer neighbors than they
would have if they were in the body of the liquid. Those molecules thus have higher potential energies than they
would have in the body of the liquid. To minimize the overall potential energy of a liquid, it naturally tends to
minimize its surface area.
When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny
droplet. The air keeps bubble large and it can't avoid having lots of molecules on its surfaces, where they have
higher than normal energies. But what the soap solution can do to minimize its total potential energy is to
minimize the number of its molecules that are on the surface. The soap solution experiences what is called
"surface tension"--an elastic tightening of its surface. This surface tension tends to minimize the surface area of
the soap solution to minimize its potential energy. The soap solution minimizes its surface area around the
trapped air by forming a spherical shape. A spherical shell makes the most efficient use of its surface area in
enclosing a volume.
Could you describe the process of an ice cube melting only from ambient (room) temperature? -- JAS, Malta, NY
An ice cube is a crystal of water molecules. It is only stable up to a temperature of 32° F (0° C). When you place
it in ambient temperature, it gradually warms until it reaches 32° F and then its surface begins to melt. As heat
from the room flows into the ice cube, its molecules begin to separate briefly from one another and to exchange
neighbors. These molecules lose their crystalline rigidity and structure and to become liquid. The liquid that
forms is still at 32° F, but it has less order than the crystalline ice had.
As more heat flows into the mixture of ice and water, the ratio of solid ice to liquid water gradually changes and
the fraction of liquid water increases. But only after all the ice has converted to water does the temperature of the
water begin to rise significantly above 32° F.
How do bicycle shocks and suspension affect the performance of a bicycle? -- D
When the wheels of a bicycle are attached directly to the frame of a bicycle, the wheels and frame must move
together. When one of the wheels hits a bump, both that wheel and the frame must accelerate upward together.
When this happens, the bump exerts a huge upward force on the wheel and everything, including the unfortunate
rider, experiences a sudden upward acceleration. A sudden jolt of this sort is unpleasant--the seat of the bicycle
pushes upward violently on the rider and the rider feels large forces throughout his or her body. Each body part
pushes upward on the body part above it so that everything leaps upward.
To reduce the upward acceleration that the rider experiences, the direct connection between the bicycle wheels
and the frame can be replaced by a spring suspension. When the wheel of a bicycle with a spring suspension
encounters a bump, the springs compress and the force on the frame and rider is much smaller. The rider still
accelerates upward, but not as rapidly as the wheel and without the abrupt jolt of a suspensionless bicycle. In
fact, by the time the rider has begun to rise much, the wheel will probably have rolled back off the bump and the
spring will return to its original shape. Overall, the rider will barely move at all and will hardly notice the bump.
But a spring suspension isn't perfect by itself. Suppose that the bicycle rolled over a curb and onto a sidewalk.
This bump doesn't end--the pavement level rises permanently. When the wheel hits the curb, it rises suddenly
and compresses the spring. But since the wheel never drops back to its original height, the only way for the
spring to decompress back to its original shape is for the frame and rider to rise. And that's what happens. But the
frame and rider don't stop moving once the spring has reached its original shape. They have upward momentum
and they continuing rising. The spring begins to stretch upward now. Eventually the frame and rider stop rising
and begin to descend again, but they continue to bounce up and down as though they were on a pogo stick. In
effect, they are on a pogo stick. When a spring is compressed or stretch, it stores energy. If there is nothing to
get rid of the energy stored in the bicycle's compressed or stretched spring, the frame and rider will continue to
bounce up and down indefinitely.
To stop the bouncing (and prevent most of it in the first place), a bicycle with a spring suspension also has shock
absorbers. These devices waste energy whenever the wheel and frame move relative to one another. Whether the
spring is compressing or stretching, the shock absorber extracts energy from the wheel, frame, and spring, and
turns that energy into thermal energy. As a result, the frame and rider don't bounce significantly after the wheel
rides up and onto the curb. Similar issues occur in cars, where shock absorbers damp out the bouncing that can
occur because the car body is suspended above the wheels on springs.
How does one prove that the earth is round?
There are many possibilities, so I'll suggest an intriguing method that is familiar to surveyors. While the overly
simple technique I suggest isn't particularly practical, it is closely related to surveying techniques that are
practical.
Take a very long string, say about 20 miles long, and attach one end of the string to a post. Now draw the string
taut and walk all the way around the post while holding on to the other end of the string. If you measure the
distance you walked while completing one full trip around the post, you would expect it to be related to the
length of the string by a factor of 2 times pi because you learn in grade school that the circumference of a circle
is 2 times pi times the radius of that circle. However, that relationship is only true if you're working on a flat
surface. Since the earth is curved, the circumference of the circle around which you walk will be somewhat less
than 2 times pi times the radius of the circle. That result is enough to prove that you're on a curved surface.
You can see this effect by performing the experiment I just suggested on the surface of a basketball. Take a short
length of string and use it, together with a pin and a pencil, to draw a circle on the surface of the ball. If you
measure the circumference of that circle and compare it to 2 times pi times the length of the string, the circle's
circumference will be a bit shorter than expected. As with the earth, the basketball is a curved surface. The larger
the circle you try to draw in this manner, the greater the discrepancy between 2 times pi times the radius and the
actual circumference of the circle.
How dangerous are plastics for storing and reheating food? I remember hearing that plastic containers can release
carcinogenic materials when reheating food in the microwave. I also heard that plastics can release "plasticizers" into
food even when cold. What studies exist about these dangers? -- CVL, Fairfax, VA
While I'm not up to date on actual studies, I would think that most food storage plastics introduce very little
contamination into the foods stored in them. We have become so concerned as a society about toxic chemicals in
recent years that we tend to overreact much of the time. While the actual polymer molecules in most plastics are
relatively inert and harmless, plastics inevitably contain some small molecules, either by accident or by design,
that work their way into food. Even if some of these molecules are toxic or carcinogenic, the quantities involved
are almost certainly insignificant. Modern chemical testing can detect incredibly small quantities of various
chemicals and we panic every time we find them in our environment. But the societal cost of banning or
avoiding all contact with or use of these chemicals may have hidden costs that are worse than the problem we're
trying to solve. Moreover, I'll bet that many of the foods put in plastic containers are greater health hazards than
the containers themselves.
Does food coloring change the color of food?
Food coloring is a solution of dye molecules--molecules that absorb light of certain wavelengths extremely
efficiently. When a particle of light--a photon--of the right wavelength encounters one of these dye molecules, an
electron in the molecule uses the photon's energy to shift from one quantum level to another. The photon
vanishes and the molecule is placed in an electronically excited state. The dye molecule's electron quickly returns
to its original quantum level by releasing this extra energy as thermal energy within the molecule and its
surroundings. Overall, the photon has vanished and the dye has become warmer. When you add these dye
molecules to food, the dye gives the food a color by preventing that food from transmitting or reflecting certain
colors of light. The dye simply absorbs those colors.
How does electricity work?
I'll assume that you are asking about moving or dynamic electricity, the type that lights the bulb in a flashlight
(as opposed to static or stationary electricity). In that case, you are referring to a flow of electric charges that is
generally called an electric current. This movement of electrically charged particles carries with it energy, both
as kinetic energy (energy of motion) in the charged particles and as potential energy in the electrostatic
attractions and repulsions of these particles. The particles typically acquire this energy from a battery. The
battery pulls opposite charges away from one another and pushes like charges together. These actions increase
the energy of those charges. The charges then rush through electrically conducting materials, generally metals, in
order to bring opposite charges closer together. This flow of charges releases the energy given them by the
battery.
In a flashlight, the batteries provide the charges with power and the light bulb makes use of the power. The
charges first flow through the battery (which gives them energy), then through wires to the light bulb, then
through the light bulb (where they give up their energy), and finally back through wires to the battery. The
charges move in a loop--a circuit--so that they don't accumulate anywhere. They travel endlessly between battery
and bulb, shuttling energy from the battery to the bulb. As is always the case in electric circuits, two wires
connect the battery and bulb--one wire to carry charges to the bulb and one wire to return them to the battery to
begin their trip over again.
March 5, 1997
What is surface tension? -- C
The molecules in a gas are independent and only collide with one another briefly before separating again. In
contrast, the molecules in a liquid cling to one another so that they always remain in contact. While their mutual
attachments aren't as strong as normal chemical bonds, these molecules have reduced their overall potential
energies by moving as close as possible to one another. However, the molecules at the surface of a liquid have no
neighbors on one side and don't benefit from the full energy-lowering effects of moving as close as possible to
other molecules on all sides. Molecules at the surface of the liquid thus have higher potential energies than
molecules within the liquid.
Because physical systems tend toward arrangements that minimize their overall potential energies, a liquid tends
to minimize its surface area in order to minimize the number of high-energy molecules it has at its surface. This
tendency to minimize surface area is the origin of surface tension in a liquid. The liquid behaves as though its
surface were a taut elastic membrane. If you poke at a liquid, you can deform its surface but as soon as you stop
pushing on it, it will spring back to its original flat or smoothly curved shape. That springiness is the result of
surface tension.
How can you speed up the process of dissolution? -- T
Dissolution occurs when a solid material is disassembled into its constituent atoms, molecules, or ions and these
particles are carried around in a solvent liquid. Since the disassembly is a statistical process, with thermal energy
allowing particles to leave the solid and chance allowing them occasionally to return to the solid again, anything
you can do to accelerate the leaving process and impede the returning process will speed dissolution. Heating the
solid and liquid will speed dissolution by making it easier for particles to leave the solid and harder for them to
stick when they try to return. Keeping fresh, pure solvent in contact with the solid will also prevent molecules
from returning.
What is the difference between apparent weight and true weight?
Your true weight is caused by gravity--it is the force exerted on you by gravity; usually the earth's gravity. Your
apparent weight is the sum of your true weight and a fictitious force associated with your acceleration. Whenever
you accelerate, you experience what feels like a gravitational force in the direction opposite your acceleration.
Thus when you accelerate to the left, you feel a gravity-like experience toward your right. It is this effect that
seems to throw you to the right whenever the car you are riding in turns toward the left. In fact, this effect is
caused by your own inertia--your own tendency to travel in a straight line at a constant speed. Your apparent
weight can be quite different from your true weight. Perhaps the most striking example occurs on the loop-theloop of a roller coaster. While your true weight remain downward throughout the ride, as it always is, your
apparent weight actually becomes upward as you pass around the top of the loop-the-loop. You are accelerating
downward so rapidly at the top of the loop that the experience you have is one of a gravity-like force that is
pulling you skyward. Since the car you are riding in is invert and above you, you feel pressed into your seat even
though the ground is in the other direction.
Why are any materials transparent? -- MZ, Peligna, Italy
Because light is an electromagnetic wave, it is emitted and absorbed by electric charges. For an electric charge to
emit light it must move--in fact, the charge must accelerate. For an electric charge to absorb light it must also
move--it must also accelerate. However, there are many materials that do not have mobile electric charges. For
example, while all electric insulators have electric charges in them, those electric charges can't move long
distances. The electric charges in many electric insulators can't even move enough to absorb light and the light
simply passes right through them. They are transparent.
Is it true that microwaves cause cancer?
I think that it's very unlikely that microwaves cause cancer. Microwaves are not ionizing radiation--they don't
directly damage chemical bonds. Instead, they heat materials, particularly those containing water. As a result,
they may cause damage to proteins in the same way that cooking damages proteins (and hardens egg protein, for
example). But while such protein damage can easily cause cell death, I wouldn't expect it to cause the genetic
damage associated with cancer.
How do windmills work to generate electricity? -- KT, Aurora, Ontario
Windmills extract energy from the wind by rotating as the wind twists them. Whenever an object rotates in the
same direction as the torque (the twist) being exerted on it, mechanical work is done on that object. In this case,
wind exerts a torque on the windmill's blades and they rotating in the direction of that torque, so the wind is
doing work on the blades. Work is the mechanical transfer of energy, so the wind is transferring some of its
energy to the blades.
The blades don't keep this newly acquired energy. Instead, they do work on a generator. The generator, which
consists of a rotating magnet that spins within stationary coils of wire, uses this energy to generate electricity.
The amount of power that a windmill generates depends on the wind speed and the windmill's size, but large
windmills can generate in excess of a million watts of electric power.
How does electricity get to my home?
The electricity you receive comes from a distant power plant. A generator in that power plant produces a
substantial electric current of medium high voltage electric charge. This current is alternating, meaning that its
direction of flow reverses many times a second--120 reversals per second or 60 full cycles of reversal (over and
back) in the United States. This alternating electric current flows through the primary coil of wire in a huge
transformer at the power plant, where it produces an intense alternating magnetic field. When a magnetic field
changes with time, it produces an electric field and, in the transformer, this electric field pushes electric charges
around a second coil of wire in the transformer, the secondary coil. The effect of this transformer is to transfer
power from the current in the primary coil of the transformer to the current in the secondary coil of the
transformer. Thus the generator's electric power moves along to the current passing through the secondary coil of
the transformer. However, the secondary coil has far more turns of wire than the primary coil and this gives each
charge passing through that coil far more energy than the charges had in the primary coil. Although the current
passing through that secondary coil is relatively small, it acquires an enormous voltage by the time it leaves the
secondary coil. The transformer has produced this high voltage power needed for efficient power transmission to
a distant city.
This high voltage electric current passes through the countryside on high voltage transmission wires. The value
of using a small current of high voltage charges is that wires waste power in proportion to the square of the
electric current they are carrying. Since the current in the transmission wires is small, they waste relatively little
power.
When this current reaches your town, it passes through a second transformer, which transfers its power to yet
another electric current. This current is large and, because it passes through a coil that has few turns of wire, it
acquires only a medium high voltage when it flows through the secondary coil of the new transformer.
Electricity from this second transformer flows toward your neighborhood through medium high voltage wires.
Finally, near your home there is a third and final transformer that extracts power from the medium high voltage
current and transfers that power to a very large current that acquires a low voltage when it flows through the
secondary coil of the final transformer. It is this very large current of low voltage charges that flows through
appliances in your home and those of your neighbors. That final transformer is often visible as a large gray drum
on a utility pole or a green box in someone's yard.
March 4, 1997
How much electric current is there in an automobile spark plug? -- DG, Brooklyn, NY
Without measuring it directly, I would guess that the current passing through a spark plug during a spark is about
10 milliamperes. I base that guess both on a calculation--assuming sensible values for the energy, voltage, and
duration of the spark--and on my experience with electric sparks. If I have a chance to measure the current
directly--I have the equipment but not the time--I'll put a more specific value here.
How do fruit machines work? Do they operate on a fixed mathematical model which governs payouts using probability or
are they totally random? -- TS, Norfolk, UK
I assume that you are referring to the gambling machines that spin several wheels when you pull a lever and that
pay you amounts that depend on the patterns of symbols that show on the faces of the wheels when they stop.
While the final arrangement of symbols that appear on such a machine when it stops is entirely random, the
patterns that pay and the amounts they pay are calculated to ensure a slight financial advantage for the house.
The mathematics of probability is well developed for such gambling machines and it's relatively simple to
determine what fraction of your money you should expect to lose if you play the game for a very long time. If
you do play long enough to sample the full statistics of the game, you are certain to lose money. It's only if you
play briefly that you can take advantage of statistical fluctuations to leave with more money than you had when
you started.
Does the creation of life and the theory of evolution violate the laws of thermodynamics? -- BY, Liverpool, NY
While the laws of thermodynamics forbid an overall increase in the order of the universe and while life is an
example of significant order, the laws of thermodynamics don't forbid some parts of the universe from becoming
more orderly at the expense of other parts of the universe becoming less orderly. Living organisms are
consumers of order and exporters of disorder--they derive their order by creating disorder elsewhere. You eat
highly ordered chemicals in your food and you eliminate those chemicals in much more disordered forms latter
on. You also emit heat, the most disordered form of energy. Thus thermodynamics has no problem with the
ongoing existence of life; it simply requires that living organisms consume order and we are doing just that at a
furious pace.
As for the creation of life, that could have been a random event and thermodynamics permits random events.
Improbable events do occur--people win the lottery, lightning strikes twice, two snowflakes are occasionally
alike--and the creation of life could have been one of those unlikely but not impossible events. Once the simplest
organism had assembled itself by chance, it could then begin the process of consuming order and exporting
disorder.
How hot is a match when it is ignited? Is the initial point of combustion hotter than when it is just burning? -- TB,
Excelsior, MN
You can usually judge the temperature of a hot object by its color--the brighter and whiter the light, the hotter the
object. A candle flame has a temperature of roughly 1700° C while an incandescent light bulb has a temperature
of about 2500° C. To my eye, a struck match briefly becomes brighter and whiter than a candle flame, so I would
guess that its peak temperature is somewhere in the mid 2000° C range. Once the chemicals in the head have
been used up, the flame temperature drops to about 1700° C.
At what temperature does paper burn? -- KR
If the title of the book "Fahrenheit 451" is correct, then the temperature is 451° F (233° C). Actually, I'm sure
that the ignition temperature depends on the exact type of paper.
Why does the tower of Pisa lean? -- CM, Edison, NJ
The tower was built long ago on unstable ground that was unsuitable for supporting such a tall and heavy
masonry structure. For an object to remain upright indefinitely, its center of gravity must lie above its base of
support and that base of support must be firm at all its edges. The tower's base of support had at least one edge
that wasn't firm and that began to sink downward under the weight of the tower. Once this edge sunk a small
distance, the tower's center of gravity shifted sideways so that it was above that weak portion of the base of
support. This shift in the tower's center of gravity put even more stress on the weak part of the ground and caused
additional sinking, additional tipping, and even more shifting of the tower's center of gravity. This process might
have toppled the tower over by now were it not for recent efforts to stop the tipping. The base of the tower has
been reinforced to prevent further tipping.
What exactly are gravity waves and how are they measured? -- AY, Wayne, PA
Gravity waves are deformations of space/time that propagate through space at the speed of light. While many
motions of matter and energy are thought to emit gravity waves, those waves are normally extraordinarily weak.
The only sources of detectable gravity waves are probably collapsing and colliding stars. Careful studies of the
dynamics of binary star systems have shown that they also emit reasonably strong gravity waves, but those
waves haven't been detected directly.
The two classes of gravity wave detectors currently in development or operation are large cryogenic bar detectors
and laser interferometric detectors. A cryogenic bar detector tries to observe gravity waves by looking for
vibrational excitations of huge metal bars. When a strong gravity wave passes through one of these bars, it
should excite various vibrations in the bar that can be detected by sensitive motion sensors. A laser
interferometric detector tries to observe gravity waves by looking at distance changes in the arms of a laser
interferometer--a huge mirror system with laser beams bouncing back and forth within it. When a strong gravity
wave passes through the mirror system, it should change the spacings of the mirrors enough to cause variations
in the optical characteristics of the interferometer (for more info, see www.ligo.caltech.edu). So far, no gravity
waves have been observed definitively.
How does a roller coaster work?
A roller coaster is essentially a gravity-powered train. When the chain pulls the train up the first hill, it transfers
an enormous amount of energy to that train. This energy initially takes the form of gravitational potential energy-energy stored in the gravitational force between the train and the earth. But once the train begins to descend the
first hill, that gravitational potential energy becomes kinetic energy--the energy of motion. The roller coaster
reaches maximum speed at the bottom of the first hill, when all of its gravitational potential energy has been
converted to kinetic energy. It then rushes up the second hill, slowing down and converting some of its kinetic
energy back into gravitational potential energy. This conversion of energy back and forth between the two forms
continues, but energy is gradually lost to friction and air resistance so that the ride becomes less and less intense
until finally it comes to a stop.
Is there a device that would provide a variable output of radiated energy in the infrared that would be obtainable to
experiment with? -- NAT, Marion, SC
You can produce a broad range of infrared lights with a heat lamp. A heat lamp looks very dim because most of
the thermal radiation it emits is in the infrared portion of the electromagnetic spectrum. Just attach the heat lamp
to a normal light dimmer and you'll be able to vary its infrared output over a wide range of intensities. Its
frequency range will also shift farther away from the visible as you lower its temperature by turning down the
dimmer. If it produces more visible light than you want, you can put a filter in front of it that absorbs visible light
while permitting infrared light to pass. Such filters are certainly available from filter companies such as Hoya or
Corning but cheaper versions (perhaps even plastic filters) may be found through scientific supply companies.
How does a computer chip work? -- JM, Austin, TX
A computer chip is also known as a digital integrated circuit. It is typically a thin wafer of silicon, cut from a
single crystal of that element. The surface of the wafer has been chemically modified and it has had intricate
patterns of aluminum wires and other structures cut and deposited photographically on its surface to form
enormous numbers of transistors and other special structures. Each of these transistors is an electronically
controllable switch. A tiny adjustment in the electric charge on the control element of one of these transistors--its
gate--can dramatically alter that transistor's current carrying ability. These transistors work together to perform
task that range from remembering one bit of information to multiplying two huge numbers together. The millions
of transistors on a typical computer chip are able to perform extremely complicated tasks, as we see everyday in
modern computers.
What is the physical nature of magnetism? Is it a wave or particle phenomenon or an undefined energy like gravity? -GA, Paisley, Scotland
Magnetism is one sector of the electromagnetic interactions of matter. From a classical perspective, magnetism
consists of an energy-containing field that surrounds magnetic poles and that exerts forces on other magnetic
poles. At a higher classical level, magnetism and magnetic fields are part of the full electromagnetic interaction,
meaning that they are inextricably mixed with electricity and electric fields. Finally, from a full quantum
mechanical perspective, magnetism is associated with energy-containing quantum fields, the fields of quantum
electrodynamics, that govern the electric and magnetic interactions of matter. These quantum electrodynamic
interactions are mediated by virtual photons, cousins of the real photons that include light and radio waves. From
this quantum viewpoint, magnets interact with one another by exchanging virtual photons and, like all quantum
objects, these photons are emitted and absorbed like particles but travel as waves. Thus magnetism is both a
wave and particle phenomenon. It isn't undefined at all; in fact, quantum electrodynamics is probably the most
well-established and precise theory in modern physics.
Does magnetism affect the growth of plants? If so, how? -- JA, Somerville, MA
I am not aware of any effects of magnetism on plant growth. The effects of magnetism on most molecular
processes are incredible slight and I don't see how any but the most extreme magnetic fields could affect plant
growth.
March 3, 1997
Could you give me the formula for figuring the wavelength of an ultrasound wave? -- BH
The wavelength of any wave is equal to the speed of that wave divided by its frequency. In air, the speed of
sound is about 330 meters per second, so an ultrasonic wave with a frequency of 50,000 cycles per second would
have a wavelength of about 6.6 millimeters. Since sound travels much faster in liquids or solids, the wavelengths
would be larger than in air.
Which substance, calcium chloride or sodium chloride, melts ice faster and why? -- MT, Fenton, MI
Without trying the experiment, I would expect sodium chloride to melt ice more quickly than calcium chloride
simply because sodium chloride is more soluble in water. Anything that dissolves easily in water can melt ice,
even sugar! A water-soluble material interferes with the crystalline structure of ice and, assisted by the tendency
of everything to maximize randomness, converts the orderly arrangement of solid ice and soluble solid to the less
orderly mixture of soluble material dissolved in liquid water. Both calcium chloride and sodium chloride are
water soluble and thus melt ice, but sodium chloride is substantially more soluble than calcium chloride and
ought to work faster.
However, molecule for molecule, calcium chloride will melt more ice than sodium chloride. That's because a
single calcium chloride molecule decomposes into three separate ions in solution (one calcium ion and two
chlorine ions). In contrast, a sodium chloride molecule only forms two separate ions in solution (one sodium ion
and one chlorine ion). Since each ion contributes to the ice melting process, calcium chloride molecules are
about 50% more effective than sodium chloride molecules. But even this increased molecular efficiency has a
price: calcium ions are heavier than sodium ions, so a kilogram of sodium chloride actually yields more ions and
more ice melting than a kilogram of calcium chloride. Still, salt is messy and corrosive so calcium chloride is
often a good alternative.
What chemical properties of water cause it to be a medium of life? -- WZ, Pacific Palisades, CA
Water is such a remarkable chemical that I hardly know where to begin. First, it is one of the lightest, simplest
molecules and yet it remains a liquid even at temperatures approaching 100° C, making it well suited as a
medium for chemistry of all sorts. Second, it is an extremely good solvent for a vast range of ionic and organic
materials, so that it is an ideal medium for the complicated chemical mixtures of biology. Third, water has
enormous latent heats of melting and vaporization that make it hard to freeze and its evaporation very effective at
cooling a hot animal.
What is a digital display and how does it work?
The term "digital display" usually refers to a system that reports the value of a physical quantity in numerical
form. A digital watch display is a good example. The physical quantity it reports is time and it makes its report in
the form of hours, minutes, and second--all in numerical form. In a digital watch, the display makes use of liquid
crystals that are sensitive to electric fields. When you look at the display, you are actually looking through a
layer of polarizing filter, some transparent electric wires, and a layer of liquid crystals. Liquid crystals are liquids
that contain molecules that naturally orient themselves relative to one another. In the display, these liquid
crystals adopt different orientations when they are exposed to electric fields than when they're not exposed to
such fields. This electrically altered orientation affects their optical properties and causes them to appear dark
when viewed through the polarizing filter. The watch can control the appearance of each segment of its digital
display by the pattern of electric charge on its transparent wires. Since it takes very little energy to change the
orientation of the liquid crystals, the watch uses almost no power for its display and can operate for years on a
button battery.
What is the frequency, amplitude, wavelength, etc. of a sound wave at the sound barrier? -- KT, Ocean Springs, MS
The sound barrier is something of a myth that dates to the early days of transonic flight. As early airplanes
approached the speed of sound, they suffered various flight instabilities--a significant rise in air drag and a
tendency for supersonic shock waves to interfere with the operations of control surfaces. Exceeding the speed of
sound appeared problematic at the time and the expression "the sound barrier" came into common use. However,
there is no real sound barrier. Once Yeager had exceed the speed of sound in an experimental plane, it became
clear that the speed of sound was not a firm barrier.
However, there is one peculiar thing that does happen once a plane has exceeded the speed of sound. You can no
longer hear the plane coming because it is outrunning its own sound waves. Instead of having its sound spread
out in front of it, the plane has its sound swept back in a cone behind it. The edges of this cone are a shock wave
and you experience a sudden pressure rise as this cone passes across you--you hear a sonic boom. A supersonic
plane carries this conical shock wave with it at all times and everyone hears a sonic boom as this shock wave
sweeps across them. What you should remember is that the sonic boom doesn't occur when the plane "breaks the
sound barrier"; the sonic boom is a continuous feature of a supersonic plane that you hear as its shockwave
passes you by.
How do different airplane wings help or hurt the airplane?
An airplane wing's main job is to generate a large upward lift force while experiencing as little backward drag
force as possible. To obtain the lift force, a wing must make the air flowing over its top to speed up while the air
flowing under its bottom slows down. The wing must also avoid introducing turbulence into the main airstream
because that will result in severe pressure drag. There are many cross sectional shapes for wings that achieve
both large lift forces and small drag forces, but some are better suited to each style of airplane than others. For
example, private propeller-driven planes travel relatively slowly and need broad, highly curved wings to obtain
enough lift to support them. In contrast, commercial jets have much narrower, less curved wings because they
travel faster and produce lift more easily. But during takeoff and landing, even jets need to increase the
curvatures of their wings. That's why many jets have slats and flaps that extend from the leading and trailing
edges of their wings to increase the wings' breadths and curvatures for low-speed flight.
Since spent fuel rods from propulsion reactors are still quite hot would it be possible to harness the heat produced for
energy needs? It seems like a possible source, and a waste not to harness what we can. -- SS, Lakewood, CO
While the radioactive decays from spent nuclear fuel rods continue to produce thermal energy, the amount of
energy released each second isn't enough to make it cost effective to use that energy. Since the power output
from a spent fuel rod would only be in the watt range, it wouldn't justify the hazardous job of trying to extract
that power without encountering the radiation. Furthermore, the laws of thermodynamics make it much harder to
use heat from a warm object than heat from a hot object and spent fuel rods would at best be warm objects.
Why can't light resolve details smaller than about half its wavelength? -- SJ, Philadelphia, PA
Suppose that you have a white card with what appears to be a black line on it. That line might actually be two
very closely spaced lines; you're not sure. To find out, you focus a beam of light to the smallest possible spot and
then move this tiny spot of light across the line. You realize that if there are two separate lines on the card, then
the spot of light should cross first one line and then the other, and you should see two changes in the reflected
light rather than just one.
It turns out that, however, that no matter how hard you try you can't focus the light to a spot much smaller than
the wavelength of the light. An equivalent problem would occur if you tried to use water waves to create a
narrow spike of water above the surface--no matter how you worked with the water waves, you would be unable
to make them to merge together into a spike that's much narrower than the wavelength of the water waves.
Because of his limitation, your spot of light can't be much smaller than the wavelength of light and you can't
distinguish between one line or two if those lines are much closer than a wavelength of the light you're using.
Since visible light has a wavelength of 400 nanometers or more, you can't use it to resolve details much smaller
than 400 nanometers wide.
Actually, there is an exception to this general rule--near-field scanning optical microscopy or NSOM uses light
emerging from the tiny tip of a glass fiber to resolve details far smaller than the light's wavelength. In NSOM,
the resolution is determined by the tip size and not the light's wavelength.
If a microscope's resolution is limited by the wavelength of the light it uses, why don't we use very short wavelength light
instead of electrons or other particles for studying very small objects? -- SJ, Philadelphia, PA
Ultraviolet light isused in microscopy to achieve higher resolution than can be obtained with visible
microscopes. But beyond ultraviolet light comes X-rays and it's difficult to build imaging optics for X-rays.
There are some X-ray microscopes, but they aren't nearly as common and practical as electron microscopes. The
electrons in electron microscopes have very short wavelengths (atomic and subatomic length scales) and yet
electron optics are easy to build. So while very short wavelength electromagnetic waves can be made, they're just
not practical for microscopy.
How does a stereo convert 110 volt electric current into the positive and negative current that is sent to power the
speakers? -- JF
A stereo contains a power supply that converts 110-volt alternating current into lower-voltage direct current.
This direct current is ultimately when powers the speakers. The stereo's power supply first lowers the voltage
with the help of a transformer. Alternating current from the power line flows back and forth through a coil of
wire in this transformer, the primary coil, and causes that coil to become magnetic. Since the coil's magnetism
reverses 120 times a second (60 full cycles of reversal each second), along with the alternating current, it
produces an electric field--changing magnetic fields always produce electric fields. This electric field pushes
current through a second coil of wire in the transformer, the secondary coil, and transfers power to that current.
There are fewer turns of wire in the secondary coil than in the primary coil, so charges flowing in the secondary
coil never reach the full 120 volts of the primary coil. Instead, more current flows in the secondary coil than in
the primary coil, but that secondary current involves less energy per charge--less voltage. In this manner, power
is transferred from a modest current of high voltage charges in the primary coil to a large current of low voltage
charges in the secondary coil.
Having used the transformer to produce lower voltage alternating current, the power supply than converts this
alternating current into direct current with the help of four diodes and some capacitors. Diodes are one-way
devices for electric current and, with four of them, it's possible to arrange it so that the alternating current leaving
the transformer always flows in the same direction through the circuit beyond the diodes. The diodes act as
switches, always directing the current in the same direction around the rest of the circuit. The capacitors are
added to this circuit to store separated electric charge for the times while the alternating current is reversing and
the diodes receive no current from the transformer. The capacitors store separated charge while there is plenty of
it coming from the transformer and provide current while the alternating current is reversing. Overall, the stereo's
power supply is a steady source of direct current.
March 1, 1997
Are some frequencies of sound more directional than others and, if so, why? -- BKZ, Dayton, OH
In open air, sound waves travel in straight lines regardless of frequency or wavelength. But low frequency (long
wavelength) sounds don't fit well in confined spaces and have less directional character to them. That's why you
only need one subwoofer for a sound system--you can't hear where the lowest frequency sounds are coming from
any way. Higher frequency sounds remain relatively directional, even in confined spaces. The same effects apply
to electromagnetic waves--in confined spaces, long wavelength radio waves are effectively less directional than
short wavelength light waves.
I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come
to a boil. But does it take longer? As a cook I've always been told to add a little salt to the water to bring it to a boil faster.
It seems to work or is that just the power of suggestion? If it does boil faster, why does it? -- ND, Ashland, OR
I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if
you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure
water will reach its boiling temperature first.
However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just
before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that
are boiling while others aren't. The edges and corners of crystals are particularly good at nucleating bubbles, so
that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at
least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil
when it really isn't.
What types of gas are used in light bulbs and how do their effects differ? -- SF, Westfield, NJ
The glass envelope of an incandescent bulb can't contain air because tungsten is flammable when hot and would
burn up if there were oxygen present around it. One of Thomas Edison's main contributions to the development
of such bulbs was learning how to extract all the air from the bulb. But a bulb that contains no gas won't work
well because tungsten sublimes at high temperatures--its atoms evaporate directly from solid to gas. If there were
no gas in the bulb, every tungsten atom that left the filament would fly unimpeded all the way to the glass wall of
the bulb and then stick there forever. While there are some incandescent bulbs that operate with a vacuum inside,
most common incandescent lamps contain a small amount of argon and nitrogen gases.
Argon and nitrogen are chemically inert, so that the tungsten filament can't burn in the argon and nitrogen, and
each argon atom or nitrogen molecule is massive enough that when a tungsten atom that's trying to leave the
filament hits it, that tungsten atom may rebound back onto the filament. The argon and nitrogen gases thus
prolong the life of the filament. Unfortunately, these gases also convey heat away from the filament via
convection. You can see evidence of this convection as a dark spot of tungsten atoms that accumulate at the top
of the bulb. That black smudge consists of tungsten atoms that didn't return to the filament and were swept
upward as the hot argon and nitrogen gases rose.
However, some premium light bulbs contain krypton gas rather than argon gas. Like argon, krypton is
chemically inert. But a krypton atom is more massive than an argon atom, making it more effective at bouncing
tungsten atoms back toward the filament after they sublime. Krypton gas is also a poorer conductor of heat than
argon gas, so that it allows the filament to convert its power more efficiently into visible light. Unfortunately,
krypton is a rare constituent of our atmosphere and very expensive. That's why it's only used in premium light
bulbs, together with some nitrogen gas.
Incidentally, the filament in many incandescent bulbs is treated with a small amount of a phosphorus-based
"getter" that reacts with any residual oxygen that may be in the bulb the first time the filament becomes hot.
That's how the manufacturer ensures that there will be no oxygen in the bulb for the tungsten filament to react
with.
How can one be fire safe while dealing with incandescent and fluorescent light bulbs? -- TJ, Woodbridge, VA
Fluorescent tubes produce relatively little heat, so they're relatively fire safe already. However, incandescent
light bulbs become very hot and you have to be careful with them to avoid fires. First, make sure that the bulb
can get rid of its waste heat. That means that you shouldn't wrap the bulb in insulation because it needs to
transfer its waste heat to the air. Second, keep flammable materials away from the bulb, particularly above the
bulb since hot air from the bulb rises upward.
What is polyester and how does it work insulating clothing? -- PGF, Seabrook, SC
Polyesters are a class of polymers, extremely long molecules that are commonly known as plastics. Each
polyester molecule is several thousand atoms long, so that a polyester fiber resembles a tiny rope made of
microscopic spaghetti strands that are all entangled with one another. Like most electric insulators, polyester
plastic is a poor conductor of heat. But in clothing its main insulating effect is to trap air. While air is a terrible
conductor of heat, it tends to undergo convection and convection allows it to transport heat pretty effectively.
However, when air is trapped by countless tiny fibers, convection is inhibited and the air becomes a great
insulator. That's why polyester fibers are such good thermal insulation--they trap air and let the air act as the real
insulation.
What are firewalls? What are they made of? -- RB
Firewalls are just insulating, non-flammable walls that prevent the heat from a fire on one side of a firewall from
initiating combustion on the other side. As far as I know, most firewalls are made from masonry block. Ceramics
such as stone or cement are already fully oxidized and can't burn. Furthermore, most ceramics are poor
conductors of heat and many can become almost white hot without melting. As long as a masonry wall is thick
enough and sturdy enough, it can tolerate having a fire on one side without conveying that fire's heat to the
combustible materials on its other side. If there aren't any holes or flaws in the firewall, it will prevent the spread
of fire between adjacent buildings.
Why does fire burn? -- PJ
Fire is a chemical reaction in which a combustible fuel reacts with oxygen to release large amounts of thermal
energy. Many atoms bind very strongly with oxygen atoms and these fuel atoms release energy when they bind
with oxygen. Initiating these combustion reactions normally requires some thermal energy to get started. This
starting energy is known as activation energy. That's why you have light the fire--you must provide the activation
energy. After that, each oxidization reaction produces the activation energy needed to start another oxidization
reaction and the fire keeps itself going until it has consumed all of its fuel.
An architect friend tells our coffee group that liquids (water, in this case) are compressible to a slight extent. We tell him
hydraulics would be impracticable under his thesis. We would appreciate comments or ammunition. -- WAW,
Brownsville, TX
I'm afraid that your friend is right--liquids are slightly compressible. A compressible material is one that
experiences a decrease in volume when it's exposed to an increase in pressure. Gases are highly compressible-they change volume dramatically with changes in pressure. Liquids are said to be incompressible--they change
volume very little with changes in pressure. But very little isn't zero. A liquid is essentially incompressible
because its atoms and molecules are touching one another and, since those atoms and molecules have relatively
fixed sizes, it's hard to pack them closer together than they already are. But increases in pressure do cause those
atoms and molecules to move slightly closer together and the liquid does becomes denser and occupies less
volume. The effect is small enough that it has almost no effect on most hydraulic systems--the pressurized fluid
loses only parts per million of its volume as you squeeze it with normal pressures. All you really care about in a
hydraulic system is that over the range of pressures used, the fluid involved doesn't change volumes much. Thus
if you keep the pressure changes small enough, even air can be used in a hydraulic system. For example,
pneumatic tube delivery systems are essentially air-operated hydraulic systems. But if the pressure changes are
large enough, even liquids and solids can be highly compressible. In fact, plutonium-based nuclear weapons use
high explosives to crush spheres of solid plutonium, already one of the densest materials in existence, to several
times solid density. You wouldn't think of plutonium as compressible, but under these astronomical pressures it
compresses almost like a gas.
Is there any gravitational force between two atoms? -- AW, Karachi, Pakistan
Yes, everything in the universe exerts gravitational forces on everything else in the universe. However, those
forces are usually so small that they are undetectable. The gravitational forces between two bowling balls are
only barely measurable in a laboratory. The gravitational forces between two atoms are so small as to be
hopelessly undetectable.
What makes heat rise? -- BN, Burlington, MA
Heat itself doesn't rise--it's a form of energy, not an object. But heated fluids often do rise. That's because raising
the temperature of a fluid usually causes that fluid to expand so that its density drops. Whenever a region of less
density fluid is surrounded by more dense fluid, the less dense region experiences a net upward force. This result
is a consequence of Archimedes' principle that less dense materials float in more dense liquid. With a net force
pushing it upward, the heated region floats upward and we say "heat rises."
What is a barometer, how does it work, and why is it useful in predicting the weather? -- HC
A barometer measures air pressure by examining the forces that air exerts on surfaces. The higher the air
pressure, the more force air will exert on a certain surface. Most barometers compare the present air pressure
with a known pressure by putting those two pressures on opposite sides of a flexible surface. The higher the air
pressure, the more that surface will bend away from it.
You can make a simple barometer by inserting a drinking straw in narrow-mouthed jar that's half full of water
and by sealing the neck of the jar around the straw (with a rubber stopper, wax, or glue). Make sure that the end
of the straw is immersed in the water and that the water level in the straw is above the top of the jar. As the
outside air pressure decreases, the trapped air inside the jar will push the water farther up the straw. As the air
pressure increases, it will push the water farther down the straw. Try to keep your barometer's temperature
constant, because temperature will also affect its water level. You can use your barometer to predict the weather
(somewhat) because storms tend to be accompanied by lower air pressures.
How do air currents flow?
Air typically rises near sources of heat and descends elsewhere. Since air doesn't normally accumulate in one
place and leave another place empty, it tends to form circulating currents. The air rises near hot objects, flows
outward above those objects, cools and descends, and finally flows back toward the hot objects from beneath
them. These circulating currents are called convection cycles.
How does water divining work? -- GD, Mansfield, Australia
I'm afraid that I remain unconvinced that water divining works at all. I believe that the whole issue is
psychological--the power of suggestion. A divining rod will twist when something exerts a torque on it but there
is no special force between the rod and water that would exert an unusual torque on the rod.
How does gravity influence the passage of time? -- AW, Karachi, Pakistan
Gravity's effects on time are the result of general relativity. Any concentration of mass/energy curves the
space/time around it, which is ultimately why objects passing near that mass/energy are deflected. This curvature
of space/time also slows the passage of time for objects that are near the concentration of mass/energy. To see
why this slowing of time must occur, imagine people operating a radio transmitter on the surface of a very
massive planet. They transmit their radio wave at exactly 100 MHz. You are far from the planet with your radio
receiver and you begin trying to find their transmission. You will find it at a lower frequency, perhaps 99 MHz.
That's because their radio wave has had to struggle to escape from the planet's gravity and has used up some of
its energy in the process. Since energy is proportional to frequency, the radio wave shifts toward lower frequency
as it climbs out of the planet's gravitational well to reach your receiver. Since the people on the planet think that
their system is operating at 100 million oscillations per second and you think that it is operating at only 99
million oscillations per second, the people on the planet are evidently experiencing time more slowly than you
are. Their second actually lasts longer than yours.
To understand how their time passes more slowly that yours, you can think of the radio wave's frequency as the
ticking of a clock. The time it takes the clock's ticks to reach your ear isn't important in measuring the passage of
time. What you care about is how often those ticks occur. When you "listen" to the ticking of the clock on the big
planet, it ticks 99 million times each second. However, to the people on the planet, it ticks 100 million times each
second. This apparent inconsistency is explained by the fact that time is passing faster for you than for the people
on the planet. Their second lasts longer than yours, which is why they count more ticks during their second than
you count during your second.
I'm doing a science experiment of what factors affect the distance a golf ball travels. One of my factors is the bounciness
of the ball. Does this have any effect on the distance the ball will go? -- EG, North Salem, NY
Yes. The bouncier the golf ball, the farther it will go after being struck by a golf club. While we normally think
of a bounce as occurring when a ball hits a stationary object, it's also a bounce when a moving object hits a
stationary ball. The golf ball bounces from the golf club and the more bouncy the golf ball is, the faster and
farther it will travel.
How are the nylon ropes of parachutes able to stop the falling parachuter? How much of a force must they over come, and
how might the ropes' elasticity be affected? -- C
When the parachuter opens the parachute and begins to slow down, the parachute's nylon shrouds briefly exert a
large upward force on the parachuter. Over a period of a few seconds, the parachuter slows from a downward
speed of about 150 mph to a downward speed of 20 mph and experiences several g's of upward acceleration. To
cause this much upward acceleration, the nylon shrouds must exert an upward force on the parachuter that is
several times the parachuter's weight. The nylon shrouds are quite strong and can easily tolerate this much
tension without exceeding their elastic limits. There should be no adverse effects on their elasticities.
Do crystals "grow"? -- JJ
Yes, crystals grow. They begin as tiny seed crystals when a few atoms or molecules manage to arrange
themselves accidentally in an orderly fashion. From that point on, new atoms or molecules that join the seed
continue the orderly pattern and the crystal grows. Some crystals grow from solutions that contain more of
particular atoms or molecules than those solutions can handle. Other crystals grow from molten materials that are
cooling off and beginning to freeze. Still other crystals form from atoms or molecules that are diffusing
randomly through solids, liquids, or even gases and encounter the proper crystal on which to stick.
How does a sound dish work? I know that it's a parabola, but I can only find drawings not explanations. -- DW, Omaha,
NE
A sound dish is actually a mirror telescope for sound. When sound waves from a distant source encounter a rigid
parabolic surface, they reflect in such a way that they focus to a point. If you put a microphone at that point, it
will detect the sound waves from the distant source. You can see this focusing effect by drawing a parabola on a
sheet of paper and directing parallel lines--the sound waves from the distant source--toward the parabola. If you
reflect each line in a mirror-like fashion from the surface it hits, you'll find that all the reflected lines pass
through a single point as they move away from the parabola.
How do you make lasers that burn?
Lasers use excited atoms or atom-like systems to amplify light. Putting mirrors around such excited atoms or
atom-like systems allows them to amplify their own light until the laser is emitted vast numbers of identical light
particles or "photons." To burn something with laser light, there must be a great many excited atoms or atom-like
systems and they must be very efficient at amplifying light. Probably the easiest to build powerful laser is a
carbon dioxide laser. This laser uses an electric discharge in a mixture of nitrogen and carbon dioxide gas to
produce excited carbon dioxide molecules. These molecules amplify infrared light at a wavelength of 10.2
microns extremely efficiently, so that a laser consuming about 1000 watts of electric power can emit
approximately 100 watts of infrared light. That's enough power to burn things very quickly. Even more powerful
carbon dioxide lasers are used in industry to cut and machine metals, including thick steel plates. But while they
are surprisingly simple to build and operate, given the right components, carbon dioxide lasers require dangerous
high voltage power supplies. There were many physics graduate students electrocuted in the 1960's while
tinkering with homemade carbon dioxide lasers.
How does a foghorn turn on and off? -- M, Brant Rock, MA
Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send
light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light
into their surroundings and turn the foghorn on when they see excessive reflection of that light.
Could you please give me a precise explanation of light scattering in relation to blue moons and red sunsets. Do dust
particles, or whatever, facilitate the transmission of some wavelengths and not others? -- DW
While the expression "blue moon" usually refers to the infrequent occurrence of second full moon in a calendar
month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or
volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the
red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the
viewer's eyes. The moon thus appeared blue.
Red sunsets are much more common and they are caused by Rayleigh scattering--the non-resonant scattering of
light by particles that are much smaller than the light's wavelength. While Rayleigh scattering is rather weak, it's
weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue
and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red,
particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your
eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and
the red sunsets are particularly intense.
Is it harmful for children to sit too close to microwave ovens? Is it possible to get "burned" opening the microwave oven
during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? -- C
As long as the microwave oven hasn't been damaged and doesn't leak excessive microwaves, there should be no
harm in having children sit near it. I wouldn't hold my face right up against the door edges because that would be
asking for trouble with leakage, but it's extremely unlikely that even doing that once in a while would cause
injury.
As for being injured by microwave radiation after the cycle has stopped, that's essentially impossible. As soon as
the high voltage disappears from the magnetron tube and it stops emitting microwaves, the microwaves in the
cooking chamber begin to diminish. Even if they bounce 1000 times off the metal walls of the chamber before
they're absorbed by those walls or the food in the microwave, that will only take about 2 millionths of a second.
You can't open the door fast enough to let them out before they're already gone.
Please explain the "Wagon Wheel Effect." How can the wheel appear to move forward, then backward, then stop, just by
viewing it differently? -- J, Davenport, IA
This effect is the result of viewing a series of stop-action frames in rapid sequence as a movie or video. Even
though a wagon wheel is turning forward, its orientation during sequential frames of a movie may make it appear
to be stopped or turning backward. For example, if the wagon wheel completes exactly one full turn between
each frame of the movie, the wheel will appear to be stopped--its orientation in each frame will be the same. If it
completes slightly less than one full turn between each frame, it will appear to be turning backward! As you can
see, a tiny change in wheel rotation rate, from slightly more than one full turn per frame to slightly less than one
full turn per frame, is enough to make the wheel appear to switch from turning forward, to stopped, to turning
backward. So it's no wonder that the wheels appear to change speeds abruptly from no apparent reason.
How do I make my own satellite descrambler/decoder?
Even if I knew, I'm sure that I'd get in trouble for telling. The encoding schemes are proprietary information and
not available to the general public. To my knowledge, most of the descrambling/decoding in a satellite receiver is
done by custom integrated circuits that are extremely difficult to reverse engineer (i.e., to open up, examine, and
duplicate) so that pirating satellite signals is nearly impossible without insider information.
Is there a homing device small enough to fit onto or inside a pc laptop? How does a homing device work?
There are homing devices small enough to fit on bugs, so there should be no problem fitting one on or into a
laptop. A homing device is simply a radio transmitter and, while it has recently become possible to build a
homing device that actually knows where it is and can tell you via its transmission, the techniques involved in
locating most normal homing devices are those of trying to find the source of a radio transmission. Using
directional receiving antennas and studying the transmission from several locations, you can figure out where the
transmission is coming from.
How do electric/magnetic linear drives work?
Linear electric motors are very much like rotary electric motors--they use the forces between magnetic poles to
push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle,
a linear motor uses these forces to push a carriage along a track. Both the carriage and the track must contain
magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed.
By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the
track smoothly and continuously.
In making an electric generator, how do different aspects of the wire affect the total voltage and amperage? What are the
effects of wire gauge, number of turns in the coils, and whether the magnets move past the coils or the coils past the
magnets? -- BLM, Houston, TX
First, it doesn't matter when the magnet moves past the coils or the coils past the magnet; a generator will work
the same way in either case. The voltage produced by the generator is determined by the number of turns in its
coils, the strength of its magnet, and the rate at which its magnet turns. The more turns in the coils, the more
work the generator does on each charge that passes through those coils and the more voltage the charges have
when they leave the generator. The current that the generator can handle is limited by the power of its engine and
by the wire's ability to handle the current without wasting too much power. In general, a generator's wire gauge
is chosen to minimize power loss while keeping the coils reasonably small and light. If you try to send too much
current through the generator, its engine may stall or its wires may overheat.
How would you construct and wire a battery recharger using solar panels as a voltage source? -- JW, Kingston, Ontario
First, you would need to put enough solar panels in series to develop a voltage greater than that of your battery.
For example, to recharge a 1.5 volt battery, you would probably have to attach three or four simple solar cells in
series because each one only provides a current passing through it with about 0.5 volts of voltage rise. Having
assembled enough solar cells, you should then attach the positive output terminal of the solar cell chain to the
positive terminal of your battery and attach the negative output terminal of the solar cell chain to the negative
terminal of your battery. When you put the solar cells in the light, they will begin to push electric current
backward through the battery and the battery will recharge. Whenever you send current backward through a
battery, its electrochemical reactions can run backward and it can recharge to some extent. Unfortunately, some
batteries recharge more effectively than others--the bad ones just turn the recharging energy into thermal energy.
The only real subtlety in this business is in stopping the charging when the battery is fully recharged. You should
check the battery voltage periodically and when it's close to the voltage of a new battery, it probably can't take
any more charging.
How do you make solar cells? -- BP
Solar cells are made in the same way that semiconductor diodes are made. Two different types of semiconductor,
p-type and n-type, are joined together to form a diode--a one-way device for electric current. When light energy
is absorbed in the n-type portion of the diode, it can propel an electron across the p-n junction between the
materials and into the p-type material. Since the electron can't return across the p-n junction to its original
location, it must flow through an external circuit to get back. Since it obtains energy from the light that sent it
across the junction, the electron can provide that energy to the circuit. The solar cell is thus a source of electric
power.
A charge coupled device converts light (photonic energy) into electric energy. What is the underlying mechanism that
makes this happen? -- PM, Belfast, Ireland
As in any photoelectric cell, the energy from a single particle of light--a photon--is used to raise the energy of an
electron in a diode and to propel that electron from one side of the diode to the other. In this process, the light
energy is partly converted to electrostatic potential energy and partly to thermal energy. Since a diode only
carries current in one direction, the electron is unable to return to its original side. In a photoelectric cell, the
electron flows through a circuit to return to the other side of the diode and provides energy to that circuit. In a
charge coupled device, a complicated charge shifting system transfers the electrons to a detector that registers
how much light was absorbed.
February 28, 1997
Why do neutrinos only spin left? -- BA, Fairbury, IL
The absence of right-handed neutrinos is simply a feature of our universe and I don't believe anyone has a good
explanation for their absence. Actually, it's still possible that right-handed neutrinos exist, but if they do exist,
then they don't interact with other matter by way of any known force other than gravity. Even left-handed
neutrinos barely interact with matter--they experience only gravity and the weak force, and usually pass through
the entire earth without being absorbed. It could be that right-handed neutrinos are also present but that they don't
even experience the weak force.
Is there an effective shield for the EMF generated from mercury vapor ballasts? -- CS, Washington, DC
An electric field can always been shielded by encasing its source in a grounded conducting shell. Electrically
charged particles in the shell will naturally rearrange themselves in such a way as to cancel the electric fields
outside the shell. But magnetic fields are harder to shield, particularly if they don't change very rapidly with time.
The difficulty with shielding magnetic fields comes from the apparent absence of isolated magnetic poles in our
universe--there is no equivalent of electrically charged particles in the case of magnetism. As a result, the only
way to shield magnetic fields is to take advantage of the connections between electric and magnetic fields.
Because changing magnetic fields are always accompanied by electric fields, the two can be reflected as a pair
by highly conducting surfaces or absorbed by poorly conducting surfaces. In these cases, the electric fields push
and pull on electric charges in the surfaces and it is through these electric fields that the magnetic fields are
reflected or absorbed. However, this effect works much better at high frequencies than at low frequencies, where
very thick materials are required. Appliances that operate from the AC power line have magnetic fields that
change rather slowly with time (only 120 reversals per second or 60 full cycles of reversal each second) and that
are extremely hard to shield with conducting material. Instead, their magnetic fields have to be trapped in special
magnetic materials that draw in magnetic flux lines and keep them from emerging into the surrounding space.
One of the most effective magnetic shield materials is called "mu metal", a nickel alloy that's like a sponge for
magnetic flux lines. Since it also conducts electricity pretty well, it is an effective shield for electric fields. So if
you wrap your mercury vapor ballasts in mu metal, there would be almost no electric or magnetic fields
detectable outside of the mu metal surface.
February 27, 1997
There is a debate amongst the teachers in our school as to what are the three primary colors. Some say Red, Green, and
Blue, others say Red, Yellow, and Blue. Do you have an explanation? -- RS, Farmington Hills, MI
The true primary colors of light are Red, Green, and Blue. This empirical result is determined by physiological
characteristics of the three types of color sensitive cells in our eyes. These cells are known as cone cells and are
most sensitive to red light, green light, and blue light respectively. Light that falls in between those wavelength
ranges stimulate the three groups of cells to various extents and our brains use their relative stimulations to
assign a color to the light we're seeing. For example, when you look at yellow light, the red sensitive and green
sensitive cone cells are stimulated about equally and your brain interprets this result as yellow. When you look at
an equal mixture of red light and green light, the red sensitive and green sensitive cells are again stimulated
about equally and your brain again interprets this result as yellow. Thus you can't tell the difference between true
yellow light and an equal mixture of red light and green light. That's how a television tricks your eyes into seeing
all colors. If you look closely at a color television screen, you'll see tiny dots of red, green, and blue light. But
when you back up, you begin to see a broad range of colors. The television is mixing the three primary colors of
light to make you see all the other colors.
Incidentally, the three primary colors of pigment are yellow, cyan, and magenta. Yellow pigment absorbs blue
light, cyan pigment absorbs red light, and magenta pigment absorbs green light. When exposed to white light, a
mixture of these three pigments controls the mixture of the reflected lights (red, green, and blue) and thus can
make you see any possible color.
When making an electromagnet, why does a hard core stay permanently magnetized while a soft core does not? -- CD,
Houston, TX
Iron and steel are intrinsically magnetic materials, meaning that at the atomic scale they exhibit magnetic order
and have magnetic poles present. Most materials, including copper and aluminum, have no such magnetic order-they are nonmagnetic all the way to the atomic scale. But while it is composed of magnetic atoms, a large piece
of iron or steel normally doesn't appear magnetic. That's because a large piece of iron or steel contains many tiny
magnetic domains. Although each of these magnetic domains is highly magnetic, with a north pole at one end
and a south pole at the other end, the metal appears nonmagnetic at first because these domains point equally in
all directions and their magnetizations cancel one another. Before the magnetic character of a piece of iron or
steel will become visible, something must align its magnetic domains.
In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil
of wire, the magnetic field of the current causes the core's magnetic domains to change sizes--the domains that
are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of
iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the
domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.
The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal,
particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily.
These materials are considered to be "soft"--they magnetize easily in the presence of a magnetic field and
demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials
because it takes only a small current in a wire coil to magnetize the electromagnet's soft core and that core
quickly becomes nonmagnetic when you stop the current from flowing.
But in other magnetic materials, the domains don't change size easily. These materials are considered to be
"hard"--they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through
the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the
current, the material will retain its magnetization and it will be a permanent magnet.
February 26, 1997
How does a thermometer work? -- DL
A common liquid in glass thermometer takes advantage of the fact that liquids generally expand more than solids
as their temperatures increase. The glass envelope of the thermometer contains a fine hollow capillary with a
sealed reservoir at its base that's filled with a liquid such as alcohol or mercury. If both the liquid and glass
expanded equally as they became warmer, the thermometer would simply change sizes slightly as its temperature
increased. But the liquid expands more than the glass and can't simply remain in place. Some of it moves up the
capillary. That's why the level of liquid in the thermometer rises as the thermometer's temperature rises.
How do rockets work?
Rockets push stored materials in one direction and experience a thrust force in the opposite direction. They make
use of the observation that whenever one object pushes on a second object, the second object exerts an equal but
oppositely directed force back on the first object. This statement is the famous "action-reaction" concept that is
generally known as Newton's third law. While it seems sensible that when you push on a wall it pushes back on
you, this situation is extraordinarily general. For example, if you push a passing car forward, that car will still
push backward on you with an equal but oppositely directed force. If you push on your neighbor, your neighbor
will push back on you with an equal but oppositely directed force even if your neighbor is asleep! In the case of a
rocket, the rocket pushes burning fuel downward and the burning fuel pushes upward on the rocket with an equal
but oppositely direct force. If the rocket pushes its fuel downward hard enough, the fuel will push up on the
rocket hard enough to overcome the rocket's weight and accelerate it upward into the sky and beyond.
How does one calculate the pressure of air flowing in a tube? My specific application is air traveling in a 1/2-inch tube at
a velocity of 14 inches/second. I know that Bernoulli would have the answer, but I cannot find it myself. -- NT,
Cambridge, MA
Without more information about the air in your tube, it's not possible to determine its pressure. Bernoulli's
equation is frequently misunderstood to say that high-speed air is low-pressure air and that low speed air is highpressure air--two observations that aren't necessarily true. Just because air is moving rapidly doesn't mean that its
pressure is low. For example, the air in an airplane cabin is moving quickly but its pressure is higher than that of
the air outside the cabin. Similarly, if you were to throw a tank of compressed air across the room, its pressure
would remain high despite its increase in speed.
What Bernoulli's equation really says is that air has three forms for its energy and that as long as that air flows
smoothly and without significant friction through a system of stationary obstacles, the sum of those three
energies can't change. The three energies are kinetic energy (the energy of motion), gravitational potential
energy, and an energy associated with pressure that I call pressure potential energy. The obstacles must remain
stationary so that they can't do work on the air and thus change its total energy. Since the sum of those three
energies doesn't change as air flows through a stationary environment, its pressure typically falls whenever its
speed rises and vice versa. If the air also changes altitude significantly, then gravitational potential energy must
be included in these energy exchanges.
So the reason why I can't answer your question about air in a pipe is that I don't know what the air's total energy
was before it flowed through the pipe. While I can calculate the air's kinetic energy from its speed and we can
neglect gravitational potential energy because the air isn't changing altitudes much in the pipe, I need to know
what the air's total energy is in order to determine its pressure potential energy and thus its pressure.
What is the definitions of a "Hanning window", a "rectangular window", and a "triangular window"? -- CV, Cape Town,
South Africa
In the days before digital signal processing, the filters that were available for audio or video systems were very
simple. These filters monitored the audio or video signal and produced an output signal that was related to the
present input signal and to that signals value's in the recent past. Such simple filters could enhance or diminish
certain ranges of frequencies and were able to perform basic tasks such as adjusting the balance between treble,
midrange, and bass in an audio system.
But with computers and digital signal processing now commonplace, filtering has become much more
sophisticated. Filters can now study an audio or video input signal over a long period of time and can even use
data about future values of the input signal when producing an output signal. The filters that you ask about are all
digital filters that produce an output signal that is related to the past, present, and future values of the input
signal. A rectangular window filter is one that determines the output signal from a certain range of past, present,
and future input signal values, all weighted evenly. A triangular or "Parzen" window filter is one that determines
the output signal from a certain range of past, present, and future input signal values, with the weighting of
values decreasing linearly with increasing time in the past or future. A Hanning window filter is one that
determines the output signal from the complete past and future input signal values, with the weighting of values
decreasing as the cosine of the time in the past or future (see for example, "Numerical Recipes" by Press,
Flannery, Teukolsky, and Vetterling). All three filtering windows and filters are used to keep filters that extract
certain frequency ranges from the input signal from affecting other frequency ranges. For that purpose, the
Hanning window is better than the Parzen window and both are better than the rectangular window. As an
example of the applications of these filters, a digital audio filter that makes good use of the Hanning window can
enhance the treble of an audio signal uniformly without coloring the midrange at all. Earlier filters that only used
past information always colored the midrange and didn't affect the treble uniformly.
Is it possible to greatly increase the speed of a roller coaster, while retaining some safety, by applying the same theory
that is used in Bullet Trains? -- JA, Henderson, NV
While roller coasters could be made faster if they used the high performance tracks of bullet trains, smoothing
out the tracks would only make the ride less jittery and wouldn't reduce the accelerations needed to complete the
turns. The faster the train moves, the faster everything must accelerate as the track bends. Doubling the speed of
the roller coaster would double the changes in velocity associated with each bend and would halve the time
available to complete that change in velocity. As a result, doubling the roller coaster's speed would quadruple the
accelerations it experiences on the same track and thus will quadruple the forces involved during the ride. A
roller coaster ride already involves some pretty intense forces and accelerations. If those forces and accelerations
were increased by a factor of 4, they would be more than most people could handle. Thus I wouldn't expect many
riders on a double-speed bullet train roller coaster.
What is the difference in distance that a soccer ball will travel if the air pressure in the ball changes? -- AB
A properly inflated soccer ball bounces well when you drop it on a hard floor because the ball stores energy by
compressing the air during the bounce and the air returns this energy quite efficiently during the rebound. An
under inflated soccer ball doesn't bounce so well because it stores energy by bending its leather surface during
the bounce and the leather doesn't return energy very efficiently during the rebound. The same result holds true
when you kick a ball rather than dropping it on the floor. Whether a moving ball hits a stationary surface or a
stationary ball hits a moving surface, the ball is still bouncing from a surface. When you kick a ball with your
foot, the ball is bouncing from your foot and a properly inflated ball will bounce more efficiently from your foot
than an under inflated ball. The properly inflated ball will rebound at a higher speed and will travel farther.
What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the
microwave that absorbs all the microwaves? -- AD, San Anselmo, CA
The walls of a microwave oven's cooking chamber are made of highly conductive metals so that they reflect the
microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by
these metal walls and virtually none of the microwaves escape into the room. However, there is a substance
inside the cooking chamber that absorbs the microwaves: water in the food! If you don't put water-containing
food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the
magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor
leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much
higher than normal.
February 22, 1997
What is gravity? We know Newton's formula but he did not answer what the true nature of gravity is. I hear talk about
"gravitons" -- is this real or just another elegant metaphor? -- BC
Newton's gravity has been superceded by Einstein's gravity; the gravity of general relativity. In this
understanding of gravity, the accelerations associated with gravity result from a curvature of space/time around
concentrations of mass & energy. The gravity of general relativity is responsible for such exotic effects as the
bending of light by gravity and the existence of black holes.
But physicists are still not satisfied with the gravity of general relativity. General relativity is what's known as a
"classical" theory of interactions--it does not include quantum physics and is thus considered to be incomplete.
All the other classical theories of interactions have given way to quantum theories. For example, the classical
theory of electromagnetic interactions, dating from the works of Oersted, Ampere, Maxwell and others in the
1800's, was replaced in the 1940's and 50's by quantum electrodynamics, through the works of Feynman,
Schwinger, Tomonaga, and others. Each time that a classical theory is replaced by a quantum theory, the
responsibility for the interactions themselves shifts from classical fields (e.g., the electric and magnetic fields) to
quantized or particulate fields (e.g., photons). These sorts of quantum field theories, theories in which
interactions between particles are mediated by the exchanges of other particles (the particles of the quantized
fields) are the bases for all modern interaction theories except gravity itself. People are still trying to quantize
gravity but so far without real success. The particles that mediate gravitational interactions have been named
gravitons, but the full theory in which these particles operate is still uncertain.
Why is a satellite dish necessary to receive satellite broadcasts? Why doesn't a conventional radio antenna work? -- AW,
Karachi, Pakistan
Normal television broadcasts use electromagnetic waves with relatively low frequencies and long wavelengths
while satellite broadcasts use waves with relatively high frequencies and short wavelengths. The short
wavelength waves from a satellite are known as microwaves while the longer wavelength waves from a normal
broadcast station are generally known as radio waves. Since the optimal antenna size for receiving a particular
electromagnetic wave is proportional to the wavelength of the wave, you need a smaller antenna to receive the
microwaves from a satellite than you do the radio waves from a normal television station. However, the
microwaves from a satellite are much weaker than the radio waves from a nearby television station and a small
microwave antenna isn't likely to absorb enough of them to produce a useable signal.
The solution to this dilemma is to concentrate the microwaves from a satellite with the help of an optical imaging
system. Although it may not look like one, a satellite dish is really a carefully shaped mirror telescope. Just as the
curved mirror of the Hubble space telescope can bring light from a distant star to a focus on an optical image
sensor, so the curved wire mesh of a satellite dish can bring microwaves from a distant satellite to focus on a
small microwave antenna. This microwave antenna sits at the focus of the satellite dish and absorbs the
microwaves that the dish collects. The dish's imaging behavior also ensures that microwaves from only one
satellite are brought to a focus on the microwave antenna. You must redirect the dish or move the antenna in
order to switch from one satellite to another.
I have experimented with passing high voltage arcs through ionic compounds and have observed different colors when I
do. An arc through salt (sodium chloride) produces a brilliant yellow light. How does this work? -- JB, Lantana, FL
When electric current passes through air as an arc, the air becomes hot enough to vaporize the compounds you
expose to it. As a result, there are individual sodium and chlorine atoms moving about in the arc itself. Like all
atoms, a sodium atom resembles a tiny planetary system. It has 11 negatively charged electrons orbiting a
massive, positively charged nucleus. But unlike our experience with the solar system, the electrons in a sodium
atom can only travel in certain allowed orbits or "orbitals." These electrons are normally found in the orbitals
with the lowest possible energy. But when charged particles in the arc collide with sodium atoms, they often shift
electrons in those atoms to orbitals with more energy. The electrons quickly return to their original orbits and
emit their excess energies as light during their returns. In the case of sodium, the final step of the most common
return path results in the emission of yellow light with a wavelength of about 590 nanometers. This yellow light
is the same one you see in the sodium vapor lamps that are used to light highways and parking lots.
While sodium tends to emit yellow light, other atoms have different orbital structures and emit their own
characteristic colors. Copper and barium atoms emit blue/green light while strontium atoms emit red light. These
colored lights are the same ones that you see in fireworks.
February 21, 1997
What happens when a speaker blows?
A speaker produces sound by using magnetic forces to push or pull a thin surface--the speaker cone--toward or
away from the listener. As the cone moves forward, it compresses the air in front of it and as the cone moves
backward, it rarefies the air in front of it. These compressions and rarefactions are what produce sound. But if
you try to drive the cone into motions that are too extreme by turning up the volume of an amplifier too high, the
cone will reach the limits of its motion. At that point, the cone may tear away from the electromagnetic coil that
pushes it back and forth or it may tear away from the supports at its outer edge. The electromagnetic coil may
also burn up because of overheating. All of these failures are lumped together as "blowing a speaker."
February 18, 1997
When you are looking at something and there is an object partially blocking your view (e.g., a fence or a railing), why
with one eye closed does the barrier block your vision but with both eyes open you seem to look through the barrier? -DS
Your brain merges the images it obtains from your two eyes so that you "see" a composite image that is
essentially a sum of what both eyes see. When you close one eye so that only the other eye is providing an image
to your brain, any object that blocks your view chops a piece out of the distant scene. No light from that portion
of the scene reaches your open eye, so you can't see that portion of the scene. But when you have both eyes open,
the image observed by one eye can compensate for any missing pieces in the image observed by the other eye.
Since the barrier you are looking through chops out a different piece of the distant scene for each of your two
eyes, the composite image that your brain assembles from these two individual images will include the whole
scene.
Why does microwave radiation affect plant seeds differently? If you microwave sunflower seeds 30 seconds, they
germinate faster than if you did not microwave them at all, and yet if you microwave them for 60 seconds, the seeds do
not germinate at all. If you do this same experiment with carrot seeds, the non-radiated seeds, the 30 second and 60
second seeds all germinate within 14 days. Why? Is it because the sunflower seeds are larger and absorb more radiation
than the smaller carrot seeds? -- ST, Mobile, AL
When you expose the seeds to microwave radiation, you are selectively heating portions of the insides of the
seeds. Fats and oils don't absorb microwaves well but water does, so the parts of the seeds that become hottest
are those that contain the most water molecules. Evidently, heating the water-containing portion of a sunflower
seeds slightly cause that seed to germinate faster, but heating that same portion too much sterilizes the seed. That
observation indicates that a moderate temperature rise causes the chemical reactions of germination to occur
more rapidly while a more severe temperature rise denatures some of the critical biological molecules and kills
the seed. The absence of any effect in carrot seeds may indicate that they don't have enough water in them to
absorb the microwaves. It may also indicate that they can tolerate higher temperatures without undergoing the
chemical reactions of germination and without experiencing damage to their critical molecules.
I heard recently of someone with a pacemaker who went near a microwave oven and his pacemaker faulted, with him
needing urgent medical attention. How did this happen? I also know of someone currently undergoing chemotherapy,
who was told by his doctor not to eat food from a microwave oven. Why?
A pacemaker contains electronic circuits and wires that can act as antennas for microwaves. If a pacemaker is
exposed to sufficiently intense microwaves, currents will begin to flow in those wires and circuits, and these
currents may cause computational errors to occur or they may cause the circuitry to overheat. But while a
pacemaker is far more sensitive to microwave radiation than say your hand is, I'm still surprised that enough
microwave radiation leaked out of the oven to cause trouble. I'd suspect a real problem with that oven.
As for the chemotherapy question, I can't think of any reason why the doctor would suggest avoiding cooking
food in a microwave oven. Unless I hear otherwise, I would suspect ignorance on the part of the doctor. The
doctor may not understand the difference between "microwave radiation" and "gamma radiation".
What is a Zobel network in an audio amplifier and how does it work? Is it an effective device or not? -- CV, Cape Town,
South Africa
My understanding is that a Zobel network consists of a resistor in series with a capacitor and that the capacitor is
normally connected to ground. When you attach the free end of this network to a wire carrying an audio signal,
the network acts like a frequency-dependent load. At very low frequencies, the capacitor has plenty of time to
charge through the resistor and the network has little effect on the audio signal--it acts as though it weren't there.
At very high frequencies, the capacitor has no time to charge through the resistor and behaves like a wire. As a
result, the network acts as though it were just the resistor connecting the audio signal wire to ground. So the
impedance of the Zobel network varies from infinite at low frequencies to become equal to the resistance of the
resistor at high frequencies. The crossover between these two behaviors is related to the RC time constant. I
think that Zobel networks are used in audio amplifiers to dampen out high frequency oscillations that might
occur in the absence of loads at high frequencies.
How does a car horn work? -- CP
While some modern car horns are actually specialized computer audio systems, the old-fashioned
electromagnetic car horns are still common. An electromagnetic horn uses an electromagnet to attract a steel
diaphragm and turns that electromagnet on and off rhythmically so that the diaphragm vibrates. In fact, it uses
the diaphragm's position to control the power to the electromagnet. Whenever the diaphragm is in its resting
position or even farther from the electromagnet, a switch closes to deliver electric current to the electromagnet.
The electromagnet then attracts the diaphragm's center. But when the diaphragm moves closer to the
electromagnet, as the result of this attraction, the switch opens and current stops flowing to the electromagnet.
Because of this arrangement, the diaphragm moves in and out and turns the electromagnet off and on as it does.
The diaphragm's tone is determined by the natural resonances of its surface.
How can you run a clock off of a potato?
The classic technique is to insert two dissimilar metal strips into the potato in order to build a simple battery.
You can then run an electronic clock with the power provided by that battery. But the energy in that battery is
coming from chemical reactions of the metals and not really from the potato. If you really want to use a potato as
the power source for a clock, you should dry the potato out and burn it. You can use the heat of the fire to run a
steam engine or to generate electricity.
What is an analog clock? How do you attach it to batteries? -- HB
An "analog" clock is a clock that has an hour hand and a minute hand. Twenty years ago, virtually all clocks
were analog clocks but nowadays electronics has made it easier to display time with digits ("digital" clocks) than
with hands ("analog" clocks). However, there are some clocks and wristwatches that still use moving mechanical
hands to display the time. Most of these devices use quartz crystal oscillators to control electronic pulsing
devices that drive electric motors that advance the hands. In such clocks, the batteries power the oscillators and
the motors. You connect them as you would any electronic device: you form a string of batteries with the correct
voltage, attach the negative lead from the clock to the negative terminal of the battery string, and attach the
positive lead from the clock to the positive terminal of the battery string.
There are also some analog clocks in which the hands are just lines on a computer display, an arrangement that
strikes me as silly. Finally, long ago there were two interesting types of analog electric clocks: the electric clocks
that used the AC power line to run synchronous electric motors to advance their hands and the electric clocks
that were used in automobiles. The automobile clocks were actually mechanical clocks, with mainsprings and
everything, but they were wound by electromagnetic devices. Every minute or two, this device would give the
spring a small wind and you would hear a click.
Why does regular water freeze faster than salt water? -- CD, Crown Point, IN
When salt dissolves in water, its individual sodium positive ions and chlorine negative ions are carried about by
the water molecules. Each of these ions is wrapped in a solvation shell of water molecules. These solvation shells
and the salt ions themselves interfere with the water's ability to crystallize into ice. The ice crystals that form
when salt water freezes rarely include the salt ions so the water molecules must abandon the salt ions in order to
crystallize. Because of the attraction between the salt ions and the water molecules, and because of the loss of
randomness that comes with forming pure ice crystals in the midst of salty water, you must lower the
temperature of salt water below the freezing temperature of pure water before that salt water will begin to freeze
into ice. When ice does begin to form, it will be relatively pure water crystals and the remaining water will
become increasingly saltier. If you're ever lost in the winter without a supply of fresh water, look for sea ice-even though it forms from salt water, it contains very little salt.
How does ammonia refrigeration work?
There are actually two answers to this question. First, like the more modern chlorofluorocarbon (Freon) and
hydrofluorocarbon refrigerants, ammonia (NH3 converts easily from a gas to a liquid near room temperature. If
you squeeze ammonia to high density, it will release heat and convert to a liquid. If you let it expand to low
density, it will absorb heat and convert to a gas. A compressor-based ammonia refrigeration unit makes use of
that easy convertibility. First, it uses a compressor to squeeze the ammonia gas outside the refrigerator. The hot
dense ammonia gas that leaves the compressor enters a condenser, where it releases heat to its surroundings and
condenses to a cool ammonia liquid. This liquid enters the refrigerator and passes into an evaporator, where it's
allowed to expand into a gas and it absorbs heat from its surroundings. The gas then returns outside the
refrigerator to repeat this cycle again and again.
But there is a second type of ammonia refrigerator that makes use of an absorption cycle--ammonia dissolves
extremely well in cool water but not so well in hot water. In an absorption cycle refrigerator, a concentrated
solution of ammonia in water is heated in a boiler until most of the ammonia is driven out of the water as a highpressure gas. This hot, dense ammonia gas then enters a condenser, where it gives up heat to its surroundings and
becomes a cooler liquid. The liquid ammonia then enters a low-pressure evaporator, where it evaporates into a
cold gas. This evaporation process draws heat out the evaporator and refrigerates everything nearby. Finally, the
ammonia gas must be returned to the boiler to begin the process again. That return step makes use of the
absorption process, in which the ammonia gas is allowed to dissolve in relatively pure, cool water. The gas
dissolves easily in this water and thus maintains the low pressure needed for evaporation to continue in the
evaporator. The now concentrated ammonia solution flows to the boiler where the ammonia is driven back out of
the water and everything repeats.
Why is an incandescent light bulb hotter than a fluorescent light? -- TJ, Woodbridge, VA
An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. At that
temperature, the thermal radiation that the filament emits includes a substantial amount of visible light. But the
filament also emits a great deal of infrared light (heat light) and it also transfers heat via conduction and
convection to the glass bulb around it. When you put your hand near the bulb, you feel both the infrared light and
the heat that has worked its way to the surface of the bulb. The bulb feels hot.
In contrast, a fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to
produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the
layer of white phosphor powders on the inside of the lamp's glass envelope. In principle, this whole activity can
be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to
convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm
rather than hot.
How does a halogen cooktop unit heat up food? -- BS, Logan, UT
A halogen cooktop unit uses thermal radiation to transfer heat to a pot or pan. All objects emit thermal radiation,
but that radiation isn't visible until an object's temperature is at least 500° C. At higher temperatures, a significant
fraction of an object's thermal radiation is visible light. In a halogen cooktop unit, an electrically heated tungsten
filament is heated to the point where it emits a large amount of thermal radiation. Since the filament is small, it
takes only a second or two for the filament to reach full temperature and begin emitting its intense thermal
radiation. Any dark object above the unit will absorb this thermal radiation and experience a rise in temperature.
When you turn off the unit, the filament cools rapidly and stops emitting its thermal radiation. The filament itself
is protected from oxygen in the air by a heat-resistant glass envelope that's filled with halogen gas. This gas helps
to keep the filament intact and prevents it from depositing tungsten atoms on the insides of the glass envelope.
Can you tell me the difference in lifting power of helium versus hydrogen? -- FL, Napa, CA
A balloon experiences an upward buoyant force that's equal in amount to the weight of the air it displaces. If that
balloon is filled with helium or hydrogen, both of which have very low densities, then this upward buoyant force
may be more than the balloon's weight and the balloon may accelerate upward. Helium weighs a little more per
cubic foot or cubic meter than hydrogen does, so replacing the helium with hydrogen will make it easier to float
the balloon. A cubic foot of hydrogen weighs 0.0056 pounds less than a cubic foot of helium and a cubic meter
of hydrogen weighs 89 grams less than a cubic meter of helium. Any weight saving made by replacing helium
with hydrogen in your balloon can be viewed as extra lifting power. As you can see, the effect is small and
hydrogen is a whole lot more dangerous than helium.
I have read that sometimes two very slick things rubbing together have more friction than two rough things. Is that true?
Why? -- A
Friction is caused by contact and collisions between the tiny projections that exist on all surfaces. When you put
one block on top of another, the tiny projections on the bottom of the upper block touch the tiny projections on
the top of the lower block. If you then try to slide one block across the other, these projections begin to collide
with one another and they oppose the sliding motion.
If the two blocks have rough surfaces, then the projections that are colliding are obvious to your eyes. But if the
two blocks have very smooth surfaces, you can't see their surface projections. However, the invisibility of these
projections doesn't make them insignificant. Even the smoothest surfaces are rough at the atomic scale. When
you press two smooth surfaces against one another, their microscopic projections still touch one another and
those projections still collide when you try to slide the surfaces across one another. In short, smooth surfaces still
experience friction.
But it's also possible for attachments to form between portions of the two smooth surfaces when they touch. This
molecular adhesion makes it even harder to slide the two surfaces across one another. You can feel this adhesion
when you press two pieces of very clean glass against one another--they form bonds that partially stick them
together. Actually, this sort of sticking would be quite common if it weren't for water. Almost all surfaces are
coated with a layer or two of water molecules. These water molecules lubricate the interface between any two
surfaces and make it hard for those surfaces to stick to one another. But if you get rid of the water molecules, the
sticking becomes quite severe. This effect causes trouble in my laboratory, where sliding mechanisms that move
easily in air stop working properly when we put them in a vacuum chamber and remove the water on their
surfaces.
We know that spinning objects on earth can lose their spin (angular momentum) due to friction (fluid or sliding) with the
air or ground. However, if an object is set spinning in space, will it lose its initial angular momentum eventually or will it
spin forever assuming no outside forces (e.g., gravity) act upon it? If it does come to rest, how does the earth maintain its
spinning motion? -- RD, Kingwood, TX
If a spinning object is truly free of outside torques--the influences that affect rotation--then it will spin forever.
Angular momentum is a conserved quantity in our universe, meaning that it can't be created or destroyed and can
only be transferred between objects. Thus if you set an object spinning (by exerting a torque on it) and then leave
it entirely alone, it will not be able to change its angular momentum. The earth is a good example of this
situation--it's almost free of torques and so it spins steadily about a fixed axis in space. Its angular momentum is
essentially unchanging.
Since gravity acts at the center of rotation of a freely falling object (which is that object's center of mass), gravity
exerts no torque on freely falling objects. Because of that fact, even objects in orbit around the earth are
essentially free of torques and satellites that are set spinning when they're launched continue to spin steadily for
centuries. The space shuttle astronauts encounter this result each time they release or catch a satellite. If they set
it spinning when they let go of it, it will still be spinning when they retrieve it years later.
February 17, 1997
How can one tell the difference between a gravitational red shift of light and a red shift caused by motion? Could the red
shift of quasars be from gravity and not speed, therefore making the quasars closer than we think they are? - FG
At astronomical distances, there is no way to tell the difference between the two red shifts. An object that is deep
in the gravitational potential well of a very massive object experiences time slowly and its light appears shifted
toward the red (low frequency and long wavelength) when it reaches us. The light from an object that is moving
away from us rapidly also appears red shifted (low frequency and long wavelength), but this time it's due to the
Doppler effect.
Quasars exhibit enormous red shifts and one explanation for those red shifts is that the quasars are located near
the other side of the universe. If so, they would be moving away from us rapidly, along with their surroundings
in the expanding universe, and their light would appear highly red shifted. Moreover, their light would have been
traveling almost since the beginning of the universe so that we would be observing very ancient objects.
However, it's also possible that quasars are much near to us and that their red shifts are caused by gravitational
effects rather than relative motion. As far as I know, this possibility can't be ruled out and remains a concern
amount the astronomical community.
How is sunlight both harmful and beneficial? - CP
Sunlight provides virtually all the energy in our world. Without it, plants wouldn't grow and we wouldn't have
food or daylight. We wouldn't even have fossil fuels such as coal and petroleum because those were formed from
vegetation that itself derived energy from the sun. However, sunlight also contains ultraviolet light, which can
damage chemicals in biological tissue. Long exposure to ultraviolet light can age your skin or cause cancer.
When you talk about the magnetic tape and recording, is it the pressure or frequency that is being recorded? Are pressure
and frequency interrelated?
Sound consists of pressure fluctuations. The stronger those pressure fluctuations, the louder the sound. The
rapidity with which the air goes between a pressure increase and a pressure decreases determines the frequency
of the sound and the pitch that we hear. So the extent of the pressure fluctuations, their amplitude, determines
the sound volume while the number of pressure fluctuations each second, their frequency, determines the sound
pitch. The tape recorder detects both and records both. The louder the sound, the deeper the recorder magnetizes
the tape. The higher the frequency of the sound, the more often the tape recorder reverses the magnetization of
the tape's surface.
How long will a magnetic tape stay magnetized? Won't it lose its magnetization very fast, like we saw with the iron nails?
At room temperature, a magnetic tape will remain magnetized for years and years. It is made of much harder
magnetic materials than the nails are made of and it is much harder to demagnetize than the nails. In effect, it is
covered with tiny permanent magnets and you have seen permanent magnets that remain magnetic for decades or
centuries.
How does a magnetic tape record the difference in timbre or sound quality of the sounds? How does it represent a piano
versus an electric guitar? Also, how does more than one tone get recorded (e.g., an entire band or symphony)?
Even a single instrument playing a single note produces a complicated sound. The air pressure fluctuations
produced by the instrument aren't as simple and smooth as you might think. While the instrument may produce
mostly the fundamental tone--the main pitch associated with the note being played--it also produces other tones
that are usually integer multiples of the fundamental tone. These higher pitched "harmonics" contribute to the
sound we hear and allow us to determine what instrument is playing that sound. We also hear the temporal shape
of the sound--the sound envelope. A piano produces a sound that starts loud and gradually becomes softer while
a violin produces a sound that starts soft and gradually becomes louder. An electric guitar offers its player even
more control over the pitch and sound envelope. The tape recorder detects the pressure fluctuations associated
with all these tones and volume changes and records them all as the magnetization of the tape's surface. When
many instruments are playing at once, the pressure fluctuations are even more complicated and they add together
to create a complicated pressure pattern at the microphone. Nonetheless, the recorder simply detects the air
pressure changes at the microphone and records them on the tape, and that's all it needs to do to keep an accurate
record of the sound. When the magnetization of the tape is used to reproduce sound, you again hear all the
instruments playing.
Can an object conduct electricity but be nonmagnetic? Are these independent properties? You said during lecture that
copper is nonmagnetic but doesn't it conduct electricity?
Electric conductivity and magnetism are pretty much independent properties. There are good conductors that are
magnetic (iron) and good conductors that are nonmagnetic (copper). There are also insulators that are magnetic
(iron oxide) and insulators that are nonmagnetic (glass).
Is it possible to mechanically connect two motors of equal speeds and powers to provide twice as much power as a single
motor? -- EG, Torrance, CA
As long as they're both AC induction motors, I don't see any reason why not. While induction motors would turn
synchronously with the power line if they had absolutely no load, they naturally lag slightly behind in normal
situations. While a line synchronous AC motor would turn at 1800 or 3600 rpm, depending on how it's wired, a
typical induction motor turns at 1725 or 3450 rpm. The more you load an induction motor, the slower it turns and
the more torque it exerts on that load. By coupling two induction motors together mechanically, you'll make
them turn at the same rate. Since the torque each motor exerts on the load depends on rotation speed, they'll both
contribute equally to the task and will together provide twice the power of a single motor.
I wouldn't try this with any kind of motor that doesn't have such a clear relationship between rotational speed and
power output. If you join two mismatched motors with one another, one may end up doing all the work and the
other motor might effectively become a generator rather than a motor!
How does steam work? -- SS, Nairobi, Kenya
Steam is the gaseous form of water. When the water molecules in liquid or solid water have enough thermal
energy, they can break free of one another and become independent particles. Even at room temperature, the air
you are breathing is several percent water molecules. But at higher temperatures, the rate at which water
molecules leave the surface of solid or liquid water increases so much that these water molecules can form a
dense, high-pressure gas. This gas is called steam.
February 13, 1997
How do record players and their needles work? - JW
As a phonograph record turns, the needle of its playing arm slides through a narrow spiral groove on the record's
surface. This groove is cut with a 90° angle at its bottom and both of its sides have undulations in them. As the
needle slides through the groove, it rides up and down on these undulations. The needle's movement causes
currents to flow in two separate pick-ups that are attached to the needle. One pick-up responds to needle motions
caused by the right edge of groove and the other pick-up responds to needle motions caused by the left edge of
the groove. The physical mechanism for converting needle motion into electric current depends on the needle
cartridge--it can involve moving magnets, moving coils of wire, or squeezed piezoelectric crystals. Since the
groove undulations represent air pressure fluctuations at the right and left microphones during recording, the
currents from the two pick-ups represent those pressure fluctuations during playback. With the help of amplifiers
and speakers, these currents are used to reproduce the sounds that were recorded at the two microphones.
What are the names of the subatomic and fundamental particles and what do they do? -- BA, Fairbury, IL
Subatomic particles and fundamental particles aren't necessarily the same--some subatomic particles are built
from several fundamental particles. That's the case for two of the most important subatomic particles: the proton
and the neutron. Each of these particles is built from three fundamental particles known as quarks. The proton
contains two "up" quarks and one "down" quark. The neutron contains one "up" quark and two "down" quarks.
However, another important subatomic particle is also a fundamental particle: the electron. Virtually all matter is
composed of these three subatomic particles: protons, neutrons, and electrons.
The list of fundamental particles--particles that are not known to be composed of other particles--is relatively
short. It includes 6 types of quarks, which are given the arbitrary names "up", "down", "charm", "strange", "top",
and "bottom". These quarks are never found by themselves but are instead used to build two major classes of
subatomic particles: baryons (including protons and neutrons) and mesons. The list of fundamental particles also
includes 6 types of leptons, which are given the names "electron", "electron neutrino", "muon", "muon neutrino",
"tau", and "tau neutrino". These leptons are found by themselves and aren't used to build any other subatomic
particles. These quarks and leptons are described as fermions and each has an associated antiparticle.
In addition to quarks and leptons, there are a number of fundamental particles that allow the fundamental
fermions to interact with one another. These interaction particles are described as bosons and include the
"photon", "W+ Boson", "W- Boson", "Z Boson", 8 different "gluons", and a particle called "Higgs" (which has
not yet been observed but is thought to exist).
The list of subatomic particles that can be formed from the fundamental particles is extremely long and listing it
here wouldn't be very enlightening. The only subatomic particles that are common in nature are protons,
neutrons, electrons, and photons. Some of the others appear through nuclear or subnuclear processes in
radioactive materials, nuclear reactors, particle accelerators, or celestial objects, but most of these exotic
subatomic particles haven't been common since moments after the big bang.
How does a transistor amplify an input signal in an audio amplifier? -- AR, Pierrefonds, Quebec
The answer depends a little on which type of transistor is used, so I'll consider only an audio amplifier based on
MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). One of these three-electrode devices allows a
tiny electric charge on its gate electrode to control a substantial current flowing between its source and drain
electrodes. In a typical amplifier, the current flowing in the input circuit is allowed to deposit or remove electric
charge from the gate electrode(s) of one or more MOSFETs. This action dramatically changes how much current
flows in a second circuit. This second circuit is ultimately responsible for the current that passes out of the
amplifier and through the speakers that reproduce sound. As the current in the input circuit fluctuates to represent
a particular musical passage, the charges on the gates of the MOSFETs also fluctuate and the MOSFETs vary the
current through the output circuit and the speakers. Because MOSFETs are so sensitive to even a tiny amount of
charge, it doesn't take much current in the input circuit to cause large changes in the current of the output circuit.
Which gives off more heat energy, an incandescent light bulb or a fluorescent lamp? Which is more efficient to use in the
summer or winter? -- TJ, Woodbridge, VA
An incandescent lamp turns its electric power completely into heat. Even the visible light it gives off is actually
thermal radiation. A fluorescent lamp tries not to produce heat--the light it produces is non-thermal (it doesn't
involve hot materials). While a fluorescent lamp is only partly successful at not producing heat, it's still several
times more energy efficient than an incandescent lamps--fluorescents produces several times as much
illumination for the same amount of electric power. This statement is true both in summer and winter, although
fluorescent bulbs lose some of their energy efficiencies in very cold or very hot weather. Fluorescent lamps work
best at temperatures between about 15° C and 40° C.
How can heat be trapped? -- PR, Brooklyn, NY
You can prevent heat from moving about with the help of insulation. The three principal mechanisms of heat
transfer are conduction (the passage of heat through a stationary material), convection (the passage of heat in a
moving fluid), and radiation (the passage of heat as electromagnetic waves or light). Good insulation doesn't
conduct heat well, doesn't support convection, and blocks radiation. Wool is a good example: its hair and trapped
air don't conduct heat well, the trapped air can't really undergo convection well, and thermal radiation can't travel
through the wool along a straight path. As a result, wearing a wool sweater keeps you from losing heat quickly-you stay warm. Wool has the added benefit of carrying water away from your skin.
What is the approximate terminal velocity for a spent falling bullet that was fired into the air? Is this velocity sufficient to
kill someone? - M
A bullet's terminal velocity is the downward speed at which the upward force of air resistance acting on it
balances its downward weight. Once the falling bullet reaches this speed, it coasts downward at a steady rate.
Because air resistance depends largely on surface area while weight depends on volume, larger bullets will drop
faster than smaller bullets (just as a piece of chalk drops faster than chalk dust). While I am not sure of the exact
speed of a dropping bullet, I expect it to be several hundred miles per hour. As to whether or not it can kill
someone, the answer is most definitely yes. In fact, a distant cousin of mine was killed several years ago during
Mardi Gras when a falling spent bullet pierced her brain. Firing bullets into the air is an extraordinarily foolish
and inconsiderate action. In cultures where it's common to fire guns during celebrations, innocent people are
frequently killed by these descending "party favors." If you ever see people shooting guns into the air, you
should immediately seek cover in a basement. Their bullets will return to earth in less than thirty seconds and
will be just as deadly when they arrive as if they had been shot right at you.
What's energy?
Formally defined as "the capacity to do work", energy is a measure of an object's ability to make things happen.
It is interesting to physicists for one important reason: it is a conserved physical quantity. By "conserved physical
quantity", I don't mean that it's something that we try not to waste. I mean that the amount of energy in an
isolated system can't change--energy can't be created or destroyed, it can only be transferred from one object to
another or converted from one form to another. Because you can't make it or consume it, energy is an important
characteristic of objects and systems. You can often watch it move from object to object and observe the
consequences of this movement. For example, the energy that I'm using now to type at my keyboard arrived at
the earth's surface as sunlight, was used by plants to build new molecules that eventually become part of my
breakfast this morning and are now being combined with oxygen in my body to allow me to move my fingers.
Nowhere along this chain was energy created or destroyed--it simply moved about and changed forms. It will
still be here tomorrow, and then next day, and even the day after that.
February 12, 1997
Why does an artificial sponge absorb more water than a natural sponge? -- JH, Angleton, TX
Water is drawn into a sponge in part because of an attraction between the water molecules and the sponge's
surface and in part because of water's tendency to minimize its own surface area. When you put a drop of water
on a waxy surface, the water beads up. That's because water and wax don't bind well to one another and the water
molecules pull toward one another instead. The water droplet tries as best it can for form a sphere, since a sphere
has the smallest surface area that a given volume of water can occupy. These forces that pull water's surface
inward are called surface tension.
But when you put a drop of water on real cellophane (a smooth form of cellulose), the water spreads out. That's
because water and cellulose bind strongly to one another and the water will permit its surface area to increase
somewhat if that increase allows it to attach to more cellulose. Similarly, water binds well with other forms of
cellulose, including paper, cotton, and Rayon. I think that most artificial sponges are either cellulose or a close
chemical relative of cellulose.
A sponge absorbs water by allowing that water to cling to an extensive surface that binds well with water. The
water spreads out along that surface while trying to minimize the surface area of any water that isn't touching the
sponge. The surface of a natural sponge interacts well with water (the sponge lives in water after all), but a
natural sponge can't compete with modern technology. A company that makes artificial sponges can adjust the
chemical structure of the sponge's plastic so that it binds nicely to water molecules; it can adjust the sizes of the
holes in the sponge to attract the water as efficiently as possible with a given mass of plastic; and it can tailor
wall thickness to give the sponge the right elasticity. Furthermore, some of the water is brought right into the
plastic and that water softens or "plasticizes" the plastic. That's why a sponge is hard when dry and soft when
wet--the water molecules are effectively lubricating the plastic molecules so that they can slide past one another.
How does luminol work? -- CW, San Antonio, TX
Luminol produces light during a chemical reaction with either molecular oxygen or a mixture of potassium
ferricyanide and hydrogen peroxide and is probably the basis for most light sticks. In an alkaline (basic) solution,
the luminol molecule becomes a dianion, a molecule with two negative charges on it. In this dianion form, the
molecule has two nitrogen atoms exposed to the solution and these nitrogen atoms are easily replaced by two
oxygen atoms. When that exchange takes place, a molecule of nitrogen gas is released and the final oxidized
luminol is left in an electronically excited state. This molecule quickly gets rid of its excess energy by emitting
light.
If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep
that generator turning, why do brownouts occur?
As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its
alternating current won't change. When the homes powered by the generator draw more current, then the
generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it.
But suppose that the turbine can't exert any more torque on the generator. In that case, the power company can
either shut down the generator or it can reduce the strength of the generator's rotating magnet. This rotating
magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a
period of excessive current demand, the power company may choose to weaken the rotating electromagnet to
prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator
becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right
frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow
relatively dimly--a brown-out.
You have mentioned the relationships between electric fields, magnetic fields, and current. Which causes which? Does
current cause a magnetic field, in turn, causing flow in the next circuit and so forth? What is this order of occurrence? -BJ
Those three items, electric fields, magnetic fields, and currents, are strongly interrelated. Here are some of those
relationships: (1) currents cause magnetic fields, (2) currents that change with time cause magnetic fields that
change with time, (3) magnetic fields that change with time cause electric fields, (4) electric fields cause currents
to flow in electric conductors. From these relationships, you can see that any time you have a changing current
through one circuit, you can end up with a current flowing through another nearby circuit. Power moves from the
first circuit to the second circuit with the help of a magnetic field and an electric field. A moving magnet also
produces a magnetic field that changes with time and it can send a current through a nearby circuit, too.
February 11, 1997
Why does a spring want to come back to the original position it started at? -- JF, Hazen, ND
When you stretch or bend a spring, you are displacing atoms within the crystals that make up that spring. Each
atom in those crystals moves a tiny bit nearer or farther from its neighbors and it begins to experience tiny forces
that would push it back toward its original position if you let go of the spring. When you do let go of the spring,
these tiny forces act together to return the spring to its original shape while returning the individual atoms in the
crystals back to their original positions. However, if you bend a spring too far, the atoms begin to slide across
one another and they can no longer find their way back to their original positions. In that case, the spring has
become permanent bent and won't return to its original shape when you let go. Good spring materials are those
that can tolerate a substantial amount of stretching or bending without allowing their atoms to slide across one
another. Many common metals don't make good springs because this sliding occurs much too easily.
What causes things to glow in the dark? Why does phosphorus glow? Why does the glow die? -- DB & EB
Most glow-in-the-dark materials store energy when they are exposed to visible light and then glow dimly as this
stored energy is gradually converted back into light. In such a material, exposure to light promotes some of the
electrons in the atoms or molecules to excited states and these electrons become trapped in lower-energy excited
states from which they have trouble escaping. It takes a very long time for each of these trapped electrons to
return to their original states by emitting light. Since that return is a random process, a glow-in-the-dark object
glows with an ever diminishing light as the excited electrons return at random moments to their original states.
Eventually almost all the electrons have returned and the glow weakens to essentially nothing.
White phosphorus also glows in the dark, but not for the same reason. You don't need to expose white
phosphorus to light to make it glow; you need to expose it to air. The chemical reaction between phosphorus and
oxygen causes the phosphorus to emit light. This reaction can also cause the white phosphorus to burst into
flames. Because of its dangerous flammability and its toxicity, white phosphorus isn't something you want to
have around.
Is there an inexpensive device for detecting leaks from a microwave oven?
Yes. You can get one from a hardware or appliance store for about $5 to $30. ComfortHouse.com sells one online at www.comforthouse.com. While I have tended to downplay the leakage issue in the past, I bought a tester
and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class
demonstrations, so it had been abused and the door wasn't properly aligned any more. I retired it. Incidentally,
the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way
that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave
Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters,
and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.
I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. - M
A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate
rapidly around the motor's rotating central component--its "rotor." The rotor consists of a cylindrical aluminum
metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The
nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and
it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into
rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications,
but you wouldn't want it in an elevator--the sudden starting of the elevator car that would accompany the sudden
starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced
by a group of wires that are connected by way of metal ring (the "slip rings") and some stationary conductive
brushes to some components outside the rotor. During the starting process, the currents that are induced in the
rotor's wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully.
When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted
together so that the rotor behaves like the aluminum cage of a normal induction motor.
Why is alternating current better than direct current? -- MK, California
The genius of George Westinghouse and Nikola Tesla in the late 1800's was to realize that producing alternating
current made it possible to transfer power easily from one electric circuit to another with the help of an
electromagnetic device called a transformer. When an alternating electric current passes through the primary
wire coil of a transformer, the changing magnetic and electric fields that this current produces transfer power
from that primary current to the current passing through another coil of wire--the secondary coil of the
transformer. While no electric charges move between these two wires, electric power does. With the help of a
transformer, it's possible for a generating plant to move power from a large current of relatively low energy
electric charges--low voltage charges--to a small current of relatively high-energy electric charges--high voltage
charges. This small current of high voltage electric charges can move with relatively little power loss through
miles and miles of high voltage transmission lines and can go from the generating plant to a distant city without
wasting much power. Upon arrival at the city, this current can pass through the primary coil of another
transformer and its power can be transferred to a large current of relatively low voltage charges flowing through
the secondary coil of that transformer. The latter current can then deliver this electric power to your
neighborhood. A transformer can't transfer power between two circuits if those circuits operate with direct
current. Edison tried to use direct current in his power delivery systems and fought Westinghouse and Tesla
tooth and nail for years. Edison even invented the electric chair to "prove" that alternating current was much
more dangerous than direct current. Still, Westinghouse and Tesla won out in the end because they had the better
idea.
I'm doing a science fair project on electricity and I need to know how to make a homemade hot dog cooker. - BE
Although I have never done it myself, I understand that it is possible to run electric power directly from the
power line through a hot dog and to use the resistive heating that occurs as electric current struggles to pass
through the hot dog to cook that hot dog. While I can't recommend doing this and caution anyone trying it to be
extremely careful with the electricity (i.e. seek adult supervision from someone who is experienced with the safe
handling of electricity), I believe that it can be done. My understanding is that you should carefully connect each
wire of an electric power cord (unplugged!) to its own nail (choose an uncoated steel nail to avoid toxic
materials). You should then insert one nails into each end of the hot dog and place that hot dog on a safe,
nonconducting surface where no one and nothing can touch it. Finally, you should plug the electric cord into an
electric socket that is properly connected to a working circuit breaker. I would recommend using a socket
protected by a ground-fault interrupter (GFI) such as are used in modern bathrooms (the ones with a "test" and
"reset" button). (As you can see, I don't want anyone hurt!) I'm not sure how quickly the hot dog will cook, but
I'd expect it to be quite fast. Be sure to unplug the cord before getting anywhere near the hot dog.
In the simplest terms, how does a basic electrical circuit work? -- CC, Port St. Joe, FL
An electric circuit is racetrack for electric charges. It must be a complete loop--a "circuit"--so that the charges
don't pile up somewhere along the track. The simplest circuit has a source of energy for the electric charges (e.g.,
a battery) and a device that takes energy away from the electric charges (e.g., a light bulb). When the charges are
in motion through the circuit, they are an electric current. By convention, current points in the direction of
positive charge flow, so you can imagine a stream of positive charges circling this circuit over and over again,
with current pointing always in the direction that those positive charges are moving. As the current passes
through the battery, entering it at the battery's negative terminal and leaving it at its positive terminal, the charges
pick up energy. The battery is converting some of its stored chemical potential energy into electric energy and
giving that energy steadily to the current flowing through it. The battery is "pumping" the charges from its
negative terminal to its positive terminal. The current continues around the circuit and then passes through the
light bulb. In the light bulb, the charges give up most of their energies to the filament and the filament becomes
white hot. The current continues out of the bulb and returns to the negative terminal of the battery to pick up
more energy. This simple circuit is present in a flashlight. The same charges complete this circuit millions of
times each second, shuttling energy from the battery to the bulb.
Are there any levitated trains in the world? - BP
At present, I believe that the only magnetically levitated trains are those undergoing development and testing.
Why does popcorn pop? - AB
Inside the hard, dry hull of a popcorn kernel is a portion of moist starch. When you heat the kernel well above
the boiling temperature of water, the water in the starch converts to hot, high-pressure steam. The hotter this
steam gets, the higher its pressure rises and the stronger the outward forces it exerts on the hull. Eventually, the
hull rips open under the stress and exposes the starch to the low-pressure air around it. The pressurized steam
then pushes the starch outward, expanding it to many times its original size. The kernel "pops."
What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of water, will I end up with 2 cups of
solution? - RT
As a crystalline solid, salt consists of a beautiful cubic lattice of sodium atoms that have lost one electron to
become sodium positive ions and chlorine atoms that have gained one electron to become chlorine negative ions.
The crystal is held together by the attractive forces between these oppositely charged atomic ions. When a salt
crystal dissolves in water, it decomposes into individual sodium positive ions and chlorine negative ions that are
then carried about by shells of water molecules. Water molecules are electrically polar, meaning that they have
positively charged ends and negatively charged ends. The water molecules line up around a positively charged
sodium ion with their negatively charged ends inward and carry that ion about. Similarly, water molecules line
up around a negatively charged chlorine ion with their positively charged ends inward and carry that ion about.
Whether you will end up with 2 cups of solution after mixing 1 cup of salt and 1 cup of water depends on how
tightly the atoms and molecules pack together in each case. Remember that your 1-cup of salt contains a fair
amount of air between the salt grains. You'll have to try it to find out the answer--I'm not sure what the answer
will be.
How fast can glaciers move? -- SF, Burton, OH
Glaciers move at a variety of rates, ranging from inches per year to many feet per day. Currently there are very
few "galloping" glaciers (those that move many feet per day) and most are either stagnant or retreating.
Can you explain gyroscopic precession? -- BW, Newport, RI
When a gyroscope is spinning rapidly, it has a large amount of a conserved physical quantity called angular
momentum. Angular momentum is a special measure of rotational motion that can't be created or destroyed--it
can only be transferred between objects. As long as nothing tries to transfer angular momentum to or from the
spinning gyroscope, it will continue to spin at a steady pace about a fixed axis in space. But when an external
torque (a twist) is exerted on the gyroscope, a transfer of angular momentum takes place. The gyroscope's rate of
rotation or its axis of rotation begins to change so that its angular momentum changes. If you apply a twist to the
gyroscope around its axis of rotation, it will either spin faster or slower, depending on which way you twist it.
But if you twist the gyroscope about a different axis, its axis of rotation will shift--the gyroscope will undergo
precession. The direction of this precession depends on how you apply the twist and tends to be very nonintuitive.
Would it be possible for a spacecraft to use electrically powered propulsion? Could it gather atoms and molecules from
space and then use an electromagnetic field to push them through a nozzle? -- JC, Burnaby, British Columbia
Not only is it possible to use electrically powered propulsion, such systems are already in use on several
spacecraft. While they don't scavenge atoms and molecules from space, these ion propulsion engines uses
electric forces to accelerate ionized atoms to enormous speeds. As the engine pushes on the ions it accelerates,
those ions push back on the engine. The ions rush out into space in one direction and the engine experiences a
modest thrust in the opposite direction. While the overall thrust from an ion engine is small, it uses its storedatom "fuel" very efficiently and can be sustained for a very long time in a solar- or nuclear-powered satellite. Ion
engines are used in spacecraft that need small but steady thrust for a long time. Scavenging atoms from space
would allow these engines to run for an even longer time, but it's probably not realistic. The atoms in space are
typically so rare and so fast-moving that they would be more trouble than they're worth.
How do thermals affect the atmosphere and air currents? -- RM, Praire Farm Schools, Wisconsin
Thermals are air currents in the atmosphere. When sunlight and exposure to warm ground raises the temperature
of surface air, that air expands--its molecules travel faster and bounce against one another more vigorously, so
they push themselves farther apart. This expanded air weighs less per cubic foot or meter than cooler air, so the
cooler air around it lifts it upward in a rising current of warm air--a "thermal." The air can't simply accumulate
way up overhead forever, so cooler air descends to take its place. The overall result is rising warm air and
descending cool air. These air currents are part of giant circulation loops or "convection cells" that also include
surface winds and high altitude winds.
If you lived on the moon, would it be easier to adapt to living with the moon's gravity, or to create an artificial
environment with the gravity of earth? -- MK, Orlando, FL
Building an environment that made you feel what appeared to be the earth's gravity would be a substantial
undertaking. The only way to simulate gravity is through acceleration and the only way to make a person
experience acceleration continuously is to swing them around in a circle. So this environment will have to swing
its occupants around in a circle. However, we are extremely sensitive to changes in orientation, so that we can
tell the difference between true gravity and the experience of being swung around in a small circle. To avoid the
dizzying feeling of having our orientations changed rapidly, the turning environment would have to be extremely
large. It would have to be a huge rotating wheel, looking like a heavily banked circular racetrack that spun at a
steady pace and completed something like one full turn per minute. The occupants would have to live on the
long, thin surface of this turning racetrack. Building such a device on earth wouldn't be easy. Building it on the
moon would be much harder. I wouldn't plan on trying to simulate the earth's gravity on the moon. So I vote for
just putting up with the moon's weaker gravity.
How can people lay on a bed of nails and still survive? -- LW, Marion, OH
If you push gently on the tip of one nail, it won't pierce your finger. When you push on the nail, it pushes back on
you, but the force pushing the nail against your finger isn't strong enough to break your skin. If you push twice as
hard on two nails at once, using two different fingers, then the force you exert on each nail will be the same as
before and each nail will push back against one of your fingers with the same force as before. Once again, the
nails won't break your skin. If you now push 100 times as hard against 100 nails, each nail won't push hard
enough against you to break your skin. In fact, a few hundred nails will be able to push on you with an overall
force equal to your weight without piercing you. That's the idea behind a bed of nails--by lying on many nails at
once, you allow so many nails to push upward on you that, while the overall force they exert on you is enough to
balance your weight, the force exerted by each individual nail isn't enough to draw blood. These nails have to be
spread out around your body so that no individual nails bear more than their fair share of your weight. If one of
the nails took too much of your weight, you'd be hurt by it.
February 10, 1997
With reference to power generation and transmission, can you please explain "Volt Amp Reactance" (VAR, kVAR,
MVAR). What is meant by "importing/exporting VAR's"? What is meant when a plant is "consuming/producing VAR's"- ID, Northern Territory, Australia
In most situations of AC electric power generation or AC electric power consumption, the current flowing
through the circuit is in phase with (or, more simply, directly proportional to) the voltage across the circuit. But
that isn't always the case. In situations involving reactive components (e.g., capacitors and inductors), it's
possible for the current and voltage to be out of phase with one another. If the current and voltage are a full 90°
out of phase, there is no average power flowing through the circuit. I believe that VAR is a reference to this
portion electricity in the circuit--the portion for which the voltage and current are 90° out of phase. While this
portion of the electricity doesn't transfer any power, it does place demands on the power transmission system. I
think that the distinctions between "importing" and "exporting" and between "consuming" and "producing" are
related to the phase ordering of the current and voltage (whether a device is acting as a capacitor or an inductor).
In one case, the voltage leads the current by 90° and in the other the current leads the voltage by 90°.
Why is it that the same transformers seem to always be hit by lightning?
Lightning tends to strike elevated objects that acquire large charges that are opposite to those of the clouds. Since
transformers are often elevated and they are connected to wires that allow them to become highly polarized when
a charged cloud passes overhead, transformers are good targets for lightning.
Is it safe to live near high-power lines?
It's probably fine. While the high-tension wires do create modest alternating electric and magnetic fields, there is
no credible evidence that these fields cause any injury and no one has proposed a convincing mechanism
whereby those fields could affect biological tissue.
If voltage shocks you, why does current kill you?
Your skin is a very good electric insulator and it prevents any current from passing through your body as long as
that current doesn't have much voltage. A higher voltage (the electric equivalent of "pressure") is required to
push charge through your skin. But once the charge is inside you body, it moves through you quite easily--your
body fluids are essentially salt solutions and are relatively good conductors of electricity.
However, a small current passing through your body won't cause injury. It takes about 0.030 amperes or 30
milliamperes to cause a life-threatening disturbance to your "electric system." The small currents associated with
static electricity are not enough to cause trouble, even through they easily pass through your skin. So high
voltages are needed to break through your protective barrier--your skin--in order to give you a shock, but large
currents are needed to injury you.
What accounts for the difference between two sounds having the same frequency, loudness, etc. but generated by a guitar
and a sitar? -- AW, Karachi, Pakistan
Different instruments sound different, even when they play the same notes at the same volumes, primarily
because they add different amounts of harmonic tones to their fundamental tones and because these various tones
change in volume with time. When you play a note on a guitar, you don't hear just one pure frequency with a
constant volume. Instead, you hear the fundamental frequency and all of the integer multiples of that frequency-the harmonics of that frequency. The relative volumes of those harmonics, and how those volumes change with
time, are characteristic of the guitar. If you listen to the same note on a sitar, the relative volumes of the
harmonics will be different and you will hear the difference. Because both instruments are plucked, the sounds
they emit both start loud and gradually grow softer. If you were to bow their strings, the sound would start soft
and gradually grow louder. That's one reason that you can distinguish a guitar or sitar from a violin.
What will be the source of energy for vehicles 50 years from now? -- AW, Karachi, Pakistan
When the earth's petroleum supply has been depleted to point where it becomes too precious and expensive to
burn, electric vehicles will probably take over. While it's possible to synthesize chemical fuels, I don't think it
will be worth the trouble. The bigger question is where the electricity needed to charge the batteries will come
from. I'll bet on solar power. Right now, electric cars don't save fossil fuels or keep the air significantly cleaner
because the electricity those cars use is obtained by burning fossil fuels. But the electric cars of the future will
probably obtain their electric power from the sun. Nuclear fission and fusion are also possibilities, but fission
power has its drawbacks and its not clear when or even if fusion power will be available.
February 9, 1997
What makes a paper airplane fly when its wings are not shaped like real airplane wings? -- JC, Idaho Falls, ID
Even though a paper airplane's wings are flat, they experience all of the aerodynamic forces found in more
sophisticated wings. For example, when the air flowing past the paper airplane encounters the lower surfaces of
its wings, this air slows down and its pressure rises above atmospheric pressure. However, while the air flowing
over a sophisticated airplane wing experiences a substantial increase in speed and consequently a drop in
pressure, this effect is very small in a paper airplane's wing. Depending on how the air flows over or around the
wing's leading edge and whether or not it breaks away from the wing's upper surface, the air pressure above the
wing will be at or slightly below atmospheric pressure. Nonetheless, the air pressure below the wing is always
slightly higher than that above the wing and the wing experiences a net upward aerodynamic force--a lift force. If
you examine the airflow around a well-designed paper airplane wing, all of the flow features that occur around a
sophisticated wing will be present but weak. Bowing the wing outward, as is done in a sophisticated wing,
simply enhances those features so that the wing can lift a larger load.
What is a vacuum? Is it filled with charges with no mass? -- AW, Karachi, Pakistan
In principle, a vacuum is a region of space containing no real particles (no atoms, molecules, electrons, or other
subatomic particles). Because the universe is filled with particles that pass easily through lots of matter
(neutrinos, for example), it's very hard to obtain a true vacuum. But let's suppose that you could actually obtain a
region of space with no real particles in it. That region of space would still contain large numbers of virtual
particles at any given moment. These virtual particles are temporary quantum fluctuations of the vacuum; brief
excursions of the quantum fields associate with various subatomic particles. These excursions are permitted by
the Heisenberg uncertainty principle, which allows temporary violations of the conservation of mass/energy as
long as those violations are extremely brief. While the presence of these virtual particles can only be detected
indirectly, they are not massless. Except for their short lifetimes, these particles have characteristics similar to
those of normal particles. In fact, if enough energy is used in the process of looking for a virtual particle, that
virtual particle can be converted from virtual to real so that it can be detected directly. The energy of detection
serves to "pay" for the mass of the particle so that it can leave the virtual realm and become a real, permanent
particle.
Einstein's famous equation E=mc2 says that mass is directly proportional to energy. Does this mean that an object that is
suspended overhead has more mass than an object located at ground level? -- ST, Denver, CO
Yes, the mass/energy of a suspended object is greater than the mass/energy of that same object at ground level.
The extreme example of this result comes with lowering an object slowly toward the surface of a black hole--as
the object descends, its mass/energy diminishes until it reaches zero at the surface of the black hole.
How can I make 1000 nanometer light waves visible to the human eye? -- DMB, Broken Aarow, OK
Although our eyes are insensitive to 1000 nanometer infrared light, there are two ways to detect it effectively.
The easiest is to use an inexpensive black-and-white surveillance video camera. Many of these cameras are
sensitive to a broader spectrum of light than are our eyes and they can see 1000 nanometer light. If you check
around, you should be able to find one that sees the light you're interested in. The other technique is to use a
phosphorescent or "glow in the dark" material. When exposed to visible light, the atoms in such a material
become trapped in electronic states that can emit visible light only after a very long random wait. But exposing a
phosphorescent material to infrared light can shift the states of the atoms in the material to new states that can
emit light immediately. Thus exposing some phosphorescent materials to infrared light causes them to emit light
promptly. You can then see these materials glow particularly brightly after storing visible light energy in them
and then exposing them to infrared light. However, they'll only glow briefly before you have to "recharge" them
by exposing them to more visible light.
What are the key components of a microwave oven?
In addition to the digital controller that runs the microwave, it contains (1) a power relay that allows the
controller to turn on and off the microwave source, (2) a power transformer that produces the high voltage
electricity needed by the magnetron, (3) a power rectifier that converts the alternating current from the
transformer into the direct current needed by the magnetron, (4) a capacitor that smoothes out ripples in the
direct current leaving the rectifier, (5) a magnetron that uses the high voltage direct current to produce an intense
beam of microwaves, (6) a wave guide that transports the microwaves from the magnetron to the cooking
chamber, and (7) a cooking chamber in which the food absorbs the microwaves and becomes hotter.
Do microwaves have no effect on gas?
While water vapor can absorb microwaves at certain frequencies, the absorption mechanism is very different
from the one that causes liquid water to become hotter. Steam isn't affected very much by a microwave oven.
Can one's health be adversely affected by the use of certain wraps, films, or containers, when heating food in the
microwave?
When various plastics become hot, their molecules become more mobile. The most obvious such case is when a
plastic actually melts. But even before it melts, a plastic can begin to lose molecules to objects that are touching
it. However, the plastics used in cooking are pretty non-toxic, so that even eating pieces of those plastic won't
cause you any significant trouble. On the other hand, I would be careful with plastics that weren't intended for
cooking. Some non-food related plastics are mixed with additives called "plasticizers" that keep them softer than
they would be if they were pure. These plasticizers have a tendency to migrate out of the plastics, giving such
things as "vinyl" their characteristic odors. Heating a plastic containing a plasticizer can drive this plasticizer out
of the plastic and into something else. I don't think that it's a good idea to eat plasticizers so I would suggest not
cooking with plastics that weren't intended for use with food. Still, not all plasticizers are bad--water is an
excellent plasticizer for such common plastics as hair and cotton.
When TV screens or computer monitors are shown on television shows, they flicker or bars of light wave across them.
Why does this happen? -- SY, Halifax, Nova Scotia
Although you can't tell it by looking at a television screen, the image on that screen is formed one dot at a time
by beams of electrons that are scanning back and forth across its surface from inside. The image is built one line
at a time, from the top of the screen to the bottom of the screen, and each line is itself built one dot at a time,
from the left side of the screen to the right side of the screen. You can't see this sequential construction process
because your persistence of vision prevents you from seeing any changes in intensity that occur in less than
about 1/100 of a second. In any short period of time, the screen will only have had time to produce a few
horizontal lines of dots. When a camera or television camera observes a television screen, it often makes its
observation in such a short period of time that only part of the screen is built. When you then look at the recorded
image, you see a horizontal bar of image--the portion of the image that was built during the observation.
How does heat affect magnetism? -- MC, Capitol Heights, MD
The magnetism we associate with a permanent magnet or with steel's response to that permanent magnet
involves the careful ordering of tiny magnetic electrons within the materials. Just as heat tends to destroy all
forms of order in a newspaper when you put it in the fire, so heat tends to destroy the magnetic order in a
permanent magnet or in steel when you bake them. Many permanent magnets lose their magnetism when heated
to oven temperatures and even steel becomes non-magnetic when heated red-hot.
Why does copper conduct electric currents better than steel and lead? Why do copper and aluminum seem to conduct
about the same? - L
A metal's conductivity is related to how far an electron can coast through the metal before suffering a collision
that reduces its kinetic energy. Since an electron can collide with an impurity in the metal or a region of local
disorder, the first task in obtaining a good conductor is to make a pure and uniform metal. Increased temperature
also enhances these inelastic collisions, so keeping a metal cool improves its conductivity. Finally, different
metals exhibit different couplings between the electrons and the metal ions from which those electrons came.
Copper and aluminum have relatively weak electron-ion couplings while steel and lead have stronger couplings.
The stronger the coupling, the more likely is a collision between an electron and an ion. Because of their weaker
couplings, the electrons in copper and aluminum suffer far fewer collisions per centimeter than the electrons in
steel and lead. That's why copper and aluminum are better conductors of electricity than steel and lead. The
coupling in copper is only slightly weaker than that in aluminum, so they have similar conductivities. However,
aluminum's tendency to form a very hard, insulating oxide coating (aluminum oxide or "alumina" is the mineral
sapphire) makes it a bit tricky to use in wiring.
Can we make an electric fence with no physical wire? -- AW, Karachi, Pakistan
No. An electric fence needs at least one real wire. When you put a large electric charge on this wire, anyone who
touches it and the ground at the same time will serve as the path through which that charge will flow into the
ground. They will receive a shock. But without either the charged wire or the ground, they won't carry any
electric current and they won't receive a shock.
What is induced polarization and what are its applications? -- PSD, Rio de Janeiro, Brazil
An electrically neutral object contains both positive and negative electric charges, however, those opposite
charges are equal in amount and therefore cancel one another. But this cancellation doesn't mean that the charges
are unaffected by another nearby charge. If you hold an electrically neutral object near an electrically charged
object, the charged object will cause a slight rearrangement of the charges in the neutral object. Charges opposite
to that of the charged object will shift toward that object while charges like that of the charged object will shift
away from that object. The neutral object will acquire an "induced polarization", meaning that it its positive and
negative charges are displaced relative to one another and that this displacement is "induced" by the presence of
nearby charge. Induced polarization is a common effect and is present whenever lightning is about to strike the
ground. As an electrically charged cloud drifts overhead, the objects on the ground acquire induced polarization.
Their tops become covered with charge opposite that of the cloud and a lightning strike may occur between the
cloud and the oppositely charged top of a tree or building.
How does a smoke alarm work? -- GL, San Leandro, CA
The most common type of smoke detector uses a tiny amount of a radioactive element called americium to inject
electric charges into the air. In the absence of smoke, these electric charges attach themselves to individual air
molecules and the resulting ions (electrically charged molecules) move rather easily through the air. However, if
there is smoke present in the air, the electric charges attached themselves to the smoke particles and the resulting
charged smoke particles don't move easily through the air. The smoke detector measures how easily the charged
items it produces move through the air. If these items move easily, then the air is clean. However if they don't
move easily, the air contains smoke and the detector signals danger.
How does a vacuum flask operate? -- MA, Altamonte Springs, FL
A vacuum or "Dewar" flask is a double-walled vessel with a vacuum between the two walls. It's effectively a
bottle inside a bottle, with their only contact at their mouths. Since there is no air in between their bodies, no heat
can flow from one bottle to the other by either conduction or convection. The only way in which heat can move
between the bottles is via thermal radiation. By coating the surfaces of the two bottles with highly reflective
materials such as aluminum, even thermal radiation can be almost prevented from transferring heat between the
bottles. Since there is virtually no way for heat to flow into or out of the vacuum flask, except through its mouth,
the flask can keep hot things hot or cold things cold for extremely long times.
Why do some parts of a house get dustier than others? -- BC, North Reading, MA
Dust particles are tiny bits of rock, ash, and organic matter that have been ground into fine pieces by the wind
and wear. Although these particles are denser than the air that surrounds them, they have trouble falling through
the air because as soon as they move faster than about a snail's pace, they experience considerable air resistance
or drag forces. A dust particle has trouble falling through the air because the upward drag force it experiences
while descending even a few millimeters per second is enough to balance its weight so that it stops accelerating
downward. Because dust particles have so much trouble descending through air, they tend to be swept along with
moving air. That's why areas of your home that have large air currents tend to accumulate relatively little dust-the dust is swept along with the air currents and doesn't have time to descend all the way to the floor or furniture.
But in areas of your home with fairly still air, the dust can slowly settle out so that it coats all the surfaces.
What do you feel g-forces when you ride on a roller coaster? - F
Whenever you accelerate, you feel a gravity-like sensation "pulling" you in the direction opposite your
acceleration. What you feel isn't really a force--it's really just your own inertia trying to keep you going in a
straight line at a constant speed. In other words, your inertia is trying to keep you from accelerating. For
example, whenever you turn left in a roller coaster, your inertia opposes your leftward acceleration and you feel
"pulled" toward the right. This "pull" of inertia is sometimes called a "fictitious force" but you should remember
that it isn't a force at all, no matter how "real" it feels. Perhaps the most striking effect of acceleration occurs
during your trip around a vertical loop-the-loop. When you are arcing around the top of the loop-the-loop, you
are accelerating downward so quickly that you feel an enormous "fictitious force" upward. This "fictitious force"
has a stronger effect on you than the real force of gravity, so you feel as though you are being pulled upward.
The result is that you feel pressed into your seat, even though your seat is actually upside-down.
February 7, 1997
Why are batteries different sizes (e.g., AAA, AA, C, and D) if they all have 1.5 volts?
Those different alkaline battery sizes are chemically equivalent, which is why they all produce the same voltage
rises for currents passing through them from their negative terminals to their positive terminals. The same
chemical reactions allow each of these batteries to pump the charges, giving each coulomb of positive charge
about 1.5 joules of energy--a voltage rise of 1.5 joules-per-coulomb or 1.5 volts. Where these batteries differ is in
how many charges they can pump each second--their maximum currents--and in how many charges they can
pump before running out of chemical potential energy--their total stored energy. The bigger cells (C and D) can
handle far more current than the smaller cells (AAA and AA) and they also contain more stored energy.
If you reverse one of the batteries in a string, does that reversed battery recharge?
Yes. While the other batteries in the string will pump positive charge from their negative terminals to their
positive terminals, the reversed battery will extract energy from the positive charge as it flows from that battery's
positive terminal toward its negative terminal. The charge will lose energy and the battery will gain energy.
Some of the battery's additional energy will go into recharging the battery--converting its used chemicals back
into their original forms. But some types of batteries are better at recharging than other. Those that aren't meant
to be recharged may turn most of this energy into thermal energy and thus waste it.
How do batteries go dead if you are only sending current through them?
As current flows through a battery, from its negative terminal to its positive terminal, the battery does work on
that current. It must pull positive charges away from the negatively charged negative terminal and push them
toward the positively charged positive terminal. An alkaline battery needs 1.5 joules of energy to transfer each
coulomb of positive charge in this manner. This transfer operation consumes the stored chemical potential
energy inside the battery and eventually causes the battery to go dead. Just because you don't see anything
moving in the wires or in the battery doesn't mean that something substantial isn't occurring inside the battery--it
undergoes electrochemical reactions whenever current is flowing through it.
February 5, 1997
How do you demagnetize a magnet?
A permanent magnet was magnetized when it was first made out of metal. It did have microscopic regions of
magnetic order--magnetic domains--but those regions all pointed in random directions and the magnet didn't
have any overall magnetic poles. To give it poles, it had to be magnetized. It was placed in a very strong
magnetic field so that its domains grew or shrank until most of them were aligned with the magnetic field. The
magnet acquired overall magnetic poles for the first time. When the field was removed, the domains remained as
they were and the magnet permanently retained its new magnetic poles.
If this same magnet were reversed and then placed in that strong magnetic field again, it would become
remagnetized in the opposite direction from before--its domains would grow or shrink until most of them were
aligned with the magnetic field again. The magnet's north poles would become south poles and vice versa.
Finally, if the magnet were wiggled back and forth in that strong magnetic field and gradually removed from the
field, its domains would grow or shrink almost randomly. The magnet's magnetic domains would become
randomized and it would end up with no overall north or south magnetic pole at all. It would be demagnetized.
February 4, 1997
Is the jet stream flowing in the same direction in the southern hemisphere as it is in the northern hemisphere? -- LS,
Ashland, OR
The jet streams flow eastward in both hemispheres. Their directions of flow are determined by the Coriolis
effect, in which high-altitude winds that are heading away from the equator veer eastward because of their
angular momentum on the spinning earth.
I have read about how black holes can emit X-rays and radiation. If they absorb light, why do they emit these other
things? -- BA, Fairbury, IL
A black hole is surrounded by an imaginary surface called the event horizon. Nothing at all can escape from
within this surface-not light, not X-rays...nothing! However, as matter falls into the black hole, and before it
reaches the event horizon, the matter can emit any type of radiation it likes. The X-rays and radiation emitted
"from a black hole" are actually coming from the area surrounding the event horizon, not from within that
surface. As matter pours into a black hole, it often heats up so hot that it emits incredible amounts of radiation of
all types so that black holes appear as very bright objects.
Does blowing on or waving a developing Polaroid picture actually speed up its development process? -- PS, Columbus,
OH
I don't think so. The speed of the development process is determined by the diffusion of molecules within the
developing film. Since blowing on or waving the film won't affect the movements of molecules in that film, it
shouldn't change the development time.
In your explanation of why microwaves don't penetrate the oven door, you said it is because the holes in the screen are
smaller than the wavelength of a microwave. Wouldn't it be the amplitude of the wave and not its wavelength? - P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to
move. The microwave's electric field fluctuates back and forth rapidly and the charges reverse directions rapidly
as a result. If the electric current made up of these charges has enough time to travel all the way around each hole
before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able
to completely reflect the microwave.
Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave
crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave
propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the
current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave
wavelength. The amplitude of the microwave doesn't matter--increasing the amplitude of the microwave just
makes more current flow.
In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? - K
In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass
through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the
magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is
typically a few inches wide and an inch or so tall, and is called a "wave guide." Finally, the walls of the cooking
chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back
and forth in the metal and this moving charge causes the microwave to reflect.
How does a crystal radio work?
A crystal radio uses a crystal diode to detect tiny fluctuating currents in its antenna system. When a radio wave
passes across an antenna, the wave's electric field pushes electric charges up and down the antenna. The crystal
diode acts as a one-way gate that allows some of this moving charge to flow onto another wire and then prevents
it from returning to the antenna. Since the charge can't return to the antenna, it flows elsewhere--passing through
a sensitive earphone and creating sound. An AM radio station encodes sound as changes in the intensity (or
amplitude) of the radio wave. As the radio wave's intensity fluctuates, the amount of electric charge flowing
through the earpiece of the crystal radio also fluctuates and you hear sound.
Magnets stick to metal, but can you make a magnet repel metal? - M
Yes, but not in the way you're thinking of. When you bring a magnet near a piece of steel, the intrinsic magnetic
character of that steel causes it to become magnetic in such a way that it attracts the magnet. There is no way for
the steel, or another similar metal, to become magnetic in such a way that it would repel the magnet.
However, if the metal is already magnetized it can repel an approaching magnet. A more interesting case is when
a magnet approaches a normally non-magnetic metal at high speeds; in which case electric currents begin to flow
through the metal and these currents do repel the approaching magnet.
Do regular magnets lose their magnetism or do they stay magnetized always? What about electric magnets, like the ones
used in wrecking yards? -- KM, Delta, British Columbia
Permanent magnets are made from materials with two important magnetic characteristics. First, these materials
are intrinsically magnetic, meaning that some of the electrons in these materials retain their natural magnetism.
While electrons are always magnetic, that magnetism is lost in most materials because of complete cancellations-each magnetic electron is paired with another magnetic electron so that they cancel one another perfectly.
However, there are some materials in which the cancellation is imperfect and these materials (including iron,
cobalt, nickel, and many steels) are the basis for most permanent magnets.
Second, the materials used in permanent magnets have internal structures that make the magnetic electrons align
along particular directions. Once the electrons are aligned along one of those directions, they stay aligned and the
material exhibits strong magnetic characteristics. It becomes a "permanent magnet."
A permanent magnet remains its magnetization as long as nothing spoils the alignments of its magnetic electrons.
These electrons can be knocked out of alignment by vibrations, heat, or other magnets. If you hit a permanent
magnet with a hammer or heat it in the oven, you will change and perhaps destroy its magnetization. This
magnetization can be recovered by exposing the permanent magnet to the magnetic influences of an electric
current. In fact, permanent magnets are originally magnetized by placing them near electric currents that align
their magnetic electrons. Moreover, even a material that doesn't have the internal structures needed to keep its
electrons aligned along a particular direction will become magnetized temporarily by placing it near an electric
current. That's how a wrecking yard magnet works-an electric current temporarily turns a large piece of iron into
a strong magnet.
What effect does ice have on potholes? - AH
Water and ice are major contributors to potholes. When water flows into cracks in the road and then freezes, it
tears the roadway apart. That's because ice takes up more room than the water from which it's formed--ice is less
dense than water. Since the water expands as it freezes, it enlarges the cracks that contain it and gradually breaks
up the roadway.
How does desiccant absorb and hold water? -- JP, Houston, TX
Water molecules from the air are continuously colliding with surfaces and sometimes one of those water
molecules will stay attached to a surface for some amount of time. That water molecule forms a weak chemical
bond with the surface and remains there until thermal energy knocks it back into the air. As a result of this
occasional sticking, most surfaces have a thin layer of water molecules on them. Desiccants are materials that
tend to keep those water molecules for a relatively long time and that have lots of surface area on which those
water molecules can stick. However, the strongest desiccants react chemically with water molecules so that those
water molecules essentially never leave.
How do balloon pilots navigate around countries that forbid overflights? -- LS, Ashland, OR
The speeds and directions of the winds vary considerably with altitude. For example, while surface winds near
the sea blow toward shore on a hot summer day, high-altitude winds blow away from the shore at the same time,
completing a huge circulation loop. Unlike a sailboat, which is at the mercy of the surface winds, a balloonist can
adjust the balloon's altitude to search for winds heading in the desired direction. The balloonist makes these
altitude adjustments by changing the balloon's weight and volume so that it sinks or rises.
February 3, 1997
You mentioned that time perception is different for different locations in the universe. Were could we find a place where
one day is equal to one thousand years of time on earth? -- AWG, Karachi, Pakistan
The perception of time is different for observers who are in motion relative to one another. The issue is not how
far away they, it's how fast they are moving relative to one another. If you were to observe a person who is
traveling past the earth at almost the speed of light, you would notice that their watch is running extremely
slowly. It might be as though you'd have to wait one thousand years for their watch to show that a day has passed
for them. Yet paradoxically, they would make the same observation about you! You would see them aging
slowly and they would see you aging slowly! The resolution to this apparent paradox lies in the differences in the
perceptions of space that these differences in the perceptions of time. In this short answer, I can hardly begin to
resolve the paradox. I'll simply point out that the mixing of space and time associated relativity are caused by
relative motion not by relative position.
How does a fax machine send written words over telephone wires? -- AM, Halifax, CA
The fax machine uses a row of optical sensors to detect dark and light spots on the original document. It scans
the document one line at a time and enters the pattern of dark and light spots into a digital controller or simple
computer. The controller or computer than encodes this pattern, together with enough information to correct
minor transmission errors if they occur, as a series of numbers. The numbers are then sent through the telephone
system in much the same way that computer information is sent through the telephone wires by a modem. The
numbers becomes specific patterns of tones and volumes. While the electric currents flowing through the
telephone system are meant to represent voice sounds, they can do a moderately good job of representing
numbers instead. Because of various limitations on the currents that the phone wires can carry well, the fax
system can only so much information each second. The receiving fax machine analyzes the tones and volumes it
receives over the telephone wires and recreates the pattern of dark and light spots. It then uses one of several
printing techniques to reproduce that pattern on a piece of paper. It recreates the document one line at a time.
How do remote garage door openers work? -- JD, Greenville, SC
The communication from the remote to the opener is done with radio waves. When you push the button on the
remote, it produces a brief burst of radio waves at a specific frequency and with a selected pattern of pulses. A
radio receiver in the opener is continuously looking for a transmission at that same frequency and with that same
pattern of pulses. While other garage door openers may use radio waves of the same frequency, it's extremely
unlikely that they will make use of the same pattern of pulses. This pattern of pulses is the security code that
prevents unauthorized opening of your garage door. These security codes have grown longer and more
sophisticated over the years. Early garage door openers had no security code at all and could be opened by
almost any radio transmission at the right frequency. You could drive around neighborhoods with a remote and
open garage doors right and left. But now the security codes are complicated enough that opening someone else's
garage door is almost impossible.
What is the difference between a single-phase electric motor and a three phase motor? Does that make one of them more
efficient, better, or longer lasting than the other? -- EJ, Houston, TX
To keep the center component or "rotor" of an electric motor spinning, the magnetic poles of the electromagnets
surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic
poles. With single-phase electric power, producing that rotating magnetic environment isn't easy. Many singlephase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These
electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of
the other electromagnets. The result is magnetic poles that seem to rotate around the rotor and that start it
turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the
starting process is clearly rather complicated in a single phase motor.
In a three phase motor, the complicated time structure of the currents flowing through the three power wires
makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the
rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general,
three phase motors start more easily and are somewhat more energy efficient during operation than single phase
motors.
What is a kVA? Can you convert watts to kVA? - M
kVA is the product of kilovolts (kV) times amperes (A) and is a measure of power. In fact, if you multiply the
voltage in volts delivered to an electric heater by the current in amperes sent through that heater, you will obtain
the electric power in watts consumed by the heater. Thus the heater's power consumption in watts is the same as
the product of its voltage times its current, or its kVA. However, there are many devices that don't behave like an
electric heater. The heater is purely resistive, while many other devices such as motors are both resistive and
reactive. Reactive devices don't obey Ohm's law and may not draw their peak currents at times of peak voltage.
Therefore, the power in watts consumed by a reactive device isn't the same as the product of its current times its
voltage, or its kVA.
How does 240-volt electricity work in house wiring? If each "hot" wire in a circuit from the central wiring panel is at 120
volts with respect to neutral/ground, how are devices that use 240 volts wired? -- GK, Ottawa, Ontario
Most homes receive power through three wires: two power wires and one neutral wire. Each power wire is at 120
volts AC with respect to the neutral wire, meaning that its electric potential fluctuates up and down with respect
to the neutral wire and behaves as though, on average, it were 120 volts away from the potential of the neutral
wire. But the fluctuations of the two power wires are opposite one another--when one power wire is at a positive
voltage relative to the neutral wire, the other power wire is at a negative voltage relative to the neutral wire. If
you compare the two power wires to one another, you'll find that they behave as though, on average, they are 240
volts away from one another. Thus home appliances that need 240 volts are powered by the two power wires,
rather than one power wire and one neutral wire.
How do you calculate the change in water pressure as the diameter of the hose changes? - JH
When water flows through a hose, it has three main forms for its energy: kinetic energy, gravitational potential
energy, and an energy associated with its pressure--which I'll call pressure potential energy. Since energy is
conserved, the water's energy can't change as it flows through the hose (we'll ignore frictional forces here,
although they really are pretty important in a hose). Let's assume that the hose is horizontal, so that the water's
gravitational potential energy can't change. When the water enters a narrowing in the hose, the water must speed
up to avoid delaying the water behind it. This increase in speed is associated with an increase in kinetic energy.
Since the water's energy can't change, the increase in kinetic energy must be accompanied by a decrease in
pressure. If the water then enters a widening in the hose, it slows down, its kinetic energy drops, and its pressure
rises to conserve energy! If the hose then rises upward, so that the water's gravitational potential energy rises, the
water's pressure must drop to conserve energy. In general, one form of energy can become another but the sum of
those three forms can't change.
January 31, 1997
If the Fermi level is the highest energy level used by an electron, how can electrons shift to conduction levels that are at
energies above the Fermi level? -- PH
The Fermi level is the highest energy level occupied when all the electrons have as little energy as possible. That
situation occurs only when all the electrons are paired two to a level and the levels are filled all the way from the
lowest energy level up to the Fermi level. At any reasonable temperature and in the presence of light or other
energy sources, some of the electrons will have been shifted out of their normal levels and into levels above the
Fermi level. The Fermi level doesn't change when these shifts occur--it's defined before the electrons shift.
January 30, 1997
How does the light emission of Wint-O-Green Lifesavers work? If you bite them, they give off light, but what are the
chemicals involved and how does it work? -- KA, Davis, CA
This phenomenon is the result of tiny electric sparks that occur when sucrose crystals in the Lifesaver crack as
they are exposed to severe stresses. A separation of electric charge occurs between the two sides of the fracture
tip and an electric discharge occurs through the air separating those two sides. The light that you see is produced
by this electric discharge.
To understand how this charge separation occurs, we must look at how crystals respond to stress. Many
crystalline materials are microscopically asymmetric, meaning that their molecules form orderly arrangements
that aren't entirely symmetric. To visualize such an arrangement, consider a collection of shoes: an orderly
arrangement of left shoes can't be symmetric because a left shoe isn't its own mirror image--you can't built a fully
symmetric system out of asymmetric pieces. Like left shoes, sucrose molecules (the molecules in table sugar) are
asymmetric so that a crystal of sucrose is also asymmetric.
Whenever you squeeze a crystal, exposing it to stress, its electric charges rearrange somewhat. In a symmetric
crystal, this microscopic rearrangement doesn't have any overall consequences. But in an asymmetric crystal
such as sucrose, the microscopic rearrangement can produce a large overall rearrangement of electric charges and
huge voltages can appear between different parts of the crystal. The most familiar such case is in the spark
lighters for gas grills, where a stressed asymmetric crystal creates large sparks. In a Wint-O-Green Lifesaver, the
large build-ups of charge cause small sparks that produce the light you see.
What happens when matter and anti-matter collide? Do they just destroy each other? I thought that matter couldn't be
created or destroyed? - S
As Einstein's famous formula points out, mass and energy are equivalent in many respects. In most situations,
mass is conserved and so is energy. But at the deepest level, it's actually the sum of those two quantities that's
conserved. When matter and anti-matter collide, they often annihilate one another and their mass/energy is
converted into other forms. For example, when an electron and an anti-electron (a positron) collide, they can
annihilate to produce two or more photons of light. There is no fundamental law that prevents matter from being
created or destroyed but there is a fundamental law that mass/energy must be conserved. In this case, the masses
of the electron and positron become energy in the massless photons. Overall, mass/energy has been conserved
but what was originally mass has become energy. The fact that when matter and anti-matter annihilate, the
product is usually energy, makes this mixture attractive as a possible super-rocket fuel. But don't hold your
breath; anti-matter is incredibly difficult to make or store.
How can we measure magnetic fields or magnetic potentials of solvent atoms that reside interstitially inside solid solutes?
-- DR, Tampa, FL
You can measure the magnetic fields in which certain atoms reside with the help of nuclear magnetic resonance
(NMR). This technique examines the magnetic environment of the atom's nucleus by determining how much
energy it takes to change the orientation of the nucleus. Since the nucleus is itself magnetic, it tends to align with
any magnetic field--like a compass. The stronger that magnetic field, the harder it is to flip the nucleus into the
wrong direction.
How do Oven Cooking Bags work? I know they are made of heat resistant nylon resin, but can you explain what that
means? -- HY, Halifax, Nova Scotia
There are two broad classes of plastics: (1) thermoplastics that can melt, at least in principle, and (2) thermosets
that can't melt under any circumstances. Thermoplastics consist of very long but separable molecules and
common thermoplastics include polyethylene (milk containers), polystyrene (Styrofoam cups), Nylon (hosiery),
and cellulose (cotton and wood fiber). Thermosets consist of very long molecules that have been permanently
cross-linked to one another to form one giant molecule. Common thermosets include cross-linked alpha-helix
protein (hair) and vulcanized rubber (car tires).
Most common plastic items are made from thermoplastics because these meltable plastics can reshaped easily.
But different thermoplastics melt at different temperatures, depending on how strongly their long molecules cling
to one another. The plastic in an Oven Cooking Bag is almost certainly a thermoplastic form of Nylon, but one
that melts at such a high temperature that it doesn't change shape in the oven. It's possible that the Nylon has
been cross-linked to form a thermoset, so that it can't melt at all, but I wouldn't expect this to be the case.
How does ultrasound detect cracks or imperfections in metal? Is this to do with density or is it just reflecting off surfaces?
-- PA, Essex, UK
Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in
speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed
of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting
sound and detecting the reflected sound, it's possible to image the imperfections. The same technique is used in
medical ultrasound to image organs or fetuses, and even to image the insides of the earth.
Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? - PTW
You can put a microwave oven anywhere that it's stable and where it has adequate ventilation. A microwave
oven has a fan and vents through which it gets rid of its excess heat. You mustn't block the vents or the oven will
overheat.
Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW
Not one that came from the microwave oven. Microwaves are all around us and are completely innocuous. Your
body emits weak microwaves all the time, as part of its thermal radiation! Like light, microwaves don't remain
still in objects so you can't eat one that was put in the food by the oven.
If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? -- IF,
Bakersfield, CA
According to a reader, most microwave oven capacitors have fuses in them so that when they fail, they usually
become open (they lose all of their ability to store separated charge and behave as a simple open circuit). You'd
need a capacitor checker to find this open circuit within the capacitor.
How can we clean the microwave oven? - PTW
Since the cooking chamber of a microwave oven doesn't get hot, there is no way to make a "self-cleaning"
microwave oven. Instead, you have to clean it by hand with a sponge and perhaps a little soapy water. As long as
you get the soap or any other cleaning agents out, you can clean the cooking chamber just as you'd clean the top
of a stove.
Don't microwaves penetrate metal at all? -- DR, Tampa, FL
If the metal is a good conductor, then the microwaves don't penetrate more than a fraction of a millimeter. That's
because the microwave electric fields push on the metal's mobile electrons and those electrons immediately
rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal
responds to the fields and it shields the rest of the metal from the microwaves.
I am interested in finding out if and what materials affect magnetic fields. -- HLD, Jacksonville, FL
Magnetic fields are associated with lines of magnetic flux, invisible structures that stretch between north and
south magnetic poles or that curve around on themselves to form complete loops. Unless a material has its own
north or south magnetic poles, it can't terminate the magnetic flux lines and can have only small effects on
magnetic fields. The few materials that do affect magnetic fields substantially are ones such as iron or steel that
are intrinsically magnetic and that can easily develop strong north and south magnetic poles. These magnetic
materials can significantly shift the paths of the magnetic flux lines. If you put an iron or steel box in a magnetic
field, the flux lines will tend to travel through the walls of the magnetic box. As a result, there will be few
magnetic flux lines inside the box and almost no magnetic field. This effect is used to shield sensitive equipment
such as the picture tubes in televisions from magnetic fields.
How long will the magnetic data last on a VCR tape before it becomes no longer useable as read data? -- KR, Urbana, IL
As long as the tape is kept cool and dry, its magnetization should remain stable for years. However, there is the
problem of magnetic imprinting from one layer of tape to the adjacent layers on a spool. With time, one layer
transfers some of its magnetization to those adjacent layers. In a videotape, this imprinting leads to a gradual
appearance of noise in the video images. As long as you're willing to tolerate a little video "snow," this
imprinting shouldn't be too much of a problem. You can reduce its severity by occasionally winding and
rewinding the tapes. But I don't see any real reason why a tape won't be reasonably useable for decades.
Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water
that is medium to very hard? I want enough to use in a steam iron regularly? -- HD, Kintnersville, PA
There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts.
The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing
detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange sodium
ions for the calcium, magnesium, and iron ions because sodium ions don't have such bad effects on soaps and
detergents. Adding salt to hard water, as you propose to do, won't exchange sodium ions for the other ions. It will
only add more metal ions to the water and the water will remain hard.
Second, a steam iron shouldn't use hard water because when hard water boils away as steam, it leaves behind all
the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply
leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for
your iron. The best way to demineralize water is to distill it.
I have read recently that achieving absolute zero is impossible. Why is this the case? What will happen to objects at this
temperature (i.e., solid, liquid, and gas)? -- BC, Ottawa, Ontario
Absolute zero can't be reached for the same reason that any perfect order is impossible. It's just too unlikely to
ever happen. For an object to reach absolute zero, every single bit of thermal energy and every aspect of disorder
must leave the object. If the object is a crystalline material, then its crystal structure must become absolutely
perfect. This sort of perfection is essentially impossible. Reducing the temperature of an object towards absolute
zero requires great effort and ends up creating a great disorder elsewhere. The closer the approach to absolute
zero, the more disorder is created elsewhere. To reach absolute zero, you'd have to create infinite disorder
elsewhere. For something to think about, imagine trying to make you lawn absolute perfect. The more perfect
you tried to make it, the more gardeners you'd need and the more food, money, and services would be consumed.
The lawn would grow more and more perfect but everything else would grow more disordered. And still you
would never have a truly perfect lawn.
How does a heat lamp work and could it be harmful to the eyes of pets from extended exposure? -- DM, Osceola, IA
A heat lamp is much like a normal incandescent lamp, except that the heat lamp's large filament operates at a
much lower temperature. Because of this lower temperature, the filament emits relatively little visible light.
Instead, it emits mostly invisible infrared light. While you can't see infrared light, you can feel it as heat. Looking
at a heat lamp is no more dangerous than looking at the glowing coals in a fireplace. Their thermal radiation
heats your skin and the surfaces of your eyes, and is likely to make you uncomfortable enough to turn away
before it causes real damage. In contrast, ultraviolet light from a sunlamp can injure your skin and eyes without
causing any immediate pain--it's only much later that you feel the sunburn on your skin and corneas. That's why
a heat lamp is relatively safe while a sunlamp is not.
How does air pressure affect the distance a soccer ball can be kicked? -- SR, Pittsburgh, PA
In general, the greater the air pressure, the greater the air resistance. As the soccer ball moves through the air, the
air in front of it experiences a rise in air pressure and pushes the ball in the direction opposite its motion. While
there are various other changes in air pressure around the ball's surface, this rising pressure in front of the ball
remains largely unbalanced and it slows the ball down. The higher the air pressure was to start with, the greater
its rise in front of the ball and the stronger the backward push of air resistance. Thus if you were to play soccer in
the Rocky Mountains, where the air pressure is much less, you'd be able to kick the ball significantly farther.
January 29, 1997
Why doesn't a helium balloon pop when it reaches the ceiling?
The buoyant force lifts the helium balloon upward--the denser air flows downward to fill the space vacated as the
balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on
the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon's skin is gentle
enough and spread out enough that it doesn't injure the rubber. The balloon simply comes to a stop and remains
suspended until enough helium diffuses out of the balloon to cause it to descend.
January 28, 1997
When I read of scientists discovering galaxies "on the edge of the universe," perhaps 15 billion light years away, I
wonder if they are including the distance the objects must have traveled in the time it took for the light to reach their
telescopes. Very distant objects are said to be receding from any other point in space at a higher rate than closer objects.
If a galaxy is discovered 15 billion light years away today, the light left that galaxy 15 billion years ago while receding at
a high rate. Where is it today, really? Twice as far away? -- DK, Missouri City, TX
This seemingly simple question has a surprisingly complicated answer. You might expect that if the earth and
one of these distant galaxies had been very near one another at the creation of the universe and had both been
moving away from one another at almost the speed of light, that after 15 billion years each would have moved
almost 15 billion light years in opposite directions and would thus be separated by almost 30 billion light years.
That's not the case. That simple view ignores the important effects of special relativity on rapidly moving
objects.
To understand these effects, suppose that there was an observer who was stationary at the creation and watched
the earth and galaxy head off in opposite directions at almost the speed of light. From that observer's perspective,
the two objects are heading away from one another at almost twice the speed of light. After 15 billion years, this
observer sees the galaxy as almost 30 billion light years away from the earth.
Now suppose that there was another observer who was on the earth at the creation. From this person's
perspective, the galaxy recedes from the earth at almost the speed of light, but no more. Nothing can move faster
than speed of light! After 15 billion years, this observer sees galaxy as almost 15 billion light years away from
the earth.
These two observations don't seem to agree. The problem lies in how the two observers perceive time and space.
According to special relativity, observers who are moving relative to one another don't perceive time and space
in the same way. Their perceptions will be so different that they will not even agree about just when 15 billion
years has passed.
With this long introduction, here is the answer to your question: no distant galaxy in the observable universe can
ever be farther from us than the distance light has traveled since the creation of the universe. Since that creation
was about 15 billion years ago, the most distant possible galaxy is almost 15 billion light years away.
How does a rotary phone switching system distinguish between the off-hook signal and the dialing signals, one through
ten? - B
It doesn't. When you dial a rotary phone, it briefly hangs itself up one time for every number on the dial. Thus if
you dial a "5", it hangs itself up briefly 5 times. In fact, you can dial the phone by tapping the switchhook briefly
one time for every number. For example, if you want to dial a "5", tap the switchhook (hang up the phone)
briefly 5 times very quickly. It takes some skill, but you can "dial" just fine without ever touching the dial. It
used to be that people installed key locks on the rotary dial to prevent unauthorized use of the telephone.
Unfortunately, this action didn't prevent someone with a nimble hand from dialing with the switchhook.
How does a dishwasher machine work? -- WW, Bochum, Germany
A dishwasher is really a number of simple machines that work together to clean dishes. These machines are
controlled by a mechanical or electronic timer and include an electrically operated water valve, a water level
sensor, one or two water pumps, a thermostat, an electric heating element, one or more rotating spray nozzles,
and a fan.
The cycle begins when the timer sends electric current through a coil of wire in the water valve, making that coil
magnetic and pulling the water valve into its open position. Water flows then flows from the high pressure in the
water line to the atmospheric pressure in the cleaning chamber. When the water sensor detects that the
dishwasher is adequately filled, it shuts off current to the valve and the valve closes.
The thermostat measures the water temperature and may delay the start of the cycle if the water is too cool. If so,
it directs electric current through the heating element, where that current's energy is converted into thermal
energy and transferred to the water. When the water is hot enough, the cycle continues.
During the cleaning cycle, one or more pumps operate. They add energy to the water and increase its pressure.
This high-pressure water flows slowly to the rotating nozzles and then accelerates to high speeds as it enters the
narrow openings and sprays out into the low-pressure cleaning chamber. As the high-speed water collides with
the dishes and slows down, its pressure rises again and begins to exert substantial forces on the food particles.
The food particles are pushed off the dishes and fall into the bottom of the dishwasher. Soap added to the
cleaning water forms tiny spherical objects called micelles that trap and carry away fats that would otherwise not
mix with water. At the end of the cycle, the water, food particles, and fat-filled soap micelles are pumped down
the drain.
The cleaning cycle may repeat with fresh water and is then followed by a rinse. A soap-like surfactant may be
added to the rinse water to lower its surface tension and prevent it from beading up on the dishes. When the
pumps have removed the last of the rinse water, a fan begins to blow air over the dishes. The heating element
may heat this air to assist evaporation. The water molecules leave the surfaces of the dishes and become gaseous
water vapor. The dishes are left clean and dry.
How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the
rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much
louder than normal. -- AA, Ontario, CA
While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum
you hear is probably an indication that something is overloading the power transformer. That suggests that the
diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and
found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail
by shorting out, the result of electromigration of the filament material. The tube would then draw excessive
currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine
is only worth what you've paid for it. I'd suggest you consult a local repairperson, who has test equipment that
can pinpoint the problem in seconds.
If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or
receive interference. How does digital RF work and does it stop interference problems? -- KG, New York, NY
Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that
overlap in space and frequency. The receivers for these two waves can't tell them apart and end up receiving both
at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same
time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can't
determine which broadcast they're supposed to follow. Instead, they lock onto whichever wave is strongest and
will often flip back and forth from one station to the other as their signal strengths fluctuate.
The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using.
That way your transmission is certain to be stronger than any other at the same frequency and your receiver will
follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some
encoding scheme such as digital transmission so that your receiver can tell when it's receiving a broadcast from
your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme
and won't respond to that of some other transmitter. However, if that other transmitter is strong enough, it will
probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a
second transmission is the principle behind jamming of a radio transmission.
What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. -- DG, Houston,
TX
In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously
variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a
tape recorder's microphone rises during one moment of a song, an electric current in the recorder will rise and a
region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall,
each value of air pressure is converted to a particular value of the physical quantity.
The problem with analog recording is that when the sound is recreated, any defect in the physical quantity
representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization
of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces
won't be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by
recording the information as bits. The physical quantity such as magnetization is representing bits (which take
only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor
changes in the physical quantity representing these bits won't change the bits. Thus imperfections in the
recording or playback process won't affect the sound quality.
I have to do an experiment for school on the electromagnetic properties of iron, steel, and aluminum. The only problem is
that I am not too sure what I should be testing. Any ideas? -- CP, Nassau, Bahamas
Iron and steel (not stainless) are ferromagnetic metals, meaning that they are intrinsically magnetic. While this
magnetism is normally hidden by the formation of millions of tiny, randomly oriented magnetic domains, it
becomes apparent when you hold a magnet near the iron or steel: they are attracted! Aluminum has no intrinsic
magnetism and is not attracted to a magnet. There are far more non-magnetic metals than magnetic ones. Why
don't you try to see which metals will stick to a magnet. Only the ferromagnetic ones will. Even common
stainless steel is non-ferromagnetic.
How much water power do you need to turn on a light bulb? How much wind power does it take to turn on a light bulb?
Can artificial light make a solar paneled car run? If so, how bright? -- BB, Stafford Springs, CT
If you are trying to light a 60 watt bulb, you must deliver 60 watts of electric power to it (unless you are willing
to have it glow relatively dimly). So the answers to your questions are 60 watts of waterpower and 60 watts of
windpower. But you are probably more interested in how much water or wind is needed to run those power
sources. An efficient water generator that produces 60 watts of power lowers about 6 liters (or one and a half
gallons) of water about 1 meter (or 3 feet) each second. An efficient wind generator that produces 60 watts of
power stops about 1 cubic meter (or 32 cubic feet) of air moving at 36 km/h (or 21 mph) each second. Finally, a
solar powered vehicle needs at least several hundred watts of power to operate. Since solar panels are only about
20% energy efficient and artificial light sources are also only about 10 to 50% energy efficient, it would take
thousands of watts of artificial lighting to operate a solar powered car. Not very practical.
How does an electronic dimmer work? I know that a regular household dimmer works through resistance coils, but I read
that electronic dimmers actually clip the A.C. cycle. Is this why you read the voltage output of an electronic dimmer the
voltage remains the same even when it is dimmed down? Why can electronic dimmers dim fluorescents and arc lamps,
but resistive dimmers cause those lamps to flicker? -- KG, New York, NY
Electronic dimmers do clip the AC cycle. They use transistor-like devices called triacs to switch on the current to
a lamp part way into each half-cycle. By shortening the time that power is delivered to the lamp, the dimmer
reduces the total energy delivered to the lamp during each half-cycle and the lamp dims. But while a triac turns
on easily, the only way to turn it off is to get rid of any voltage drop across it. The dimmer uses the alternating
current itself to turn off the triac--the voltage of the power line naturally goes to zero at the end of each halfcycle and the triac turns off. The triac then waits until the dimmer restarts it, sometime into the next half-cycle.
Since the dimmer messes up the waveform of the electric current flowing through the lamp circuit, what you
measure with a voltage meter depends on how that meter works. Since many AC voltmeters just measure peak
voltage and assume that they are looking at a pure sinusoidal current, they don't give you an accurate sense for
what is really happening to the voltage of this clipped waveform as a function of time. Unless an electronic
dimmer is turned way down, the peak voltage it delivers will be close to the normal power line peak, a fact which
tricks the voltage meter into reading a high value and which allows a properly designed fluorescent lamp to
continue operating normally but at a dimmer level.
How does a light switch work? -- AB, Tulsa, OK
A light switch controls the flow of electricity through a circuit--a complete, unbroken loop through which
electric charges can move. When the light switch is on, these electric charges can move in an endless loop. This
loop starts with a trip to the power company--actually to the power transformer near your home--where the
charges pick up electric energy. They then flow through wires to the light switch, then to the light bulb where
they deliver their electric energy, and finally back to the power company to obtain more energy. The same
charges complete this loop over and over again. The loop is called a circuit.
But when you turn off the light switch, you open or break the circuit. One of the wires connecting the power
company to the light bulb suddenly has a gap in it and the current of electric charges can no longer flow. The
switch itself actually contains two separated wires and a mechanical device that connects them only when the
switch is in its on position. The precise structure of the mechanical switching device differs from switch to
switch, but the behavior is always the same: the switch disconnects the two wires--and thus breaks the circuit-whenever you turn the switch off.
How does a magnetic train work? How can I make an experiment with it for a school project? -- AASE, Quito, Ecuador
There are many techniques for supporting a train on magnetic forces, but the simplest and most promising
involves electrodynamic levitation. In this technique, the train has a strong magnet under it and it rides on an
aluminum track. The train leaves the station on rubber wheels and then begins to fly on a cushion of magnetic
forces when its speed is high enough. Its moving magnet induces electric currents in the aluminum track and
these currents are themselves magnetic. The train and track repel one another so strongly with magnetic forces
that the train hovers tens of centimeters above the track.
To demonstration this effect, you can lower a very strong magnet above a rapidly spinning aluminum disk. In my
class, I spin a sturdy aluminum disk with a motor and lower a 5 cm diameter disk magnet onto its surface. I hold
the magnet firmly with a strap made of duct tape, so that the magnet won't fly across the room or flip over as it
descends. Instead of touching the spinning disk, the magnet floats about 2 cm above it. If you try this experiment,
don't spin the aluminum disk too fast or it will tear itself apart. It should spin about as fast as an electric fan on
high speed. Also, be careful with the magnet, because it will experience magnetic drag forces as well as the
magnetic lift force. If you don't hold tight, it will be yanked out of your hand.
For a simpler experiment that anyone can do, float an aluminum pie plate in a basin of water and circle one pole
of a strong magnet just above its surface. The pie plate will begin to spin with the magnet. You are again
inducing currents in the aluminum, making it magnetic. While the forces here are too weak to lift the magnet in
your hand, they are enough to cause the pie plate to begin spinning, even though you never actually touch it. This
technique is used in many electric motors. That's physics for you--the same principles just keep showing up in
seemingly different machines.
In steam generation, wouldn't it be more economical to heat a small boiler and feed it just enough water for it to maintain
its optimal steam generating temperature than to heat a huge boiler as is normally done? -- MF, Gillette, WY
Not really. Once you have heated the water to its steam generating temperature, all of the heat you add goes into
converting water into steam. The presence of more or less water just doesn't make any difference. The extra
water requires no extra heat while the boiler is making steam. And having that extra water does act as a buffer in
case you add too much or too little heat for a short while. That's probably why most boilers have a bit more water
than they need over any short period of time. Furthermore, it's not always easy to add water to a boiler when the
boiler's pressure is very high.
How does reverse osmosis work? - MC
Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a
concentrated solution on the other side. It is driven by statistics--it's much more likely for a water molecule on
the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution
side to pass through the membrane. There are simply more water molecules trying to cross the membrane from
the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated
solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows
the passage of water and brings it to a halt.
Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water
reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water
molecules from that solution through the semi-permeable membrane and into the fresh water on the other side.
The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from
the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water
in desert countries.
How do stalactites and stalagmites form in caves? -- GS, Conroe, TX
They form when various minerals come out of solution in water and crystallize on the surfaces of a cave. To
understand how this process occurs, we must look at the interface between the water and the cave surface.
Whenever water is in contact with a mineral surface, there is a chance that an atom of the surface will suddenly
leave the surface and dissolve in the water. If there are atoms already dissolved in the water, there is also a
chance that one of them will suddenly come out of solution in the water and attach to the surface. Atoms leave
and return to cave surfaces all the time as water drips from the ceiling of a cave to its floor.
What is important for the growth of stalactites and stalagmites is that more atoms stick to the cave surfaces than
leave those surfaces. That is exactly what happens and it does so because the water has already picked up more
than enough dissolved atoms before it reaches the stalactite. Either because of temperature changes or because of
evaporation, the water that runs across the cave roof and down the sides of a stalactite deposits more atoms on
the stalactite's surface than it removes. The same goes for the stalagmite after the water drips down to the cave
floor. As the atoms build up on the cave surfaces, the stalactites grow down and the stalagmites grow up.
At times a very thin invisible layer of ice forms on road surfaces. The road surface appears dry and does not have the
telltale reflections of ice. Many people refer to this as "black ice." How is this ice formed? What are the crystal properties
that make it invisible? - BK
Black ice is a layer of ice that is almost free of internal defects or air bubbles and that does not have a smooth
surface. The absence of internal defects or air bubbles is what makes it transparent rather than white. Snow and
crushed ice appear white because they contain countless tiny surfaces. Whenever light changes speed, as it does
in going from ice to air or air to ice, some of that light reflects. Since snow and crushed ice contain many ice/air
interfaces, they reflect light extensively and appear white. In contrast, black ice contains no internal ice/air
interfaces and doesn't reflect any light from inside. Any light that makes it into the black ice goes all the way to
the roadway. If the roadway reflects any of this light, it again passes unscathed through the black ice. The only
evidence that the black ice exists at all comes from its surface, but here again the ice offers little that you can see.
Since true black ice is microscopically rough, the small amount of light that reflects as it enters the ice from the
air is reflected randomly in all directions. So little of that reflected light travels in any one direction that you can
barely see it at all. Overall, black ice reflects so little light that you see only the roadway itself. While I am not
sure, I think that it forms when moisture in the air condenses to dew on the roadway and then freezes into ice.
Whatever process forms it must leave it almost without holes and therefore invisible.
I have a thermometer made of a column of fluid containing seven spheres of fluid that rise and fall according to the
temperature (commonly known as a Galileo thermometer). How does this work? -- LS, Conroe, TX
A Galileo thermometer combines Archimedes' principle with the fact that liquids generally expand faster with
increasing temperature than solids do. Each sphere in the thermometer has an average density (a mass divided by
volume) that is very close to that of the fluid in the thermometer. As stated in Archimedes' principle, if the
sphere's average density is less than that of the fluid, the sphere floats and if the sphere's average density is more
than that of the fluid, it sinks. But the fluid's density changes relatively quickly with temperature, becoming less
with each additional degree. Thus as the temperature of the thermometer rises, the spheres have more and more
trouble floating. Each sphere's density is carefully adjusted so that it begins to sink as soon as the thermometer's
temperature exceeds a certain value. At that value, the expanding fluid's density becomes less than the average
density of the sphere and the sphere no longer floats. The spheres also expand with increasing temperature, but
not as much as the fluid.
Here is a picture of a combined Galileo thermometer and simple barometer.
In
addition to measuring the temperature with floating spheres, this device measures the outside air pressure with a
column of dark liquid. It has a trapped volume of air that pushes the liquid (visible at the bottom of the unit) up a
vertical pipe when the outside air pressure drops. The owner of this unit would like to know its history and
origin, so if you have any information about it, please let me know.
How do fletchings stabilize an arrow in flight after it is shot from a bow? -- SH, Newton, TX
Like all isolated objects, the arrow naturally pivots about its own center of mass, a point located near its
geometric center. If the arrow had no fletchings (or fins) it would tend to rotate wildly in flight. But the
fletchings experience substantial aerodynamic forces whenever the arrow isn't flying point first and these
aerodynamic forces twist the arrow back toward its proper orientation. Thus whenever the arrow begins to rotate
so that its point isn't first, the air pushes hard on the fletchings and returns the arrow to its point-first orientation.
The same effect keeps airplanes and birds flying nose (or beak) forward.
January 27, 1997
Piling sandbags in the back of a truck would increase friction between the wheels and the ground, but wouldn't it also
increase the truck's inertia, making it harder to stop on an icy road?
Adding sandbags to the back of a pickup truck increases the truck's traction and adds to the truck's mass.
Fortunately, the truck's traction increases more dramatically than its mass and it becomes easier to start and stop
the truck, rather than the reverse. That's because even a modest amount of sand can double the force pressing the
rear wheels against the road and thus double the frictional forces the wheels can experience. That same amount
of sand won't double the total mass of the truck.
In class, you sat motionless on a cart with a ball in your lap. You said that your momentum was zero. You then threw the
ball in one direction and you began moving in the other direction. You said that your momentum was still zero. How can
your momentum be zero if you are moving?
In both cases, I was referring to the total momentum of the ball and me. The total momentum of the ball and me
was zero before I threw the ball and it was still zero after I threw the ball. However, before I threw the ball
nothing was moving and after I threw the ball the two of us were moving in opposite directions. It was our total
momentum that was zero after the throw, not our individual momenta. While the ball and I each had a nonzero
momentum after the throw, our momenta were equal in amount but opposite in direction--the ball's momentum
was exactly opposite mine. If you were to add our momenta together, they would sum to zero. Since momentum
is conserved and we couldn't exchange momentum with anything around us, the ball and I began and ended with
the same total amount of momentum: zero.
If there were no friction or air resistance, would the bowling ball pendulum continue in motion forever?
Yes. If the pendulum had no way to convert its energy into thermal energy (e.g., via friction) and no way to
transfer that energy elsewhere (e.g., via air resistance), it would continue to swing forever. While its energy
would transform from gravitational potential energy (at the ends of each swing) to kinetic energy (at the middle
of each swing) and back again, over and over, the total amount of energy it has won't change.
If energy is always conserved, why does a pendulum eventually stop swinging if you leave it alone?
The pendulum experiences friction and air resistance, both of which extract energy from the pendulum. Friction
turns that energy into thermal energy and air resistance transfers the energy to the air.
January 26, 1997
How fast is the earth moving through space? Does this movement affect our perception of time? -- GR, Grabil, IN
Because there is no preferred reference frame for the universe, we can only talk about the earth's speed in
reference to other objects. For example, the earth is moving at about 5 kilometers per second relative to the sun
and about 30,000 kilometers per second relative to the center of the galaxy. These speeds do affect our
perceptions of time, so that times passes at a different rate for us than for someone closer to the sun or to the
galactic center. However, gravitational wells also affect the perception of time, so that the effects are
complicated. The earth is also receding extremely rapidly from objects at the far side of the universe; so fast that
time passage is dramatically affected. Those distant objects appear to be aging very slowly and their light is
shifted substantially toward the red.
How does an infrared sensor faucet work? -- DD, Sacramento, CA
The sensor has two lenses: one that emits a beam of infrared light and the other that looks for a reflection of that
light. As long as there is nothing beneath the faucet, there is very little infrared light reflected back toward the
sensor and the sensor prevents any water from flowing out of the faucet. But when you hold your hands under
the faucet, the infrared light reflects from your hands and some of it returns to the sensor. The sensor detects this
light and opens an electronic valve to permit water to flow out of the faucet. The lenses are aimed so that only
objects under the faucet itself will reflect the infrared light back toward the lens. A more distance object may
reflect some of the infrared light, but the light won't pass through the sensor at the proper angle and won't be
detected.
Can you explain how the telephone wiring in my home works for the telephone? My touch-tone phone has 4 wires, but I
understand that only 2 wires are used. Does the phone use the other 2 wires for the light on the phone pad, etc.? -- DS,
Larkspur, CA
Your telephone performs all of its functions using only those 2 wires. The 2 extra wires are virtually never used
by a single-line telephone. The only exception that I'm aware of is the old "Princess Telephone," which had a
special light powered by the extra pair of wires. In most telephones, even the power for the lighted keys comes
from the 2 main wires. While the telephone is off the hook, the telephone company sends a constant DC current
through those two wires. This current powers the telephone's electronics and its lights. When you talk, the
microphone causes the telephone's electric impedance to fluctuate up and down and this variation causes sound
to be reproduced in your friend's earpiece. Pressing the dialing buttons causes similar fluctuations in impedance
and the telephone company uses these tones to make the proper connections. When the telephone company rings
your telephone, they send a higher voltage AC current through the two wires and the telephone's bell rings.
Radioactive elements' half-lives are fixed and they decay at a constant rate. Their decay rates have been determined
thanks in part to our nuclear weapons research. Under what circumstances can a radioactive element have its decay rate
changed? Can the element's radioactivity be destroyed (cancelled) by applying high temperatures? If so, how high would
the temperature have to go to achieve this? -- RD, Humble, TX
Since radioactivity is a feature of atomic nuclei, the only way to alter radioactivity is to alter atomic nuclei. But
there aren't many ways to change atomic nuclei. Of various atomic and subatomic particles, only a neutron can
enter a nucleus easily and cause it to rearrange. However, it's more common for a neutron to increase
radioactivity than to destroy it, so that's not a good approach. Furthermore, the only practical way to obtain
neutrons is with radioactivity.
Heating a collection of nuclei can cause them to collide and rearrange. However, this process is also fraught with
problems. The products of the fusion and fission events that occur when nuclei collide will probably be
radioactive themselves, so that it's unlikely that heating radioactive materials will make them less radioactive.
Instead, it's likely that heating radioactive materials will make them more radioactive. Furthermore, the
temperatures at which nuclei will begin to collide are extraordinarily high. Even the smallest nuclei repel one
another fiercely so that they need temperatures of 100 million degrees C or more to begin colliding effectively.
Larger nuclei, such as those common in nuclear wastes, won't collide until their temperatures exceed 1 billion
degrees C. The only way to reach these temperatures is with nuclear weapons and they certainly don't reduce the
radioactivity of nearby materials. In short, the only way to get rid of radioactivity is by waiting patiently.
Why do colors fade in the sun? - RD
While light travels as electromagnetic waves, it's emitted and absorbed as particles called "photons." Each
photon carries with it a tiny bit of energy. The amount of energy in a photon depends on the wavelength of the
light associated with it. While a photon of red light contains too little energy to cause chemical processes to
occur in most molecules, a particle of violet or ultraviolet light contains enough energy to cause significant
chemical damage to a typical molecule. Since sunlight contains a substantial amount of violet and ultraviolet
lights, it can cause a fair amount of chemistry to occur in the molecules that absorb it. That's why colors often
fade in sunlight. Many colored molecules are relatively fragile and are damaged by photons of ultraviolet light.
The portion of a dye molecule that gives it its color is called a "chromophore" and is usually the most fragile part
of the molecule. Destroying its chromophore will often leave a dye molecule colorless. Exposure to sunlight was
the traditional way to bleach fabrics and make them white.
What would things look like if I could see wavelengths of the spectrum other than just visible light (e.g., X-rays, radio
waves, ultraviolet, infrared, gamma rays, etc.)? -- SH, Hurricane, UT
As you looked around, you would see a general glow of radio waves, microwaves, and infrared light coming
from every surface. That's because objects near room temperature emit thermal energy as these long-wavelength
forms of light. While we don't normally see such thermal radiation unless an object is hot enough for some of it
to be in the visible range, your new vision would allow you to see everything glow. The warmer an object is, the
brighter its emission and the shorter the wavelengths of that emission. People would glow particularly brightly
because of their warm skin.
You would also see special sources of radio waves, microwaves, and infrared light. Radio antennas, cellular
telephones, and microwave communication dishes would be dazzlingly bright and infrared remote controls
would light up when you pressed their buttons.
You would see ultraviolet light in sunlight and from the black lights in dance halls. But there wouldn't be much
other ultraviolet light around to see, particularly indoors. X-rays and gamma rays would be rare and you might
only see them if you walked into a hospital or a dentist's office. Gamma rays would be even rarer, visible mostly
in hospitals.
How do the 2" diagonal color LCD screens used in some of the new digital video cameras work? -- M, Waynesboro, MS
Like most liquid crystal displays (LCD), these devices use liquid crystals to alter the polarization of light and
determine how much of that light will emerge from each point on the display. Liquid crystals are large molecules
that orient themselves spontaneously within a liquid--much the way toothpicks tend to orient themselves parallel
to one another when you pour them into box. The liquid crystals used in an LCD display are sensitive to electric
fields so that their orientations and their optical properties can be affected electronically. The liquid crystals in
the display occupy a thin layer between transparent electrodes and two polarizing plastic sheets. Light from a
fluorescent lamp passes through a polarizing sheet, an electrode, the liquid crystal layer, another electrode, and
another polarizing sheet. The orientation of the liquid crystal determines whether light from the first polarizing
sheet will be able to pass through the second polarizing sheet. When electric charges are placed on the two
electrodes, the liquid crystal's orientation changes and so does light's ability to pass through the pair of polarizing
sheets.
To create a full color image, the display has many rows of electrodes on each side of the liquid crystals and a
pattern of colored filters added to the sandwich. In "active" displays, there are also thin-film transistors that aid in
the placement of charges on the electrodes. Overall, the display is able to select the electric charges on each side
of every spot or "pixel" on the screen and can thus control the brightness of every pixel.
Is heating milk by microwave advisable? - I
Microwave cooking leaves no permanent mark on the food. It causes virtually no chemical damage and
absolutely no radioactivity. The only drawback with heating milk by microwave is that the heating may be
uneven and may denature some protein molecules in regions of the milk that become excessively hot. Since most
protein molecules are disassembled by your digestion anyway, this treatment probably has no effects worth
worrying about. Even with infant formula, my only concern would be the hot spots. If you carefully shake the
milk after heating, so that its temperature is uniform, it should be just fine. I suspect that companies warn you not
to heat milk in a microwave because they are worried that you will either not shake the milk to distribute its
temperature evenly or that you will overcook it until it boils and the bottle explodes.
Please explain the concepts of magnetism pertaining to ferromagnetism, diamagnetism, and paramagnetism. - SC
A ferromagnetic material is one that contains intrinsic magnetic order. Iron, for example, is a ferromagnetic
material--meaning that if you were to examine a microscopic region of the iron, you would find that it was
highly magnetic. The magnetism in a ferromagnetic material is often hidden by a domain structure, in which
microscopic magnetic regions or "domains" all point in random directions to give the material no apparent
magnetism. Only when you expose the ferromagnetic material to a magnetic field does its magnetic character
suddenly reveal itself. A ferromagnetic material becomes strongly magnetic when it's exposed to a magnetic
field.
A diamagnetic material is one in which the electrons begin moving when it's place in a magnetic field. These
moving electric charges create a second magnetic field that partially cancels the original field. A diamagnetic
magnetic field partially shields itself from magnetism when it's exposed to a magnetic field.
A paramagnetic material is one in which individual magnetic electrons respond magnetically to any external
magnetic field. It becomes weakly magnetic when it's exposed to a magnetic field. Unlike a ferromagnetic
material, a paramagnetic material has no intrinsic magnetic order before it's exposed to an external field.
In our busy trial court we have preserved the original cassette tapes since 1989. They are kept in a relatively constant
room temperature environment in our modern courthouse. Should we take any further precautions to extend the life of
these tapes, considering the possibility that they may need to be replayed one day, such as in the retrial of a death penalty
case that is reversed a decade after trial? I've heard of the practice of unwinding and rewinding tapes for this purpose, but
haven't attempted it yet. The time involved is daunting! What is your opinion? -- JD, Bryan, TX
A magnetic recording tape is usually a Mylar ribbon, coated with a thin layer of plastic that's impregnated with
tiny permanent magnets. As long as it's store away from heat and moisture, the Mylar film itself shouldn't age.
However, the layer of permanent magnets can change slightly with time. When a tape is left tightly wound on its
reel for a long time, the magnetic layers can begin to affect one another--the magnetic fields from one layer of
tape can alter the magnetization of the layers above and below it. The result is that sounds from one layer of tape
can gradually transfer themselves weakly to the adjacent layers, creating faint echo effects. The solution to this
problem is to unwind and rewind the tape, so that the layers shift slightly relative to one another. But while these
echoes may be annoying in a recording of classical music, they probably aren't important in a recording of a
noisy courtroom. Unless I hear otherwise from someone reading this note, I wouldn't worry about unwinding and
rewinding your tapes. The slight imperfections that will result from transfers between layers shouldn't affect their
utility in later trials. Properly stored, I'd expect the tapes to outlive everyone involved with the trials, even
without any unwinding and rewinding.
How much does it cost to run a regular 60 to 100 watt light bulb per minute or per hour? -- JM, Smithfield, ME
Electricity typically costs about 7 cents per kilowatt-hour. Over the course of an hour, a 100-watt light bulb will
use 100 watt-hours or 0.1 kilowatt-hours, at a cost of about 0.7 cents. That's about 0.012 cents per minute.
How does electricity get from the generating station to the outlet in my living room? -- JJ, Arlington, MA
The generating station uses a large generator to transfer energy from a giant turbine to an electric current flowing
through a coil of wire. Current from this generating coil then flows through the primary coil of a huge
transformer, where it transfers its energy to the magnetic core of the transformer. The current then returns to the
generator to obtain more energy.
The magnetic core of the transformer transfers its energy to a second current--one that is passing through the
secondary coil of the transformer. Because this current consists of far fewer electric charges per second, each
charge receives a very large amount of energy. This large energy per charge gives the current a high voltage and
it flows very easily through a high voltage transmission line. Because the amount of power that a wire loses is
proportional to the square of the current passing through it, this high-voltage, low-current electricity wastes very
little power in the transmission line on its way across country to your city. When the current reaches your city, it
passes through another transformer and its energy is transferred to a third current. The cross country current then
returns through the transmission line to the original power station to obtain more energy from the first
transformer.
This third current involves more charges per second, so each charge carries less energy and the voltage is lower.
This medium voltage electricity travels to your neighborhood before passing through a final transformer. This
final transformer is probably either a gray metal can on a utility pole or a green box on a nearby lawn. In passing
through the final transformer, the current transfers its energy to a current which then enters your home. This last
current delivers energy to your appliances and lights and then returns to the final transformer to obtain more
energy.
Is it true that you can get lead poisoning in your home more easily from hot water than from cold water? -- WH, Erial, NJ
Yes, assuming that your home has either lead pipes or copper pipes that were joined with lead-containing
solders. That's because lead compounds are more soluble in hot water than they are in cold water. The amount of
lead that was permitted in pipe solders has diminished over the years until now, when pipe solders can't contain
any lead at all. While very little lead actually leaches out of the solder joints and enters the water, the effect is
slightly more significant in hot water pipes than in cold water pipes. That's why it's recommended that you not
use water from hot water pipes in cooking.
How do flashing lights, chasing lights, and any type of Christmas lights work? - N
Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series,
electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The
result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the
entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in
a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny
bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the
bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb
and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would
repeat endlessly.
But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized
switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully
timed switching can make patterns of light that appear to move or "chase." As for the problem with one failed
bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a
bulb's filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires
inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a
little more brightly than normal and their lives are shortened slightly as a result.
How do you determine the volume of water passing through a weir? - R
If the speed of the water were uniform as it passes through the opening, you could measure that speed and
multiply it by the cross-section of the weir to obtain the volume of water passing through the weir each second.
However, since the flow is faster near the center of the flow, it's difficult to calculate the volume flowing each
second. Your best bet is probably to divide the opening into a number of regions and then to measure the water's
velocity at the center of each region. Multiply each velocity by the cross-sectional area of that region and then
sum up all the products to obtain the overall volume flow per second.
If air in a rigid 80 cubic foot scuba tank is pressurized to 3000 psi, giving the diver a certain amount of breathing time,
then why does bottom time decrease with depth? I know about external pressure, but how does the pressure affect air
inside the tank? - RJ
The deeper a scuba diver goes, the greater the water pressure and the more the water presses in on the diver's
chest. To be able to breathe, the air in the diver's mouth must have roughly the same pressure as the water around
the diver's chest. That way, the diver will be able to use chest muscles to breathe the air into the diver's lungs.
But the pressure of the air in the diver's mouth is proportional to its density and thus to the number of air
molecules contained in each liter of air. At great depths, the diver must breathe dense, high-pressure air and this
air contains a great many air molecules per liter. Since the scuba tank contains only so many air molecules, these
molecules are consumed more rapidly at great depths than they are at shallow depths. The scuba regulator
automatically controls the density of air entering the diver's mouth so that the air pressure is equal to the
surrounding water pressure. That way, the air is easy to breathe. The deeper the diver goes, the more air
molecules the regulator releases into each of the diver's breaths and the faster the air in the scuba tank is
consumed.
January 22, 1997
In Exercise #9 on pg. 33: If you are riding on an escalator, with a suitcase, doesn't the escalator supply the upward force?
Doesn't this also mean that the forces of the suitcase and escalator cancel one another to produce a net force of zero?
First, let's suppose that the suitcase is resting directly on the escalator and you are not touching it (I had intend
that you hold the suitcase in your hand). Because the suitcase is traveling at constant velocity, the net force on it
must be zero. Since the suitcase has a downward weight, the escalator must be pushing upward on the suitcase
with a force exactly equal in magnitude to the suitcase's weight. As you suggest, the force of the suitcase's
weight and the support force of the escalator cancel one another to produce a net force of zero on the suitcase.
Now, if you are holding the suitcase, it's your job to exert this upward force on the suitcase. Once again, that
upward force is equal in magnitude to the weight of the suitcase.
With Newton's first law, the word "tends" seems a bit ambivalent. Does this word suggest there are exceptions to the
rule?
The statement of inertia contains the word "tends" (an object in motion tends to continue in motion and object at
rest tends to remain at rest) because it doesn't deal with the presence or absence of forces. If forces were
outlawed, then the word "tends" could be dropped from the statement.
However, Newton's first law is not ambivalent and does not contain the word "tends." It states directly that an
object that's free of outside forces moves at constant velocity. No ifs, ands, or buts. If I have inserted the word
"tends" into this law in class, it was a mistake on my part.
January 21, 1997
Why is the element mercury a liquid at room temperature when none of its neighbors on the periodic table are? -- BZ,
Trenton, NJ
The answer to that question lies at least partly in the electronic structure of the mercury atom. The mercury atom
is the largest member of the third row of transition metals, meaning that it is the atom at which the 5d shell of
electrons is finally filled completely. Whenever a shell of electrons is filled, that shell can no longer assist in
forming chemical bonds. While the d shell electrons normally help hold transition metal atoms together, making
these metals strong and hard to melt, the filling of the 5d shell makes it hard for mercury atoms to stick to one
another. In contrast to metals like tungsten and tantalum, which melt only at very high temperatures, mercury is a
liquid at room temperature. Actually, the zinc atom is the atom at which the 3d shell is filled and the cadmium
atom is the atom at which the 4d shell is filled. While those two metals are solid at room temperature, they have
very low melting points.
What happens to gas in a gas mask? -- TF, Auburn, WA
Most gas masks remove toxic molecules from the air by allowing those molecules to react with or stick to a
surface inside the mask. Molecules are generally too small to remove from the air with simple filters, so they
must be removed by chemical processes. Highly reactive molecules, such as chlorine, fluorine, and ozone,
naturally attack and bind with many chemicals and are easily removed by a mask containing those chemicals.
Other molecules aren't so reactive and must be collected in a more complicated manner. Sometimes the gas mask
will contain a reactive chemical that seeks out specific toxic molecules in the air and binds chemically to those
molecules. But some mask simply use activated carbon, which just sticks molecules to its surface. The molecules
don't stick very tightly to the carbon surface, so they can be driven off by baking the carbon. But the carbon is
finely divided so that it has an enormous amount of surface area and can accumulate a great many molecules
before it becomes "full." Finally, some gas masks contain catalysts that decompose certain toxic molecules,
chopping them up before they enter your lungs.
Could you see a laser beam in outer space since it can't reflect off of anything? -- RM, Rochester, NY
No. The reason that you can see a very intense laser beam as it passes through the air is that light can scatter off
of dust particles and air molecules. When it does, some of the laser light is sent toward your eyes and you see the
light coming toward you from the laser beam's path. But if there is no air in the path of the laser beam, the light
will travel without scattering and you won't see the path at all.
How do neon lamps work? -- TF, Auburn, WA
A neon lamp consists of a neon-filled tube with an electrode (a metal wire) at each end. When you put enough
electrons on one of the electrodes and remove enough electrons from the other, electrons will begin to leap off
the first electrode and accelerate toward the other electrode. Because the density of neon atoms in the tube is
relatively low, only about 1/1000th that of air molecules in normal air, the electrons can travel long distances
without colliding with a neon atom. As the electrons accelerate, their kinetic energies increase. However, these
electrons occasionally collide with neon atoms and, when they do, they can give up some of their kinetic
energies to those atoms. The neon atoms then end up with excess energy and they often emit this energy as light.
The color of this light is determined by the structure of a neon atom and tends to be the familiar red of a neon
sign.
Why do we see colors when light strikes atoms? -- GN, Marine City, MI
When white light strikes a molecule, that molecule may absorb some of the light. Light interacts with molecules
as particles called "photons" and whether a particular photon is absorbed depends on the structure of the
molecule and the color of the photon. Each molecule has the ability to absorb only certain colors of light. For
example, a particular molecule may absorb only red photons. As a result, your eye will see only green and blue
light photons coming from that molecule when it's exposed to white light and you will perceive that molecule as
having a blue-green color known as cyan. In general, the colors that you see coming from molecules that are
illuminated by white light are the colors of light that the molecules don't absorb.
On really cold winter days at temperatures well below zero, I've noticed that sunlight is brighter and whiter than on days
that are a little below freezing. Why does this happen? -- CP, Madison, WI
The colder the air is, the less humidity it can hold. That's because at low temperature, water molecules in the air
are much more likely to land on a surface and stick than they are to break free from a surface and enter the air.
Thus cold air is relatively free of water molecules. Water molecules in the air tend to bind together briefly and
form tiny particles that scatter light. The sky is blue because of such scattering from tiny particles. With less
water in the air, there is less scattering of sunlight. As a result, the sky is a darker blue, almost black, and the
sunlight that reaches you directly from the sun retains a larger fraction of its blue light. The sun appears less red
and more blue-white than on a warmer, more humid day.
Does an electric blanket produce enough EMF to affect the body and possible increase the risk of cancer? - FL
The electromagnetic fields (EMF) produced by the currents in an electric blanket are very weak and it takes a
pretty sensitive electronic device to detect them. You body is not nearly so sensitive and I still haven't seen any
credible explanation for how these fields could cause any injury to biological tissue. I strongly suspect that all the
concern about EMF is just hysteria brought about by a few epidemiological flukes or mistakes.
How do an ammeter and voltmeter work? Why must the former be connected in series while the latter goes in parallel? -SK, New Haven, CT
The answer is somewhat different for older electromechanical meters than for modern electronic meters. I'll start
with the electromechanical ones and then briefly describe the electronic ones. An electromechanical meter has a
coil of wire that pivots in a nearly friction-free bearing and has a needle attached to it. This coil also has a spring
attached to it and that spring tends to restore the coil and needle to their zero orientation. Because the spring
opposes any rotation of the coil and needle, the orientation of the needle depends on any other torque (twist)
experienced by the coil of wire--the more torque the spring-loaded coil experiences, the farther the coil and
needle will turn away from the zero orientation. The needle's angle of deflection is proportional to the extra
torque on the coil.
The extra torque exerted on the spring-load coil comes from magnetic forces. There is a permanent magnet
surrounding the coil, so that when current flows through the coil it experiences a torque. Because a currentcarrying coil is magnetic, the coil's magnetic poles and the permanent magnet's magnetic poles exert forces on
one another and the coil experiences a torque. This magnetic torque is exactly proportional to the current flowing
through the coil. Because the torque on the coil is proportional to the current and the needle's angle of deflection
is proportional to this torque, the needle's angle of deflection is exactly proportional to the current in the wire.
To use such a meter as a current meter (an ammeter), you must allow the current flowing through your circuit to
pass through the meter. You must open the circuit and insert this ammeter in series with the rest of the circuit.
That way, the current flowing through the circuit will also flow through the meter and its needle will move to
indicate how much current is flowing.
To use such a meter as a voltage meter (a voltmeter), some current is divert from the circuit to the meter through
an electric resistor and then returned to the circuit. The amount of current that follows this bypass and flows
through the electric resistor is proportional to the voltage difference across that resistor (a natural phenomenon
described by Ohm's law). The voltmeter system thus diverts from the circuit an amount of current that is exactly
proportional to the voltage difference between the place at which current enters the voltmeter and where it
returns to the circuit. The needle's movement thus reflects this voltage difference.
In an electronic voltmeter, sensitive electronic components directly measure the voltage difference between two
wires. Virtually no current flows between those two wires, so that the meter simply makes a measurement of the
charge differences on the two wires. An electron ammeter uses an electronic voltmeter to measure the tiny
voltage difference across a wire that is carrying the current. Since the wire also obeys Ohm's law, this voltage
difference is proportional to the current passing through the wire.
How does a dehumidifier know when to turn on and off? The one I bought from Sears doesn't use the "wet-bulb/drybulb" method (of which I could use a better understanding, too). How does its on-off switch work? -- JS, Amherst, NY
Most humidity sensing switches or "humidistats" use the expansion or contraction of certain materials to measure
humidity. The more humid the air is, the more water molecules there will be in those materials and their shapes
and sizes will be affected. For example, human hair becomes longer when wet and it makes an excellent
humidity sensor. On a dry day, a hair will contain relatively few water molecules and its length will be shorter.
On a humid day, the hair will contain more water molecules and its length will be longer.
A wet-bulb/dry-bulb system measures humidity by looking at the temperature drop that occurs when water
evaporates. As water evaporates from the bulb of the wet thermometer and the bulb's temperature drop, the rate
at which water molecules leave the bulb's surface decreases. The bulb temperature drops until the rate at which
water molecules leave the bulb is equal to the rate at which water molecules return to the bulb from the air. At
that point, there is no net evaporation going on. In humid air, water molecules return to the bulb more often so
that this balance is reached at a higher temperature than in dry air. The wet bulb temperature is thus warmer on a
humid day than it is on a dry day.
How does a thermostat regulate temperature? -- TF, Auburn, WA
A typical thermostat turns on the furnace whenever the temperature falls below a certain temperature and turns
the furnace off whenever the temperature rises above another temperature. Those two temperatures are slightly
separated so that the furnace doesn't turn on and off too rapidly. In a typical home thermostat, a bimetallic coil
tips a small mercury-filled glass bottle. The bimetallic coil is made from two different metal strips that have been
sandwiched together and then rolled into a coil. As the temperature changes, the two metals expand differently
and the coil winds or unwinds. As it does, it tips the glass bottle and the mercury rolls from one end of the bottle
to the other. When the mercury falls to one end, it allows an electric current to flow between two wires and the
furnace turns on. When the mercury falls to the other end of the bottle, the current stops flowing and the furnace
turns off. So the winding and unwinding of the coil controls the furnace and the home temperature tends to hover
at the point where the bottle of mercury is almost perfectly level. When you adjust the set point of the thermostat,
you tilt the whole coil and bottle so that the average temperature in your home must shift in order for the bottle to
be almost level.
Some friends and I are having a debate. They maintain that if a person sleeps on an unheated waterbed, heat might be
drawn from their body to the point that hypothermia would occur. Is it possible for a waterbed to do this? -- JS, College
Park, MD
The answer depends on how cold you allow room temperature to become. Without a heater, the water
temperature in the bed will be very close to room temperature. When you then lie on the bed, you will be in
contact with a surface that's at room temperature and heat will flow out of you and into the water. Your heat will
warm the water and it will tend to float upward and remain at the top surface of the waterbed, forming an
insulating layer that will slow your heat loss. However, heat will continue to diffuse into the water as a whole
and you will continue to lose heat. As long as the water isn't too cold, your metabolism will be able to replace the
lost heat and you'll stay warm. But if the room and waterbed are very cold, your temperature will begin to drop.
I'm not sure how cold the water would have to be for this to happen, but if the room and water were almost ice
cold, you'd probably have trouble.
January 20, 1997
In "Empire Strikes Back", when Luke learns that Darth Vader is his father, he falls/jumps off a platform in Cloud City
without his hand. Given the fact that objects reach terminal velocity, which would have a faster terminal velocity and
which would hit the ground first if in the movie that fell from a height of 1000 meters?
Luke would probably reach the ground before his hand. An object reaches a terminal velocity as it fall because
the upward force of air resistance becomes stronger as the object's downward speed increases and this upward
force eventually stops the object from accelerating downward. The object's downward speed at the point when it
stops accelerating is its terminal velocity. Since air resistance is what sets this terminal velocity, an object that
experiences a great deal of air resistance relative to its weight will have a smaller terminal velocity than an object
that experiences relatively little air resistance relative to its weight. Because Luke is much larger than his hand,
he has lots of weight relative to his surface area. Since surface area largely determines air resistance, he
experiences relatively little air resistance relative to his weight. His hand has less weight relative to its surface
area and it experiences a lot of air resistance relative to its weight. So Luke's terminal velocity is larger than that
of his hand. He reaches the ground first. This tendency for large objects to descend faster than small objects
explains why small animals, such as insects, can fall from incredible heights without injury. They reach their
terminal velocities quickly and descend rather slowly to the ground.
You said that if I push on a friend they will push back (even if they are asleep). But if I push hard enough, they will fall to
the ground, whereas I will not. Therefore, I don't see how the reaction is equal. Can you please explain this? - JK
Newton's third law only observes that the forces two objects exert on one another are equal in amount but
opposite in direction. The law doesn't make any statement about the consequences of those forces on the objects
involved. Moreover, it doesn't say that those forces are the only forces on the objects. When you push on an
awake friend, your friend will obtain additional forces from the ground or a nearby wall, and will manage to
avoid falling over. Even though you push your friend away from you, your friend will see to it that the ground
pushes them toward you. As a result, they will probably stay in one place. But when your friend is asleep, they
won't be able obtain the additional forces necessary to compensate for the force you exert on them and they may
accelerate away from you or fall over.
Would a small-mass hammer that accelerated rapidly exert more horizontal force on a nail than a large-mass hammer that
didn't accelerate very much?
Yes. Since the only horizontal force acting on the hammer is that exerted on it by the nail, the hammer's
acceleration is entirely determined by that force. The force on the hammer is equal to the hammer's mass times
the hammer's acceleration (Newton's second law). If both hammers experienced the same acceleration, then the
large-mass hammer would have to be experiencing the larger force from the nail and would therefore be exerting
the larger force on the nail. But because the small-mass hammer is experience a larger acceleration, the force that
the nail is exerting on it may be quite large. If the small-mass hammer's acceleration is large enough, the force on
it may exceed the force on the large-mass hammer.
What would it be like if Newton's third law weren't true? Can we imagine that?
Many strange things would happen. For example, suppose that you pushed on your neighbor and your neighbor
didn't push back--you wouldn't feel any force pushing against your hand so you wouldn't even notice that you
were pushing on your neighbor. Your neighbor would feel you pushing on them and they would accelerate away
from you.
Among the many consequences of such a change would be that energy wouldn't be conserved--you would be
able to create energy out of nowhere. To see how that would be possible, imagine lifting a heavy object and
suppose that as you pushed upward on it, it didn't push downward on you. As you lifted it upward, you would do
work on it--you would exert an upward force on it and it would move upward. But it wouldn't do negative work
on you--it would exert no force on you as your hands lifted it upward. As a result, its energy would increase but
your energy wouldn't decrease. Energy would be created. In fact, you wouldn't even notice that you were lifting
it because it wouldn't push on you as you lifted it.
If the net force on an object is zero and it has no acceleration, then what causes it to have velocity? Doesn't a force give it
velocity? And doesn't this make the gravitational and support forces unequal? - EH
The great insights of Galileo and Newton were that an object doesn't need a force on it to have a non-zero
velocity. Objects tend to coast along at constant velocity when they are free of forces, or when the net force on
them is zero. Inertia keeps them going even though nothing pushes on them. While it takes an acceleration and
thus a non-zero net force to get an object moving in the first place, it will continue to move even if the net force
on it drops to zero. So while I was lifting the bowling ball upward at constant velocity, the net force on the
bowling ball was truly zero--it was coasting upward because its weight and the support force from my hand were
canceling one another. However, to start the bowling ball moving upward, I had to push upward on it harder than
gravity pushed downward. For a short time, the bowling ball experienced an upward net force and it accelerated
upward. After that, I stopped pushing extra hard and let the bowling ball coast upward at constant velocity.
If forces are always equal but opposite, how can a hammer drive a nail into a wall? Don't the forces on the nail cancel?
Although forces always appear in equal but oppositely directed pairs, the two forces in each pair act on different
objects. The nail and hammer experience one of these force pairs--the hammer pushes on the nail just as hard as
the nail pushes on the hammer. Because the nail's force on the hammer is the only force that the hammer
experiences, the hammer accelerates away from the nail and the wall. The nail and wall experience the other
force pair--the wall pushes on the nail just as hard as the nail pushes on the wall. The nail thus experiences two
horizontal forces: the hammer pushes it toward the wall and the wall pushes it away from the wall. As long as all
the forces are gentle, the two forces on the nail cancel and it doesn't accelerate at all. But if you hit the nail hard
with the hammer, the wall can't exert enough support force on the nail to prevent it from enter the wall. The two
forces on the nail no longer cancel and it accelerates into the wall.
Can you explain once again how the bowling ball and the tennis ball drop at the same time. Are weight and mass
proportional? If mass is the resistance to acceleration and weight is a gravitational force pulling down on the ball, doesn't
the weight of the bowling ball make it fall faster? Or does the bowling ball's increased mass in a way cancel out the
bowling ball's increased weight? - HC
Weight and mass are proportional to one another and the bowling ball's increased mass does effectively cancel
out its increased weight. Let's suppose that the bowling ball is 100 times as massive as the tennis ball--meaning
that it takes 100 times as much force to make the bowling ball accelerate at a certain rate as it does to make the
tennis ball accelerate at that same rate. Because weight is proportional to mass, the bowling ball also weighs 100
times as much as the tennis ball. So if the only force on each ball is its weight, each ball will accelerate at the
same rate. The bowling ball will experience 100 times the force but it will be 100 times as hard to accelerate. The
two factors of 100 will cancel and it will accelerate together with the tennis ball.
January 17, 1997
What are the two chemical in glow sticks? -- JW, Westport, CT
I believe that the glow sticks contain luminol and hydrogen peroxide, which mix when you crack the glass
ampoule and begin to emit light. There are several other chemicals present in the sticks to assist and control the
process, but the principal reaction is one in which the hydrogen peroxide oxidizes ("burns") the luminol
molecule. The result is a product molecule that is initially in an excited state--its electrons have more energy than
they need--and it emits a particle of bluish-violet light. Since our eyes aren't particularly sensitive to that bluishviolet light, it's often converted into more visible light with the help of a fluorescent dye. The green light sticks
probably contain sodium fluorescein molecules, each of which can absorb a photon of bluish-violet light and
reemit some of its energy as a photon of green light. Other dyes, probably rhodamines, are used to make red or
orange light sticks.
Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR
Like all electromagnetic waves, microwaves are composed of electric and magnetic fields. Since an electric field
exerts forces on charged particles, a microwave pushes electrons back and forth through any metals it encounters.
It is this motion of electrons back and forth through the metal walls of the microwave oven that allow that metal
to reflect the microwaves and keep them inside the oven. If you leave a spoon in you cup of coffee as you heat it
in the microwave, electrons will move back and forth through the spoon. This motion of charge will cause no
problems so long as (1) the spoon can tolerate this flow of charge without overheating and (2) the spoon doesn't
allow the charges at its ends to leap into the air as a spark. To keep the spoon from overheating, it must be a good
conductor of electricity. Since most spoons are pretty thick, the modest currents flowing through them in the
microwave will leave little energy inside them and they won't overheat. But a thin twist-tie or small bit of
aluminum foil may well overheat and begin to burn. To keep the spoon from sparking, it should have smooth
ends. Electrons are more likely to leave the end of a metal surface at a sharp point, so avoiding points is
important. Most spoons are smooth enough that no sparks will occur. But a fork, a sharp piece of foil, or a twisttie may well begin to emit electrons into the air as those electrons pile up at one end of the wire while the
microwave oven is on. Like a spoon, the walls of the oven are good conductors of electricity and they have no
sharp points. While electrons move back and forth in these walls, they simply reflect the microwaves without
becoming very hot and without emitting any sparks. You'll note that the light bulb for the microwave is always
outside the cooking chamber because it contains small bits of metal that would have trouble inside a microwave
oven.
How do steam generators produce electricity? -- KA, North Platte, NE
In a steam generating plant, water is boiled in a confined container (a "boiler") to produce very high-pressure
steam. This steam is allowed to flow through a turbine to the low-pressure region beyond the turbine. A turbine
resembles a fan, but one that is turned by the gas that flows through it rather than by a motor. The steam flows
through the blades of the turbine and exerts forces on those blades to keep the turbine rotating. The steam loses
energy as it twists the turbine around in a circle and this energy is transferred to the rotating turbine. The lowpressure steam is recovered from the end of the turbine. It is then condensed back into liquid water with the help
of a cooling tower and then returned to the boiler for reuse.
The rotating turbine is connected to the rotating portion of a generator. This rotating component is an
electromagnet and, as it spins, its magnetic field passes across a set of stationary wire coils. Whenever the
magnetic field through a coil of wire changes, any current flowing through that coil experiences forces that may
add or subtract energy from it. In this case, the rotating magnet transfers energy to the current passing through
the wire coils and "generates" electricity. The current in these stationary wires carries away energy from the
generator and it is this energy that eventually arrives in your home through the power lines. Overall, the energy
flows from the boiler, to the steam, to the turbine, to the generator, to the current, and to your home.
Is there any way to make a homemade fog machine, like they use in clubs? -- JW, Westport, CT
While it's pretty clear that fog machines fill the air with tiny water droplets, I'm not sure how all of them work.
Some probably use high-frequency sound waves to break up water into tiny droplets and then blow these droplets
into the room with a fan. That technique is used in some room humidifiers and you can see a stream of fog
emerging from them as they operate. An easier way to make fog is to mix water and liquid nitrogen. While liquid
nitrogen is harder to find, all you have to do is put them together and they'll start making fog. The boiling
nitrogen shatters the water into tiny droplets, which flow out of the mixture in a layer of cold nitrogen gas.
If heat rises, how come snow accumulates on mountains? Why is it colder up there instead of down here? -- HG, Grand
Prairie, TX
On a local scale, hot air does rise through cold air. That's because when hot air and cold air are at the same
temperatures, the hot air has fewer air molecules per liter than the cold air and so each liter of hot air is lighter
than each liter of cold air. In short, hot air is less dense than cold air and it floats upward in cold air. But when
hot air rises a long way through the atmosphere, something begins to happen to the hot air. It cools off! That's
because the air pressure decreases with altitude. The air pressure that's around us on the ground is only present
because the air down here must support the air overhead. The air down here must push upward on the air
overhead and it does this by developing a high pressure. But as you move upward in the atmosphere, there's less
air overhead and therefore less air pressure around you.
So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to
expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward
forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy--the
hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air
becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per
kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the
air at lower altitudes, such as the air in valleys.
Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises!
This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect
explains the special "Katabatic" winds that blow warm and dry out of the mountains--including the Santa Ana
winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in
Argentina.
How does a refrigerator work? - SK
A refrigerator uses a material called a "working fluid" to transfer heat from the food inside the refrigerator to the
air around the refrigerator. This working fluid moves through the refrigerator's three main components--the
compressor, the condenser, and the evaporator--over and over again, in a continuous cycle. I'll begin as the fluid
enters the refrigerator's compressor, which is usually located on the bottom of the refrigerator where it's exposed
to the room air. The working fluid enters the compressor as a low-pressure gas at roughly room temperature. The
compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since
squeezing a gas involves physical work (a force exerted on an object as that object moves in the direction of the
force), the compressor transfers energy to the working fluid and that fluid becomes hotter as a result.. The
working fluid leaves the compressor as a high-pressure gas that's well above room temperature. The working
fluid then enters the condenser, which is typically a snake-like pipe on the back of the refrigerator. Since the
fluid is hotter than the room air, heat flows out of the fluid and into the room air. The fluid then begins to
condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy
also flows as heat into the room air.
The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into
the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like
pipe that's wrapped around the freezing compartment (in a non-frostfree refrigerator) or hidden in the back of the
food compartment (in a frostfree refrigerator). When the fluid goes through the narrowing in the pipe, it's
pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately
begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from
one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the
evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin
the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor
consumed electric energy during this process and that energy has become thermal energy in the room air.
What is heat? -- PM, Princeton, NJ
Heat is thermal energy that's flowing from one object to another because of a temperature difference between
those two objects. Whenever an object contains thermal energy--which it always does--the atoms and molecules
in that object are jittering about microscopically. Each atom or molecule isn't completely stationary; instead it is
vibrating back and forth, and pushing or pulling on its neighbors. The object's thermal energy is the sum of the
tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy),
and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its
atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects
together. When that thermal energy is flowing from the hotter object to the colder object, we call it "heat."
In today's lecture, you stated that a person accelerating downward OR UPWARD does not feel the effects of gravity.
How do you explain the g-forces felt by astronauts at escape velocity? - TH
In the lecture, I said that a person who is falling does not feel the effects of gravity, even when they are traveling
upward. But when they are falling, they are accelerating downward at a very specific rate--the acceleration due
to gravity, which is 9.8 meters/second2 at the earth's surface. When an astronaut is accelerating upward during a
launch, they are not falling and they do feel weight. In fact, because they are accelerating upward, they feel
particularly heavy.
You said that from the moment the ball leaves your hand (after you threw it upward), it accelerates downward even
though you threw it upward. However you then said that the ground (gravity) pushed on your foot to make you
accelerate, so why would you also not be accelerating in the opposite direction, like the ball? Why would you not
accelerate in the direction in which you were pushed?
I got ahead of myself by using forces I had not yet introduced. I was using friction to push me horizontally across
the floor! Here is the complete story:
When I tossed the ball upward and it was rising, gravity was pulling downward on it and it was accelerating
downward. But when I obtained a force from the ground, it was not gravity that exerted that force on me; it was
friction! As we will discuss in a few days, whenever you try to slide your foot across the floor toward the left,
friction pushes your foot toward the right. In class, I traveled toward the right because I was being pushed by
friction toward the right. I was actually accelerating in the direction I was pushed, just as you expect.
When accelerating, can you decelerate by going in a direction that is not opposite (your velocity)? For example, going
north can you decelerate by going east?
Decelerating is a very specific acceleration--always in the direction opposite your velocity. If you were heading
north and accelerated toward the east, your velocity would soon point toward the northeast. It would have some
northward aspect because you were initially heading north and hadn't yet accelerated toward the south. It would
have some eastward aspect because you had initially been heading neither eastward nor westward and had since
accelerated toward the east.
On the other hand, if you were heading north and then turned toward the east, you would have lost your
northward velocity and obtained an eastward velocity. This "turning" would have involved a southward
acceleration (to get rid of the northward velocity) and an eastward acceleration (to acquire an eastward velocity).
Warner Brothers has been misleading children! The coyote and the anvil hit the ground at the same time!
You're exactly right. Occasionally one of those cartoons shows the coyote falling with the anvil directly above
his head and the distance between them remaining constant, which is what should happen (ignoring air
resistance). But more often, the coyote falls much faster than the anvil, hits the ground first, and is then pounded
by the anvil. It sure would be neat to live in a cartoon--the laws of physics just wouldn't apply.
January 15, 1997
How do rotary telephones work? -- JG, DeSoto, Kansas
As your finger turns the dial of the telephone, you wind a spring and store energy in that spring. When you
remove your finger, the spring unwinds and its stored energy drives the dialing mechanism. This mechanism
consists of a cogged wheel and a switch, as well as a centrifugal governor. As the dial unwinds, the cogged
wheel turns and it's cogs close and open a switch one time for each number on the dial. For example, if you dial a
"6", the switch closes briefly 6 times. For a "0", the switch closes 10 times. Each time the switch closes during
this action, it "hangs up" the telephone briefly. The switching system at the telephone company recognizes these
brief hang-ups as signals for establishing the connection. The centrifugal governor controls the rate at which the
dial unwinds and makes sure that the pulses coming from the telephone occur at a uniform rate.
How are the paints made that artists (like Rembrandt and Monet) used in the past? -- SB, Oedenrode, The Nederlands
These paints consisted principally of a pigment and a drying oil binder. The pigment was usually a colored
powder that didn't dissolve in the oil. Historically, these pigments were materials collected from nature. The
drying oil binder was usually linseed oil, obtained from the seed of the flax plant and a byproduct of the linen
industry. Like most organic oils, linseed oil is a triglyceride--it consists of a glycerin molecule with three fatty
acid chains attached to it. But while in typical animal or tropical plant oils the carbon atom chains of the fatty
acids are completely decorated with hydrogen atoms (saturated fats) or almost completely decorated
(monounsaturated fats), the carbon atom chains in linseed oil are missing a significant number of hydrogen
atoms (polyunsaturated fats). The polyunsaturated character of linseed oil makes it vulnerable to a chemical
reaction in which the chains stick permanently to one another--a reaction call polymerization. With time and
exposure to air, the molecules in linseed oil bind together forever to form a real plastic! This "drying" process
takes weeks, months, or years, depending on the chemicals present in the paint. It can be accelerated by the
addition of catalysts--chemicals that assist the polymerization process but that don't become part of the final
molecular structure of the plastic.
What is an electric field and how does it affect us? -- MT, Brampton, Ontario
Electrically charged particles exert forces on one another. For example, a negatively charged particle attracts a
nearby positively charged particle and repels another negatively charged one. These attractions and repulsions
are mediated by electric fields that are created by those charges. By this statement, I mean that the negatively
charged particle creates an electric field around itself and this electric field is what ultimately exerts forces on the
other two charges--attracting the nearby positively charged particle and repelling the negatively charged one.
Whenever an electrically charged particle finds itself in an electric field, it experiences a force. The direction of
that force depends on its electric charge (either positive or negative) and on the direction of the electric field
(which may have somewhat different directions at different points in space). The strength of that force depends
on the amount of electric charge on the particle and on the strength of the electric field (which can vary from
nothing at all to extremely strong).
But while electric fields always exist around charged objects and exert forces on any other charged objects that
enter them, electric fields can also exist far away from charges. Electromagnetic waves contain electric and
magnetic fields (the magnetic equivalents of electric fields) and these two fields sustain one another as the wave
travels. Although electromagnetic waves are created and destroyed with the help of charged particles, they can
travel alone and without any nearby charged particles to assist them.
While electric fields exert forces on the charged particles in our bodies, the response of those charges isn't likely
to injure us. When you are exposed to an electric field, there is a subtle rearrangement of electric charges on the
surface of your body that then creates its own electric field. The result is that there is essentially no electric field
inside you. Only when you are exposed to extremely strong electric fields, and spark and currents begin to flow
through you, is there any significant effect to you.
Where does the wax from a burning candle go? Also, why do beeswax candles burn virtually completely, leaving no wax
behind at all? -- SC, Rhode Island
The wax molecules in the candle react with oxygen in the candle flame and are converted into water molecules
and carbon dioxide molecules. That reaction is associated with combustion and it releases energy so that the
candle produces light and heat. The molecules formed by this combustion drift off into the air.
Normal candle wax (paraffin wax) consists of relatively large hydrocarbon molecules. Each molecule in paraffin
is a chain of between 30 and 50 carbon atoms that are surrounded by hydrogen atoms. Because its molecules are
fairly long and they stick together reasonably well, paraffin is a firm, crystalline solid. If the chains were shorter,
say 20 to 30 carbon atoms long, the material would be softer--it would be a liquid-like wax known as petroleum
jelly. If the chains were much longer, say 2000 to 3000 carbon atoms long, the material would be firmer--it
would be a solid known as polyethylene. Still shorter chains are used in machine oil, diesel fuel, unrefined
gasoline, and finally petroleum gases such as propane and methane. The shorter the chain, the softer, thinner, and
more volatile the hydrocarbon is at any given temperature. All of these hydrocarbon molecules can burn
completely, leaving only water molecules and carbon dioxide. In a candle, the heat of the flame vaporizes the
wax molecules--they become a gas--and they then burn completely in the flame itself. As long as the wax doesn't
drip away from the flame, the flame will consume it all completely and leave no ash or wax. Although the
structure of the molecules in beeswax is slightly different from that in paraffin, beeswax also vaporizes from the
heat of the flame and then burns completely.
When friction is made by two atoms rubbing -- it makes heat. But how and why? -- GN, Marine City, MI
When two surfaces slide across one another, some of the mechanical energy in those surfaces is converted to
thermal energy (or heat). That's because the surfaces are microscopically rough and their atoms collide as the
surfaces slide pass one another. Each time a collision occurs, the atoms that collide begin to vibrate more
vigorously than before. In this process, the surfaces lose some of their overall mechanical energy but the atoms
gain some randomly distributed local vibrational energy--more thermal energy. Those surface atoms become
hotter. As the sliding continues, large regions of the surfaces become hotter and the surfaces lose much of their
energy. If you don't push them to keep them sliding across one another, they'll come to a stop as all their
mechanical energy is converted into thermal energy.
How was Newton able to prove inertia with gravity and friction still being present? Why didn't people think he was
crazy? Did he have some type of vacuum or something? - JP
Actually, it was Galileo who first realized that objects have this tendency to continue moving at a steady rate in a
straight-line path--what we call "inertia." He deduced this fact by studying the motions of balls on ramps. He
noted that a ball rolling down a slight incline steadily picked up speed while a ball rolling up a slight incline
steadily lost speed. From these observations he realized that a ball rolling along a level surface would roll at a
steady speed indefinitely, where it not for friction and air resistance. He was aware that friction, air resistance,
and gravity were disturbing the natural motions of objects and had figured out a way to see beyond them. But it
wasn't until Newton took up this sort of study that the idea of forces and their effects was properly developed.
Overall, it took almost two thousand years, from Aristotle to Newton, for the incorrect idea that objects tend to
remain stationary when free of forces to be replaced with the correct idea that objects tend to continue at constant
velocity when free of forces.
January 13, 1997
How does a TV or VCR remote control work? Is it infrared light or a laser? How does the TV or VCR know what to do
with the light it receives from the remote? -- FC, Lafayette, CA
The remote unit communicates with the TV or VCR via infrared light, which it produces with one or more light
emitting diodes (LED). The most remarkable feature of this communication is that the TV or VCR is able to
distinguish the tiny amount of light emitted by the LED from all the background light in the room. This
selectivity is made possible by blinking the LED rapidly at one of two different frequencies. Since it's unlikely
that any other source of light in the room will blink several hundred thousand times per second and at just the
right frequency, the TV or VCR can tell that it's observing light from the remote. The remote sends information
to the TV or VCR by switching back and forth between the two different frequencies. For example, it may use
the higher frequency to send a "1" bit and the lower frequency to send a "0" bit. The remote sends a long string
of these 1's and 0's, and the TV or VCR detects and analyzes this string of bits to determine (1) whether it's
directed toward the TV or VCR (an address component in the information) and (2) what it should do as the result
of this transmission (a data component in the information). Assuming that the string of bits was intended for the
TV or VCR, its digital controller (a simple computer) takes whatever action the data component of the
transmission requested.
What is white noise? - AT
Acoustic "white noise" is a collection of random sounds that together have the same volume at every frequency
or pitch. It's defined more accurately as having the same amount of power in each unit of its bandwidth, so that
the acoustic power between 20 and 21 cycles per second is the same as the acoustic power between 500 and 501
cycles per second.
Is it true that Tesla invented a way to send electrical power without the use of power lines? If so, how? - BS
Yes. Tesla found that the alternating electromagnetic fields around a large high frequency transformer could
propel currents through wires or lamps that were lo