http://www.nearingzero.net (nz235.jpg)
Announcements
 We will meet in room 104 Physics (the big lecture hall) on
Monday, January 23. Physics 8 will meet Friday of this week.
 I’ll pass graded homework sets around. Please take yours.
If you forgot to put your name on it, do so and hand it back
in.
I will read everything you hand in. If you want more than a
check mark ...
“If we knew what it was we were doing, it would not be called research,
would it?”—A. Einstein
Old Business: Occam’s Razor
Suppose I put a saucer of milk out before I go to bed. The
next morning the milk is gone.
I can think of two theories that explain why the milk
disappeared:
1. My cat drank it.
2. The Milk Fairy took it.
Discussion, anybody?
Old Business: Room Length
Here’s our data from last time:
552 552
552.5 553
Room length in inches:
552 552 552 558 551.5
490 553 549 531.6875
287.5
Discussion, anybody?
Old Business: Tacoma
What did you learn about the Tacoma Narrows Bridge?
Make sure your name is on your homework paper, and turn it
in when we take a break.
Next, I’ll talk briefly about Environmental Issue 2.
In case I didn’t make it clear what I am trying to
accomplish discussing these issues, the point is…
1) to remind you it’s not too early to start thinking
about which Issue you are going to do…
2) … and to clarify in some cases how much I expect
you to do.
Environmental Issues
Issue 2:
Carrying Capacity and
Ecological Footprint
If you choose this Issue, you’ll investigate the number of
individuals that can be supported by an area’s resources.
You’ll also investigate “sustainability;” whether a population is
using resources faster than they can be replenished.
This issue is medium length, so I would also ask that you
estimate your own personal “ecological footprint.” You can find
many sites online that will do this. Be sure to report what site
you use, and comment on whether or not you think that site
has any ulterior motives for helping you do the calculation.
Physical Science:
Matter and Mass
Please bear with me if this seems ridiculously simple. I need
to see where we all stand here.
What is matter? Write your definition on a piece of paper and
turn it in when we take a break, along with your Tacoma
Narrows Bridge homework. Please don’t discuss your definition
with anyone else yet. Put your name on the “matter” paper so
I can use it to take today’s attendance.
Here’s the grade school definition of matter: something that
has mass and takes up space.
During a summer
workshop for middle
school teachers on
“properties of matter”
that I helped run a few
years ago, the best the
teachers could come up
with was “the stuff that
the universe is made of.”
The four scientists who helped run the project may have
snickered a bit at that definition, but couldn’t do much better.
Mass tells us how much stuff there is in an object. Weight (on
earth, of course) is a measure of the earth's gravitational
attraction for an object.
If I go to the moon, do I
weigh more or less?
Is my mass more or less?
If we were doing physics with
math in this class, I might use
Newton's second law, F=ma,
to “define” mass.
Mass is measured in grams or kilograms (metric),
or slugs (our English system). The kilogram is
the SI unit.
If I tossed you a kilogram mass, could you catch it
easily, or would it be too heavy?
Forces are measured in dynes or newtons (metric), or pounds
(English). The newton is the SI unit.
If I pushed on you with a 1 N force, would you ignore me, or
would you fall backwards?
Matter has charged particles in it.
-
+
www.nearingzero.net
Most of the time
the net charge on
an object is zero…
there are the same
number of positive
and negative
charges, and the
unlike charges
“neutralize” each
other.
The earth, to which we are usually
"connected" will either take our excess
electrons (if we have any) or give us
electrons if we have a shortage, until we
become neutral. You've experienced that
effect on winter days with a carpet and
doorknob.
Later on in this course we will develop some degree of
familiarity with electric charge and electrical units in order to
understand environmental issues related to electrical power.
Environmental Issues
Issue 7:
Oil and Natural Gas
If you choose this Issue, you’ll investigate oil and natural gas supply and
cost.
This is one issue that will have a direct impact on your lives!
The Environmental Issues book was written before the current global
recession.* Instead of answering the “for further thought” questions,
answer these: if economies recover soon, what impact will it have on your
answers in this issue? If the if economies take years or decades to recover,
what impact will it have on your answers in this issue?
There is some math in this Issue (natural logarithms)! (You might need to
review Issue 0.)
*Economists
seem to think the recession has ended, but ordinary people might have different ideas.
Atoms
What can you tell me about atoms?
Written records of the idea that matter consists of tiny,
indivisible “building blocks” date back to the 5th century BC
Greek philosophers Leucippus and Democritus.
Of course, being philosophers, they specialized in “thought
experiments.” They felt no need to verify that their thoughts
were correct.
The modern idea of atoms was slowly (and sometimes
painfully) developed over the century between the late 1700’s
and late 1800’s.
“I would rather discover one scientific fact than become King of Persia.”—Democritus
As late as 1895, the great chemist Wilhelm Ostwald (1909
Nobel Prize winner for work on catalysts, equilibria, and
reaction rates), publicly ridiculed the idea of atoms.*
The “discovery” of the electron in 1897 by J.
J. Thomson led him to suggest that atoms
are positively charged matter with small,
negatively charged electrons embedded.
Plum pudding.
Some textbook pictures of the plum
pudding model do not show what Thomson
was thinking of. This one is OK.
*And contributed greatly to the suicide of Boltzmann, who we will learn of later.
Ahh, the power of
plum pudding.*
You may laugh at the plum pudding model today, but it was
taken seriously in the early 1900’s.** The atomic model you
probably believe right now is just as laughable.
*There are not now, nor have there ever been, plumbs in plum pudding. And no, that is not a
typo. See http://www.whatscookingamerica.net/Cake/plumpuddingTips.htm.
**“Plum Pudding” Thomson won the 1906 Nobel Prize for his work on the conduction of
electricity by gases.
Forgetting about plum pudding for now… if you are a scientist
and have a mental picture of an atom, what are you going to
do with it?
See if it matches reality!
How?
Develop a theory. Experiment. Compare with theory.
Where do you start?
The simplest atom. Hydrogen.
The plum pudding model for hydrogen.
+
A lump of + charge (colored
like pudding, just for your
viewing enjoyment).
Where does the electron go?
-
A small, negatively
charged electron.
What if you pull the electron away from the center?
Even though the + charge is
uniformly distributed, in this case
it “acts” like it is all concentrated
at the center of the atom.
+
Restoring force!
-
If you let go, what does the electron do?
Oscillates!
-
What does an oscillating (and therefore accelerated) electron
do?
Emits radiation!
In this case, visible light.
You can calculate the energy of the emitted radiation (visible
light), and the calculated value agrees nicely with some of the
radiation emitted by hydrogen.
You think you’re on to something. What do you do next?
More complex atoms!
Unfortunately, for anything beyond hydrogen, the modeling
and calculations are too difficult! So what else do you try.
Study hydrogen some more?
When you pass a current at high voltage through hydrogen
gas, the gas glows. Several colors of light are emitted. Of all
these colors, only the single one mentioned on the previous
slide is predicted by Thomson’s model.
Dang! We don’t understand what’s in the pudding.
What next?
If you want to find out what’s inside the pudding, do what
Rutherford (one of the Plum Pudding believers) suggested…
Stick your finger in the pudding and see what’s there!
Ernest Rutherford, a physicist, won the 1908 Nobel Prize in
Chemistry for his work on radioactive decay.
Rutherford was the world expert on
alpha particles, which are produced in
radioactive decay. An alpha particle is
a helium nucleus—two neutrons, two
protons, tightly bound together.
Alpha particles have 8000 times the mass of an electron and
Rutherford’s alpha particles were highly energetic. An alpha
particle colliding with an electron would be like a human
colliding with an ant.
“I have dealt with many different transformations with various time-periods, but the quickest I have met
was my own transformation from a physicist to a chemist.”—paraphrase of a quote by Rutherford
In 1906, Rutherford discovered that alpha particles were only
slightly deflected when they passed through matter. This was
expected.
Analyzing this weak scattering would give insight into the
distribution of charge and mass inside the atom. Rutherford
pioneered the technique of studying matter by scattering
beams of particles (or waves).
Rutherford’s co-worker was Hans Geiger (“Geiger
counter”).
Rutherford and Geiger had an undergraduate named
Ernest Marsden working for them in 1909. They
assigned Marsden the job of measuring the
scattering at large angles* of alpha particles by a
very thin gold foil.
*“Nothing interesting can possibly happen, but let’s have poor Marsden do
it anyway. Ha ha ha...”
In the Thomson model, electric charge is smeared out over the
atomic volume, and minimal interaction is expected between
the charged alpha particles and the gold atoms. That’s because
there’s no local electric field to deflect a charged particle.
Indeed, most of Marsden’s alpha particles passed straight
through the gold foil.
A few were scattered at large angles.
Some even bounced straight back.
“It was as if you fired a 15-inch
shell at a sheet of tissue paper
and it came back to hit you.”—
Ernest Rutherford
15 inch guns
USS Washington
OK, I lied on the previous slide
(for dramatic effect). The
battleship Washington had a main
battery of nine 16-inch guns.
I spent a long time looking for
pictures of 15-inch guns and
shells, with humans in them to
illustrate the scale.
Lots of 16-shell pictures, like this
one. No 15-inch. But if you see
one of these coming at you, are
you going to argue about that
extra inch?
“It was as if you fired a 15-inch
shell at a sheet of tissue paper
and it came back to hit you.”—
Ernest Rutherford
15-inch shell: weight—2700
pounds; muzzle velocity—1570
miles per hour; range—21 miles
Splat!
Scattering experiments “demanded” the model that
Rutherford then invented for the atom.
Expected.
Observed. There must be
concentrations of massive,
highly charged matter to
deflect the alpha particles.
figures from: http://www.as.utexas.edu/astronomy/education/spring01/lambert/classnotes9.html
See http://micro.magnet.fsu.edu/electromag/java/rutherford/
for a “toy” to play with.
So a “snapshot” of Rutherford’s atom
at some instant in time looks like this.
Of course, the electrons and nucleus are
very small, but I had to draw them big
enough to see.
-
++
++
-
Rutherford didn’t use the word “nucleus” in describing this
model. He used the term “charge concentration.”*
Rutherford was confident he had the atom figured out. "The
question of the stability of the atom proposed need not be
considered at this stage, for this will obviously depend upon
the minute structure of the atom, and on the motion of the
constituent charged parts."*
*See http://dbhs.wvusd.k12.ca.us/AtomicStructure/Rutherford-Model.html
http://www.valleystream13.com
/Wheeler/science/jan/atom
About 16 slides back, you probably told me atoms are made
of neutrons, protons and electrons. The neutrons and
protons are "squeezed" together inside very small nuclei.
The electrons "orbit" somewhere outside the nucleus.
Protons are positively charged particles, and neutrons are
neutral.
So your atom looks something like this…
http://www.csmate.colostate.edu/cltw/cohortpages/viney/atomhistory.html
You have a problem here!
Did it ever bother you when a science teacher told you that
the nucleus was made of neutrons and protons? How can lots
of positively charged protons be squeezed together inside a
small volume? I thought like charges repel.
+ +
Did any of you raise your hand and
protest that finding a bunch of protons
crammed together is not consistent with
“like charges repel?”
It is true that matter is made of atoms, which are made of
neutrons, protons, and electrons.
It is also true that like charged protons repel each other, like
charged electrons repel each other, and unlike charged
electrons and protons attract each other.
The force between charged particles is called the "electrostatic" force; "electro-" for obvious (I hope) reasons and
"static" because the force exists between particles when they
are stationary (as well as when they are in motion).
What does the fact that atomic nuclei are held together tightly
imply to you?
There must be some other force in operation besides the
electrostatic force.
This force must be stronger than the electrostatic force.
The force that hold nuclei together is, in fact, extremely
strong. That's why atomic bombs and nuclear reactors are so
"powerful." I'll talk about it later.
The force that binds electrons to atoms is
much weaker. That's why you can easily
"scrape" electrons off atoms.
I wonder what role the neutrons in atoms play. It seems now
as if they are excess baggage. Anybody ever wonder about
them? (You’ll have to wait for the answer.)
Atoms can bond together to form molecules or crystals. The
bonds that hold atoms together in crystals are a result of the
behavior of electrons.
The bonds can be very weak, as
between oxygen molecules (at
room temperature it is far too
warm for oxygen molecules to
form solid oxygen), or quite
strong, as in diamonds.
Environmental Issues
Issue 18: Grain Production
and Beef Consumption
If you choose this Issue, you’ll investigate global grain
production and the effect of beef consumption on grain usage.
You don’t need to do the “for further study.”
As you do these Issues, the text author will refer you to
external sources. You might wish to investigate these sources
further. Do these sources of information have a particular axe
to grind?