Physics 1002 – Magnetic Fields
(Read objectives on screen.)
Instructor
Hello again. At the end of our last program, before I got attached to this magnet, I asked you how this
cow magnet got its name. Did you find out what, if anything, it has to do with a cow? Those of you
who live in rural areas probably already knew. But did you city kids find out?
Well, a cow magnet is actually fed to a cow in order to attract and hold pieces of iron objects that the
cow may pick up while grazing. If the sharp objects were allowed to go through the digestive system
or be regurgitated up when the cow chews its cud, severe damage could occur. So the magnet keeps
the objects in the cow’s stomach. If and when the cow is slaughtered, the magnets are retrieved and
recycled. That’s probably more information than you wanted to hear, huh?
Now that we’ve cleared up that question, let’s get right to today’s topic, magnetic fields. When we
studied electrostatics, you learned to draw lines of force to represent electric fields. We can do the
same to represent a magnetic field. So while I get down from here you need some guidelines.
(green chalkboard on screen)
VO
To represent a magnetic field, magnetic field lines are drawn.
• The direction of a magnetic field is defined as the direction the north pole of a compass needle
would point in the field
•
Magnetic field lines outside a magnet are always drawn from north to south.
Instructor
Since the direction of a magnetic field line is the direction a north pole would move in a magnetic
field, we can use a compass needle to help us map the field around a bar magnet. If you have small
compasses and bar magnets in your school, your teacher may want you to do this lab with us. If not,
watch as our students do the work.
(students on screen)
VO
We’ll start close to the north pole of the magnet and make dots on the paper to mark the positions of
the north and south poles of the compass needle. Naturally, the north pole of the needle points away
from the north pole of the bar magnet.
Next, we’ll move the compass a little farther from the magnet so that the south pole of the needle lines
up with the last dot made. We’ll make another mark where the north pole points.
As we continue this procedure, notice how the compass needle’s position is changing in response to
the magnetic field around the large magnet. Finally, our marks return to the magnet at its south pole.
We’ll start the next field line closer to the middle of the magnet. When we’ve mapped out the field
line, you can see that the line doesn’t extend as far out from the magnet as the one originating closer
to the pole.
(diagram on screen)
VO
If we repeated this process closer to the middle of the magnet, the field line would look like this. And
if we did the same thing on the other side of the magnet, the lines would be identical. At the poles of
the magnet, the field lines would look like this, with the arrows pointing away from the north pole and
toward the south pole.
Inside the magnet, the field lines continue to form closed loops, like this. However, you won’t need
to draw these because we’ll concentrate on the field surrounding the magnet.
Instructor
Mapping magnetic fields using the compass method would take a long, long time. But there’s an
easier way. Remember in the last program, how we used iron filings in a test tube to represent
domains in a nail? Well, we can use them in a different way to map a magnetic field. Each little
piece of iron, if it is free to move around, will act like an individual compass needle. The only
problem is that no north or south pole is marked, so you’ll have to add your own arrows, from north
toward south.
Again, your teacher may want you to do this lab along with our students. Stop the tape now and get
everything ready for the lab or, if you are just going to watch the lab, get a copy of the lab sheet so
you’ll be ready to draw the magnetic fields as you see them. Local teachers, give your students the
lab report sheet from the facilitator's guide.
(Pause Tape Now graphic)
(students on screen)
VO
In this part of the lab, we’ll use iron filings to map the same magnetic field around a bar magnet.
Each small filing will act as a temporary magnet and line up with the magnetic field.
A bar magnet is placed in the holder, and a sheet of paper is put on top, with the outline of the bar
magnet marked on the paper.
Iron filings are sprinkled on top of the paper, evenly all around the magnet. The paper is tapped to
help the filings line up with the magnetic field. Notice where the field lines are most concentrated.
On your lab sheet, draw 10 to 12 field lines to represent the field around the magnet. Don’t forget to
use arrows to show the direction of the field lines, away from north and toward south.
(magnet and iron filings on screen)
VO
Here is another view of the field around a strong bar magnet suspended between two sheets of
Plexiglas. The filings on top of the magnet are responding to the field above the magnet.
Although we commonly represent magnetic fields as being flat, the field actually surrounds the
magnet in three dimensions.
Instructor
If you sprinkle the iron filings evenly all around the magnet and then tap to help the filings align to
the field, you’ll see that the filings will be more concentrated around the poles. What does that tell us?
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Well, watch this demonstration. I’ll bring this paper clip close the north pole of a magnet and let it
go. You can see that it defies gravity and jumps toward the magnet. The same thing happens at the
south pole.
Now watch what happens when I release the paper clip under the middle of the magnet. Evidently,
the magnetic field is strongest at the poles and weak in the middle.
(green chalkboard on screen)
VO
When we draw magnetic field lines to represent a magnetic field, the closer together they are, the
stronger the field.
The number of lines in any given region is called magnetic flux.
Magnetic flux is greatest around the poles of a bar magnet.
(Read fact or fiction statement on screen)
(dollar bill on screen)
VO
This strong magnet is made of a rare ferromagnetic element, neodymium. The magnetic flux is so
great that the magnet can attract the ink on the dollar bill, which contains small amounts of iron
compounds.
Instructor
Now you should be able to answer this question. What is the advantage of using a horseshoe magnet
instead of a bar magnet? Tell your teacher.
If magnet fields are strongest around the poles, the advantage of a horseshoe magnet is that the poles
are close together. So you can use both poles at once to double the pick-up force of the magnet.
Let’s go back to the lab to see the magnetic field around a horseshoe magnet and the interaction
between the magnetic fields of two magnets. In each case, we will give you time to draw about eight
field lines to represent the magnetic field. Don’t forget to use arrows. If you need more time to draw
what you see, ask your teacher to pause the tape or rewind so that you can see it again.
(students on screen)
VO
This time a horseshoe magnet is placed in the holder. Iron filings are used as before to map the
magnetic field.
On your lab sheet, draw six to eight field lines to represent the field, concentrating on the area around
and between the poles.
(students on screen)
VO
Next, we’ll line up two bar magnets with the north pole of one facing the south pole of the other,
about five centimeters apart. We’ll sprinkle the iron filings between the magnets and around them.
And we’ll tap the paper as before.
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On your lab sheet, draw six to eight field lines to represent the field, concentrating on the area
between the magnets.
(students on screen)
VO
Finally, we’ll line up two bar magnets with the north pole of one facing the north pole of the other,
about five centimeters apart. We’ll sprinkle the iron filings between and around the magnets and tap
the paper as before.
On your lab sheet, draw six to eight field lines to represent the field, concentrating on the area
between the magnets.
(diagrams of magnets on screen)
VO
Here’s what the magnetic field around the poles of a horseshoe magnet looks like. The magnetic flux
is greatest at the two poles.
Next, you saw the magnetic field between the north pole of one bar magnet and the south pole of
another. The lines are drawn from the north to the south pole, with the flux being great between the
two magnets.
And last, you saw the field around the north poles of two magnets. The fields repel each other, with
the lines bending away from each other. The magnetic flux between the two magnets will be small.
Now, you draw the field between two south poles.
You should have drawn the same shaped magnetic field lines as in this case. Only the directions are
different, with the arrows pointing toward the south poles.
Instructor
Here’s a challenge for you.
(close-up of apparatus on screen)
VO
Under this paper, I’ve placed a magnet, a wood block, and a stack of soft iron washers. I’ve also
sprinkled iron filings on top of the paper. Now I’ll tap the paper to help them line up with the
magnetic field.
Can you tell me where the three objects are? Tell your teacher.
Well, it’s easy to locate the magnet, isn’t it? And did you say that the washers are here because the
magnetic field is distorted in this area? You’re right.
But where is the wood block? It’s impossible to tell.
Instructor
That’s because certain substances don’t affect a magnetic field. Let’s go into the lab and compare
different materials and how they affect a magnetic field. Then we’ll talk about some practical
applications.
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(students on screen)
VO
This apparatus tests for the ability of a material to affect a magnetic field. A magnet is suspended
close to the opposite pole of another magnet. The attraction keeps the suspended magnet in place.
When one index card is moved through the space between the magnets, nothing happens.
But let’s try five index cards, filling more space. Nothing happens. The paper has the same effect as
air.
Now we’ll try a razor blade. The suspended magnet wiggles, so we know that the razor blade has
affected the magnetic field.
Next, we’ll run a stream of water between the magnets. No effect.
When we move a straight pin between the magnets, we see that the pin has more affect than air. Is it
the metal that affects the magnetic field?
Let’s try another metal, aluminum foil. Again, no effect. So only some metals have high
permeability. Which metals do you suspect? Tell your teacher.
Instructor
Did you conclude that only ferromagnetic metals can affect a magnetic field? You’re right. Materials
like paper, wood, and cotton have the same affect as air. You can shield electrical forces by placing
insulators around charged objects, but you can’t insulate against magnetic forces. So hiding a gun
inside a suitcase, surrounded by towels and cotton underwear, won’t keep it from being detected in a
metal detector.
How do metal detectors, like the ones in airports, work? They contain a magnetic field that is
produced by current flowing through coils of wire around the opening. There is a link between
electricity and magnetism, and we’ll discuss that in our next program. When a ferromagnetic
substance goes through the detector, it alters the magnetic field, which can then be detected by the
electronics in the device. We’ll learn more about that in the next lesson, too.
(physics challenge on screen)
VO
Here’s a physics challenge question for you?
Some traffic lights are designed to turn green only when a car approaches the intersection. Hoes does
the traffic signal know that your car is there?
Instructor
You should have used what you learned about metal detectors to answer the challenge question
because they operate on the same principle. This time, the coils of current carrying wire are buried in
the street. When the steel body of the car passes over the coils, the magnetic field is altered, and a
signal is sent to the traffic light to turn green.
Once again, physics explains so many things in the world around us. But can it explain the magnetic
world itself, namely the planet earth and the magnetic field that surrounds it?
Where does the earth’s magnetic field come from anyway?
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(diagram of earth on screen)
VO
We know that a magnetic field surrounds the earth, and its shape is like one that would surround a
strong bar magnet placed at the earth’s center, like this.
But to answer to the question, “Where does the earth’s magnetic field come from?”: We’re not sure.
We know there is not a magnetized hunk of iron at the center of the earth because it is too hot for
domains to remain aligned. So that’s out.
Another idea involves the molten, or liquid, part of the earth’s core. Some scientists believe that there
are currents in this region that surrounds the solid center. And the slow movement of charged
particles would create a magnetic field.
There are other theories, but whatever the cause, we know that the earth’s magnetic field is not stable.
In fact, we know that the field has reversed poles at least 170 times, with the most recent reversal
occurring about 700,000 years ago.
Instructor
Now how do we know that the earth’s pole have flip-flopped that many times? Well, earth scientists
have studied rocks that have formed from molten iron-containing minerals. It seems that the iron
atoms in the molten material tend to line up with the earth’s magnetic field. So when the iron
solidifies, a record is left of the direction of the field. As different layers formed throughout geologic
time are dated and analyzed, we get a record of the earth’s changing magnetic field. There is no
regular pattern, so we can’t predict when the next reversal will occur. So don’t throw away your
compasses just yet.
And don’t touch that dial because I want to show you some beautiful pictures of a phenomenon that is
related to the earth’s magnetic field. It’s called the aurora borealis or northern lights. Enjoy the
photographs now. We’ll explain later.
(no audio while pictures are being shown)
Instructor
You can imagine all the myths associated with these strange and beautiful lights. If you lived in
Alaska or in other northern latitudes, you could see them often. But even if you can’t see the northern
lights, you can still “see the light” about what really causes them. As I tell you the story of these
lights, fill in the blanks in your notes.
It all starts on the sun. Now the surface of the sun is very hot.
(Sound Effect: people saying, “How hot is it?”)
It’s sooo hot that solids, liquids, and gases, don’t exist. It’s so hot that electrons jump right out of
gaseous atoms, producing the fourth phase of matter, PLASMA.
Plasma consists of charged particles, and these particles rush out from the sun and through the solar
system as SOLAR WIND.
When solar wind reaches the region of the earth’s outer atmosphere called the MAGNETOSPHERE
their motion is controlled by the earth’s magnetic field.
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(diagram of earth on screen)
VO
Because moving charged particles are surrounded by their own magnetic fields, they are repelled by
the earth’s magnetic field here. So most of the people on earth never see an aurora.
But the charged particles can slip through the earth’s magnetic field at the poles and enter the
atmosphere.
Instructor
In the atmosphere, the charged particles collide with electrons in atoms of oxygen, nitrogen, and other
gases, and transfer energy, making electrons jump to higher energy levels. If you remember your
chemistry, you know that when the electrons fall back down they give off different colors of visible
light.
(pictures of aurora borealis on screen)
VO
The aurora borealis is the result of billions of electrons jumping and falling at the same time. The
colors depend on the type of gases in the atmosphere.
(text on screen)
VO
Here’s another physics challenge for you. Another aurora appears in regions close to the south pole.
What is the other name given to these southern lights? See if you can find this information and tell
your teacher.
Instructor
You may have thought magnetism was just something to play with. Now you know it is all around us
and affecting us all the time. Magneto Man will be back in a minute to share even more magnetism
trivia with you.
But first, it’s time to …SHOW WHAT YOU KNOW!!
Jot down your choice for each question. After the program, your local teacher will go over the correct
answers with you.
(Read Show What You Know questions on screen)
Instructor
There is nothing trivial about magnetism but there is some magnetism trivia. In fact, this one comes
from a textbook used by many schools participating in this course. Scientists have studied a type of
anaerobic bacteria that lives in swamps. Do you remember that anaerobic organisms live and grow
without oxygen? Well it seems that these bacteria have magnetized chains of magnetite, the mineral
in lodestones, as part of their internal structure. When the bacteria find themselves out of the mud at
the bottom of the swamp, the chains acts like compasses, leading them back to the oxygen free
environment. The bacteria simply follow the earth’s magnetic field lines that don’t stop at the
surface but dip down toward the molten core.
And have you ever wondered how homing pigeons find their way home? Well, x-rays reveal chains
of magnetized iron imbedded in the pigeons’ brains. These also act as compasses, helping the pigeons
return home. You might want to read more about this and share what you learn with your class.
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Next time we meet, we’ll explore the magical link between electricity and magnetism. It’s one of my
favorite topics because almost every convenience in our lives..
(Sound Effect: telephone rings) including the telephone, depends on it.
Magneto Man here. Oh, hi Electromagneto Woman. Good to hear from you. No, no… Just because I
haven’t called, it doesn’t mean I’m not attracted to you.
A little privacy here ---- See you next time.
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