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Magnetic Mysteries
P8-1155
BACKGROUND:
The Magnetic Mysteries kit was put together to give students access to hands-on activities that focus
on the basics of magnetism. Here are a few recommended activities.
KIT CONTENTS:
5
1
1
1
1
2
Medium size ceramic disc magnets
Neodymium magnet
Small ceramic disc magnet
Ceramic bar magnet
Ceramic square bar magnet
Steel plates
2
3
1
1
1
Coin A
Coin B
Ball bearing
Paper clip and thread
Piece of magnetic field viewing film
3" x 6"
ACTIVITIES:
1. The Floating Magnet
a. Get a length of copper pipe several feet long and 3/4" in inner diameter (any hardware
store will have one). Prove that there is no magnetic attraction to the copper pipe by
touching the neodymium and an ordinary ceramic magnet to its side.
b. Have a student hold his or her cupped hands by one end of the pipe as you drop the
ordinary magnet down the other end. What happens? It should fall out immediately, no
surprise.
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c. Again have the student cup his or her hands in anticipation as you drop the neodymium
magnet down the pipe. You may want to prep the trick by asking the student one or two
times if he or she is ready to catch it! What happens this time? It slowly wobbles and
floats down the pipe. It takes the neodymium magnet about three or four times longer to
fall through the pipe than the other magnet.
Explanation
This is a demonstration of two things: electromagnetic induction and Lenz's Law. Copper is
not magnetically sensitive, but it is an excellent conductor of electricity. When either magnet
falls through the copper pipe, there is a current of electricity that is induced. The neodymium
magnet creates a greater amount of current than any other kind of permanent magnet since
it is much stronger magnetically.
An induced current of electricity in a conductor always, in turn, creates its own magnetic field.
As the neodymium enters the pipe it encounters this magnetic field (even though it helped
create it.) Lenz's Law states that this magnetic field will always oppose the direction in which
a magnet moves. If the north pole of the neodymium magnet enters the copper pipe and
drops downward it encounters the magnetic north pole of the electricity it induces. The
opposing magnetic field of the induced electricity pushes against the neodymium magnet,
while gravity forces it down. Now ask your students, why doesn’t the magnet just stop? The
answer lies in the behavior of induced currents. A current will be created only if the magnet
is moving relative to the current carrier (or vice versa.) So, there will only be an induced
current and opposing magnetic field if the magnet is falling down. If the magnetic repulsion
becomes too great and stops the magnet the opposing magnetic field disappears and the
magnet resumes falling. Which, of course, creates another opposing magnetic field. In this
case, gravity and the induced magnetic field find an equilibrium point and the magnet
mysteriously floats down the pipe.
2. The Moving Coin
a. Place coin A on a piece of ordinary paper. Have your students hold the neodymium
magnet next to the coin so that they can confirm that it is not attracted to the magnet.
b. Place the magnet under the coin and paper. Quickly move it back and forth. What
happens? The aluminum coin should slide back and forth with the movement of the
magnet.
c. Now touch the magnet to the coin and quickly lift up. What happens? The coin should
be pulled into the air.
Explanation
This is another demonstration of Lenz's law. When the magnet passes under the coin it
induces eddy currents (aluminum, like copper, is a good conductor of electricity) on the
bottom of the coin. This electricity creates a magnetic field which opposes the direction of
the magnet's movement beneath it.
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If the top of the neodymium magnet is north the opposing magnetic field of the electrical
current created on the bottom of the coin will also be north. When the magnet passes under
the coin its north pole is opposed by the north pole of the coin and therefore causes the coin
to move.
3. The Ambivalent Copper Wire
a. Take about one meter of copper and wrap one end around the thick part of your thumb 8
times so as to make a coil. Wrap the very end of the wire around one side of the coil so
as to secure it. Take the other end of the wire and wrap it around your thumb the same
number of times. Do not tie off this coil it should look like an open spring. Find the
middle point between the two coils and make a small loop so that a pen or pencil can fit
into it. Suspend the entire apparatus with a pencil or pen.
b. Move the neodymium magnet into the coil with the closed loop. What happens? As the
magnet approaches the coil it should be repelled and move away.
c. Try approaching the coil from the other direction. What happens? From either direction,
the magnet should cause the coil to push away.
d. Next, move the neodymium magnet into the open coil. What happens? Nothing should.
When the magnet approaches the open coil, the coil does not move away.
Explanation
This is a demonstration of Lenz's Law that emphasizes the requirements for an induced
current. The closed coil represents a closed circuit. When the magnet moves into the closed
coil the induced current of electricity can move all the way through the circuit path. Where
there is an induced moving current, there is an induced magnetic field. Lenz’s Law tells us
that the induced magnetic field will repel the magnet, and, hence, the coil gets pushed away.
The open coil, however, can’t hold a current because there is no closed circuit path. Without
a current there is not magnetic field and, hence, no repulsion.
4. The Strange Dollar Bill
a. Fold a U.S. paper dollar bill in half and lay on a table. (don’t make the fold too creased,
just try to lay the bill over.)
b. Bring the neodymium magnet close to the top half. What happens? The paper bill
should be attracted to the magnet and move up and down with your strokes.
c. Hold the bill up so that your students can see that there is nothing on the bill and that it
is ordinary currency.
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Explanation
Inside the ink used to print all American paper currency there are very fine particles of iron.
Unlike other magnets, the neodymium magnet is strong enough to attract these particles. If
you hold the magnet to areas of the bill where there is little or no printing the bill will not be
attracted.
5. The Magnetic Money
a. Hold up coin B and explain that it has no iron in it. Bring the neodymium magnet close to
the coin and pick it up.
b. Ask your students to guess what might be in coin B that would cause it to be attracted to
the magnet. See how many of the 3 coins can be picked up.
Explanation
Coin B has a high amount of nickel in it. Nickel is magnetically sensitive, and therefore, it is
immediately attracted to the powerful neodymium magnet.
6. The Mysterious Magnetic Paper Clip
a. Lay the small disc magnet, the ball bearing, and the paper clip with thread on a table so
that your students can see them. Unwrap the thread from the paper clip. Hold the paper
clip by the thread (so that it dangles) and touch it to the bearing to show that there is no
attraction. Then ask your students which is magnetically stronger, the magnet or the
paper clip.
b. Place the bearing on the magnet, then lower the paper clip by the thread until it touches
the bearing. Wait for a moment. Again ask your students which is magnetically stronger,
the paper clip or the magnet.
c. Pull on the string to separate the paper clip from the magnet. The bearing should come
along with it attached to the paper clip!
d. Ask if there is any way that the magnet can pull the bearing away from the paper clip.
Investigate several of the suggested ways to do this.
e. Next, turn the magnet over and bring the bearing and paper clip close to this opposite
side. The bearing will immediately be pulled from the paper clip.
Explanation
This trick concerns the nature of magnetic lines of force (flux lines) and how a magnet
polarizes a magnetically sensitive material. When the paper clip, bearing and magnet are all
in contact, both the paper clip and bearing are magnetized because they lie in a particularly
strong part of the magnet’s magnetic field. That is, directly touching one of it’s poles.
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We say the field is strong near the poles because of the relatively dense number of flux lines.
The bearing, after becoming magnetized, also has its own poles which the paper clip is
affected by. You could say that the lines of flux are most dense where they go through the
bearing and touch the paper clip.
The ball bearing can't be pulled from the paper clip because both have a rounded shape.
When they become magnetized, their rounded edges cause the magnetic lines of force to
become concentrated. Unlike the flat surface of the disc magnet, their magnetic lines of
force are packed into a smaller area. Magnetic strength is measured in terms of how many
magnetic flux lines there are in a given area. The disc magnet may be stronger because it
has more magnetic lines of force, but when the lines are spread out, it is too weak to pull the
ball bearing from the paper clip.
The reason the bearing will spin when it is brought close to the magnet is because the thread
is slightly twisted (if it isn't, then twist it a little yourself when lowering the bearing to the
magnet). As the bearing approaches the disc magnet, it begins to wobble slightly inside the
magnet's magnetic field. This causes torque on the thread which causes more spinning. (Be
sure that the thread is not tightly twisted before you do the trick).
(NOTE: This trick will not work if you use either a very weak or very strong magnet. A weak
magnet will not magnetize the bearing and clip enough to keep them together as they are
pulled away from the magnet's surface. A very strong magnet, like the neodymium, is too
strong for even the magnetized clip to pull the bearing away. The neodymium has so many
lines of force in such a wide area of space that even when the paper clip concentrates its
magnetic lines, it has fewer in a given area than the neodymium magnet).
7. The Confused Bar Magnet
a. Show the bar magnet to your students. Let them place it on a metal surface so that they
can see that it is magnetically strong on all four of its sides.
b. Use the piece of magnetic field viewing film to show that the poles are located on the two
faces.
c. Place a steel plate on each face of the bar magnet.
d. Have a student place the magnet on a metal surface to see what happens. The magnet
now seems much stronger than before. The magnet is much stronger along its length,
not its width. In fact, when the magnet is placed on a metal surface along its width, it
appears weaker, except for the outer most edges.
e. Have your students use the magnetic field viewing film again to see that although the
magnet is stronger along its length than along its width its magnetic orientation is still
through the width (each face is a pole).Use the magnetic field viewing film again to see
that although the magnet is stronger along its length than along its width its magnetic
orientation is still through the width (each face is a pole).How can the orientation of the
magnet change and how can it be stronger without having received any extra energy into
it?
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Explanation
When the metal plates are placed on the bar magnet’s poles, then the magnet's flux lines
become concentrated into a smaller areas of space. This results in stronger magnetic
attraction where the flux lines are concentrated.
Magnetic flux lines are conducted by anything that is magnetically permeable. Steel is very
magnetically permeable and the flux lines move through it to the outer most reaches of the
material (the edges of the steel plates). Since the edge of each plate has become a
magnetic pole, and the distance from one plate to the other is very small, the lines of force
are very dense. With the steel plates, the edges of the magnet have now become like the
closely spaced poles of the u-shaped magnet.
Is the magnet really stronger than before? No! The same amount of force is still present. It
has just been concentrated into a smaller area.
8. The Repulsive Coins
a. You will need one medium size disc magnet and two B coins. Place the two B coins on
one pole and hold the magnet and coins upright.
b. Now bring the neodymium magnet above the disc magnet and coins so that the poles
repel each other. What will happen when you bring the neodymium magnet close to the
disc magnet and coins?
c. Quickly bring the neodymium magnet straight down towards the top coin, but do not
touch it. As the neodymium magnet approaches, the top coin will suddenly jump from
the coin below it and attach itself to the neodymium magnet. If the coin should slide to
one side, simply touch your finger tips to the edge of the coins so that it goes straight up
and not out to the side.
d. Ask your students why coin B(the top coin), which is the furthest extension of the disc
magnet's pole, should be attracted to a magnet that is exposing it to the same magnetic
pole.
Explanation
This demonstration helps to explain the nature of magnetic polarity. It is similar in nature
and cause to the magnetic paper clip trick. The neodymium magnet is much stronger than
both the disc magnet and the magnetized coins. If the north pole of the neodymium magnet
comes close enough, it changes the polarity of coin B so that it and the coin below it face
each other with opposing poles. The top B coin is repelled away from the coin below it.
9. The Hidden Magnet
a. Place two of the medium size disc magnets, the bar magnet and a neodymium magnet
under a piece of paper, a manila envelope, or a piece of ordinary cardboard. Be sure to
space the magnets far enough apart that they do not snap together.
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b. The magnetic field lines pass right through magnetic nonconductors without being
absorbed or rearranged at all. Explain to your students that paper and cardboard are
nonconductors and that the lines of force of the magnets go right through without
hindrance. Run the piece of magnetic field viewing film over the page and let them see
the images of the magnets beneath.
c. Next, rearrange and/or add a few more of the medium size disc magnets.
d. Place both of the steel plates over the small square bar magnet and place it underneath
the cover, too. Replace the paper or cardboard and hold it so that the students can not
tell what you have done.
e. Let the students run the magnetic field viewing film over the paper or cardboard. Ask
them how many magnets are under the cover and what kind they are. They will not be
able to see the small square bar magnet that is covered by the steel plates.
Explanation
This is a demonstration of how magnetic lines of force are affected as they move through
material that is either magnetically conducting or non-conducting.
Magnetic lines of force pass completely through nonconductors without any hindrance. With
magnetically conducting material, however, the lines of force are partly pass through and
partly absorbed and redirected. The small square bar magnet is the smallest and weakest of
all the magnets in the kit. Its lines of force are conducted into the steel plates and outward
to their edges, just as was done with the steel plates on either side of the larger bar magnet
in 7.
The magnetic field viewing film is two pieces of plastic with an inner coating of oil in which
there are microscopic metal flakes. When the two steel plates are over the small square bar
magnet, its lines of force are partially absorbed and partially redirected through the plates.
However, there are very few flux lines that manage to pass upward through the manila
envelope or piece of cardboard. Try looking at the edges of the steel plates with the viewing
film. You should see some faint patterns. The flux lines have spread out on the edges and
run between the plates, not upward.
If the steel plates are placed over one of the bigger and more powerful magnets, this trick will
not work. All the other magnets have a sufficient number of magnetic flux lines, so that even
though some of them are redirected by the steel plates, there are enough to pass through a
nonconductor and make the representational field in the magnetic field viewing film.
RELATED PRODUCTS:
The Magnaprobe (P8-8005) is a gimbal-mounted bar magnet that acts as a three dimensional
compass. Use it to show magnetic lines of force in three dimensions.
PO Box 2750
ANN ARBOR, MI 48106 T 800-367-6695
WWW.ARBORSCI.COM
©2009 ARBOR SCIENTIFIC ALL RIGHTS RESERVED