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Exercise 4B: Interactions
Equipment:
neodymium disc magnets (3/group)
compass
triple-arm balance (2/group)
clamp and 50-cm vertical rod for holding
balance up
string
tape
scissors
Your goal in this exercise is to compare the forces
two magnets exert on each other, i.e. to compare
magnet A’s force on magnet B to magnet B’s force
on magnet A. Magnet B will be made out of two of
the small disc magnets put together, so it is twice
as strong as magnet A.
1. Note that these magnets are extremely strong!
Being careful not to pinch your skin, put two disc
magnets together to make magnet B.
2. Familiarize yourself with how the magnets be-
have. In addition to magnets A and B, there are
two other magnets that can come into play. The
compass needle itself is a magnet, and the planet
earth is a magnet. Ordinarily the compass needle
twists around under the influence of the earth, but
the disc magnets are very strong close up, so if
you bring them within a few cm of the compass,
the compass is essentially just responding to them.
Investigate how different parts of magnets A and
B interact with the compass, and label them ap-
propriately. Investigate how magnets A and B can
attract or repel one another.
3. You are ready to form a hypothesis about the
following situation. Suppose we set up two bal-
ances as shown in the figure. The magnets are
not touching. The top magnet is hanging from a
hook underneath the pan, giving the same result
as if it was on top of the pan. Make sure it is hang-
ing under the center of the pan. You will want to
make sure the magnets are pulling on each other,
not pushing each other away, so that the top mag-
net will stay in one place.
The balances will not show the magnets’ true
weights, because the magnets are exerting forces
on each other. The top balance will read a higher
number than it would without any magnetic forces,
and the bottom balance will have a lower than
normal reading. The difference between each
magnet’s true weight and the reading on the bal-
ance gives a measure of how strongly the magnet
is being pushed or pulled by the other magnet.
How do you think the amount of pushing or pull-
ing experienced by the two magnets will compare.
In other words, which reading will change more,
or will they change by the same amount. Write
down a hypothesis:________________________
Before going on to part 4, discuss your hypoth-
esis with your instructor.
4. Now set up the experiment described above
with two balances. Since we are interested in the
changse in the scale readings caused by the mag-
netic forces, you will need to take a total of four
scale readings: one pair with the balances sepa-
rated and one pair with the magnets close together
as shown in the figure above.
When the balances are together and the magnetic
forces are acting, it is not possible to get both bal-
ances to reach equilibrium at the same time, be-
cause sliding the weights on one balance can
cause its magnet to move up or down, tipping the
other balance. Therefore, while you take a read-
ing from one balance, you need to immobilize the
other in the horizontal position by taping its tip so
it points exactly at the zero mark.
You will also probably find that as you slide the
weights, the pointer swings suddenly to the oppo-
site side, but you can never get it to be stable in
the middle (zero) position. Try bringing the pointer
manually to the zero position and then releasing
it. If it swings up, you’re too low, and if it swings
down, you’re too high. Search for the dividing line
between the too-low region and the too-high re-
gion.
If the changes in the scale readings are very small
(say a few grams or less), you need to get the
magnets closer together. It should be possible to
get the scale readings to change by large amounts
(up to 10 or 20 g).
magnet A taped to pencil
magnet B
pencil