Physics 144 – Chowdary
How Things Work
Spring 2006
Group Exercise #29 – Generators & Eddy Currents
Goals:
 Observe that mechanical energy can be turned into electrical energy.
 See the effects of changing a magnetic field in a conductor.
Instructions:
 Work in groups of 2 on the various tasks below. Please take and return equipment as generated.
Task #1:
From your kit: magnets.
From center table: copper tube, plastic tube.
(a) Drop the magnet down the plastic tube; make sure to catch it when it comes out the other end! From the
moment you release the magnet to the moment you catch the magnet, discuss all the energy changes involved.
(b) Drop the magnet down the copper tube with the N pole end down; make sure to catch it when it comes out the
other end. Repeat, but this time with the S pole end down. Describe how this motion differs from the case when
the magnet was dropped down the plastic tube. What’s a key difference between the plastic tube and the copper
tube (the tubes themselves)? How do you know that the energy changes involved when dropping the magnet
down the copper tube must be different than the energy changes when using the plastic tube?
Task #2:
Keep magnets, plastic tube. Return copper tube.
From your kit: red LED
From center table: 2 banana plug leads. 2 alligator clip connectors (that turn a banana plug lead into an alligator
clip). Small coil (3200 turns).
(a) Plug the banana plug leads into the small coil. Connect the alligator clip leads to the free ends of the banana
plug leads. Clip one lead to one end of the red LED. Clip the other lead to the other end of the red LED. Make
sure that the alligator clip leads don’t touch each other. You’ll drop the magnet so that it falls through the small
coil. We’ll use the plastic tube to guide the magnet. See the figure on the board. Carefully watch the red LED.
Drop the magnet down the plastic tube (catch it before it hits the ground). If nothing happens, repeat, but drop the
magnet with the other pole facing down. Do this several times. Record your observations.
Task #3:
Keep magnets, red LED connected via alligator clip leads to banana plug leads. Return plastic tube and small coil.
From center table: Large coil. 2 binder clips. Rubber band (cut).
Take a length of rubber band and two binder clips from your lab bench. Stretch the rubber band along a diameter
of your big coil using the binder clips to secure it in place (see picture below on left).
red LED
Simple generator. Magnets squeeze rubber band
between them; coil is hooked up to red LED
Set things up as shown in the figure to above on the right. Two magnets should be placed together so that they
pinch the rubber band between them. Obtain a light emitting diode (LED) from the front table and hook it to your
coil as shown. Twist the magnets around about 15 times, which should wind up the rubber band as well. Be
careful about winding much more than 15 time– if the magnets spin too fast they’ll come flying apart! Now,
release the magnets and watch the LED. What do you see? Write down your observations.
You’ve just made a simple generator! You’ve turned mechanical energy into electrical potential energy. In many
ways, this is how most of the electricity that you use every day is generated (some differences may be that the coil
is rotated instead of the magnet, that electromagnets are used instead of bar magnets, and that there probably
aren’t any rubber bands involved).
Return large coil, banana plug leads, alligator clip connectors, rubber band, and binder clips
to the appropriate places. You can put away your red LED and magnets.
The remaining tasks involve shared equipment. The tasks can be done in any order, so if some equipment is
being used, please just move on to another task until that equipment becomes available. Work in your pairs,
but every person should perform each task.
Task #4: Flashlight
Make sure the blue plastic flashlight is on. Pay attention to the brightness of the light. Examine it so that you can
see the magnet and the coil of wires. Turn the flashlight off, and shake it a few times (the number of times will
vary; if you’re one of the first people to use the flashlight, you’ll need to shake it lots of times; later people will only
need to shake it a few times). Turn the flashlight back on, and compare the brightness of the light bulb to the
previous brightness: is it brighter? Make sure to leave the flashlight ON. Write down some observations,
including a discussion of energy changes and the role of the magnet and the coil of wire (you can answer these
questions later; for now move on to another task.)
Task #5: Lightbulbs
(a) Make sure that all 4 light bulbs are loose in their sockets so that they don’t make a complete circuit. Hold the
yellow base or metal part firmly, and turn the crank as fast as you can. Pay attention to how hard it is to turn the
crank.
(b) Now, screw one light bulb firmly in its socket. Again holding it firmly, turn the crank as fast as you can. What
do observe about the light bulb? What do you observe about how hard it is to turn the crank?
(c) Have two light bulbs screwed firmly into their sockets. Again holding it firmly, turn the crank as fast as you
can. What do observe about the brightness of the light bulbs (especially compared to part (b)? What do you
observe about how hard it is to turn the crank?
(d) Repeat one more time, with three light bulbs. Again holding it firmly, turn the crank as fast as you can. What
do observe about the brightness of the light bulbs (especially compared to part (b)? What do you observe about
how hard it is to turn the crank?
Make sure all light bulbs are unscrewed before moving on to the next task.
Task #6: Pendulum & Magnet
You should see a magnet assembly that looks like the following:
Above it should be a rod from which you can hang one of the
following pendula which look like:
Each pendulum is labeled: Pendulum (1) looks sort of like a comb or a fork; Pendulum (2) is a solid sheet;
Pendulum (3) is kind of combination of (1) and (2).
(a) Verify that pendulum (1), pendulum (2); and pendulum (3) by themselves are nonmagnetic, using a bar
magnet.
(b) Next, hook up pendulum (1) as shown (even though the figure
shows pendulum (3), use pendulum (1)). Make sure that the pendulum
can swing freely and that it passes between the poles of the magnet
assembly. Lift up the pendulum and release it from rest. Do you
observe the pendulum to swing back and forth pretty freely?
(c) Now, replace pendulum (1) with pendulum (2). Again, make sure that the pendulum can swing freely and that
it passes between the poles of the magnet assembly. Lift up the pendulum and release it from rest. Repeat several
times. Write down your observations, and specifically contrast this motion to that of pendulum (1).
(d) BEFORE doing anything else, examine pendulum (3). BEFORE doing the experiment, PREDICT what you
think will happen when pendulum (3) swings through the magnet. BEFORE doing the experiment, WRITE
DOWN your prediction.
(e) Now, do the experiment. What do you observe?