On the exhibit you should have two wooden cylinders and two cylinder magnets of about the same size. Starting from the right, the tubes are plastic, aluminum and copper.
Try dropping the wooden cylinders down different pairs of tubes to see if there is any difference in how fast they fall. Now try dropping the cylinder magnets down different pairs of tubes to see if there is any difference in how fast they fall.
Finally, see if the magnets attract any of the three types of tubes.
Utility companies generate electricity by moving a coil of conducting wire (usually copper) within a magnetic field. Thus the "ingredients" to making electricity are motion, a conductor and a magnetic field. When a magnet is dropped down a copper or aluminum tube, two rings of electricity are being created, one immediately below the magnet and one immediately above. Moving electricity creates an electromagnetic field. These resulting electromagnetic fields oppose the fall of the magnet, slowing it down.
Residential electric meters have a spinning aluminum disk which measures the electricity being used. To keep the disk from over spinning, it is moving in a magnetic field which serves to retard or dampen its motion without touching it. Some chemistry balance scales include an aluminum disk, again in a magnetic field, to slow down or dampen the motion of the scale when weighing materials.
Many airplanes have an outer skin of thin aluminum sheets. These sheets can be severely weakened by invisible cracks and corrosion. (Part of the skin came off a commercial airplane in flight over Hawaii because of such weakness.) Mechanics check for these types of defects by passing a magnet over the skin in a carefully controlled manner, and reading the amount of induced electric current. Decreased current indicates a crack or corrosion.
You can make your own version of this exhibit by obtaining a strong magnet and a piece of copper water pipe. (An alnico magnet may not be strong enough to work.) You can also try dropping your magnet alongside a bar or u shaped piece of aluminum, or even a square tube of aluminum.
Why not use a piece of iron pipe for this exhibit?
A magnetic field attracts iron, so the magnet would stick to the side of an iron pipe rather than fall through it.
Did you notice any difference when two magnets are dropped at the same time through the two copper tubes? Why would we be less interested in comparing the two aluminum tubes?
One copper tube is slightly thicker than the other, and the magnets fall more slowly through the thicker pipe. The magnetic field is passing through more conductor in the thicker tube, more electricity is being generated, and stronger electromagnetic fields are the result.
Looking at the two aluminum tubes, both the thicknesses and inside openings or diameters of the tubes differ. Therefore we would not be sure whether thickness or diameter or both made a difference in the speed of the falling magnet.
With a stronger magnet, would the magnet ever completely stop falling?
The magnet's fall is slowed by the electromagnetic fields. Remember that three ingredients are necessary for the electromagnetic fields to exist: motion, magnetic field and conductor. If the magnet were to stop moving, electricity would not be created.
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This exhibit was suggested by the Electrodyne Company.
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