PURPOSE: To demonstrate Lenz's law dramatically.

DESCRIPTION: This is a commercial device purchased from Pasco Scientific. Two arrays of magnets, containing five strong disc magnets each with small aluminum spacers between the magnets, are dropped through a vertical aluminum tube. One set, having its poles North-to-South (at center in right photo above), has very little external field, and falls very quickly through the tube. The other set, having its poles arranged North-to-North, then South-to South, etc. (at right in right photo above), has a large external field. As it falls it induces large currents in the aluminum tube. According to Lenz's law, these currents interact with the falling magnet array so as to oppose its (falling) motion, and the array takes several seconds to fall about two meters through the tube. A simple aluminum rod (at left in right photo above) is included for comparison. This is very dramatic, and is a real attention-getter for teaching about eddy currents.

Click your mouse on that photograph to see the magnetic fields of the two magnet arrays (Magnetic field photographs made by Jack Horne.).

SUGGESTIONS:

See Question of the Week #126 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 5K20.25) See Demonstration Reference File for several articles about this device.

EQUIPMENT: Two linear magnet arrays, aluminum rod, mounted six-foot aluminum tube with magnet catcher.

SETUP TIME: None.