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, 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, 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 is included for comparison. This is very dramatic, and is a real attention-getter for teaching about eddy currents.
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.
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