Magnetic Levitation
Electricity and Magnetism
Level
1
Compare the speeds at which a permanent magnet and a piece of metal of the same shape and mass fall through a conducting tube.
The permanent magnet will:
Fall slower
Fall equally as fast
Fall faster
Levitate rather than fall
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1 solution
The magnetic field of the permanent magnet induces a current in the conducting tube. Below the magnet, the magnetic flux is increasing and so that induced magnetic field from Lenz's Law points in the opposite direction from the permanent magnet's field. Above the permanent magnet, the opposite is true, and the induced field points in the same direction. On either side this causes a magnetic dipole-dipole force: parallel fields attract and antiparallel fields repel, so the permanent magnet is attracted upwards from above and repelled upwards from below. Both effects combine to slow the fall of the magnet.
The piece of ordinary metal has no magnetic field, so none of these effects occur. Therefore the magnet falls slower than the metal.