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TEAL Electricity & Magnetism

SECTION : Falling Magnet  

SUBJECT: The Falling Magnet with Finite Resistance Ring  
see the experiment  

A magnet is dropped through a conducting cooper ring. As the magnet falls under gravity, a current is induced in the ring that tries to keep the magnetic flux through the area of the ring constant. This corresponds to a field that produces a force that opposes the motion of the magnet: as it approaches from above, and the induced current generates a field that pushes the magnet upwards from below.

Once the magnet falls through the ring, the direction of the current changes to produce a field that tries to pull the magnet upwards from above. Since the resistance of the ring is finite and the magnet is heavy, the flux through the ring does not stay constant, and the relatively small magnitude of the induced field does not generate a force strong enough to prevent the magnet from falling through.

The first animation shows the magnetic field configuration around the magnet as it falls. The current in the ring is indicated by the small moving spheres. The motions of the field lines are in the direction of the local Poynting flux vector.

The second animations a 3D version of the same process.

Click on either image to see the animations.




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