Here we show an example of Faraday's Law, as a magnet
is dropped through a conducting 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, the induced current generates
a field that pushes the magnet upwards. Once
it falls through the ring, the direction of the current
changes to produce a field that tries to pull
the magnet upwards. In this case, since the ring has
zero resistance, the flux through it remains constant.
This is evidenced by the fact that the fieldlines from
the magnet never cross the ring. In addition, the magnet
is light enough that the force from the induced field
balances that of gravity, causing it to float above
The first animation shows the magnetic field configuration
around a magnet as it falls and rebounds. 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 animation shows a three-dimensional fieldline
representation of the same thing.