| DESCRIPTION:
Suppose we have five rings that carry a number of free
positive charges that are moving counter-clockwise.
This current results in a magnetic field that is strong
inside the rings and weak outside. Now suppose a set
of external agents come along (one for each charge)
and simultaneously spin down the charges as seen from
above, at the same time and at the same rate, in a manner
that has been pre-arranged. Once the charges on the
rings start to deccelerate, the magnetic field begins
to decrease in
intensity.
As the magnetic flux through the rings decreases, Faraday's
Law tells us that there is an electric field induced
by the time-changing magnetic field that is circulating
clockwise as seen from above. The force on the charges
due to this electric field is thus opposite the direction
the external agents are trying to spin the rings down
in (counter-clockwise), and thus work is done on those
agents.
As the strength of the magnetic field decreases, the
magnetic energy flows from the field back to the path
along
which the charges move, and is now provided to the agents
trying to spin down the moving charges. The energy provided
to those agents as they destroy the magnetic field is
exactly the amount of energy that they put into creating
the magnetic field in the first place, neglecting radiative
losses. This is a totally reversible process if we neglect
such losses. That is, the amount of energy the agents
put into creating the magnetic field is exactly returned
to the agents as the field is destroyed.
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