| DESCRIPTION:
Suppose we have five rings that carry a number of free
positive charges that are not moving. Since there is
no current, there is no magnetic field. Now suppose
a set of external agents come along (one for each charge)
and simultaneously spin up the charges counterclockwise
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 accelerate, there is a
magnetic field in the space between the rings, mostly
parallel to their common axis, which is stronger inside
the rings than outside. This is the solenoid configuration.
As the magnetic flux through the rings grows, 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 up
in (counterclockwise), and thus the agents have to do
additional work to spin up the rings because they are
charged. This is the source of the energy that is appearing
in the magnetic field between the rings-the work done
by the agents against the "back emf".
Over the time when the magnetic field is increasing
in the animation, the agents moving the charges to a
higher speed against the induced electric field are
continually doing work. The electromagnetic energy that
they are creating at the place where they are doing
work (the path along which the charges move) flows both
inward and outward. The direction of the flow of this
energy is shown by the animated texture patterns. This
is the electromagnetic energy flow that increases the
strength of the magnetic field in the space between
the rings as each positive charge is accelerated to
a higher and higher velocity.
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