| We showed you the "jumping
ring" trick with respect to Faraday's law. This
is an aluminum ring sitting atop a coil that has an iron
rod in the center. When I turn on the current to the coil,
the magnetic field goes from zero to some maximum. This
induces a magnetic field in the aluminum ring that opposes
the change, which means if the coil's field points "up",
the ring's field points down. North-to-north pole (or
south-to-south) repels the ring from the coil and it jumps. |
 |
By now you also know that the magnetic field has an
energy associated with it. If the energy density of
an electric field is proportional to the E-field squared,
the energy density of a magnetic field is proportional
to the B-field squared.
|
| With the coil energized, there is a magnetic
field through it. When I cut the current, that field releases
its stored energy. If a light bulb connected in parallel
to the coil glows dimly when the coil is energized, as
soon as the current is cut, a very bright flash is seen
in the light bulb -- attributed to the stored magnetic
energy being released. |
