Re: jumping ring demo

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

On Mon, 12 Apr 1999 08:35:07 +0000 Martha Takats
<mtakats@ACAD.URSINUS.EDU> writes:
> Would someone point me to an explanation of the jumping ring demo?
> This is the one where you put a nonmagnetic (aluminum) ring over the
> core of an electromagnet and plug the electromagnet into an AC
> outlet. The ring jumps vigorously off. My attempts at analysis
> do not give me a net force in the direction of decreasing field,
> though >there clearly is such a force--if you try to hold the ring in
place,
> there is a noticeable force trying to make it jump off.

When energized by an alternating current, the coils in the electromagnet
set up a changing magnetic field that induces a changing current the
aluminum ring. According to Lenz' Law, the direction of this
instantaneous current in the alumininum ring is such that it creates a
magnetic field that opposes the magnetic field of the coils.

Thus, regardless of the alternating reversing field created by the coils,
the induced magnetic field surrounding the aluminum ring always repels
that of the coils.

Herb Gottlieb from New York City
(Where the electromagnetic coils and the aluminum ring were divorced
because they never had any attraction for each other)