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jumping ring demo



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.
--
Martha Takats
Department of Physics and Astronomy
Ursinus College
Collegeville, PA 19426