Re: jumping ring demo

["Robert W. Harris" <rwharris@CATH-MEM.ORG>]

Lenz's law says that an induced current will flow in such a way as to oppose
by its magnetic field the change which produced it.  The key word here is
CHANGE.

When current begins to flow in the coil its  magnetic field strengthens.
This increasing of a magnetic field is the change which is  opposed by the
ring.  The current in the ring flows so as to set up its own field to oppose
the change.  Therefore, you have two similar poles facing one another and a
repulsive force.  The ring jumps up.

Does this help?
Br. Robert W. Harris
Catholic Memorial High School
rwharris@cath-mem.org
http://www.cath-mem.org/physics/contents.htm


From: Martha Takats <mtakats@ACAD.URSINUS.EDU>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Monday, April 12, 1999 9:36 AM
Subject: 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