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jumping ring demo



Sorry for my ignorance, but what's PIRA 5K20.30 and where can I find the
demo?
Is it possible for me to set the designing of this demo as a project for
students?

Rgs,
Chow

----- Original Message -----
From: Karl Trappe <trappe@PHYSICS.UTEXAS.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Monday, April 12, 1999 10:14 PM
Subject: Re: jumping ring demo


Actually, its not directly Lenz' Law. If you use that alone, you get no
repulsion when B=0. You can get away with explaining/using it that way in
introducing Lenz' law, but it is due to the phase lag in an inductor being
90 degrees out of phase with the voltage. The demo is PIRA 5K20.30, but
its
analysis is 5K20.31. Here you will find the references: AJP 39(3), 285,
and AJP 54(9), 808. It is also explained in Meiners 31-2.9.

And yes, I explained it as Lenz'law for years and years (and still use it
there in the class). Karl

Martha

It's due to Lenz's law which states, "The induced current is always in a
direction opposite to what you calculate."

poj

Martha Takats wrote:

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

Dr. Karl I. Trappe Desk Phone: (512)
471-4152
Physics Dept, Mail Stop C-1600 Demo Office: (512)
471-5411
The University of Texas at Austin Home Phone: (512)
264-1616
Austin, Texas 78712-1081