Re: Disectable Capacitor

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

I think the word "dielectric" in this discussion is a red herring. The
insulating cup is a dielectric, that's true, but its role in this demo is
as a repository for the excess charge from the capacitor plates. The
polarization of the dielectric increases the capacitance of the device,
that's all. In any explanation of the dissectible capacitor demo, you need
only call the inner glass (or plastic) cup an "insulator".

On Wed, 3 Mar 1999, Herbert H Gottlieb wrote:

> When the capacitor is charged, the atoms in the dielectric are stresssed
> and are aligned by the electric field that is applied.   It's like a gun
> being cocked.

But as soon as the applied electric field is removed, they unstress. But
they don't "go anywhere" so the polarized atoms are not the source of the
electrons which make up the current which produces the big spark and the
audible "snap" which wakes up the students so they say after lecture "I
got a charge out of that demo!"

We know that we can put an excess or deficiency of charge on the surface
of insulators. We do it when we rub cat's fur on plastic, or silk on
glass.  That's what's happening to the insulating cup in this dissectible
capacitor demo. Indeed, one can put the glass cup near an electroscope to
verify that this is so, and find the sign of the surface charge. We know
it when we put a hand inside the glass cup, or rub a hand on the outside.
We hear and feel the crackling of charge transfer to the hand. In this way
one can "wipe off" the surfaces of the glass safely, reassemble the cup
and outer cans, and find it produces very little spark (due to charge you
missed when wiping).

        -- Donald

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Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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