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Re: dissectible capacitor

Responding to Carl (see below) Ludwik Kowalski wrote:

The contact between the metal and the dielectric is good at
only view points and this seems to be essential. Otherwise
surface charges on the dielectric would disappear at the
first discharging.

Let me add some speculations. Suppose the contacts with the
dielectric were perfect. In that case the surface layer, as
stated above, would disappear at the first discharging. The
metallic plates and the dielectric would become macroscopically
neutral, as they were before the first charging. Why? Because
each layer of the dielectric slab, except at the outer surfaces, is
macroscopically neutral.

After discharging this "perfectly contacting" system the
electrodes are separated. What should we expect? The
process of separating two dissimilar substances electrifies
them to some extent (like "friction", when scotch tape is
pulled away from something). Ideally each metallic plate
should acquire the same amount of charge (of identical
sign). And two identical surface layers should be formed
on the slab. Take away the dielectric slab and what do
you have?

An anticapacitor, two plates with identical (not opposite)
charges. Like a regular capacitor it would behave like a
compressed spring; it would store potential energy when
we do work on it (pushing plates toward each other) and
it could be used to do work (by pushing something).

There is a more practical way to construct an anitcapacitor.
Two identical capacitors are connected in parallel to a
battery and charged. Disconnect them and separate them.
Then connect the negative terminal of C1 with the negative
terminal of C2. What do you have? Three metallic pieces
and two slabs. Each of the outer plates has the +Q charge
on it. Take away the rest (inner plates with slabs) and you
have an anticapacitor.

Could an anicapacitor be leaky? Yes, but not in the usual
way. It would discharge gradually by attracting ions of
the opposite sign from air.

"Carl E. Mungan" wrote:

A demo is shown on the 25-laser-disc set of the Video Encyclopedia
of Physics Demonstrations #18-25 which baffles me.

Here is how it goes. A cylindrical capacitor with a glass dielectric
is charged up using a Wimshurst. The inner plate of the capacitor is
then disconnected from the Wimshurst. The inner plate and the glass
are then carefully disassembled. The two plates are then shorted
together momentarily. (The video claims no spark occurs, but I think
they must have missed a small one. Anyhow, let's leave that aside
for the moment.) Next the capacitor is reassembled.

The video claims the reassembled capacitor has a potential difference
between the plates. This seems impossible to me based on the
information I provided you above. Nevertheless, they use a wire rod
to nearly short the two terminals and do get a spark!

So the experimental evidence for a potential difference is
compelling. The claim in the video is that all of the charge is
stored on the glass and none on the plates. That's why shorting the
plates of the disassembled capacitor had no effect and reassembling
the capacitor restored its potential difference. This is complete
mumbo jumbo to me.

Can someone explain? If some part of my explanation was not clear,
I'd be happy to try to provide additional details if the video
provides them. Carl
--
Dr. Carl E. Mungan, Assistant Prof. of Physics http://uwf.edu/cmungan/
Univ. of West Florida, Pensacola, FL 32514 850-474-2645 cmungan@uwf.edu
moving this fall to: US Naval Academy, Annapolis, MD 21402 410-293-6650