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dissectible capacitor



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