Re: dissectible capacitor

[William Beaty <billb@ESKIMO.COM>]

On Thu, 1 Jun 2000, Carl E. Mungan wrote:

> (2) Supposing we do spray the charges off the plates onto the glass
> (entirely?), what happens when we reassemble the capacitor? How does
> the charge move back onto the plates? (Since V=Q/C, nonzero V
> requires net Q on each plate, right?)

Hey, don't trust me entirely, since I'm just spouting off based on visual
analogies without experimental verification.

I'm imagining it like this:  Two parallel plates with opposite charge on
their inner surfaces and no dielectric, then imagine that the sheets of
charge break loose from the plates and slowly move together.  This should
cause the stored energy and the voltage across the plates to fall
continuously, no?  By this reasoning I would predict that it makes little
difference whether the layers of net-charge are on the surface of the
dielectric or on the surface of the plates, as long as the plates are up
against the dielectric.  Even if the charge was immobilised upon the
dielectric surfaces, the metal plates would be charged electrophorus-like
by induction, and would act just like a conventional charged capacitor
after reassembly.

I vaguely recall some old Amateur Science column about "electret" motors,
and something called the "channel effect."  If a slab of polarized
electret material is slipped into the slot between neutral metal plates, a
force appears which pulls the electret slab into the slot.  This suggests
that after reassembly, the Dissectable capacitor could be hooked to the HV
power supply, and if the polarity of the supply was backwards, the
repulsive force would cause the capacitor to dissect itself!  If the
voltage was high enough, maybe this would become the Electric Cannon
demonstration.  Or how about this: set up two Dissectable capacitors,
charge and discharge them as per the usual demo, then hook them in
parallel.  Pulling one of them apart would charge its plates, and cause
the other one to push itself apart via electrostatic forces.  It would be
an electrostatic motor/generator pair.  Franklin could have used them to
send Morse code across town long before Henry did it with coils, and today
our electric motors might all resemble tuning capacitors...


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