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Re: dissectible capacitor

On Wed, 31 May 2000, Carl E. Mungan wrote:

Ah, another flamewar topic!

Well that's not my intention. I really want to know what's going on here.


Not really "flamewar". More like " flamewar :) "

The topic has caused controversy here before (or perhaps it was on TAP-L)


Why would the edges of the plates start spraying charges into the air? As
the plates are separated, the capacitance drops to a low value. The
charge on the plates doesn't change, and by V = Q/C it causes the
potential difference to go through the roof.


Assuming we have an ideal parallel-plate capacitor, the potential
difference goes through the roof as you separate the plates, but the
electric field does not.

This is true if you separate the plates in the perpendicular direction,
so the increasing distance stays proportional to P.D. But if you slide
them like this:

<-- ----------------------------------
---------------------------------- -->

...then the capacitance falls and the P.D. rises, yet the distance between
the edge of the plate and the adjacent plate does not change. The e-field
at the edge would grow continuously until limited by electrical breakdown
of the air. The demo involves nested metal cups, so the above geometry
does apply.

Here's another way to get to the bottom of this effect: perform the demo
at a variety of different voltages. Rather than looking for sparks, use
an electrometer to measure the voltage between the plates. If the
corona-spraying of the charges is the origin of the effect, then the
effect will go away at low voltages. Example:

1. Charge the Dissectable Capacitor to 10V

2. Dissassemble it and discharge the plates

3. Reassemble it, then measure the potential with a
high-impedance voltmeter (i.e. 10+17 ohms impedance)

Repeat at 100v, 1kv, 10kv. If I am wrong, then the voltage in step 3 will
always be a significant percentage of the voltage in step one, regardless
of the voltage used. If I am right, then when low voltages are used, no
charges will be sprayed onto the dielectric, and the voltage in step three
will be very close to zero.

((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
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