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Re: A funny capacitor.

On Mon, 26 Feb 2001, Ludwik Kowalski wrote:

The capacitance is C1. Then bring a third
plate and position it as in B below.


* *
* *
******* ******* C2
* *
* *


Will the new capacitance, C2, be larger, smaller
or the same as C1? In my opinion it will be smaller.

I think it will be larger, since such a plate will be attracted and
accelerate towards the other plates, exchanging electromagnetic PE for
mechanical KE just as when two oppositely-charged plates fall together and
voltage measured between the plates falls (and since charge is constant,
capacitance value must rise as the 3rd plate is sucked inwards.)

Or, change the geometry of the 3rd plate a bit, and obviously it increases
the capacitance as it's inserted:

* *
* *
******* ******* ******* C2
* * * *
* * * *
* *
* *

Inserting a thick metal plate surely increases the capacitance. Does
capacitance increase when a plate is simply held near the other plates?
Here's a possible rule of thumb: if charge does not vary, yet something
allows the flux lines to shorten themselves, then capacitance must rise.
Another: if an object is attracted, then as it moves inwards the
capacitance must rise. Is there ANY way to lower the capacitance by adding
metal? Perhaps only a dielectric with permittivity less than that of air
would lower the capacitance when brought near. (And it would be

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