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Re: A funny capacitor.

William Beaty wrote:

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.)

OK, one more condition should have been specified: positions of
plates are fixed. But that does not prevent us from thinking in
terms of virtual quasi-static displacements (acceleration ~ 0).

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

* *
* *
******* ******* *******
* * * *
* * * *
* *
* *

Assuming the totally inserted plate has the thickness d1 while the
distance between the original plates is d2, and ignoring the bulging
of E lines near the edges, the capacitance is C=eps_o*A/(d2-d1),
where A is the area. In this case C after inserting the plate is indeed
larger than C before inserting it. But what does this have to do
with the origibal problem?
Ludwik Kowalski