Re: How many volts ?

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Tue, 25 Mar 1997, LUDWIK KOWALSKI wrote:

> Yes; this presents a problem. Suppose we accept that dV=62 volts. How do
> we calculate C when |Q1|=-10 nC and |Q2|=+100 nC? Which Q should we use
> in C=Q/dV?

Just reverse the logic John used to get the 62 volts.

> But, if I undertood him properly, he took it for granted that final charge 
> distributions must be uniform, at least approximately. 

I don't think it matters if those distributions are not uniform on the
outside surfaces of the plates, for what John calls the "excess"
on the outer surfaces of the plates produces a field which doesn't
penetrate between the plates. Therefore it doesn't affect the distribution
of the equal and opposite charges on the inner surfaces of the plates. And
the field within the plates doesn't extend out to affect the distribution
of charges on the outer surfaces of the plates. Thus the field within is
that of a normal capacitor, and obeys approximately the formula C = Q/V.
The excess charge on the outer surfaces of the plates I expect is *not*
uniform in charge density, but it simply doesn't matter, since whatever
field it produces doesn't penetrate between the plates sufficiently to
affect the field there.

Draw a Gaussian surface through the midplanes of the plates, and close it
at the edges of the plates.

> How much would you
> bet, Don, on the outcome of an experiment? I know where to borrow a very
> good electrostatic voltmeter (20 pF and 3 to 300 volts). You win if the
> result is between 52 and 82 volts, I win otherwise. We would have to agree
> on the method. How many people would like to participate in betting? This 
> is not a formal offer.

I wouldn't bet much on it, because I have little confidence that
real-world experiments can be counted on to correctly replicate gedanken
experiments. :-) 

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
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