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Re: a question from a student



On Mon, 07 Apr 97 Alex. F. Burr wrote:

Q1 = Q2 on unequal sized plates because of conservation of charge.
Charge into battery = charge out of battery.

You are absolutely correct, Alex, in stating what actually happens. The
question is "why". The law of conservation of charge, by itself, does
not prevent a situation in which a biger capacitor plate, connected to
the plus of my battery, for example, takes away more electrons than what
is received by the smaller plate connected to the minus. Why should the
neutrality of the source be taken for granted? Why should we assume that
"what comes in" and "what comes out" of a battery must always be exactly
equal? An assymetry of several hundreds of nC would not lead to an
explosive situation.

We need something else beside the law of conservation of Q. What is it?
Something specific inside each battery? A general law? Gauss's law can be
satisfied by unequal charges on the plates, and on the electrodes. Should
a desire to find an explanation be supported or should it be suppressed
with a comment like "do not ask silly questions"?

By the way, Alex (or anybody else), where can I order supercaps with
C larger than 1F. I have two students collecting data on deviations from
exponentiality in discharging; we want to see if deviations are more
pronounced when C becomes larger. Several months ago you made us aware
that 50F capacitors are now commercially available.
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: Gedanken-ing is not enough; physics is an experimental science! :
: Inspired by thinking about phys-L messages on capacitors :
: Ludwik Kowalski :
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kowalskiL@alpha.montclair.edu http://www.csam.montclair.edu/~kowalski