Re: a question from a student

[William Beaty <billb@eskimo.com>]

On Sun, 6 Apr 1997, LUDWIK KOWALSKI wrote:

> OOP, an older file was mailed a minute ago by mistake. It could be worse !
>
> Last Thursday a student asked me why Q1 and Q2 are equal on a capacitor 
> connected to a battery. Would this also be true if the area of one plate
> were twice as large as the other? How should I answer? Keep in mind this
> is a non-calculus course and Gauss's law is not even mentioned in our text, 
> except in the appendix (at the end of the book).

You could point out that a capacitor in a circuit acts like a resistor in
the sense that the current in one lead of a capacitor equals the current
in the other lead during charge and discharge.  When in a circuit, every
time you push a charge into one end of a capacitor, the electrostatic
force pushes one charge out the other end, and the net charge Q1-Q2 stays
zero. 

I think the above only works if the value of the capacitor is >> than the
capacitance between its plates and other parts of the circuit.  But then
one can imagine "parasitic" capacitances existing between each plate of
the above capacitor and other parts of the circuit.  The Q1 and Q2 on the
plates may then not be equal, but we can imagine this as being caused by a
Q3 and Q4 of a parasitic capacitor which happens to have one of its plates
melded in with one plate of the main capacitor. 

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