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Re: [Phys-l] defn of capacitance



At 03:46 PM 2/7/2008, Carl, you wrote in part....

>[Brian] Capacitance is the property of a two terminal device to store
>external electrical charge whose magnitude is measured by the reciprocal
>of the rate of change of voltage difference between
>the terminals, which is proportional to the amplitude of a constant current
> impressed between the terminals.

Sounds reasonable. Can you actually lay out the calculations for the
two example cases I proposed? Show me the money.


Perhaps it would be better if I baldly stated that no one would
reasonably disagree with what *I* wrote. The problem lies
in what *you* asserted Carl :-)
here it is.....

"My initial stab at a definition was the following: Capacitance is
uniquely defined if I specify the geometry of the two conductors
(shapes, sizes, separations, etc) as delta(V)/Q where delta(V) is the
absolute value of the line integral of E between the two conductors
and Q is the absolute value of the average difference in charges of
the two plates (ie. half of the difference in charges on the two
plates)."

Here you are describing two capacitors: They happen to be
concentric: an inner cap and an outer cap.
You cannot in fact uniquely define the capacitance of the
outer capacitor, because you do not mention (or it seems
to me even consider), the geometry of the outer capacitor's distant counter-electrode. It follows that your calculations lack a certain something. Necessarily. No offense.



Brian Whatcott Altus OK Eureka!