Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] RC Discharge



I am confused by phrases such as: the Q plate, where the Q actually
lives, the plate whose absolute charge is represented by q, etc.

Certainly we can imagine starting with a capacitor in a field-free
region with shorted and grounded lead wires. This capacitor will not
have a potential difference between its leads, and by convention we
would say neither plate has any net charge.

We can disconnect from ground, un-short the leads, move charge from one
plate to the other, and end up with positive charge on one plate,
negative charge of equal magnitude on the other plate, and a potential
difference between the plates. The charge magnitude on each plate would
be |q| = (C)*|(deltaV)|. I suspect this is what most students think of
when thinking of a charged capacitor.

But we can also remove, say, negative charge from each plate, such that
both plates end up with positive net charge. If we remove different
magnitudes of negative charge from the two plates, both plates have net
positive charge, but there is a potential difference between the plates.
One plate would be at higher potential than the other even though both
are positive. I would designate the higher-potential plate with a plus
and the lower-potential plate with a minus where these signs are used to
indicate which plate is at higher/lower potential.

Now... If deltaV = q/C, which plate has charge q? I would say neither.
I would say q in the capacitor equation represents half the difference
in the net charges on the plates. I would say deltaV = q/C with q = (Q+
minus Q-)/2.

Am I missing something here?


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu