I agree with John Mallinckrodt that when I came up with deltaV = q/C
where q = (Q+ - Q-)/2 I was thinking of a parallel-plate capacitor, and
it would be different for another type of capacitor, such as coaxial.
Of course real capacitors used in real circuits are pretty
complicated... For example, some sort of start out as flexible parallel
plates with flexible dielectric and then get rolled up.
In these cases it may be best just to say that q = C*deltaV, and in
general we don't know the actual charges on the plates.
I do a similar thing when I talk about batteries and/or battery circuits
that have no ground reference. I tell students that the positive
terminal of a 1.5-V battery has a potential that is 1.5 volts higher
than the negative terminal, but I have no clue what the potential of
either terminal is. I don't even know if the absolute potential of the
positive terminal is positive. I only know that the potential
difference is 1.5 volts, and the positive terminal is the higher of the
two potentials.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu