Here is a related idea which may or may not be relevant!
For an "ideal" parallel-plate capacitor, i.e. with plate separation much
less than the dimensions of the plates, I maintain that the potential
difference between the plates equals |q1-q2|/2C. For example, if q1=Q
and q2=-Q (the usual case), V=Q/C. If q1=q2 (same magnitude and sign),
V=0.
This relationship is not true for other geometries. A counter-example is
a capacitor consisting of two concentric spherical shells. The potential
difference depends on the electric field in the space between the
shells, which depends ONLY on the charge on the inner shell. (Same goes
for coaxial cylinders).
This is probably why we don't deal much with cases where q1 and q2 are
not equal and opposite: we can't use a general equation involving only
the capacitance C rather than details of the geometry.
--
Martha Takats
Department of Physics and Astronomy
Ursinus College
Collegeville, PA 19426