There was a thread a while ago regarding a capacitor where the capacitance was changed while it was part of a circuit. I won't link the thread here as it was kind of a train wreck... Now the explanation given was that in order to change the capacitance work would have to be done to cause that, which would allow the voltage to remain fixed. I initially questioned that explanation and was left wondering what forces the voltage to remain fixed. Let's say we charge a capacitor, disconnect it, and hook up a voltage probe. We then double the capacitance(say by moving the plates closer). There isn't a path from one terminal to the next, so no charge can flow between the plates, and as such we must hold charge fixed. So that leaves energy and voltage that can vary. Now, I think I have an answer, but I'm a little iffy on it. Say we move the two plates closer; you then have a stronger E field and a smaller distance. IIRC, Voltage between two points involves integrating E field over the distance, so 2X stronger E field times 1/2 the distance still gives the same voltage, but work was done by moving the plates with the force from the charge separation? Is that close, or way off?