pebe,
A state of balance can be considered as being ‘in equilibrium’, or stable. If I balance my budget it ensures that my finances are stable; that my income equals my expenditure – two opposite movements of finances.
No, in accounting, balancing a budget means that an income or expenditure is accounted for, not that money spent is replaced. Accounts can be balanced and still show a surplus or debt.
An imbalance only occurs when one exceeds the other.
That is true for caps. When one side has an excess of electrons, the other side has a deficiency of electrons, and the cap is unbalanced.
But all this is a red herring.
No, I believe it is important to understand.
Assuming for a moment that charges exist on the two capacitor plates that are equal but opposite in polarity, ie. +2C and –2C.
As I said before, a cap has only one mobile charge of negative polarity. If you say it has a +2 C charge on one plate, that means that plate has 2 C fewer electrons than the other plate.
They can only be considered as opposite relative to some fixed datum point that has no charge. The ground or earth (depending on which country you live in) is such a body.
No, they are considered relative to what they were when the cap was at equilibrium with no voltage across its terminals. The earth is nothing special, except a common point of conduction. If a cap is hanging out somewhere isolated from earth, no voltage can be measured and no current will exist if only one plate is connected to earth.
So if the negatively charged plate is connected to ground it now has a charge of 0C. No charge has left the capacitor because the voltage between the plates is unchanged, so the charge on the positively charged plate must have increased to +4C.
Nothing is going to change on the plate if it is connected to ground with no conduction path to the other plate. Try this. Take a cap and energize it to say 10 V. Disconnect the voltage supply and verify 10 V across the cap. Measure the voltage from the + terminal to ground leaving the - terminal isolated. Nothing, right? Now only touch only the + terminal to a good ground. Then measure the voltage across the cap again to read 10 volts as before. Nothing has changed as long as there is no conduction path between the cap's terminals.
It should be obvious that the absolute charge on each plate is not important – it is the differential charge that matters. That is what constitutes the charge, C.
Yes, but the differential charge is always an equal surplus and equal deficiency. If you say a surplus constitutes a charge, then I can say the deficiency constitutes a discharge. So when a cap has a voltage across it, is it charged or discharged? That is an ambiguity that the word "energized" eliminates.
So I maintain that when a voltage exists between the plates of a capacitor, a charge exists in it, ie. it is charged.
And I could just as logically maintain that when a voltage exists, the cap is discharged due to its deficiency of electrons on one plate.
Ratch