# Capacitors - what does same potential mean?

1. Nov 14, 2011

### logearav

Capacitors -- what does same potential mean?

1. The problem statement, all variables and given/known data

Revered members,
When the capacitor is fully discharged, the plates of capacitor are at same potential.
I want to know the interpretation of the term same potential.
2. Relevant equations

3. The attempt at a solution

Does it mean both the plates having the same charge, that is either + or - ? And the other possibility is if one plate is having 6C(say) of + charge, the other plate has 6C of negative charge. Thanks in advance, members

2. Nov 14, 2011

### cepheid

Staff Emeritus
Re: Capacitors -- what does same potential mean?

It means that the two plates have the same electric potential. Electric potential is a specific physical quantity with a specific definition that you are probably meant to know already. If you don't know about it, then I recommend that you look it up:

http://en.wikipedia.org/wiki/Electric_potential

In any case, if the two plates are at the same potential, then there is no difference in potential between them. We call the difference in potential between two points the "voltage" between those two points. So, saying that there is no difference in potential between the two plates is the same as saying that there is no voltage between them.

For a capacitor with +6 C on one plate and -6 C on the other plate, the two plates would most certainly NOT be at the same potential. The charge would give rise to an electric field. An electric field points in the direction in which the electric potential decreases the most steeply. So if the potential were the same everywhere in a region of space (i.e. there are no gradients in the potential), then there would be no electric field in that region. If you have a field, you must have a potential difference, and vice versa.