# Circuit with a capacitor problem

1. Feb 17, 2004

### broegger

I've just been reading about capacitors and I have some (stupid?) questions... First, how can there be a current in a (series) circuit with a capacitor when there is an insulating material or vacuum between the plates (or whatever makes up the capacitor)?? i don't get it.. Second (pretty much the same as the first), how can you apply Kirchoff's closed loop law to a circuit in which there is a capacitor - it isn't closed when there can be no current through the insulator or vacuum in the capacitor??

I know I posted this twice, but just encountered the College Level Help forum..

2. Feb 17, 2004

### NateTG

First off:

How can there be a current in a circuit that is in series with a capacitor. For DC circuits, the amount of charge that can flow in a circuit like that is effectively limited by the amount of charge that can be stored in the capacitor. (This is effectively the reverse of discharging the capacitor)
In an AC circuit things are a bit more complicated. The capacitor can store charge, so that when the source inverts, charge will flow from the source to the capacitor, or from the capacitor to the source.

I'm not familiar with Kirchoff's closed loop formula, but hopefully the comment above was helpfull.

3. Feb 17, 2004

### broegger

thanks for the answer.. no charge can flow through a capacitor, but there's still a current in the circuit determined by the charge on the capacitor?? i.e. when the capacitor is fully charged I = 0??

4. Feb 17, 2004

### NateTG

Technically it's when the potential difference between the capacitor plates is equal to the potential difference on the capacitor leads. ('Fully charged' might require a higher potiential difference that the circuit supplies.)

There is also leakage, butt you shoudn't have to worry about that.