# Current in a (series) circuit with a capacitor

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??

2. Feb 17, 2004

### chroot

Staff Emeritus
Alternating current can flow through a capacitor (but direct current cannot). Consider that in an alternating current, the electrons don't actually flow all the way around the circuit -- they basically just wiggle back and forth in place. The capacitor couples its plates together through the electric field, which is exactly what causes electrons to move through wires.

- Warren

3. Feb 17, 2004

### broegger

but in my physics book they discuss these things before even introducing alternating current.. in fact, i think it's in a chapter called Direct Current Circuits..

4. Feb 17, 2004

### chroot

Staff Emeritus
Well, capacitors do not conduct DC. So I don't know what to tell you.

- Warren

5. Feb 17, 2004

### broegger

So, you're saying that capacitors can't be used in (seriel) dc circuit?

6. Feb 17, 2004

### chroot

Staff Emeritus
No, I'm not saying they can't be "used." There are many ways a capacitor could be used in a series DC circuit -- but conducting current is not one of them.

- Warren

7. Feb 17, 2004

### NateTG

I've posted this in the other thread - there is current flow sufficient to charge/discharge the capacitor.

8. Feb 17, 2004

### Integral

Staff Emeritus
The current flow into a uncharged cap is like

$$i(t)= i_0 e^{-kt}$$

where the k is determined by the capacitor.

9. Feb 19, 2004

### jlenard

condom capacator analogy

The mechanical analogy of a capacator, is a water pipe with a condom traped in it. If water tries to flow in one direction (DC), the condom will streach until it stops the flow of water. If the water oscilates back and forth in the pipe (AC), the condom stretchs back and forth, representing little or no effect on the oscilating water.

All the best
john

10. Feb 19, 2004

### turin

Ironically, a capacitor kind of violates KCL, at least significantly more so than the other two passive elements. Charge builds up on the plates. To apply KCL, you have to "draw a black box" around the capcitor and treat it as an isotropic 2-port device. The same can be done with the other two passives. You can still apply KCL and KVL outside the elements, but, instead of Ohm's law, you have to realize that capacitors are reactive and a differential equation or Laplace transform must be used.