# Homework Help: Capacitor Energy question

1. May 8, 2007

### t_n_p

In an RC circuit, current flows from the battery until the capacitor is completely charged. Is the total energy supplied by the battery equal to the total energy stored by the capacitor? If not, where does the extra energy go?

I'm a total physics newbie, but taking an almost wild guess I think that some of the energy in the wires as heat due to resistance. Can anybody confirm/deny my theory?

Thanks :tongue2:

2. May 8, 2007

### Hootenanny

Staff Emeritus
3. May 9, 2007

### t_n_p

I'm still stuggling to understand.
So U stands for stored energy in the capacitor and U=.5QV

but how can that be used to answer my question?

Last edited: May 9, 2007
4. May 9, 2007

### Hootenanny

Staff Emeritus
So the capacitor stores half the energy supplied by the battery, the other half is the work done placing charge on the plates, irrespective of the resistance.

5. May 9, 2007

### Weimin

Hmm, then how about the heat released from the resistance?

6. May 10, 2007

### t_n_p

Yeah, that's what I am wondering, or is it negligible?

7. May 10, 2007

### Hootenanny

Staff Emeritus
Perhaps it would be better if I put it this was. For any RC circuit with a finite resistance, half the energy supplied by the battery is dissapated as heat through the resistor. This is independant of the value of the resistance.

8. May 10, 2007

### t_n_p

So of the energy supplied by a battery, half is stored in the capacitor and half is lost as heat through resistors?

9. May 10, 2007

### Weimin

It seems that we have more than two halves:

- half is stored in the capacitor
- half is the work done placing charges on the plate
- half is lost on the resistor

And I also wonder why we did not take into account the energy of the electro-magnetic field: as the charges are gradually filled up the capacitor there is a variable current in the circuit. This induces a variable magnetic field and so on, finally we have emf propagating in the space, meaning that we are losing the energy. I am a bit confused on this.

10. May 10, 2007

### t_n_p

Way to confuse me...

11. May 11, 2007

### Hootenanny

Staff Emeritus
Correct! This is always the case, irrespective of the values of R or C.

12. May 11, 2007

### t_n_p

Thanks, that's all I was after!