# Homework Help: Energy in capacitors

1. Mar 26, 2006

### dan greig

A 10 micro farad capacitor is fully charged by connecting a 24v battery.

How much energy is stored in the capacitor?

How much energy would be required from the battery to charge the capacitor to the 24v level?

Explain why the answers are different.

2. Mar 27, 2006

### Galileo

What you have got so far? Any ideas?

3. Mar 27, 2006

### dan greig

I think I have found the answer to the first question using,

E = 1/2 CV^2 or E = 1/2 QV

and I think the reason the two answers wouldn't be the same is because the rate of charge decreases as the charge increases therefore the capacitor can never fully charge.

I don't see how I can get a different answer in the second question?

4. Mar 28, 2006

### Staff: Mentor

This is a trick question that comes up often in basic capacitor problems. Assume some source impedance for the battery, and calculate the energy that is burned in that resistance as the battery charges the cap. Then try a different source resistance and re-calculate....do you see a pattern?

5. Mar 29, 2006

### rammstein

well, the second answer is different coz there might be energy dissipation in the form of heat in the capacitors and the wires connecting them...Thats all I can think of.Otherwise the energy given out by the battery should be equal to the energy gained by the capacitor-energy conservation. What do u say???

6. Mar 30, 2006

### dan greig

This is the route i think i have to go down, how would i calculate the amount of energy dissipated?

Is it half the amount of energy procuced by the battery is converted into charge in the capacitor? If so is there a formula to calculate this?