# Homework Help: Power of a capacitor

1. Feb 27, 2006

### Punchlinegirl

The energy (in joules) stored in a capacitor with capacitance C (in Farads), having a potential difference V (in volts), is given by W= 1/2 CV^2. One application is in the electronice photoflash or strobe light. In such a unit, a capacitor of 10.7 $$\mu F$$ is charged to 311 V. Find the energy stored. Answer in units of J.
I got this part.. the answer was 0.513 J.
Next, it says, find the power required to charge the capacitor to this energy level in 43 s. Answer in units of W.
Since W is given in J/s, do I just do 0.513/43? which gives me .0119. We didn't do much of this in class, so I don't know how else to do it.

2. Feb 27, 2006

### fargoth

yep, what you did seems right to me.

3. Feb 27, 2006

### Punchlinegirl

this wasn't right... is there another way to do it?

4. Feb 27, 2006

### Staff: Mentor

Hmmm. Looks tricky. Maybe try a few examples with a real >=311V power supply and some source impedance. Try a source impedance that gives you one time constant at 43 seconds with that capacitance, and calculate the total power delivered, and the power lost in the source resistor. Then try a different source voltage and source resistance to see if you get the same answer or a different answer (I don't know which it will be offhand). The problem may be trying to point out that you can't charge a capacitor without burning some energy in the source power supply....