# Homework Help: Energy and Capacitors of metal spheres

1. Jun 23, 2010

### Mitchtwitchita

1. The problem statement, all variables and given/known data

Two uncharged metal spheres, spaced 15.0 cm apart , have a capacitance of 24.0 pF. How much work would it take to move 12.0 nC of charge from one sphere to the other?

2. Relevant equations

V = Q/[4(pi)eor]
(delta)K + (delta)U = 0 *I think

3. The attempt at a solution

I'm not quite sure how to do this problem. I think I would first have to find out the potential difference between the spheres?

V = Q/[4(pi)eo*r]
=(12.0 x 10^-9)/[4(pi)(8.85 x 10^-12)(1.5 x 10^-1)
=719 V

Can somebody please tell me if I'm going in the right direction and, if not, could you please steer me in the right direction?

2. Jun 23, 2010

### hikaru1221

From the law of energy conservation, the work needed = the change in the energy of the capacitor. So all you need to find is the initial energy and the final energy.

3. Jun 23, 2010

### Mitchtwitchita

So, U = Q^2/2C would be the equation to use?

4. Jun 23, 2010

### hikaru1221

Yes :)

5. Jun 23, 2010

### Mitchtwitchita

Would the initial energy be 0 because they are originally uncharged?

6. Jun 23, 2010

### hikaru1221

What do you think? :)

7. Jun 23, 2010

### Mitchtwitchita

I think so because the electric field depends on the charge of the spheres. Therefore, if there is no charge, there can't be any electric field.

8. Jun 23, 2010

### hikaru1221

There you go :)

9. Jun 23, 2010

### Mitchtwitchita

Why is the distance given?

10. Jun 23, 2010

### hikaru1221

It's redundant.

11. Jun 23, 2010

### Mitchtwitchita

Oh, ok. Thanks for your time.