# Extra energy from capacitor problem

1. Oct 3, 2007

### indigojoker

a parallel plate capacitor is charged by a battery and then disconnected from the battery. the separation between the plates is doubled...

the E-field stays the same since charge is constant
the voltage doubles since V=Ed
the energy doubles since U=0.5QV

I was wondering, where does this extra energy come from?

does total energy increase?

2. Oct 3, 2007

### Hootenanny

Staff Emeritus
Hint: There will be a force of attraction between the two plates...

3. Oct 3, 2007

### indigojoker

the force should stay the same since it's F=EQ

how does that play into the increase in energy?

4. Oct 3, 2007

### learningphysics

It takes work to increase the distance between the plates. You have to do work on the plates to get them apart further.

5. Oct 3, 2007

### indigojoker

so the work done on the plates translate to energy stored in the plates? so the total energy does increase.

6. Oct 4, 2007

### Hootenanny

Staff Emeritus
Energy stored in the electric field increases, the energy isn't actually stored in the plates. And yes, the total energy stored increases.