# What happens if plates of charged capacitor gradually separated?

1. Jun 13, 2013

### Rolliet

Say you have a charged, parallel plate capacitor, isolated from any external circuit. If you were then to gradually seperate the two plates (or thicken the dielectric) what would happen to the charge on the two plates ?

2. Jun 13, 2013

### phyzguy

What do you think would happen? Where could the charge go?

3. Jun 13, 2013

### Rolliet

My guess is that excess charge must radiate. perhaps arcing or coronal discharge

4. Jun 13, 2013

### phyzguy

What "excess" charge? Why should it go anywhere?

5. Jun 13, 2013

### Crazymechanic

If you think of a capacitor like that in a perfect vacuum (theoretical conditions) there is no where for the charge to go and just by separating the plates doesn't make the charge to vanish.
Indeed what you would get is a increase in charge strength because now the energy that was stored in the electric field goes back to the plates.A capacitor works by that as it stores energy in electric field , hence separating the plates further and further would diminish the field between the plates.
+ you are doing work on the plates as you are separating them.

6. Jun 13, 2013

### Rolliet

Thanks crazymechanic. Yes, an increase in charge strength makes sense. Is there a quantum physics description of what would be going on here ?

7. Jun 13, 2013

### Alfred Cann

The correct answer is that the charge remains constant. The capacitance decreases as the plates are separated, and the voltage increases in inverse proportion. I.e., if you halve the capacitance, the voltage doubles.
Since the energy stored is proportional to capacitance and to voltage squared, the energy is increased! Where does the extra energy come from? It comes from the work you do in separating the plates because you are working against the attractive force. This is the principle of the Electrophorus and of the Wimshurst Machine (q.v.).

8. Jun 13, 2013

### Rolliet

Thanks. I never thought it'd add energy. I have many questions about capactance.