# Homework Help: Parallel plate capacitor voltage question

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1. Feb 13, 2017

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

A parallel-plate capacitor is connected to a 100 V battery until it is fully charged. The distance between the plates is d and the space between the plates is filled with air (k=1.0). Then, the battery is disconnected. If the distance between the plates is increased to 3d, the voltage of the capacitor will be ?
2. Relevant equations
C=ɛ0*A/d
C=Q/V
U=QV/2
E=V/d

3. The attempt at a solution
So i know that capacitance is inversely prop. to distance. so if distance is tripled then capacitance will be 1/3 C but i cannot relate that to voltage. I know the correct answer is 33V but i cannot get my head about it. I know that if i multiply 100*1/3 i will get 33V but how does multiplying capacitance (F)by volts will give me volts.

2. Feb 13, 2017

### cnh1995

Which quantity remains the same in both the cases?

3. Feb 13, 2017

Area i believe?

4. Feb 13, 2017

### cnh1995

..and??

5. Feb 13, 2017

Area and k (dielectric)

6. Feb 13, 2017

### cnh1995

7. Feb 13, 2017

Charge is not related?

8. Feb 13, 2017

### cnh1995

Charge has nowhere to go I believe. So wouldn't it remain consatnt?
V=Q/C.

9. Feb 13, 2017