# Parallel Plate Capacitor True or False

1. Jul 7, 2008

### ysk1

I keep getting the questions wrong. I'm so frustrated. I'd appreciate it if anyone can tell me what's wrong.

Which statements are correct/incorrect for two oppositely charged, isolated parallel plates (Isolated means that the charge stays constant ):
(C=capacitance, U=stored energy, +Q and -Q = charge on the plates).

1. When the distance is doubled, U increases. T
2. Inserting a dielectric decreases U. F
3. Increasing the distance, decreases the E field. F
4. When the distance is halved, Q stays the same. F
5. Inserting a dielectric increases C. T
6. Inserting a dielectric increases Q. T
7. When the distance is doubled, C increases. F

2. Jul 7, 2008

### dynamicsolo

It will be of help to you to write down:

1) the expression for finding C for a parallel-plate capacitor

and

2) the expression for the electric potential energy U in a capacitor in terms of C and Q (the versions involving V will complicate things) .

#5 is correct. Re-read the condition for the charge on this capacitor to understand why you got #4 and #6 wrong. The equations I suggested will help you figure out why the others are incorrect. (Oh, for #3, how do the field for a sheet of uniform charge density depend on distance from it?)