Parallel Plate Capacitor True or False

[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

 

[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?)

 

[Moetasim]

I can understand your frustration, but don't worry, let's go through each statement one by one to see what's wrong.

1. When the distance is doubled, U increases. - This statement is true. The stored energy in a parallel plate capacitor is directly proportional to the distance between the plates. So when the distance is doubled, the stored energy also doubles.

2. Inserting a dielectric decreases U. - This statement is false. A dielectric material increases the capacitance of a parallel plate capacitor, which in turn increases the stored energy. So, inserting a dielectric actually increases the stored energy.

3. Increasing the distance, decreases the E field. - This statement is false. The electric field between the plates of a parallel plate capacitor is directly proportional to the voltage and inversely proportional to the distance. So, increasing the distance actually increases the electric field.

4. When the distance is halved, Q stays the same. - This statement is false. The charge on the plates of a parallel plate capacitor is directly proportional to the capacitance and inversely proportional to the distance. So, when the distance is halved, the charge on the plates also halves.

5. Inserting a dielectric increases C. - This statement is true. As mentioned before, a dielectric material increases the capacitance of a parallel plate capacitor.

6. Inserting a dielectric increases Q. - This statement is true. When a dielectric material is inserted between the plates, it increases the capacitance, which in turn increases the charge on the plates.

7. When the distance is doubled, C increases. - This statement is false. The capacitance of a parallel plate capacitor is inversely proportional to the distance between the plates. So, when the distance is doubled, the capacitance decreases by half.

I hope this helps to clarify any confusion and improve your understanding of parallel plate capacitors. Keep up the good work in your studies!