# Homework Help: Physics true/false questions v. isolated parallel plates

1. Mar 4, 2005

### Gonger

Which of the statements below are true for two oppositely charged, isolated parallel plates? (C is the capacitance, U is the stored energy, +Q and -Q are the charges on the plates.) Note: isolated plates can not lose their charge. (Enter ALL correct statements, e.g., BCD)

A) When the distance is doubled, U increases.
True. Im thinking that if the distance is doubled there is more to store energy in.

B) Inserting a dielectric increases C.
I have no idea on this one.

C) When the distance is halved, Q stays the same.
True. I dont think distance will affect the charge.

D) Increasing the distance increases the electric field.
True.

E) When the distance is doubled, C increases.
False. It doesnt have any affect on the capacitance

F) Inserting a dielectric decreases U.
Again no idea.

G) Inserting a dielectric increases Q.
I just have no clue with dielectrics.

Thanks for any help in advance.

2. Mar 4, 2005

### Corneo

Some forumlas you need to know is

$$C = \frac {Q}{V} = \epsilon_0 \frac {A}{d}, \ \ U = \frac {Q^2}{2C}$$

So when d increases, C decreases, then U increase.

When ever you are dealing with a dielectric, the capacitance is given by

$$C_d = \kappa \epsilon_0 \frac {A}{d}$$

Where $\kappa$ is a constant called the dielectric constant. This is always greater than 1. This says that $C_d > C$. Which is exactly why dielectrics are inserted, to increase the capacitance.

That should get you started.

3. Mar 4, 2005

### Gonger

cool. I got it thanks.

4. Mar 5, 2005

### Andrew Mason

True. $C \propto \epsilon > \epsilon_0$

False. The field is uniform between the plates and is independent of the separation where the plate size is large compared to separation. The potential depends upon distance (V = Ed)

True. C is proportional to d.

True. The dielectric reduces the field between the plates. So it reduces the potential (V=Ed),

False. Charge is constant.

So: ABCEF

AM

5. Mar 5, 2005

### Corneo

I think $C \propto \frac {1}{d}$.

6. Mar 5, 2005

### Andrew Mason

Right you are. So ABCF.

AM