# What is the voltage across the capacitor plates

1. May 30, 2006

### flower76

Am I overthinking this problem?

A 15uF capacitor is connected ot a 55V battery and becomes fully charged. The battery is removed and the circuit is left open. A slab of dielectric material is inserted to completely fill the space between the plates. It has a dielectric constant of 4.8. What is the voltage across the capacitor plates after the slab is in place?

I'm thinking that I just use V = Vo/k = 55/4.8 =11.5V

This seems too easy and doesn't use the value for the capacitor.

Can anyone tell me if I'm totally off in my thinking?

2. May 30, 2006

### chroot

Staff Emeritus
Other way around.

The introduction of a dieletric increases the capacitor's capacitance by a factor of k.

Q = CV and Q remains constant. As a result, if C increases by a factor of k, V must decrease by a factor of k.

You are correct that the actual value of the capacitance is irrelevant, as it cancels out.

- Warren

3. May 30, 2006

### flower76

Sorry I'm confused, you said "Other way around" but based on what you wrote I have it correct that the voltage decreases by k, ie V = Vo/k

4. May 30, 2006

### chroot

Staff Emeritus