# Dielectrics and Capacitors, Very confused.

ryan8642

## Homework Statement

A 25pF parallel plate capacitor with an air gap b/w the plates is connected to a 100v battery. A Teflon slab is then inserted b/w the plates and completely fills the gap. What is the change in charge on the positive plate when the Teflon is inserted?

## Homework Equations

Q=CV
C=eA/d
C=keA/d

e=permittivity constant

## The Attempt at a Solution

I am very confused and dont know what to do, none of the formulas make sense for this question... :(
But

C=25x10^-12F
Kteflon=3.0
permittivity constant=8.85x10^-12
v=100v

what do i do??? soo confused

## Answers and Replies

cupid.callin
find the two capacitances after and before the plate is inserted

find the charge using Q=CV as you wrote

then just subtract the charge!!!!

ryan8642
i dont get it, how do i find the capacitance when the teflon is inserted? do i just multiply the C (up there) by 2.0?

ktef should be 2.0 not 3.0

Mentor
i dont get it, how do i find the capacitance when the teflon is inserted? do i just multiply the C (up there) by 2.0?

ktef should be 2.0 not 3.0

Yes. By 2.0