# Homework Help: Charge on cylindrical capacitor

1. Oct 3, 2009

### esmmajor

1. The problem statement, all variables and given/known data

A cylindrical air capacitor with a length of 12.2 m stores an amount of energy equal to 3.00E-8 J when the potential difference between the conductors is 4 V. Calculate the magnitude of the charge on each conductor.

2. Relevant equations

V=(lambda)/(2*pi*epsilon_0)*ln(r_0)/(r)

energy density=1/2*epsilon_0*E^2

3. The attempt at a solution

I know the energy, length, and potential. I'm not given either the charge on the cylinder or the ratio of the radii of the inner/outer conductors, both of which are necessary in the formula. I suspect the additional information needs to come from the equation involving the energy, some new input from knowing the electric field. Any help would be greatly appreciated.

Last edited: Oct 4, 2009
2. Oct 4, 2009

### kuruman

You don't need the dimensions of the capacitor. What relations do you know about the energy stored in a capacitor?

3. Oct 4, 2009

### esmmajor

U=1/2Q*V gives me a charge. but I need the magnitude of charge on each conductor. Is this the correct value?

4. Oct 4, 2009

### esmmajor

yes it is. thank you :) was overthinking it.