# Cylindrical Capacitor

1. Oct 15, 2009

### rey242

1. The problem statement, all variables and given/known data
Hey guys, My prof asked us to find the radius of a cylinder within a cylindrical capacitor with 2 radii a and b. I know we use the Capacitance formula for a cylinder but I cant seem to get pass the q that shows up when I plug it into the 1/2 c*V^2

EDIT: He wants us to find a cylinder's radius that contains half the stored electric potential energy.

Last edited: Oct 16, 2009
2. Oct 15, 2009

### ideasrule

I'm afraid you'll need to be a lot more specific than that. Can you give us the full question (word-for-word), plus all of the equations you used and all of the calculations you tried?

3. Oct 16, 2009

### rey242

Oh I'm sorry, I forgot to add something. He wants us to find a cylinder's radius that contains half the stored electric potential energy.

Yea theres a picture he showed us, a long cylindrical capacitor. The picture shows a cross section of the cylinder. The smaller cylinder within the bigger one has a radius a and the bigger one has a radius b. The radius of an arbitrary cylindrical Gaussian surface between a and b is r.

I tried to use C= 2pi E0 (L/ ln(b/a)) for the capacitance and use it with both forms of Electric Potential energy equation(q^2/r and 1/2cV^2). But I get stuck on the charge q variable. and I even tried to use the Integral used to derive Elec. Potent. Eng. but I cant really figure that out what to do with that.

4. Oct 17, 2009

### rey242

Even though the homework assignment passed, I would still like an answer to this. :)