Two wires in parallel and capacitence

[mattysimins]

I have to show that the capacitance per unit length for two identical wires of radius d separated (at the center) a distance D as:

C/l = (pi epsilon0) / ( ln [( D - d ) / d]

And I'm clueless. How do I go about solving this?

I started by trying with C = q / V to C = lambda * l / V .. but I didn't get far as I didn't know what to do with lambda.

Edit: they are oppositely charged, and the charge is distributed evenly along the surface of each wire.

 

[Meir Achuz]

Calculate the potential difference V between the two wires
using phi=2\lambda ln(r/d) (in Gaussian units) for each wire.
Then C/l=\lambda/V