# Homework Help: Electric field of a coaxial cable

1. Nov 13, 2015

### balanto

We have a coaxial cable with inner radius a and outer radius b. The coaxial cable is modelled as two very long circular metal cylinders. I'm supposed to calculate the electric field E, the electric potential V and the charge enclosed Q when a voltage is applied between the metal cylinder, meaning between the inner conductor and the outer conductor

I know that if I can calculate the electric field the electric potential 'V and Q should not be hard to find. But how do i set up the problem? Is there any symmetry that allows me to use gauss law or am i stuck with using superposition?
If the case is that we have symmetry(which is a hard thing for me to figure out?) then we can use cylindrical coordinates and set up the problem as the integral of E dot ds and E=Er*r^ (because the electric field is pointing radially?) and ds=r^rdzd(theta)
But i have a hard time understanding the voltage that is applied, how do I take that into account when it comes to the integral above? Maybe I'm thinking totally wrong

2. Nov 15, 2015

### Alettix

Hi! :)
I would not like to say anything wrong, but I think the following:
Because the cable consist of two metal cylinders, I think we have a cylindrical symmetry.
It should be possible to find an expression for the electrical field (with dependance on r) from Gauss's law.
Then, one should remember that the integral of E along a distance (the difference in radius between the two cylinders) is the potentialdifference, which is the same as voltage.