# Need help finding the electric fields in a coaxial cable with charged conductors!

physicsidiot1

## Homework Statement

A coaxial cable consists of a long solid inner cylindrical conductor, radius a, and a thin outer cylindrical conductor of average radius b>a. Suppose the inner conductor has a line charge density of +lambda while the outer conductor has a uniformly distributed charge of -lambda. Find the electric field (as a function of r) in a) inside the inner conductor, b) between conductors and c) the outside cable

## Homework Equations

E=q/Epsilon0 E=q/(4piEpsilonr^2)

## The Attempt at a Solution

I would imagine using Gaussian cylindrical symmetry could help find the field for the inside and outside cable. However, I cannot figure out how to find the field between the conductors. A full explanation of this problem would be so greatly appreciated, as I am struggling to grasp the effect of oppositely charged conductors in electric fields. Thanks.

Homework Helper
You must define a cylindrical closed "container" for the inner conductor's charge at radius r between a and b and apply Gauss' law. If you make the cylinder infinitely long you can ignore what happens at the ends.

Homework Helper
Gold Member
Hello physicsidiot1,

Welcome to Physics Forums!

a) Remember, the inner conductor is a conductor. That little fact turns out to be quite important.

b) Consider a cylindrical Gaussian surface, with radius r, such that a < r < b (and the Gaussian surface and the cable share the same axis). Given the symmetry, the Gaussian surface vector is always parallel to the electric field lines (except for the end-caps which are perpendicular).

What is the surface area of the Gaussian surface (ignoring end-caps)? What is the charge enclosed within the Gaussian surface?

What does Gauss' law have to say about that?

c) Do you mean the electric field outside the cable (as opposed to "the outside cable")?