# Quick question regarding gauss' law

1. Sep 8, 2004

### jimithing

A long coaxial cable consists of an inner cylindrical conductor with radius $$a$$ and an outer cylindrical shell of inner radius $$b$$ and outer radius $$c$$. The cylindrical shell is mounted on insulating supports and has no net charge. The inner cylinder has a uniform positive charge per unit length $$\lambda$$.

Calculate $$E$$
a) at any point between the cylinders
b) at any point outside the cylindrical shell
c) Find the charge per unit length on the inner surface and outer surface of the shell.

a) wasn't a problem, found $$E = \frac{\lambda}{2\pi rh}$$
b) and c) i'm having problems with

would $$E$$ outside the shell be 0 since the insulating shell carries no charge?

2. Sep 8, 2004

### Claude Bile

You answer to a is wrong for one simple reason (among others), it is dependant on the Gaussian Surface you chose, when it shouldn't be. The total charge, q is not lambda, but lambda times the height of your Gaussian surface (think about the units, lambda is in C/m, you need to multiply by a distance to get C). Also there should be an epsilon 0 in there if you are working in S.I. units.

Claude.