# Flux out of a Cylindrical Cable

1. Sep 14, 2011

### jegues

1. The problem statement, all variables and given/known data

A coaxial transmission line has an inner conducting cylinder of radius a and an outer conducting cylinder of radius c. Charge ql per unit length is uniformly distributed over the inner conductor and -ql over the outer. If dielectric $\epsilon_{1}$ extends from r=a to r=b and dielectric $\epsilon_{2}$ from r=b r=c, find the electric field for r<a, for a<r<b, for b<r<c and for r>c. Take the conducting cylinders as infinitesimally thin.

2. Relevant equations

3. The attempt at a solution

For r<a there is no charge enclosed thus the flux is 0 so E is also 0

Here is the solution they give for a<r<b. (see figure attached)

I am confused as to how he determines the components of the vector $d\vec{S}$. Is there a picture that will explain this?

Thanks again!

#### Attached Files:

• ###### Sol1.4a.JPG
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2. Sep 14, 2011

### quietrain

3. Sep 14, 2011

### jegues

The wiki gives me the line, surface and volume elements but I don't want to have to memorize them.

Isn't there a more intuitive way of understanding this as opposed to memorizing it?

4. Sep 15, 2011

### quietrain

you are not suppose to memorize them, they should be given in the exams.

you are suppose to understand how to get them