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