# The Magnetic Field of Coaxial Cables

1. Feb 22, 2010

### Ghassan_2k

1. The problem statement, all variables and given/known data
A coaxial cable of length l, has a thin wall cylindrical inner conductor of radius a carrying current I. All current returns along the outer conductor whose inner radius is b.

Assuming that the current flows only in a thin layer on each conductor, determine the magnitude and direction of the magnetic induction B at radius r.

a) when r<a
b) when a<r<b
c) when r>b

2. Relevant equations
$$\oint(B.dl) = \mu0 I$$

3. The attempt at a solution
Ive just started studying magnetostatics and am a little bit confused. I think using Ampere's Law in each of the three cases is the right thing to do, but I am unsure on what the value for I is in each case.

For part a), i think B * 2 * pi * r = $$\mu$$0 * I
but then i dont know how it differs for part b).

Thanks for any help

2. Feb 22, 2010

### gabbagabbahey

The $I_{enc}$ in Ampere's Law is the net current flowing through your Amperean loop. So, what are you using for your Amperian loop?