# Amperian loop technique

1. Sep 17, 2012

### shebbbbo

4. Consider a coaxial cable consisting of two infinitely thin conducting cylindrical surfaces
with radii R1 < R2. A current I flows in one direction in the inner cable and in the
opposite direction in the outer cable. Using the Amperian loop technique, determine the
magnetic field
a)inside the inner cable : r < R1 ,
b) between the two shells : R1 < r < R2,
c) outside of the wire : R2 < r.

a) I had a go at this one and thought it you place the amerian loop at R1, you would simply get amperes law:

∫B.dL=u0I

B = u0I/(2πR)

but i had a friend who said if you change where the loop is you can show that there is no current and therefore the field goes to zero? im not sure which is the correct angle to tackle this question.

i would use the same method for b and c, but i would then find some reduction of the B field due to the radius?

thanks for any advice!

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