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I have failed to show that the magnetic flux outside a cylindrical conductor is zero.

The problem goes like this:

a coaxial cable consists of a solid inner cylindrical conductor of radius a and an outer cylindrical conductor of inner and outer radius b and c. Distributed currents of equal magnitude I flow in opposite directions in the two conductors. Derive expressions for the magnetic flux density B(r) for each of these regions

1) 0<r <a

by amperes law I showed: [tex] B = \frac {K *l*r} {2*pi*a^2} [/tex]

where K is the permeability of free space, don't know how to write that with latex

2) a<r<b

again by amperes law: [tex] B = \frac {K *l} {2*pi*r} [/tex]

3) b<r<c

again by amperes law: [tex] B = \frac {K *l*(r^2-b^2)} {2*pi*r*(c^2-b^2)} [/tex]

4) c<r

I got stuck here, surely it must be zero....but how can that be shown?

thanks for your help - it's very much appreciated

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# Homework Help: Magnetic flux outside cylindrical conductor

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