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roam
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Homework Statement
The figure below shows a long, hollow copper pipe.
The inner radius of the pipe is a and the outer radius is b. A uniform current I flows in the walls of the pipe. You may assume that the permeability of copper is the same as free space, that is, ##\mu_0##. Use the integral form of Ampere's law to find:
(i) The magnetic induction B for r<a
(ii) The magnetic induction B for a<r<b
(iii) The magnetic induction B for r>b
Homework Equations
Ampere's law in integral form
The Attempt at a Solution
(i) Since I enclosed is 0
##\oint B . dl = B 2 \pi r = \mu_0 I \implies B=0##
(ii) ##B 2 \pi r = \mu_0 I##
Here I think the I enclosed is ##\frac{I'}{I}= \frac{\pi r^2}{\pi (b-a)^2} \implies I' = \frac{I r^2}{(b-a)^2}##. So
##B 2 \pi r = \mu_0 \frac{I r^2}{(b-a)^2} \implies B= \frac{\mu_0 I r}{2 \pi (b-a)^2}##.
(iii) For the magnetic induction outside
##B 2 \pi r = \mu_0 I \implies B = \frac{\mu_0 I}{2 \pi r}##
Is my working correct? I'm mostly doubtful about (i) and (ii). For (ii) shouldn't B be equal to zero inside the material, since copper is diamagnetic?
Thanks.
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