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Homework Help: The Magnetic Field of Coaxial Cables

  1. Feb 22, 2010 #1
    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
    [tex]\oint(B.dl) = \mu0 I[/tex]

    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 = [tex]\mu[/tex]0 * I
    but then i dont know how it differs for part b).

    Thanks for any help
  2. jcsd
  3. Feb 22, 2010 #2


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    Homework Helper
    Gold Member

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