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Homework Help: Ampere's aw-conductor.

  1. Feb 1, 2012 #1
    1. The problem statement, all variables and given/known data
    I have to find the magnetic field inside a long conductor of radius a.

    2. Relevant equations
    Ampere's law.

    3. The attempt at a solution

    Problem has cylindrical symmetry and B is along θ direction.


    How to find what fraction of current inside the area considered?

    I just saw in a book that it is ∏r2/∏a2.But can't figure out the idea.:(

    I know it is very simple and donno why i forget basics always..:(
  2. jcsd
  3. Feb 1, 2012 #2


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    You have a total current I in the wire. Assume it's evenly distributed across the cross-sectional area of the wire. What would be the current density?
  4. Feb 2, 2012 #3
    Yeah!now got it.
    Current density is constant.

    Ienc/∏r2= Itotal/∏α2
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