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Homework Help: Problems with electromagnetic fields inside matter

  1. Aug 31, 2011 #1
    1. The problem statement, all variables and given/known data
    see attachment/picture
    im having problems with fields inside matter. i get the left side of gaussian surfaces/ampere loops. but for example this first equation's right side i cant understand.

    3. The attempt at a solution
    i would think the right side should be
    since its the total enclosed current by my ampere loop

    but the book says
    [itex]I\frac{\pi s^2}{\pi R^2}[/itex]
    it has something to do with the dependence of the current enclosed and the distance, which you dont have outside the cylinder. but i cant figure it out. can someone explain?
    1. The problem statement, all variables and given/known data

    Attached Files:

  2. jcsd
  3. Sep 2, 2011 #2
    It says [itex]s\leq R[/itex]. The loop that you draw is inside the wire, so you get a fraction of the total current. That fraction being the fraction of area of your loop compared to the total area of the wire. Outside the wire, you would just set [itex]s=R[/itex] and get back the total current you were talking about.
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