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Homework Help: Another problem from magnetism

  1. Jan 16, 2007 #1
    1. The problem statement, all variables and given/known data

    An alternating current I cos wt flows down a long wire and returns along a coaxial conducting tube of radius a. Find E assuming E→0 at ∞.

    2. Relevant equations



    3. The attempt at a solution

    I did the problem from Griffiths but have some problem to understand the physical situation. I did using ∫E∙dl = (-dΦ/dt) and proceeded straight away as B=μn I cos wt.
    How much it is justified as I am simply neglecting the current returning through the conducting tube?
     
  2. jcsd
  3. Jan 16, 2007 #2

    Meir Achuz

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    B comes from Ampere's law, which includes only the current within the loop.
    The current in the outer tube is outside the loop.
     
  4. Jan 17, 2007 #3
    How do you know where to construct Ampere's loop?At first you are to justify that the B field must be 0 outside the tube.
     
  5. Jan 21, 2007 #4

    Meir Achuz

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    Ampere's loop is a circle within the tube, where you need to know B.
    The field outside the tube ils irrelevant.
     
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