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Really weird application of Lenz's Law (please help!)

  1. Nov 14, 2005 #1
    Two loops, A and B, are aligned parallel to each other. There is a current in loop A, which induces a current in loop B. How does the current in loop A change with time?

    By Lenz's Law, the current in loop B creates a magnetic field which opposes the change in magnetic flux which created it, in other words, it opposes the increase of A's magnetic field. This much makes sense. But apparently this also implies that the change in A's field must reduce the magnetic field of B. So A's field increases with time.

    I cannot understand why the change in A's field must reduce B's field. What law is being employed here, and why does it follow from the fact that B opposes the change in A?

    Thanks.

    James
     
  2. jcsd
  3. Nov 14, 2005 #2

    Physics Monkey

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    If loop A has a changing current then the flux through loop B is changing. This changing flux creates an electric field which drives a current in B. But this current in B produces a new field which is also changing so A is now experiencing a changing field as well. In fact, the changing current in A already affects A even without the B loop (this is called self inductance). So an extra current in A develops that tries to resist the change in flux due to the new current in B. You should try to convince yourself that this current will add (rather than subtract) to the current already present in A because the B loop will tend to make the magnetic field passing through the A loop smaller (just get your current directions right and use the right hand rule).
     
  4. Nov 14, 2005 #3
    Thankyou so much!
     
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