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Lenz's Law

  1. Aug 26, 2011 #1
    1. The problem statement, all variables and given/known data
    a). If the resistance of the resistor in Fig. 21-46 is slowly increased, what is the direction of the current induced in the small circular loop inside the larger loop? b). What would it be if the small loop were placed outside the larger one, to the left?

    f21-46.jpg

    2. Relevant equations

    3. The attempt at a solution
    a) Since the current in the outer loop induces a magnetic field, which in turn induces a current in the inner loop, and Lenz's Law states that this must flow in the opposite direction, shouldn't it flow clockwise? According to the answer key this is incorrect.

    Any help would be much appreciated.
     
  2. jcsd
  3. Aug 26, 2011 #2

    PeterO

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    Homework Helper

    Lenz's law says the induced current is in a direction to opposite the change in field, not necessarily the field itself.

    If the field created by the outer loop is getting stronger, the induced current will be in the opposite direction as the current in the outer loop
    If the field created by the outer loop is getting weaker, the induced current will be in the same direction as the current in the outer loop.

    So what is happening in this example?
     
  4. Aug 26, 2011 #3
    Thanks, that makes sense.

    So in essence, this stems from the negative sign in Faraday's Law, doesn't it?

    [itex]\varepsilon=-N\frac{\Delta(BA)}{\Delta t}[/itex]

    Since the magnetic field B is weakening, the delta B is negative, thus cancelling out the negative sign.
     
    Last edited: Aug 26, 2011
  5. Aug 26, 2011 #4

    PeterO

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    Now you thinking! Those minus signs are always there for a reason.
     
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