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Homework Help: Electromagnetic induction of a closed circular wire

  1. Apr 2, 2015 #1
    1. The problem statement, all variables and given/known data
    (See image) a closed circular wire A lies in the plane of the wire B in the circuit. Deduce the direction. Of the induced current in A when
    (A) the switch is closed
    (B) the switch is opened
    (C) the wire A is raised out of the plane of the paper.
    2. Relevant equations

    3. The attempt at a solution
    (A)I used lenz's law and I think that as the field from the circuit will be pushing Down on the wire it will produce a revolving field which opposes the circuit field. I think it is anticlockwise.
    (B) no idea
    (C) trick question, no change in flux so no current.

    Attached Files:

  2. jcsd
  3. Apr 2, 2015 #2


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    Homework Helper
    Gold Member

    You are correct for A though your justification is not 100% accurate. Lenz's law says that the induced current will be such as to oppose its cause. The cause of the induced current is not exactly the magnetic field but that the magnetic field produced by wire B is increasing. Therefore the induced current will oppose the increase of the magnetic field and the way to do it is to be such as to produce a field that opposes the circuit field as you said too.

    After this explanation i believe you should be able to answer B, having in mind that the magnetic field of the circuit is decreasing when u open the switch (cause the current of wire B is decreasing down to zero).

    For C it depends what one can assume aditionally. IF we assume that the magnetic field does not decrease in the z-direction (and this would be true for small values of z) then you are correct. But for medium to large values of z the magnetic field will decrease so what will be the answer similar to (B)
  4. Apr 6, 2015 #3
    Thanks, that clears the topic up for me.
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