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Homework Help: How to determine the direction of current

  1. Oct 22, 2016 #1
    1. The problem statement, all variables and given/known data
    A horizontal circular wire loop of radius 0.5 m lies in a plane perpendicular to a uniform magnetic field pointing from above into the plane of the loop, has a magnitude of 0.19 T. If in 0.15 s the wire is reshaped from a circle into a square, but remains in the same plane, what is the magnitude of the average induced emf in the wire during this time? Answer in units of V.

    The current in the loop during the deformation:
    1. does not arise. 2. flows counter-clockwise when viewed from above. 3. flows in a direction that cannot be determined from given information. 4. flows clockwise when viewed from above.

    2. Relevant equations
    Faraday's Law
    Lenz' Law

    3. The attempt at a solution
    I already solved for the first question, I only included it since it is relevant to the next question. After solving for the emf during the reshaping, I'm uncertain of the current. Since V=IR, I am leaning to thinking there is no current since I am not given any thing related to resistance. Also I'm thinking that a current would only arise if the magnetic field is varying. I'm not sure if the change of magnetic flux will induce a current or not. I'm looking in my textbook and reading as well, but I'm not seeing a definitive answer.

    Thanks in advance!
  2. jcsd
  3. Oct 22, 2016 #2


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    Since they don't ask you to determine a numerical value for the current, they haven't bothered to specify the resistance of the loop. But you can assume that the loop does have some resistance R.

    V = IR is applicable here, where V is the induced emf.
  4. Oct 22, 2016 #3
    First, thank you for replying.

    Second, would using the right hand rule apply here? I initially thought the current was clockwise because if I wrap my hand in the direction of the current, my thumb would point in the direction of the magnetic field. Since Lenz' law states the induced current is going to oppose the change of flux, the current I thought would be the opposite of this, counter-clock wise.

    Am I interpreting the right hand rule in this case?
  5. Oct 22, 2016 #4


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    Yes, the induced current will oppose the change in flux. You need to think carefully about how the flux changed when the wire changed its shape. Did the amount of flux increase or decrease?
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