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Magnetic flux through a loop

  1. Dec 10, 2016 #1
    1. The problem statement, all variables and given/known data

    Screen Shot 2016-12-10 at 8.52.38 PM.png
    2. Relevant equations
    Flux = DeltaB * Area

    3. The attempt at a solution: Since the magnetic fields are going in opposite directions through the loop, I assumed that they would have different signs, so I chose 2.0 T to be positive and 1.0 to be negative, so when calculating the change in magnetic field, I did -1.0-2.0 = -3.0. Why is this not the case? Thanks!
    Last edited: Dec 10, 2016
  2. jcsd
  3. Dec 10, 2016 #2


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    Welcome to PF!

    You are not asked to find a change in flux. You just need to find the total flux through the entire loop.

    Imagine dividing the loop into left and right halves.
  4. Dec 10, 2016 #3


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    The change in field with respect to spatial variable is indeed -3 as we move from right to left, BUT

    Faraday's law wants us to calculate the change in flux with respect to the time variable.
  5. Dec 11, 2016 #4
    Thank you! I'm starting to see it, but isn't the flux due to a change in magnetic field? So I'm still not so sure why you wouldn't choose one to be negative. Is this because flux is a scalar quantity?
  6. Dec 11, 2016 #5


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    Flux is essentially just the number of magnetic field lines piercing through the loop. Field lines in one direction count positive while field lines in the opposite direction count negative.

    If the magnetic field changes, or if the size of the loop changes, or if the orientation of the loop changes, then the flux through the loop can change. A change in flux induces an emf in the loop.

    But this problem is not concerned with emf or with a change in flux. It just asks for the flux for the situation shown in the figure.
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