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Current and Lenz's Law

  1. Feb 14, 2009 #1
    1. The problem statement, all variables and given/known data

    Two conducting loops carry equal currents I in the same direction as shown in the figure. If the current in the upper loop suddenly drops to zero, what will happen to the current in the lower loop according to Lenz’s law?

    (a) The current in the lower loop will decrease.
    (b) The current in the lower loop will increase.
    (c) The current in the lower loop will not change.
    (d) The current in the lower loop will also drop to zero.
    (e) The current in the lower loop will reverse its direction.
    http://img21.imageshack.us/img21/8688/lenzrz5.jpg [Broken]

    2. Relevant equations

    Lenz's law states "Induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change."

    3. The attempt at a solution

    I know that the answer is (b). I am just trying to figure out why.

    Lenz's law is just conservation of energy, so if both currents are going in the same direction, than forces are going upwards? If the top one were removed, wouldn't the force acting on the bottom one decrease? I think I am confusing a bunch of things, but trying to understand how/what to approach this problem.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 14, 2009 #2


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    From your statement of lenz's law, the change in current in the top loop will induce an emf and thus a change in current in the lower loop. This change in current creates a magnetic field. The magnetic field will oppose the change in flux caused by the first loop.

    So, to answer this question, you need to answer the following smaller questions:

    1. What direction does the change in flux caused by the top loop point?

    2. The field created from the change in current in the lower loop must point opposite the change in flux. What direction is this?

    3. So, what direction around the loop does the change in current point? HINT: Right hand rule.

    4. Does this change in current increase or decrease the current in the lower loop?

    Can you answer these questions?
  4. Feb 14, 2009 #3
    Well, if I use the right hand rule, and curl my fingers towards I, my thumb points straight up.



    I think I am missing something conceptually, perhaps with the application of RHR.
  5. Feb 14, 2009 #4
    I'm trying to look at the book and trying to visualize the forces, but I don't quite get it.
  6. Feb 15, 2009 #5


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    That's the direction of the flux, but the question asks about the change in flux.

    I is initially counterclockwise. If it drops to zero, it's change is in the opposite direction. So curl your fingers in that direction.

    The question is about the magnetic field or flux. This will be either up or down. So the question is, what direction--up or down--is opposite to the change in flux you got for question #1?
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