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Simple problem on Lenz's Law application using Right Hand Rule

  1. Aug 4, 2012 #1
    Hi everyone! I'm trying to work out a physics problem using Lenz's Law, but I'm having trouble with the Right Hand rule, which as I understand is supposed to be used to solve the problem.

    1. Basically, there are 2 copper wire loops, Loop A and Loop B. They are lying flat on the table. There is a magnetic field, B, inducing current, I, in both loops. However, for Loop A magnetic field lines point downward and for Loop B magnetic field lines point upward.

    The question is:

    Determine the direction of the induced current in (a) Loop A and (b) Loop B. Specify the direction of each induced current to be clockwise or counterclockwise when viewed from above the table. Provide a reason for each answer.

    2. No equations needed

    3. It seems to be pretty straightforward, since Lenz's Law says that current will try to oppose the magnetic field. So I tried the right-hand rule: thumb points in direction of the current, and fingers curl in direction of magnetic field... But I'm confusing myself, since it's the current that's supposed to curl arround, not the magnetic field :confused: The magnetic field just goes up or down...

    Please help! I just don't get it :(

  2. jcsd
  3. Aug 4, 2012 #2

    Doc Al

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    Staff: Mentor

    The induced current will be such as to create a field that opposes the change in the magnetic field.

    Did you provide the complete problem statement? I see no mention of a changing magnetic field.
  4. Aug 4, 2012 #3
    There are several different varieties of the right hand rule I'm afraid. This page may help. http://www.unm.edu/caps/caps-handouts/rh-rules.html [Broken] In this scenario I would use my right hand fingers curled in the direction of the induced current and my thumb in the direction of the field created by the current. I hope that helps.
    Last edited by a moderator: May 6, 2017
  5. Aug 4, 2012 #4
    Doc Al: I neglected to mention the fact that there was indeed a change in the magnetic field. In the beginning of the problem, the current I generates the magnetic field lines because the switch connecting it to the battery is closed. The switch is then opened and the current goes to 0.
  6. Aug 4, 2012 #5
    If the current is trying to counteract the change in the magnetic field, then seeing as the field is going to zero, would the current flow in such a way as to strengthen the magnetic field? Then if the field points down, the current would want to keep it that way -- and it would flow clockwise, according to the Right Hand rule that Rooted describes in is post.
  7. Aug 4, 2012 #6

    Doc Al

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    Staff: Mentor

  8. Aug 4, 2012 #7
    Thank you for your help! I obviously needed a push in the right direction.
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