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Help with Lenz's law

  1. Aug 11, 2011 #1
    1. The problem statement, all variables and given/known data

    Please click on the following link for the question.

    http://i1008.photobucket.com/albums/af204/John132456/physics.jpg


    2. Relevant equations

    Lenz's law. This states that when emf produces current, the magnetic field will oppose change.

    3. The attempt at a solution

    I understand I have to use the right hand rule #1 to find the direction of the current. In order to use the right hand rule, I need to know the direction the magnetic field will point to oppose the change. The problem I am having is that I don't understand what change is happening so I don't know which direction the magnetic field will point.

    Any help will be appreciated

    (the answer is clockwise)
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 11, 2011 #2

    PeterO

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    Those crosses in the diagram indicate the field present is into the page.

    Now you have to consider the change: as the loop moves, will there be more field or less field threading the loop - that "more" or "less" is the change.
     
  4. Aug 11, 2011 #3
    Thanks for your reply.

    How do you know whether there will be more field lines going in or more field lines going out?
     
  5. Aug 11, 2011 #4

    PeterO

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    The field lines are only going in - you just need to know whether there will be more or less of them soon.

    If the loop moves a long way down, how much flux will there be?
     
    Last edited: Aug 11, 2011
  6. Aug 12, 2011 #5
    Magnetic flux is the number of field lines passing through the loop. It is equal to the magnetic field multiplied by the area. The area doesn't change but the magnitude of the field decreases so magnetic flux will decrease

    So because Lenz's law states the magnetic field will oppose change, more field lines will be going in. And using the right hand rule, the current to cause field lines going in would be clockwise.

    Is my thinking correct?
     
  7. Aug 12, 2011 #6

    PeterO

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    Reasoning a little backwards, the reasoning and answer correct.

    The area does change while the Magnetic field strength doesn't. The area is changing because less and less of the loop is in the region of the field.
    Flux is the magnetic field multiplied by that part of the area in the field.
     
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