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Homework Help: Question on induced EMF

  1. Jun 10, 2012 #1
    1. The problem statement, all variables and given/known data
    A student is experimenting with a bicycle wheel. He turns the bicycle upside down and spins the wheel in a vertical plane at a constant rate. The diagram shows the wheel. At the place where the experiment is performed, the Earth‟s magnetic field is in a horizontal direction. It acts into and perpendicular to the paper.

    A constant e.m.f. is induced across the length of each spoke.
    (i) Label the hub and rim either plus or minus to show the polarity of the e.m.f.

    The figure shows a wheel with the direction of motion clockwise. The hub is the center and the rim are the outerpart

    2. Relevant equations
    Lenz's Law

    3. The attempt at a solution
    Well, in this problem, it is said that the magnetic field lines point inside the paper. So if this is cut, it tries to oppose it... Hence, magnetic field lines should be outside. We should use flemings right hand rule but i have no idea about what would be the new direction of the spokes of the wheel (i.e the motion) and indeed upon that i don't know the direction of E.M.F..... I need help urgently.. Thanks!
  2. jcsd
  3. Jun 10, 2012 #2
    Guys!! I urgently need help. Thanks!
  4. Jun 10, 2012 #3
    People Here!! This is one important question... Please help whoever can contribute with a good reply... I will owe him/her big time!
  5. Jun 11, 2012 #4
    You didn't post the diagram.
    Lentz's law isn't the appropriate one. You need to use Faradays law: "change in magnetic flux induces a current"(tries to oppose the change in magnetic flux according to lenz law)
    So it induces an EMF which should result in an opposing magnetic flux.
    So Lenz's law only gives a hint of the direction.
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