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Homework Help: Determine the direction of induced current in a wire

  1. Jan 8, 2012 #1
    1. The problem statement, all variables and given/known data

    A wire carrying a current experiences a force when it is placed in a magnetic field. This phenomenon forms the basis of many devices including the electric motor and the gavlvanometer.

    The diagram shows a uniform magnetic field B with magnitude 25mT directed towards the right. in the plane of the page. Initially, the coil ABCD is in the plane of the page. Each arm of the coil has length 1 = 50mm.

    c) The current is turned off and the coil is rotated through 360 degrees about the axis shown. Sketch a graph showing the current flowing in the coil because of the induced emf as a function of the angular position of the coil. Explain how you determine the magnitude and direction of the current at each stage of the motion of the coil.

    2. Relevant equations

    EMF = 2NBlvsinx, where N = number of loops of wire, B = strength of magnetic field, l = length of wire, v = velocity of wire and x = angle subtending the magnetic field lines and coil of wire.

    3. The attempt at a solution

    If the velocity is constant, then the magnitude of the EMF is is determined by the angle x, and will be at a maximum when x = 90 degrees and at a minimum when x = -90 degrees.

    I don't know how I can determine the direction of the current at each stage of the motion of the coil, however. The equation for EMF will tell me when it flows from positive to negative, but how can I tell which direction positive and negative are in?
  2. jcsd
  3. Jan 8, 2012 #2


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    Homework Helper

    It is a force on the electrons in the wire that causes the current to flow.
    If you know your hand rule for that force F = q*v*B, you can use it to find the direction of electron flow.
  4. Jan 9, 2012 #3
    I think I got it now, I used Fleming's right hand rule. Thanks :)
  5. Jan 9, 2012 #4


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    Homework Helper

    Most welcome. Interesting, I didn't know it was attributed to Fleming.
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