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Homework Help: Rotating coil

  1. May 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A flat square coil of N turrns and area A is rotated at an angular velocity w in a uniform magnetic field of strength B. The rotation axis is perpendicular to the magnetic field direction. The coil is connected to a Galvonometer using suitable slip ring connectors and the total resistance in the coil and meter is R. Show that the detected current is I = -NBAcos(wt) / R

    2. Relevant equations

    Fardays 's law of induction *N
    I = V/R

    3. The attempt at a solution
    I get I = NBAwsin(wt) / R
    I dont see why i am wrong. The B field and area is uniform. So I only differentiate cos(wt) and divide the emf by R to obtain the current.

    Any help or suggestions would be very welcome.
  2. jcsd
  3. May 9, 2010 #2


    User Avatar
    Science Advisor

    Both answers can be right, depending on the orientation of the coil at time t=0. Since this isn't given in the problem, all you can say is that I ~ Cos[w(t+t0)]. If t0 = 0, you get a Cos solution, if t0=-pi/(2w), you get a Sin solution, and if t0 is something else, you get something in between.
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