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Magnetics question!

  1. Dec 9, 2005 #1
    A long piece of wire with a mass of 0.1kg and a total length of 4m is used to make a square coil with a side of 0.1m. The coil is hinged along a horizontal side, carries a 3.4A current, and is placed in a vertical magnetic field with a magnitude of 0.01T.
    a) Determine the angle that the plane of the coil makes with the vertical when the coil is in equilibrium.

    For this part I was thinking, mg = F = BILcos phi, where phi is the angle with respect to the vertical...
    phi = arccos (mg/BIL)
    Now, I thought, "Which L can I use?" and I couldn't find the answer using different values.. can anyone please show me where I went wrong?

    b) Find the torque acting on t he coil due to the magnetic force at equilibrium.
    I'm guessing a) will give me what I need for this part.

    The answers are:
    a) 3.97deg
    b) 3.39 x 10^-3 Nm

    Thanks for any pointers!
  2. jcsd
  3. Dec 9, 2005 #2


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    Did you account for multiple turns of the wire? (total length is 4 meters but the perimeter of the square is only 0.4 meters)
  4. Dec 9, 2005 #3
    Isn't that only relevant to the torque?
  5. Dec 9, 2005 #4


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    The current runs through each loop and the total current interacts with the applied field.
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