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Homework Help: Potential energy for magnetic fields

  1. Nov 6, 2008 #1
    1. The problem statement, all variables and given/known data

    A circular 10 turn coil that has a radius of 0.05 m and current of 5A lies in the xy plane with a uniform magnetic field B = 0.05 T i + 0.12 T k (i and k are the unit vectors). What's the potential energy for the system???

    2. Relevant equations

    U = -m*B where m is the dipole moment = I*A

    3. The attempt at a solution

    B = sqrt((0.05 T)^2 + (0.12 T)^2) = 0.13 T

    So for this I would get U = -0.00511 J, but the answer key says -0.000472 J... where's the mistake???
  2. jcsd
  3. Nov 6, 2008 #2
    I'm assuming from the answer that the magnetic moment must not be aligned with the field, but how do you know this given the problem? And how do you calculate this?
  4. Oct 5, 2010 #3
    Well, we have to keep in mind that -m*B is actually a "dot product". ;)
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