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Homework Help: Force on Magnetic Dipole

  1. May 3, 2013 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    I found dB/dr to be constant near r = 0, so it's a straight line, implying B increases linearly with r near r = 0.

    For the second part, I assume the setup is as such, if not placing the dipole with its axis parallel to the coil would make B = 0, and hence no force at all..Not sure what energy has to do in this case.

  2. jcsd
  3. May 3, 2013 #2
    There is a relationship between potential energy and force.
  4. May 3, 2013 #3


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    Sounds good.

    [Edited to correct a misstatement] Although there is only an r-component of field on the axis of the Maxwell loops (r-axis), off the axis there will be a radially inward field (perpendicular to the r-axis and toward the r-axis). A dipole in the form of a small current loop would experience a force due to this radially inward field.

    But, it's easier to find the force by using the idea that force is the negative gradient of potential energy: F = -∇E. See if you can use this to find the force. [I see MisterX posted this suggestion before me. Sorry.]
    Last edited: May 3, 2013
  5. May 5, 2013 #4
    Then, wouldn't the force simply be F = - ∂U/∂r = m ∂Bext/∂r at r = 0?

    This is simply the earlier part of the question of finding ∂B/∂r at r = 0.
  6. May 5, 2013 #5


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    Yes, that's right.
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