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Magnetic dipole in an external magnetic field

  1. Jun 12, 2008 #1
    I've run across a diagram that really is driving me crazy. The torque on a mag. dipole should be simply be τ = m X B, where B is the external field as usual.

    In the image I've attached, there is a large external field, not shown, in the direction of Z which is holding the dipole in the +Z direction. We then switch on a mag. field in the +X direction. What's shown is the dipole moving towards +Y... :bugeye:

    I would expect it to be deflected down towards +X. Am I right in thinking that the torque vector really shouldn't be the direction of movement. Rather, you should use the right hand rule to determine the force and direction of movement. If I'm completely wrong let me know. I've been staring at this for days now. :)

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  3. Jun 12, 2008 #2
    Actually I should have placed this thread in classical physics. To late now :(
  4. Jun 12, 2008 #3


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    The picture is hard to read. My guess is that the dipole also has angular momentum and will precess like a gyroscope.
  5. Jun 12, 2008 #4
    It does have angular momentum, but how would that cause a deflection to +y? From the "NMR techniques" at the top you probably already know that precession is going on... but we're in a rotating fram of reference. We're treating everything as static.

    To add more information:
    The picture doesn't capture it, but in the text just following the diagram the author cites the "motor rule" for this deflection to +y.

    The motor rule(left hand): (field)X(current). However we're talking about two magnetic fields. If you do a quick thought experiment about the effect of a current loop producing a dipole and how it would be affected by being immersed in a field you should still come to the conclusion that it would deflect to +x.
    Last edited: Jun 12, 2008
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