Magnetic dipole in an external magnetic field

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Discussion Overview

The discussion revolves around the behavior of a magnetic dipole in the presence of external magnetic fields, specifically focusing on the torque experienced by the dipole and its resulting motion. Participants explore the implications of different magnetic field orientations and the associated forces and torques, with references to classical physics concepts such as angular momentum and precession.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant asserts that the torque on a magnetic dipole is given by τ = m X B, questioning the direction of movement shown in a diagram where the dipole moves towards +Y instead of +X.
  • Another participant suggests that the dipole has angular momentum and may precess like a gyroscope, although the implications of this on the dipole's deflection are questioned.
  • A further contribution mentions the "motor rule" (left hand rule) relating to the interaction of magnetic fields and currents, suggesting that despite the presence of two magnetic fields, the dipole should deflect towards +X based on a thought experiment involving a current loop.

Areas of Agreement / Disagreement

Participants express differing views on the expected direction of the dipole's movement and the role of angular momentum and torque, indicating that multiple competing interpretations exist without a clear consensus.

Contextual Notes

The discussion highlights potential limitations in the diagram's clarity and the assumptions regarding the static treatment of the system, as well as the complexity introduced by the presence of multiple magnetic fields.

rndnrg
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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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Actually I should have placed this thread in classical physics. To late now :(
 
The picture is hard to read. My guess is that the dipole also has angular momentum and will precess like a gyroscope.
 
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:

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