Magnetic dipole in an external magnetic field

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SUMMARY

The discussion centers on the behavior of a magnetic dipole in the presence of two external magnetic fields, specifically addressing the torque equation τ = m X B. The user expresses confusion over a diagram showing the dipole moving towards +Y when an external field is applied in the +X direction, while they expect it to deflect towards +X. The conversation highlights the importance of the right-hand rule in determining the force direction and mentions the "motor rule" for magnetic fields. The user also notes the role of angular momentum and precession in this context.

PREREQUISITES
  • Understanding of magnetic dipoles and their behavior in magnetic fields
  • Familiarity with the torque equation τ = m X B
  • Knowledge of the right-hand rule for magnetic forces
  • Basic concepts of angular momentum and precession in classical physics
NEXT STEPS
  • Study the application of the right-hand rule in electromagnetic contexts
  • Explore the implications of the motor rule in magnetic field interactions
  • Investigate the effects of angular momentum on magnetic dipoles
  • Learn about precession in rotating frames of reference in classical physics
USEFUL FOR

Students and professionals in physics, particularly those focusing on electromagnetism, classical mechanics, and magnetic field interactions. This discussion is beneficial for anyone looking to deepen their understanding of magnetic dipole behavior in external 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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