Can a magnetic field create angular momentum?

AI Thread Summary
Magnetic fields can align the spins of electrons in materials, potentially leading to observable changes in angular momentum. The discussion references the Einstein-de Haas effect, which demonstrates that total angular momentum is conserved, causing an object to rotate in response to aligned spins. The conversation distinguishes between the proposed model and a permanent magnet motor, emphasizing the absence of alternating current in the model. The historical context of the Einstein-de Haas experiments, conducted before the discovery of electron spin, is noted for its relevance to the topic. Overall, the interaction of magnetic fields with electron spins raises intriguing questions about angular momentum in suspended objects.
BucketOfFish
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I know that magnetic fields can align objects with spin. In that case, if we suspend an object and turn on a magnetic field such that a significant number of electrons become aligned with the field, could we observe a macroscopic change in angular momentum?
 
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Aren't you describing a permanent magnet motor with the magnet as the rotor? The short answer is yes.
 
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I don't think I'm describing a motor. For one thing, there is no alternating current in my model, which is required even in a permanent magnet motor. I'm proposing to just dangle a block of some material in midair and then to apply an extremely strong stationary magnetic field. I know the field does interact with spins, causing the Zeeman effect, but I don't know if it's possible to see a change in angular momentum as a result.
 
This is the Einstein-de Haas effect. The total angular momentum is conserved so the object will rotate to counteract the spin angular momentum.
 
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Thanks, Truecrimson! This is exactly what I was thinking of!
 
See also the Feynman Lectures:

http://www.feynmanlectures.caltech.edu/II_37.html#Ch37-F3

in particular Fig. 37-3 and the paragraph preceding it (you'll probably have to scroll the page up a bit).

Note that the Einstein-de Haas experiments were in 1915-16, about ten years before the invention/discovery of electron spin! They must have analyzed their results in terms of the surface bound current of the magnetized cylinder, in classical electromagnetism:

http://www.physicspages.com/2013/06/29/magnetization-bound-currents/

See Example 1 which discusses a cylinder with uniform longitudinal magnetization. If there really were a macroscopic current running azimuthally around the cylinder, it would have angular momentum due to the circular motion of the electrons.
 
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Ah great, Feynman always catches the interesting stuff.
 
BucketOfFish said:
I don't think I'm describing a motor. For one thing, there is no alternating current in my model, which is required even in a permanent magnet motor.

A homopolar motor uses DC and a permanent magnet.
 
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