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I Can a magnetic field create angular momentum?

  1. Jul 31, 2016 #1
    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?
  2. jcsd
  3. Jul 31, 2016 #2
    Aren't you describing a permanent magnet motor with the magnet as the rotor? The short answer is yes.
  4. Aug 1, 2016 #3
    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.
  5. Aug 1, 2016 #4
    This is the Einstein-de Haas effect. The total angular momentum is conserved so the object will rotate to counteract the spin angular momentum.
  6. Aug 1, 2016 #5
    Thanks, Truecrimson! This is exactly what I was thinking of!
  7. Aug 1, 2016 #6


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    Staff: Mentor

    See also the Feynman Lectures:


    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:


    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.
    Last edited: Aug 1, 2016
  8. Aug 1, 2016 #7
    Ah great, Feynman always catches the interesting stuff.
  9. Aug 2, 2016 #8
    A homopolar motor uses DC and a permanent magnet.
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