Electron spin and conservation of angular momentum

  • Thread starter JonoF
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Hi there,

This is my first post here, although I have been haunting the forums for a few weeks :smile:

I have a couple of questions regarding electron spin and conservation of angular momentum that have arisen from my research into the 'Einstein-de Haas' experiment. (Please bear with me as my knowledge of this topic area is only what I have read - not yet at university).

My understanding of this effect is that when a uniform magnetic field is applied to an unmagnetised ferromagnetic rod (suspended by a wonderful magical wire that doesn't apply any restorative torque when the rod begins to rotate, for the sake of ease :approve: ), the randomly oriented magnetic moments (which are proportional to the angular momentum of the electrons ??) align parallel to the magnetic field. Thus the angular momentum (i.e. spin) of the individual electrons has changed, and because there is no initial resultant torque on the cylinder, it gains angular momentum in order to conserve angular momentum.

The reason I have explained my own understanding is because there is a (very) good chance that I've got it all horribly wrong, and ought to return under the rock whence I came...

Now, my questions - Why is it that if you calculate the change in ang. momentum due to every single electron changing its spin, this results in a horrendously small resultant angular velocity of the rod (in the region of 1 rotation every few months or so :grumpy: haha). Clearly, to me anyways, this is not the right way to go about it, as I have set up the experiment myself, and it certainly looks like it rotates a little quicker than that... - but why is it that this approach doesn't work?

Secondly, it is my understanding that there would also be a magnetic moment due to the electron orbits. Everything I have read has said that the gain in ang. momentum of the rod is due purely to the electron 'spin', rather than the orbital angular momentum of the electrons. I cannot really come up with a reason/explanation for why the orbital angular momentum is not relevant, so any input would be appreciated.

Please, go easy on the newbie :smile:
Cheers,
Jono
 

DrClaude

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Why is it that if you calculate the change in ang. momentum due to every single electron changing its spin, this results in a horrendously small resultant angular velocity of the rod
Error in the calculation?

Secondly, it is my understanding that there would also be a magnetic moment due to the electron orbits. Everything I have read has said that the gain in ang. momentum of the rod is due purely to the electron 'spin', rather than the orbital angular momentum of the electrons.
The effect depends on the total angular momentum of the electron. It would depend on spin only if the orbital angular momentum of the electrons is zero.
 

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