# Electron spin and conservation of angular momentum

• JonoF
In summary: Therefore, the orbital angular momentum cannot be neglected in this experiment.In summary, the conversation revolves around the 'Einstein-de Haas' experiment and its relation to electron spin and conservation of angular momentum. The main question is why the calculated change in angular momentum due to electron spin results in a small angular velocity of the rotating rod, and why the orbital angular momentum of the electrons is not taken into consideration. The possibility of an error in the calculation and the importance of considering the total angular momentum of the electron are discussed.
JonoF
Hi there,

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

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 ), 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 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
Cheers,
Jono

JonoF said:
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?

JonoF said:
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.

## 1. What is electron spin?

Electron spin is a fundamental property of an electron, similar to its charge and mass. It refers to the intrinsic angular momentum of an electron, which causes it to behave as if it is spinning on its axis.

## 2. How is electron spin related to angular momentum?

Electron spin is a form of angular momentum and follows the laws of conservation of angular momentum. This means that the total angular momentum of a system remains constant unless an external force acts upon it.

## 3. Why is the conservation of angular momentum important in regards to electron spin?

The conservation of angular momentum is important because it explains the stability of atoms. Without the conservation of angular momentum, electrons would rapidly lose energy and spiral into the nucleus, causing atoms to collapse.

## 4. Can electron spin be changed or manipulated?

Yes, electron spin can be changed or manipulated through various processes such as magnetic fields or interactions with other particles. This allows for applications in fields such as quantum computing and spintronics.

## 5. What are the consequences of violating the conservation of angular momentum in regards to electron spin?

If the conservation of angular momentum is violated, it would lead to a breakdown of our fundamental understanding of the behavior of electrons. It would also have significant implications for the stability of matter and the functioning of various technologies that rely on electron spin.

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