# Conservation of angular momentum and spin

1. Aug 26, 2012

### Khashishi

My understanding is that spin angular momentum is just as real as bulk angular momentum. So, if we get the spin of some electrons in an object to flip, then the object should start spinning in the opposite direction to conserve angular momentum. Right?

If we mount a permanent magnet in an external magnetic field in such a way as to allow the magnet to rotate along the field direction, and allow the magnet to align with the external field, and let the magnet come to rest, then reverse the polarity of the external magnetic field. Then, the magnet should start turning, right?

Or, if we heat up the magnet past the Curie point, we should see some change in bulk rotation, right?

2. Aug 27, 2012

### Simon Bridge

Assuming individual electron spins are coupled to the bulk rotation of the whole magnet.
Certainly, to conserve angular momentum, flipping the spin of an electron does require something else to also flip.

In the experiment described, the action of the external field would be to align the magnetic moments of the susceptible components of the magnet. Having achieved that, releasing the magnet, then reversing the field, does, indeed, result in a rotation of the magnet. That's how you make a compass needle. But I don't think this is what you mean.

Do you have something specific in mind?

3. Aug 27, 2012

### Khashishi

I mean, if we constrain the magnet to rotate only along the z axis. Then apply an external field strong enough to flip the poles of the magnet. Has this experiment been done already?

4. Aug 28, 2012

### Simon Bridge

You mean if we get all the dipoles in the magnet aligned predominantly one way, and set the magnet to pivot only about that axis, then flip the dipoles?

I do not know if that has been attempted.

5. Aug 28, 2012

### Khashishi

Ok, I found the answer. It's called the Einstein-de Haas effect.
What's with this Einstein guy. Frickin everywhere.

Last edited: Aug 28, 2012
6. Aug 28, 2012

### Simon Bridge

Oh well done! I was trawling for just that myself.
The trick is figuring out the right search term.