# Spin, charge, and magnetic moment

1. Aug 8, 2008

### Monocles

I've read that the properties considered intrinsic to a particle are its mass, spin, and charge. However, unless I am mistaken, both spin and charge can be determined from a particle's magnetic moment. Why are spin and charge considered to be intrinsic instead of magnetic moment then? Sorry if this is a silly question, I'm a physics major but the only physics class I've taken is physics 1.

EDIT: I just realized that you'd be trying to determine two quantities with just one equation. In which case, why isn't it that you can't call any two of those properties to be intrinsic and the third to be the one "left over"? Is it arbitrary?

2. Aug 8, 2008

### ZapperZ

Staff Emeritus
How do you propose to determine the "charge" of a neutron from its "magnetic moment"? What about the "magnetic moment" of a photon? Where do you get that to determine that it has a spin of 1?

Zz.

3. Aug 8, 2008

### Monocles

Whoops, I just looked up the equation again, and I missed the first time that it related magnetic moment, charge, spin, AND mass.

$$\mu = g\frac{q}{2m}S$$

I am under the impression that magnetic moment, charge, and mass are determined experimentally, while spin is found from an eigenvalue problem or something like that. So, you can determine any of the quantities from the other 3, which still leaves the question of why spin, mass, and charge are intrinsic and magnetic moment is not. Because of experimental error getting a integer or half-integer value for spin would have to be fudged, I think.

So, I am saying to determine a particle's magnetic moment experimentally, and use that, along with its mass and spin to determine its charge (for example).

4. Aug 9, 2008

### ZapperZ

Staff Emeritus
It still doesn't explain how you would use that to apply to neutrons and photons (for example).

Zz.

5. Aug 9, 2008

### clem

Magnetic moment is a fourth intrinsic property of a particle.
The four are mostly independent. A particle must have spin to have a magnetic moment,
and there are theoretical relations between spin and magnetic moment for
some particles, where the g in your equation can be determined.

6. Aug 9, 2008

### Monocles

Oh ok, so they are all intrinsic properties? I see now that the g-factor is different from particle to particle. I didn't know that the equation wasn't valid (yet?) for all particles. That makes me wonder though, why does the neutron have magnetic moment if it is electrically neutral? Is it because of the quarks inside of it not being electrically neutral?

Also, that answers my question, clem, if magnetic moment is a fourth intrinsic property of a particle. Thanks!

7. Aug 9, 2008

### humanino

You can use your formula for g only for fundamental particles. Indeed the neutron is not a point particle. The details of how to construct its magnetic moment from the quarks is still under investigations.