# Magnetic Moment

## Main Question or Discussion Point

This is a question about proton magnetic moment. Does magnetic moment mean moment of inertia, as in resistance to change? Also it seems like this must be measured on an accelerated proton. Is that so, and if so, how is the proton accelerated? Is it a bare proton, or part of a larger system? (I have no training in physics.)

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blechman
A *classical* magnetic moment is a loop of wire with a current running through it. It has a very well-defined interaction with a magnetic field. It is quite different than moment of inertia.

Protons have spin (a form of angular momentum in quantum mechanics), and as a result, they have a magnetic moment. The proof of this is not hard, but it does require a little training in physics so you can either take my word for it (always a bad idea! ) or go read a book. Feynman's lectures are quite nice, but might be a little advanced. You can try any intro to physics text.

Because the proton has a magnetic moment, it can be measured by passing a proton (ionized Hydrogen) through a magnetic field and watching what happens. This is (roughly) the idea.

Thank you blechman. I believe you about the spin (and textbooks scare me). Passing the proton through the magnetic field, this means the proton is accelerated, right? So it will gain mass?

clem
Thank you blechman. I believe you about the spin (and textbooks scare me). Passing the proton through the magnetic field, this means the proton is accelerated, right? So it will gain mass?
No and no. The proton spin will precess in a magnetic field.

Thank you clem. What does magnetic moment refer to in this precession (going by wikipedia)? I read that it is measured in joules per tesla, I take that to mean joules per weber per square meter, with a weber being a change in magnetic flux. So the strength of the field changes, and that makes the proton wobble? If the proton is “ionized hydrogen”, I take that to mean it is a hydrogen nucleus. How is the proton separated from the electron?

blechman
If the proton is “ionized hydrogen”, I take that to mean it is a hydrogen nucleus. How is the proton separated from the electron?
quite so. it is not hard to ionize hydrogen. just pass it through a strong enough electric field. The binding energy is only 13.6 eV - so passing a hydrogen nucleus through a potential of 13.6 Volts or more will do it (more or less).

Thank you blechman. If a potential difference of 13.6 volts is required to free the electron, do the particles gain some energy when they are free?

Trying to visualize what actually happens, do the protons go one way and the electrons go the other way when the potential is on? Then won’t the particles make magnetic fields around them?

How do you get the proton into a position where it is not accelerated and has no magnetic field except the one from the intrinsic spin (if it has one)? (Sorry for speaking from a place of near-total ignorance.)

blechman