# Definition of spin?

## Homework Statement

The spin is opposite in direction to the magnetic moment of the electron. Is that because the magnetic moment, u is defined as Ia where a is the area vector and spin is defined with the same direction as if it had components around it that swirl around with the angular momentum being (r)x(v) so is always oppsite in direction to the magnetic moment. So the spin of the proton would always have the same sign as its magnetic moment?

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Dox
Hello.

I don't understand what is your question... perhaps you can rearrange it a little bit.

So...

The spin is opposite in direction to the magnetic moment of the electron.
This is an statement.

Is that because the magnetic moment, u is defined as Ia where a is the area vector and spin is defined with the same direction as if it had components around it that swirl around with the angular momentum being (r)x(v) so is always oppsite in direction to the magnetic moment.

So the spin of the proton would always have the same sign as its magnetic moment?
This seems to be a guess of yours, Isn't it?

If you put it clearer, so we can help

Regards.

Meir Achuz
Homework Helper
Gold Member
The electron has negative charge, the proton positive charge.
Mag mom is proportional to charge. Spin isn't.

The key part of my question is

'...spin is defined with the same direction as if it had components around it that swirl around with the angular momentum being (r)x(v) so is always oppsite in direction to the magnetic moment [since as Meir Achuz said, the spin does not take into account the charge].'

The rest is trying to draw a comparison with something that is understandable classically which is the magnetic moment.

So spin is like the area vector of a spinning particle. However in the electron nothing is spinning. But we still give this area vector. This vector is different to the mangetic moment when the charge is negative.

So I have given a go at answering my own question. Is it correct?