# Magnetic moment of electron

Since magnetic moment come from the circulation of charge, what is the origination of the electron's magnetic moment? Because spin of electron is not the classical spin of particle. Can we say that corresponding to every angular momentum there is a magnetic moment and since spin angular momentum is conformed from Stern-Gerlach experiment so there is a magnetic moment.

Since magnetic moment come from the circulation of charge, what is the origination of the electron's magnetic moment?

It's not true that a magnetic moment always comes from the circulation of charge. An electron's magnetic moment cannot originate entirely from circulation of charge in some internal structure of the electron, because then its g-factor (essentially the ratio of magnetic moment to spin) would then have to equal 1, when in fact it's approximately 2. Similarly, an electron's angular momentum can't come entirely from this type of internal motion, since it would then have to be a whole-number multiple of $\hbar$.

As far as we know, the electron is a pointlike particle without any internal structure such as preons.

Can we say that corresponding to every angular momentum there is a magnetic moment

I don't think so, since photons have spin, but they have no magnetic moment. In general, QED is the theory that predicts $g$ factors, and the $g$ factors are deduced from the more fundamental principles of QED, not just postulated.

• Ben, how the photon does not have magnetic dipole moment? A photon has both a varying electric and a varying magnetic field component. That we can't deflect the photon with an external electric or magnetic field has to do with the frequency of this field components. But both field components exist, aren't they? – HolgerFiedler Jun 26 '16 at 13:36

In classical mechanics, a particle with angular momentum $L$ and a charge $q$ will possess a magnetic dipole given by $$\mu = \frac{q}{2m}L$$ Now, $L$ refers to any type of angular momentum, orbital or rotational. In quantum mechanics, its the same idea - up to an additional constant $g$. So, the intrinsic angular momentum of an electron results in an intrinsic magnetic moment.

Also, the Stern-Gerlach experiment detected the magnetic moment of the electron by observing the deflection of silver atoms in an inhomogeneous magnetic field. The concept of spin can then be introduced to explain the result.

• if spin of electron is not the classical analogy of spin then how we can relate the classical magnetic moment formula with electron spin magnetic moment to describe the spin angular momentum. – Rahul kumar walia Aug 17 '13 at 11:19