B Is Quantum Mechanics necessary for understanding electromagnetic phenomena?

[brianhurren]

TL;DR Summary

link between electron spin and magnetism

Is there a link between electron spin and magnetism? can a magnetic field also be thought of as a 'spin' field. If the eletron spin is linked to the magnetic field then can magnetic repulsion be caused be the pauli exclusion principle?

 

[Nugatory]

TL;DR Summary: link between electron spin and magnetism

Is there a link between electron spin and magnetism?

There is a link between spin and the magnetic moment of a subatomic particle, but if by “magnetism” you mean magnets attracted to iron and steel and all that…. No. That’s all from molecular magnetic moments dominated by the orbital angular momentum of electrons. In general, magnetic fields are caused by moving electrical charges. So…

can a magnetic field also be thought of as a 'spin' field.

No.

If the electron spin is linked to the magnetic field then can magnetic repulsion be caused be the pauli exclusion principle?

It is not. Magnetic repulsion is an example of the Lorentz force, in which charged particles moving in a magnetic field experience a force.

It’s probably best not to try quantum mechanical explanations of electromagnetic phenomena until you have a solid grasp of classical electromagnetism. For this you’ll want a good undergraduate text.

 

[vanhees71]

There is a link between spin and the magnetic moment of a subatomic particle, but if by “magnetism” you mean magnets attracted to iron and steel and all that…. No. That’s all from molecular magnetic moments dominated by the orbital angular momentum of electrons. In general, magnetic fields are caused by moving electrical charges. So…No.It is not. Magnetic repulsion is an example of the Lorentz force, in which charged particles moving in a magnetic field experience a force.

How do you come to this idea? Ferro magnetism in, e.g., iron is a quantum effect ("exchange forces") making the alignment of the spins and thus the magnetic moments of the electrons energetically favorable compared to the unpolarized state, which is an example for the spontaneous breaking of symmetry (rotational symmetry). Empirically that becomes clear by the famous fact that the gyrofactor in the Einstein-de Haas effect is 2 and not 1, as falsely claimed by Einstein and de Haas, before it was corrected by a repetition of the experiment by Barnett:

https://en.wikipedia.org/wiki/Einstein–de_Haas_effect#History

It’s probably best not to try quantum mechanical explanations of electromagnetic phenomena until you have a solid grasp of classical electromagnetism. For this you’ll want a good undergraduate text.

Well, it depends. If it comes to the prediction of material properties (constitutive relations) you cannot do without QT, as this historical example of a theoretical prejudice letting de Haas to ignore measurements indicating the gyrofactor close to 2, which of course in 1916 was unknown since there indeed they thought that magnetism is due to Amperian "molecular circuit currents". The corresponding "currents" due to spin were not known since spin has been discovered only in 1925 (Goudsmit and Uhlenbeck) and the correct gyrofactor of 2 in 1926 (Thomas). Nowadays it's all derived through the Dirac equation + QED radiative corrections (leading to deviations from g=2 for elementary spin-1/2 particles, which are among the best theoretical predictions in comparison to experiment ever).