# Static magnetic fields [duplicate]

My textbook says:

All static magnetic fields are produced by moving electric charge.

Is this true? What about the magnetic field produced by a bar magnet?

Of course Emilio Pisanty is right, but let me add a few things.

Originally the word Magnet comes from Magnesia region of Asia Minor. In those times, people found those first stones that repelled or attracted each other.

Now in that case those stones had as per QM electrons in them, and the electrons' and nuclei's magnetic dipole moment and spin were all directed in the same direction, they were aligned. The reason for that is the Earth's magnetic field, and that it took ages for those stones to gather that quality.

Now you can create permanent magnets with EM fields, and in that case the magnetization takes much less time, and in that case the EM field, that has moving electrons in a certain direction will affect the electrons in the magnet and will cause the same thing, those electrons and nuclei in the magnet will align their magnetic dipole moment and spin in a certain direction.

Together those tiny magnets (electrons) will create a bigger magnetic field, their magnetic field's forces will add up.

You are correct - the claim is false. The magnetic field produced by a bar magnet is ultimately (via several layers of coordination) due to the intrinsic magnetic dipole moment of electrons inside the material, and this magnetic dipole moment is directly proportional to the electron spin.

And, as far as we can tell, there really is no useful sense in which electron spin can be seen as 'movement' of any sort, and nor can the magnetic dipole moment it produces be usefully interpreted as 'moving electric charge'.

• Can't really tell they are not moving either, though? Jul 16 '18 at 22:14
• @JMLCarter We can't really prove they're not tiiiiny pink elephants with frizzy electric hair, either. When you say "the magnetic field is caused by moving charge", there's a reasonably wide gamut of possible interpretations as to what it might mean, and all of those are known to have serious flaws that render them incompatible with reality as experimentally observed. Is it possible that there's a super-extra-non-standard model of "moving charge" that will take us all by surprise next month? probably, yes, but I'm not holding my breath. Jul 16 '18 at 22:17
• I'm not competetive - I'm interested. What's the best theory for why quantum spin is associated with a magnetic dipole moment? Jul 16 '18 at 22:21
• There isn't one. As far as we can tell, the magnetic dipole moment is a fundamental property of the particle and there's no way to 'unpack' it. However, that said, given the fact that the electron possesses spin 1/2, in the quantum mechanical sense, and a (vector) magnetic dipole moment, it is a strict theorem (explained here for the electric dipole moment, but the theorems are identical) that the two must be proportional and with a fixed constant. Jul 16 '18 at 22:25
• Wow a 1/2 spin particle rotated $360^o$ becomes a -1/2 spin particle? This must be something other than classical spin. Jul 16 '18 at 22:58