B What is the principle of a magnet?

[tomlib]

I cannot explain that if I divide a magnet with a north and a south pole into 2 parts, the force in it rearranges itself into 2 poles. Where is the center of power? How can I explain the principle of magnetic force if I don't take it as a matter of fact.

 

[PeroK]

There are no magnetic monopoles. Instead, magnetism comes from loops of current within a material that all have the same orientation. When you divide a magnet, you simply end up with two smaller magnets.

 

[tech99]

You can also build a long magnet using a row of shorter magnets, quite instructional to do this as the reverse process.

 

[Vanadium 50]

Where is the center of power?

What is the center of power?

 

[Drakkith]

I cannot explain that if I divide a magnet with a north and a south pole into 2 parts, the force in it rearranges itself into 2 poles. Where is the center of power?

For permanent magnets, each atom is itself a small magnet and the sum of the magnetic fields from all of the atoms adds up to give you the macroscopic magnetic field. So dividing a magnet in two creates two parts that each have half the number of tiny magnets, giving you two weaker macroscopic magnets.

 

[Meir Achuz]

There are no magnetic monopoles. Instead, magnetism comes from loops of current within a material that all have the same orientation. When you divide a magnet, you simply end up with two smaller magnets.

The magnetism of a magnet comes from the magnetic dipole moments of the electron's spin, and not from current loops.

 

[Hyperfine]

The magnetism of a magnet comes from the magnetic dipole moments of the electron's spin, and not from current loops.

Do not neglect the contribution from the orbital angular momentum of the electron.

See for example: Electron Orbit Magnetic Moment posted at Hyperphysics.

 

[vanhees71]

This question can in fact not be answered at B-level, because ferro-magnetism is a quantum phenomenon. For normal metallic ferromagnets it's indeed the alignment of the electron spins, which can be measured by, e.g., the Einstein-de Haas effect. Ironically de Haas through away his result that the gyro-ratio was not unanimously 1, but Einstein of course thought that it must be 1 by assuming classical "Amperian ring currents" to be the source of the magnetic field, and indeed under that assumption you get a gyro-ratio of 1, but the modern quantum theory of spin-1/2, including the minimal-coupling assumption of the electromagnetic interaction when "gauging" the Dirac equation's symmetry under multiplication with a phase factor, leads to a gyro-ratio that is close to 2 (with the deviation from 2, the socalled anomalous magnetic moment, being due to radiative corrections, leading to one of the most accurately tested predictions of QED).

 

[Meir Achuz]

Do not neglect the contribution from the orbital angular momentum of the electron.

See for example: Electron Orbit Magnetic Moment posted at Hyperphysics.

Iron has 26 electrons, and the orbital magnetic moments of the electrons cancel.

 

[Hyperfine]

The ground state electron configuration of ⁵⁶Fe I is ... 3d⁶4s² and the term symbol is ⁵D₄. That tells me that L=2. Data taken from the NIST Atomic Spectra Database.

And I see nowhere that restricted the discussion to iron.

 

[malawi_glenn]

https://phys.org/news/2014-05-exploring-magnetism-atom.html
In an atom, magnetism arises from the spin and orbital momentum of its electrons.

 

[martinbn]

I cannot explain that if I divide a magnet with a north and a south pole into 2 parts, the force in it rearranges itself into 2 poles. Where is the center of power? How can I explain the principle of magnetic force if I don't take it as a matter of fact.

If you have a horizontal stick, so that one end is the left end and the other the right end. If you brake it in two. The smaller sticks will also have a left end and a right end. How do you explain that? Where is the centre of leftness?

 

[Meir Achuz]

The ground state electron configuration of ⁵⁶Fe I is ... 3d⁶4s² and the term symbol is ⁵D₄. That tells me that L=2. Data taken from the NIST Atomic Spectra Database.

And I see nowhere that restricted the discussion to iron.

Yes, I was thinking of pure iron, where the spin dominates.

 

[nasu]

How can I explain the principle of magnetic force if I don't take it as a matter of fact.

Why dismiss the matter of fact part? If magnetism were not a fact nobody would have bothered to try to make a model of it. There would had been to "it" to start with.

 

[vanhees71]

Ferromagnetism can fully understood only within quantum theory. The first thing is that electrons not only are point charges but also carry a spin and with it also a magnetic moment, which makes it to a microscopic dipole magnet. In a ferromagnetic material it is energetically advantageous that the spins sponaneously allign in one direction, such that the entire piece of matter has a magnetic moment, i.e., it becomes a permanent magnet. This also explains, why you can't cut a magnet in the middle to get a single north and a single south pole, but the spins of the electrons in both pieces still align in one direction, and you get two new magnets with a noth and south pole. Macroscopically that's described by one of Maxwell's fundamental equations that describes all electromagnetic phenomena: $\vec{\nabla} \cdot \vec{B}=0$, which says that there are no magnetic monopoles, i.e., the magnetic field lines all form closed loops, and the magnetic field's multipole expansion starts with the dipole term.