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Physics
Classical Physics
Electromagnetism
What is the principle of a magnet?
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[QUOTE="vanhees71, post: 6863564, member: 260864"] 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. [/QUOTE]
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Classical Physics
Electromagnetism
What is the principle of a magnet?
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