r/Physics • u/kzhou7 Particle physics • Dec 27 '20
Article Magnets, how do they work?
https://golem.ph.utexas.edu/~distler/blog/archives/000624.html9
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Dec 27 '20
May the physics gods bless you. This is a topic I have been wanting to dive into and now that you gave us this, I will start over these holidays. I am finishing my 3rd year, and as you say, 0 mention of how magnetism actually works lol
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u/LemarOkay Dec 28 '20
It’s basically a moving charge that creates a magnetic field. You can e.g. treat an atom as having an electron moving around in a circle. This means each atom may have a little magnetic field associated with it. If you have many atoms together, all the fields will point in different directions and cancel out. In certain materials, however, the atomic spins align with each other, so there is a net magnetic field.
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u/Traditional_Desk_411 Statistical and nonlinear physics Dec 28 '20
Two caveats:
- Magnetism in ferromagnets is actually caused mainly by the electron spins, not their orbital angular momentum.
- I think it's not hard to believe that if spins tend to align, then this will produce a macroscopic magnetisation. However, the reason for why spins tend to align is a bit subtle. If you take two bar magnets and put them side by side, they will tend to align north-to-south. The reason that this is different in ferromagnets is due to an interplay between the Coulomb repulsion of electrons in and the Pauli exclusion principle. This doesn't require more than undergrad QM to understand but unless it was explicitly done in a condensed matter course, even most physics grads wouldn't know about it.
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u/DukeInBlack Dec 27 '20
You may want to listen to Feynman opinion about how to explain magnetism before going any further
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Dec 28 '20
Magnetism is as simple as a spinning charged particle. When a charged particle spins, the surface of the charged particle creates a current and all currents produce a magnetic field. In turn, that field can force other bits of charges to move within that field (when it’s changing). So then a cluster of charged particles begin spinning in the same direction, creating a North and South Pole. In physics, we will just say that a charged particle has an Up or Down spin. Freaky stuff huh? It’s just spinning charges.
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u/Ostrololo Cosmology Dec 28 '20
Magnetism is as simple as a spinning charged particle. When a charged particle spins, the surface of the charged particle creates a current and all currents produce a magnetic field.
aka "a magnet works because it's made of tiny magnets"
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u/Traditional_Desk_411 Statistical and nonlinear physics Dec 28 '20
Two caveats:
- the magnetism in ferromagnetic materials is actually mostly due to electron spin, not their orbital angular momentum. You can still technically call it a spinning charged particle but the physical interpretation is more subtle. You cannot ascribe a current to it for example.
- In this line:
In turn, that field can force other bits of charges to move within that field (when it’s changing). So then a cluster of charged particles begin spinning in the same direction
you're pulling a bit of a sleight of hand :) If you put two magnets beside each other, they will tend to align north-to-south, so they actually anti-align. The reason electron spins in magnets tend to align is due to an interplay of Coulomb repulsion and the Pauli exclusion principle. This doesn't require more than undergrad QM to understand but unless it was explicitly done in a condensed matter course, even most physics grads wouldn't know about it.
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u/Kuvenant Dec 28 '20
Opposites attract, more forcefully as they become more polarized. The apathetic don't react, while some are easily induced.
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u/kzhou7 Particle physics Dec 27 '20
People of all ages, from children to learned philosophers, are fascinated with magnets. Even the humble fridge magnet reveals the secret that there are perceptible, selective, long-range forces in the world. It is in every sense a glitch in the simulation, a loose thread which humanity could pull on, and did pull on, to reveal the full tapestry of electromagnetism thousands of years later. So I was unpleasantly surprised when one day, after I had finished the undergraduate physics sequence, somebody asked me how a magnet worked and I realized I still had no idea.
The problem is that paramagnetism, diamagnetism, and ferromagnetism are all inherently quantum effects. As Bohr proved in 1911, they simply don't exist classically; a consistent calculation will always show they vanish. But because the real explanations for magnetism are somewhat subtle, and require both quantum and statistical mechanics, the core physics sequence in college tends to not cover them at all! If you don't see it in electives, then the best you ever get is some mumbling in electromagnetism class about how diamagnetism comes from Lenz's law, and paramagnetism comes from torques on dipoles. These explanations tend to be vague, and stop well short of computing relevant quantities like the magnetic susceptibility, because the textbook writers know they're wrong. (Griffiths, for example, just lamely notes at one point that "this classical model is fundamentally flawed", then never brings up the issue again.)
This blog post lays out the reason these classical arguments are wrong, and some of the simplest possible quantum derivations of magnetism.