The Nonlinear Library LW - on neodymium magnets by bhauth
Jan 31, 2024
06:50
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: on neodymium magnets, published by bhauth on January 31, 2024 on LessWrong.
Neodymium magnets are the main type used in modern motors. Why are they good? Are there any good alternatives?
review of ferromagnetism
Magnetic fields contain energy. In an inductor, that energy comes from an increase in voltage when current is first applied. When a magnetic core is added to an inductor and a stronger field is produced from the same added energy, that extra energy has to come from somewhere.
The energy that ferromagnetic cores add to magnetic fields comes from their crystal structure fitting together better in a magnetic field. This implies that ferromagnetic cores should spontaneously magnetize to some extent, and they actually do; it's just that the spontaneously generated magnetic fields are curled into microscopic 3d loops.
The microscopic internal field strength is approximately the saturation field of a ferromagnetic material, which is often greater than the field generated by a Nd magnet. Applying an external magnetic field causes those microscopic magnetic loops to partly unroll.
The actual field is generated by unpaired electrons of atoms; individual electrons are very magnetic. But ferromagnetism isn't a property of atoms, it's a property of crystals; without particular crystal structures that favor magnetic fields, those unpaired electron spins of iron atoms would just cancel out. For example, stainless steels contain a lot of iron, but most aren't ferromagnetic.
Atoms of crystals fitting together better in a magnetic field implies that iron cores slightly change shape when a magnetic field is applied. This effect is responsible for the humming noise transformers make, and has been used for eg sonar.
common misconceptions
Fucking magnets, how do they work? And I don't wanna talk to a scientist. Y'all motherfuckers lying, and getting me pissed.
Insane Clown Posse
The Insane Clown Posse is sort of right there: a lot of explanations of magnets given to people by teachers and media scientist-figures have been partly wrong.
Magnetic flux was originally thought to be a flow of something like electric current, with ferromagnetic materials having lower resistance for that flow than air. It's even still taught that way sometimes. But no, it's a complex emergent phenomenon.
I remember being taught that "iron is magnetic because it has an unpaired electron". But again, ferromagnetism is a property of crystal structures, not atoms or elements.
A lot of people think the magnetism of neodymium magnets comes from the neodymium, but the actual magnetism comes from the iron in them.
The title of the quoted song is "Miracles". The physical constants that allow for the complex emergent phenomenon of ferromagnetism are the same physical constants that allow for the complex emergent phenomenon of life; most values of them wouldn't do either. The universe having values allowing for those is indeed a miracle that nobody really knows the reason for; thanks for reminding us of that, ICP.
neodymium magnets
In permanent magnets, the crystal structure is such that the magnetic fields of crystals can't rotate around to form closed loops very well.
Neodymium magnets (Nd2Fe14B, "Nd magnets") are the strongest permanent magnets currently available. Looking at the crystal structure we can see rings of iron atoms with Nd in the middle and some boron at the 3-way vertices. When a magnetic field is applied through that (tetragonal) pattern, the atoms fit together better. You can see how the magnetic field would be unable to smoothly rotate through directions.
Strong Nd magnets are made by cooling inside a strong magnetic field, so that the crystal structures are aligned in one direction.
alternatives
An obvious idea is using the same structure but replacing the neodymium with a different element. Th...
Neodymium magnets are the main type used in modern motors. Why are they good? Are there any good alternatives?
review of ferromagnetism
Magnetic fields contain energy. In an inductor, that energy comes from an increase in voltage when current is first applied. When a magnetic core is added to an inductor and a stronger field is produced from the same added energy, that extra energy has to come from somewhere.
The energy that ferromagnetic cores add to magnetic fields comes from their crystal structure fitting together better in a magnetic field. This implies that ferromagnetic cores should spontaneously magnetize to some extent, and they actually do; it's just that the spontaneously generated magnetic fields are curled into microscopic 3d loops.
The microscopic internal field strength is approximately the saturation field of a ferromagnetic material, which is often greater than the field generated by a Nd magnet. Applying an external magnetic field causes those microscopic magnetic loops to partly unroll.
The actual field is generated by unpaired electrons of atoms; individual electrons are very magnetic. But ferromagnetism isn't a property of atoms, it's a property of crystals; without particular crystal structures that favor magnetic fields, those unpaired electron spins of iron atoms would just cancel out. For example, stainless steels contain a lot of iron, but most aren't ferromagnetic.
Atoms of crystals fitting together better in a magnetic field implies that iron cores slightly change shape when a magnetic field is applied. This effect is responsible for the humming noise transformers make, and has been used for eg sonar.
common misconceptions
Fucking magnets, how do they work? And I don't wanna talk to a scientist. Y'all motherfuckers lying, and getting me pissed.
Insane Clown Posse
The Insane Clown Posse is sort of right there: a lot of explanations of magnets given to people by teachers and media scientist-figures have been partly wrong.
Magnetic flux was originally thought to be a flow of something like electric current, with ferromagnetic materials having lower resistance for that flow than air. It's even still taught that way sometimes. But no, it's a complex emergent phenomenon.
I remember being taught that "iron is magnetic because it has an unpaired electron". But again, ferromagnetism is a property of crystal structures, not atoms or elements.
A lot of people think the magnetism of neodymium magnets comes from the neodymium, but the actual magnetism comes from the iron in them.
The title of the quoted song is "Miracles". The physical constants that allow for the complex emergent phenomenon of ferromagnetism are the same physical constants that allow for the complex emergent phenomenon of life; most values of them wouldn't do either. The universe having values allowing for those is indeed a miracle that nobody really knows the reason for; thanks for reminding us of that, ICP.
neodymium magnets
In permanent magnets, the crystal structure is such that the magnetic fields of crystals can't rotate around to form closed loops very well.
Neodymium magnets (Nd2Fe14B, "Nd magnets") are the strongest permanent magnets currently available. Looking at the crystal structure we can see rings of iron atoms with Nd in the middle and some boron at the 3-way vertices. When a magnetic field is applied through that (tetragonal) pattern, the atoms fit together better. You can see how the magnetic field would be unable to smoothly rotate through directions.
Strong Nd magnets are made by cooling inside a strong magnetic field, so that the crystal structures are aligned in one direction.
alternatives
An obvious idea is using the same structure but replacing the neodymium with a different element. Th...