![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
It was amazing. He explained superconductivity and there were no complicated formulas and it made sense. I shall explain but first, some review.
Electrons are negatively charged. Thus, they repel.
Atomic nuclei are positively charged. Thus, they attract electrons.
In most conducting materials, the electrons aren't bound to any particular atom. Instead, they float there waffling indecisively between all the fine, fine nuclei hanging around. This decentralization of electrons is what allows them to drop whatever they are doing and form a current.
In a current, electrons don't move in lines. They move kind of in zigzags, bouncing from one atomic nucleus to the next, but overall heading in one direction.
Because they are not moving in lines, electrons in a current collide. A lot. With atomic nuclei, and other electrons. These collisions are what causes the wire to get hot when current is moving through it.
In conducting materials, electrons aren't in bonds holding the metal nuclei in a rigid crystal structure, so these atoms jiggle and spin. What this means is that they, or the electrons in the orbital (the orbitals rotate when the nucleus spins) tend to get in the way of moving electrons. See above about collisions and heat.
Heat is way to measure average molecular motion. Atoms are not moving as much when they are cooler, and move more when they are hot. If you extend this down, you get to absolute zero, the temperature at which (theoretically) all molecular motion stops and a pure crystal is in a perfect lattice. This is a physical impossibility, but we can get to within a billionth of a billionth of a degree of it.
Okay, review time done.
Pretend you have a metal that is a conductor; as an example, aluminum. At normal temperatures, aluminum is a solid, and behaves as described above. It has lots of electrons floating around. In an electric field, the electrons will move and you'll get a current, of the type that runs light bulbs. The atomic nuclei are vibrating and spinning, because apparently they are high.
Cool it down.
Keep cooling it down.
Don't stop.
Okay. Now you are at 1.2K. This is 1.2 degrees Centigrade above absolute zero. It's colder than liquid helium (4K) and it's colder than space (3K). It's nearing the point when the atoms stop moving an align into a perfect crystal. But it's not quite there yet. So instead of ceasing to move entirely, the atoms slow way down. They quit vibrating and spinning quite so much, and interesting things happen.
The electrons still hate each other, but because the aluminum nuclei aren't moving as much, they have to settle down into proper orbitals. This means that the orbitals all line up into planes so the electrons can get as far from each other as possible. The crystal becomes more rigid, so the distance between the not-spinning, not-jiggling nuclei becomes very nearly constant. And so on.
Now, pretend that in this aluminum wire, you have exactly as many electrons as protons; the net charge is 0. Drop one electron in. It's going to head toward the nearest nucleus, and be repelled by the nearby electrons. But the nucleus will also be attracted to it. In normal temperatures, in fact, this helps dictate the way the nuclei vibrate. At really cold temperatures, it can't move that easily because that would break the nice grid it has going with all the other nuclei. It moves as much as it can, though.
Drop in another electron. It's going to head to the nucleus, which is in turn heading to extra electron number one. Therefore, even though the electrons hate each other, they look as though one is following the other, although it's such a small difference that they might as well be moving in unison. This is called Cooper pair, and the electron-nucleus-electron interaction is called electron-phonon-electron interaction (phonon here meaning the vibration frequency of the aluminum, a property that has to do with how far the nucleus may move).
Another thing happens. The Cooper pair is repelled by the now-regular and lined-up orbitals, so although it moves, it moves in the spaces between orbitals. This means that the pair does not ever hit anything. There is no apparent resistance to the electrons moving. The wire doesn't get hot. The wire has, in fact, become a superconductor. If you heat it up again, it the lattice will fall apart and it will stop with the superconductivity; but below the critical temperature of 1.2K, it works.
Superconductivity was first observed way back in 1911. Over the next sixty years, other metals and alloys were found that superconduct at temperatures as high as [gasp] 20K. (It was a big deal at the time.) And then someone found a superconductor that works at 36K, the best they'd gotten yet. Further advances have resulted in superconductors that work at 150K, or about -120C This is important because liquid nitrogen is at 77K; you can cool the material down to its critical temperature fairly easily, because liquid nitrogen is (comparatively) cheap. And so on.
Today in lab we watched a superconductor levitate a magnet. They do this because, for reasons I'm not going to explain, they generate little magnetic poles of exact equal strength and polarity but opposite direction to any magnet around them. It's like putting two North ends together. But unlike regular magnets, ,you can poke, prod, move, and even spin the levitated magnet - and the superconductor will adjust to keep the magnet floating, without them ever flipping and snapping together. This is how mag-lev trains work.
Yes, I did play with liquid nitrogen in lab today :D
Electrons are negatively charged. Thus, they repel.
Atomic nuclei are positively charged. Thus, they attract electrons.
In most conducting materials, the electrons aren't bound to any particular atom. Instead, they float there waffling indecisively between all the fine, fine nuclei hanging around. This decentralization of electrons is what allows them to drop whatever they are doing and form a current.
In a current, electrons don't move in lines. They move kind of in zigzags, bouncing from one atomic nucleus to the next, but overall heading in one direction.
Because they are not moving in lines, electrons in a current collide. A lot. With atomic nuclei, and other electrons. These collisions are what causes the wire to get hot when current is moving through it.
In conducting materials, electrons aren't in bonds holding the metal nuclei in a rigid crystal structure, so these atoms jiggle and spin. What this means is that they, or the electrons in the orbital (the orbitals rotate when the nucleus spins) tend to get in the way of moving electrons. See above about collisions and heat.
Heat is way to measure average molecular motion. Atoms are not moving as much when they are cooler, and move more when they are hot. If you extend this down, you get to absolute zero, the temperature at which (theoretically) all molecular motion stops and a pure crystal is in a perfect lattice. This is a physical impossibility, but we can get to within a billionth of a billionth of a degree of it.
Okay, review time done.
Pretend you have a metal that is a conductor; as an example, aluminum. At normal temperatures, aluminum is a solid, and behaves as described above. It has lots of electrons floating around. In an electric field, the electrons will move and you'll get a current, of the type that runs light bulbs. The atomic nuclei are vibrating and spinning, because apparently they are high.
Cool it down.
Keep cooling it down.
Don't stop.
Okay. Now you are at 1.2K. This is 1.2 degrees Centigrade above absolute zero. It's colder than liquid helium (4K) and it's colder than space (3K). It's nearing the point when the atoms stop moving an align into a perfect crystal. But it's not quite there yet. So instead of ceasing to move entirely, the atoms slow way down. They quit vibrating and spinning quite so much, and interesting things happen.
The electrons still hate each other, but because the aluminum nuclei aren't moving as much, they have to settle down into proper orbitals. This means that the orbitals all line up into planes so the electrons can get as far from each other as possible. The crystal becomes more rigid, so the distance between the not-spinning, not-jiggling nuclei becomes very nearly constant. And so on.
Now, pretend that in this aluminum wire, you have exactly as many electrons as protons; the net charge is 0. Drop one electron in. It's going to head toward the nearest nucleus, and be repelled by the nearby electrons. But the nucleus will also be attracted to it. In normal temperatures, in fact, this helps dictate the way the nuclei vibrate. At really cold temperatures, it can't move that easily because that would break the nice grid it has going with all the other nuclei. It moves as much as it can, though.
Drop in another electron. It's going to head to the nucleus, which is in turn heading to extra electron number one. Therefore, even though the electrons hate each other, they look as though one is following the other, although it's such a small difference that they might as well be moving in unison. This is called Cooper pair, and the electron-nucleus-electron interaction is called electron-phonon-electron interaction (phonon here meaning the vibration frequency of the aluminum, a property that has to do with how far the nucleus may move).
Another thing happens. The Cooper pair is repelled by the now-regular and lined-up orbitals, so although it moves, it moves in the spaces between orbitals. This means that the pair does not ever hit anything. There is no apparent resistance to the electrons moving. The wire doesn't get hot. The wire has, in fact, become a superconductor. If you heat it up again, it the lattice will fall apart and it will stop with the superconductivity; but below the critical temperature of 1.2K, it works.
Superconductivity was first observed way back in 1911. Over the next sixty years, other metals and alloys were found that superconduct at temperatures as high as [gasp] 20K. (It was a big deal at the time.) And then someone found a superconductor that works at 36K, the best they'd gotten yet. Further advances have resulted in superconductors that work at 150K, or about -120C This is important because liquid nitrogen is at 77K; you can cool the material down to its critical temperature fairly easily, because liquid nitrogen is (comparatively) cheap. And so on.
Today in lab we watched a superconductor levitate a magnet. They do this because, for reasons I'm not going to explain, they generate little magnetic poles of exact equal strength and polarity but opposite direction to any magnet around them. It's like putting two North ends together. But unlike regular magnets, ,you can poke, prod, move, and even spin the levitated magnet - and the superconductor will adjust to keep the magnet floating, without them ever flipping and snapping together. This is how mag-lev trains work.
Yes, I did play with liquid nitrogen in lab today :D
