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I don’t understand the physics, but it feels like a truly zero resistance conductor would violate some law of thermodynamics or something. Is a superconductor
by Waterluvian 3y ago
I don’t understand the physics, but it feels like a truly zero resistance conductor would violate some law of thermodynamics or something.
Is a superconductor truly zero impedence or just very very very low? Because I’m seeing a lot of these graphs with something like: “0.00001ohm” as the y-axis floor.
- ramraj07 3y agoMRI machines work using superconducting magnets as does the LHC. So you better believe it buddy.
- Waterluvian 3y agoWhy do MRIs need zero resistance vs incredibly low resistance?
- wbl 3y agoThe more you crank up the field and gradients the better resolution you can get. Being able to take MRI of a beating heart depends on that sort of thing.
- jacquesm 3y agoBecause the resistance is the limiting factor in how strong a magnetic field can be made in a given volume using the conducting material. Copper across an area of 1 cm ^2 does about 300A, compare with https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ for what some superconductors can do.
- robotresearcher 3y agoLink says 90 MA per cm^2, or 30,000X more than copper. Gulp!
- patrec 3y agoI think you are missing a zero.
- robotresearcher 3y agoYou are right! 300,000X more than copper. Typo converting from brain to screen. I should have used exponential notation and the mistake would have been more obvious.
- yreg 3y agoThat's not an answer to that question.
- thelazyone 3y agoI think it's a fair question. I guess the answer is "because we have superconductors, and no material with 'very small resistance' instead". Also note that while there is zero resistance you still have parasitic currents and general interaction with the rest of the environment. So no perpetual motion for us today!
- jacquesm 3y agoEven a 'very small resistance' would likely be prohibitive. Pure silver for instance is an excellent conductor, but when you're talking about the last little bits it's orders of magnitude difference between that and a superconductor, and those orders of magnitude difference in resistance translate into orders of magnitude more current. So even a small resistance would cause your MRI machine to have a resolution so low as to be unusable.
- jychang 3y agoActually, there are infinite orders of magnitude between the resistance of silver and that of a superconductor.
- deleted 3y ago[deleted]
- obrix 3y agoMR imaging is possible with permanent magnets and resistive (copper wire wound) magnets at low field strengths, below 0.35T or so. Above that the heating of the magnet windings becomes excessive and it would be very difficult to maintain a stable enough field strength. Superconducting magnets are very nice as long as there is no quenching. The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular...
- stcredzero 3y agoThe material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular I was going to ask: Why don't we use the current high temperature superconductors, and start building grid interconnects? I guess part of the answer lies in the cost that would be incurred because of the mechanical properties of the existing superconductors.
- userbinator 3y agoTo create very, very strong magnetic fields.
- scythe 3y agoThe current is ridiculously large and would lead to an unacceptably high power consumption and thermal expansion with the resistivity of even a good metal (copper).
- crote 3y agoThey don't! MRI scanners work just fine with regular electromagnets, or even plain old normal magnets. However, work better with stronger magnetic fields. In practice normal magnets end up being extremely large and heavy, and electromagnets end up using massive amounts of power. Both options are also limited in their field strength because getting enough stuff close together is tricky. A superconducting magnet is an electromagnet which is way smaller and uses orders of magnitude less power. For extremely high field strength MRIs they are the only viable option, and for regular MRIs they are often the best option.
- distortionfield 3y agoIt doesn’t though, that’s what’s so tantalizing about it. It’s counterintuitive but as I understand it, the current best theory of superconductor behavior properly conserves mass and energy and the math says a room temperature one should be perfectly possible. It’s just not been found. And yes, it’s truly zero resistance as far as we understand the concept of “resistance”.
- yongjik 3y agoNot an answer, but if you think superconductor is weird, look up superfluid, now that's going to make your head spin.
- LightMachine 3y agowhy there are no good videos of superfluids in action in the whole internet?
- 55555 3y agoThanks for the interesting new thing. If they suffer no loss in kinetic energy and a wave can move forever, could we use a circular pool of superfluid as an an energy storage mechanism?
- n2d4 3y agoMaybe, but it's probably easier to just use a superconductor for the same: https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage https://en.wikipedia.org/wiki/Superconducting_magnetic_energ...
- yongjik 3y agoWell, the whole thing must be at liquid helium temperature, so probably not practical ...
- alex_suzuki 3y ago
- marcosdumay 3y agoThe electrons are always moving in some direction. What a superconductor does is "just" making an electron pull others with it, making a cascade effect that biases the movement permanently. There is no work or state change, so no reason at all to conflict with the 2nd law of thermodynamics.
- deleted 3y ago[deleted]
- klyrs 3y agoKey point, resistance is the real part of impedance. The imaginary part, or inductance and capacitance, are not zero in a superconductor.
- _ph_ 3y agoYes, as others already said, superconductors are zero resistance. Which is easy to check if you have enough of the substance to run meaninful currents through it. There might be miniscule amounts of resistance for contacting and impurities, but the resistance doesn't grow e.g. with wire length. So how is this possible? The explanation is acutally reasonably easy, but requires the strangeness of quantum dynamics. One basic principle of quantum dynamics is, that at least most things are quantisized. Especially energy of a state comes in discrete amounts. That is the reason we have orbitals of electrons in atoms. They can only take very specific values, which creates these separate orbitals. There is no in between state, they have always to absorb or emit exactly the amount of energy which is the difference between orbitals when moving between them. Which is a very easy effect to literally see: take glowing phosphors as you find them on your watch etc. These are transitions bound to a specific photon energy. With red light, you cannot "charge" them, as red photons have to little energy, and you can only absorb single photons. Any green or blue light would work though. And whatever light you used to "charge" them, they always glow in the precise same color, coming from their destinct energy state. The resistance an electron encounters while moving through a conductor is also quantisized. In superconductors we have a situation like trying to charge a watch dial with red light: the amounts of energy an electron could release cannot be absorbed by the material. And an interaction would require this. The consequence is: no interaction, no resistance. The situation is like trying to buy a $1 bottle of water with a $100 bill. That could turn out to be impossible, because no one is willing to give you back $99, and you can of course not pay $100 for the bottle. So even when having the money, you can't buy the bottle. This is, in a very naive way, the principle how superconductivity and superfluidity work. The trick now is to prepare the conditions which allow for superconductivity. One way is to make things increadibly cold. All metals become superconductive, if the temperature is close enough to 0. But that is with single digit degrees or below, even fractions of a Kelvin. Konsequently it was a huge sensation when the first complex substance was presented which showed the effect at larger temperatures. Since then the hunt is up to find better substances.
- Jyaif 3y agoFascinating. Let me get this straight: If you have a piece of phosphor, you can't heat it up with red light, regardless of the amount of red light you shine at it? If so, does the red light bounce off? Go straight through?
- daxfohl 3y agoSimilar to how a magnet works. A magnet can hold something forever, so seems like a violation of thermodynamics. But "holding" isn't work, it's just force. (Which is still kinda spooky, admittedly) Similarly, conducting itself isn't work, it's just conduction.
- huthuthike 3y agoIt's a lot less spooky when you consider that your desk can hold your coffee mug forever. It's exerting force on the mug, but not doing any work.
- flavius29663 3y agoeven more so as the mug literally levitates above the desk, the atoms of the mug are not really touching the atoms of the desk.
- snowwrestler 3y agoThe atoms do touch, by interacting electrically--which is the physical definition of how anything touches anything.
- daxfohl 3y ago(for fermions)
- thehappypm 3y agoAsk yourself this question: Why does resistance occur at all? Why is it that electrons moving through a material cause them to heat up? What is actually happening to cause that? At micro levels, like molecules bouncing around in a gas, collisions are lossless. What makes electrons moving through materials different?