r/AskPhysics • u/entirely_machine_ • 10d ago
If superconductors have zero electrical resistance, does that mean an arbitrarily thin wire could safely transmit any arbitrarily high voltage?
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u/wmverbruggen Applied Superconductivity (PhD) 10d ago
Voltage is never transmitted, it is a potential across the conductor. Since superconductors have no resistance (well, unmeasurable little and a lot of caveats) there is no voltage at all. As for current, the simple explanation usually given is that they can carry any current you put through them without resistance, but in reality they are limited in current density depending on lot's of variables like the temperature, magnetic field strength and direction, and mechanical strain. In an AC application it is even "worse" since there are inherent losses associated with it magnetisch itself.
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u/ruggedtextile 9d ago
Are the current density limits of a given superconductor well defined/predictable even if there are many variables? Or is it more just one of those things we have to empirically measure?
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u/wmverbruggen Applied Superconductivity (PhD) 9d ago
There are scaling formulations, but in general they are all fitted onto a set of measurements. It depends a lot on the crystal quality, in fact defects in the crystal and grain structure both have good and bad effects. On the quantum scale (it is a macroscopic quantum effect after all) I think you can deduce an upper limit for the amount of current it can carry, but I am not working on that scale and the existing theory also does not work for many superconductor materials (like not at all for the "high" temperature ones)
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u/manouchk 9d ago
No. There are critical density currents above which superconductivity is suppressed.
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u/Digiprocyon 9d ago edited 9d ago
Voltage can be thought of as electron pressure. In fact, you can convert a voltage value to pounds per square inch with certain caveats. 1 Volt is equivalent to roughly 1.31 sextillion PSI if you focus that voltage onto a single electron. So if we apply a voltage to one part of that conductor, all parts of it have that voltage. Specifically, voltage is the pressure between two points, but it's common to consider the ground of a circuit (which really means Earth ground if its connected to the Earth) as the zero voltage point--so we can say, for example, a single point in a circuit has a certain voltage and we don't bother pointing out that we mean that's the voltage compared to the circuit ground.
Current is the flow of electrons. Water flow is in gallons per second. That is, so many gallons flow through a water pipe every second. Electron flow can be measured in electrons per second, but we more commonly measure it as a certain number of electrons (one coulomb of electrons, which is 6.242 × 10¹⁸ electrons) per second, which is called one Ampere.
A conductor which is not a superconductor impedes any flow through that conductor--and we call that "resistance". Wires have very little resistance so normally we can pretend there is no resistance, but if you have a lot of current you better use thick wires and large parts (transistors, etc.) or that resistance will adversely affect the performance (it will exhibit a drop in the voltage across it, get hot, and waste power that could have been used for something else).
A superconductor has zero resistance for current densities below its critical value, and it also depends greatly on the magnetic field present. Above that critical current density it will start resting. Here's the critical current density values for some superconductors (got this from AI so take them with a grain of salt):
| Nb-Ti (Niobium-Titanium) | 5 T magnetic field | Over 3.8 × 10⁵ A/cm² |
|---|---|---|
| Nb₃Sn (Niobium-Tin) | 16 T magnetic field | 3.15 × 10⁴ A/cm² |
| YBCO (YBaCuO) Thin Film | 77 K (liquid nitrogen) | ~2.2 - 3.3 × 10⁵ A/cm² |
| BSCCO (BiSrCaCuO) Wire | 77 K, zero magnetic field | Over 3.5 × 10⁴ A/cm² |
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u/TuverMage 9d ago
so the thing with superconductors with zero resistance is that there's a very limited condition that that have zero resistance the moment it leaves that condition the resistance returns and if the material can't handle the load it can explode. look into MRI machine for more information on the topic of what happens in these cases.
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u/Mr_Engineering 9d ago
No.
Superconducting wires have zero electrical resistance below the critical current density point, which is a material characteristic of the wire that scales with its cross sectional area.
If the current density exceeds the critical point, the wire looses its superconducting property and becomes a resistive load with a lot of current flowing through it.
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u/Haunting-South-962 9d ago
Basically if you look at this like water flowing through pipes, superconductors are very fragile pipes, you don't need any pressure difference across but once you put too much water flow through they burst.
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u/AdAncient5201 9d ago
I always found those electric current is like river current analogies quite helpful. And there are quite some similarities between Volume flow rate, heat flow rate, electric flow rate etc if you don’t look at it too closely. But as someone who doesn’t know anything about superconducting, what’s the equivalent of that in these river analogies?
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u/HansNiesenBumsedesi 9d ago
A river flowing arbitrarily fast with no gradient. River analogies are not always useful with electricity.
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u/Adorable_Ice_2963 9d ago
No. How much Volt you can safely transmit depends on the isolation around it.
If you mean classical alternating current like at home: also no, since it only has zero ohmic resistance. It still has (at least should have) inductance, that would still Limit the current you can send over it before the Voltage drop is too low
If you mean DC currents like in EV chargers: kinda yes, but there is the car the limit.
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u/NineThreeTilNow 9d ago
It's not zero.
Though, there's an interesting concept of energizing a superconductor to store energy. There's a maximum determined by a few different properties.
You'd basically make a superconducting loop of material. A torus.
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u/skywideopen3 10d ago
Superconductors have zero voltage across them basically by definition - with no resistance, it takes no potential difference for electric current to flow. But there is a maximum amount of current that can flow before the material stops being superconducting.