Superconductive materials - zero resistance or merely near-zero

[Keeve]

Superconductive materials supposedly have zero electrical resistance. In the things I’ve read, they seem to really mean this - that the resistance is truly zero, not merely something extremely close to zero. For example, the Wikipedia article states, near the beginning, that

An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.

Sure sounds to me like a perpetual motion machine of the third kind. I’d like to take these articles literally, but I’d feel a lot more comfortable about it if they would say something like “truly zero, not merely an infinitesimal amount”. Which is it?

[BrandonR]

I’ve always understood that everything has some amount of electrical conductivity given the right conditions. There are no such things as perfect insulators or perfect conductors – they’re idealizations. Hell, even inert air becomes a conductor when conditions create lightning.

[crazyjoe]

“Indefinitely” just means that we can’t accurately predict it, but it’s a long-ass time. So, it is probably going to peter out at some point, but it’s not a predictable formula.

[DrCube]

From the OP’s link:

Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe, depending on the wire geometry and the temperature. Thus, a superconductor does not have exactly zero resistance, however, the resistance is negligibly small.

Emphasis mine.

[SenorBeef]

Perpetual motion machines have to do some work. That’s the machine part. If the electricity just floats around not actually doing anything, that’s not a machine.

Edit: I guess that’s accounted for in the “of the third kind” part. Although the article does specifically refer to friction and mechanical systems.

[DrCube]

@SenorBeef

Perpetual motion machines have to do some work. That’s the machine part. If the electricity just floats around not actually doing anything, that’s not a machine.

Edit: I guess that’s accounted for in the “of the third kind” part. Although the article does specifically refer to friction and mechanical systems.

Yeah, I’ve never even heard of perpetual motion machines of the third kind. But when I was a kid, I always wondered why the planets could orbit the sun indefinitely without breaking some sort of “perpetual motion” law. It seems like “the third kind” of perpetual motion machine is at least theoretically possible, unlike the other two, so I guess that’s why it isn’t a big deal that superconductivity approaches this ideal. Or rather, that’s why it is a big deal.

[acetylene]

I think that a superfluid helium fountain may be an example of perpetual motion of the third kind. According to this video, a superfluid has zero viscosity and can produce a frictionless fountain. The fountain can be seen at 1:30 of the video.

So it may flow forever, but it can’t do any work. Also, you have to keep it rather cold: around 3 Kelvin, so about -270 Celcius or -454 Fahrenheit.

I know it’s slightly OT but it’s still pretty cool.

[beowulff]

@acetylene

I think that a superfluid helium fountain may be an example of perpetual motion of the third kind. According to this video, a superfluid has zero viscosity and can produce a frictionless fountain. The fountain can be seen at 1:30 of the video.

So it may flow forever, but it can’t do any work. Also, you have to keep it rather cold: around 3 Kelvin, so about -270 Celcius or -454 Fahrenheit.

I know it’s slightly OT but it’s still pretty cool.

As I recall, the fountain requires external energy input, in the form of light.

[Half_Man_Half_Wit]

@DrCube

Yeah, I’ve never even heard of perpetual motion machines of the third kind. But when I was a kid, I always wondered why the planets could orbit the sun indefinitely without breaking some sort of “perpetual motion” law.

Just as a note, they don’t orbit perpetually – even in the absence of friction and tidal effects, a system of two orbiting masses will radiate energy in the form of gravitational waves, though only at a very very small rate (about 200 watts in the case of the Sun-Earth system, according to wiki).

[acetylene]

@beowulff

As I recall, the fountain requires external energy input, in the form of light.

But does the light need to be shined (shone?) on it continuously, or just long enough to get it started. If so, then you’re right, it is not perpetual motion. If not, then I’m right.

Either way, it’s pretty cool.

[Exapno_Mapcase]

@Keeve

Superconductive materials supposedly have zero electrical resistance. In the things I’ve read, they seem to really mean this - that the resistance is truly zero, not merely something extremely close to zero. For example, the Wikipedia article states, near the beginning, that
Sure sounds to me like a perpetual motion machine of the third kind. I’d like to take these articles literally, but I’d feel a lot more comfortable about it if they would say something like “truly zero, not merely an infinitesimal amount”. Which is it?

In that same Wiki article,

Although it is impossible to make such a machine,[3][4] as dissipation can never be 100% eliminated in a mechanical system, it is nevertheless possible to get very close to this ideal (see examples in the Low Friction section). Even if such a machine could be built, it would not serve as an endless source of energy, since the amount of available energy is still finite: if we could build a frictionless flywheel, it would eventually slow down and stop if its kinetic energy were tapped for useful work, and we would get no more energy out than the amount that was initially put in to spin up the flywheel.

Low Friction
In certain quantum-mechanical systems (such as superfluidity and superconductivity), dissipation-free “motion” is possible.

Superconductivity therefore is classified as a third kind and it makes no difference. No work can be done without tapping the energy and that would quickly deplete the system. No paradoxes were harmed in the making of this example.

[Small_Clanger]

Can I complicate this a bit? My understanding is that in a superconducting system there is no way for the conducting medium to lose the energy it has (because of Quantum), like the liquid Helium doodad it has no lower state to move into, it has no option but to keep the current flowing.

Is this wrong? Or is this correct for flowing liquid Helium (I remember an experiment using a little spinning ‘bucket’ of Helium) but not for superconducting which is all to do (IIRC) with electron pairing weirdness and could behave completely differently?

[Chronos]

Although there is energy loss in superconductors, it is not resistive energy loss. For instance, if your superconductor forms a closed loop (as it must, if you expect to keep current flowing in it indefinitely), then you’ll get some electromagnetic radiation produced from the charges moving along a curved path. You can mitigate this by making the loop very large and the curvature therefore very small, but of course you can’t make a loop of infinite size, so you’ll always lose some energy this way.