The discussion revolves around the behavior of current in a superconducting loop, particularly focusing on whether a current pulse generated in such a loop can persist indefinitely without diminishing. Participants explore theoretical implications, the effects of radiation, and the nature of superconductivity.
Participants express multiple competing views regarding the persistence of current in superconducting loops and the effects of radiation. The discussion remains unresolved, with no consensus reached on the implications of classical versus quantum mechanics in this context.
Participants note limitations in their arguments, including assumptions about the nature of superconductivity, the definitions of radiation in different contexts, and the specific conditions under which currents can be induced in superconducting materials.
ashokanand_n said:The superconductor itself is in the form of a ring and if we throw a magnet through the center of this ring current should inevitably be formed in the ring right??
I am sorry for framing the question that way. But the point in the question was not that at all. If there is a dc current in a circular superconducting ring, will it die out due to radiation effects or will it persist forever??
hello ashokakand-ashokanand_n said:The superconductor itself is in the form of a ring and if we throw a magnet through the center of this ring current should inevitably be formed in the ring right??
Vanadium 50 said:You don't have radiation from the ring. To get radiation you need a changing moment, and the ring's moments are all constant. For a perfect circle, you have a magnetic dipole moment that's non-zero and all the other moments are zero.
Yes, this is another example of how classical physics is wrong. In a similar manner classically electrons orbiting a nucleus should continuously radiate thereby losing energy and falling into the nucleus.ashokanand_n said:The electrons moving in the Ring are not moving in a straight line (on an average). In other words they are accelerating. Any accelerating charge should radiate em field. This is a standard result in classical electrodynamics.
They do. This is called synchrotron radiation. A 10 GeV electron in a 1 Tesla field radiates x-rays. But the radiation energy varies as gamma-cubed. Seeashokanand_n said:The electrons moving in the Ring are not moving in a straight line (on an average). In other words they are accelerating. Any accelerating charge should radiate em field. This is a standard result in classical electrodynamics. So invariably these electrons also should radiate em energy. Shouldn't they?.
Bob S said:They do[/url]
I agree that you can get a current in a superconducting ring, if you use a heat switch. If you try to induce a current in a superconducting ring without a heatswitch (e.g., by dB/dt), the current is actually on the surface of the superconductor. If you can induce 10-GeV electrons in a superconducting ring (see my previous post) they will radiate.f95toli said:No, they don't. Read Dalespams reply.
The reason is that we are not talking about normal electrons here; some of the properties of a condenstate in a superconducting ring are very different from what you would expect from a system of normal electrons. The fact that that normal electrons radiate when accelerated in a ring is irrelevant.
Also, introducing a current in a superconducting ring is not very difficult; all you need is some form of inductive coupling. You only need a heatswitch if you want do e.g. drive a solenoid from a dc-source.
Bob S said:I agree that you can get a current in a superconducting ring, if you use a heat switch. If you try to induce a current in a superconducting ring without a heatswitch (e.g., by dB/dt), the current is actually on the surface of the superconductor. If you can induce 10-GeV electrons in a superconducting ring (see my previous post) they will radiate.
Very nice argument. I am going to shamelessly steal it whenever this question crops up again.Vanadium 50 said:I've got to disagree with a lot of people here.
What is the radiation from a purely classical charged ring spinning around its axis? I maintain that it is zero.
Radiation requires a changing multipole moment. This system has exactly one non-zero moment, the magnetic dipole moment, and it's static.
The argument that a single accelerating charge radiates neglects a very important fact: this is not a single accelerating charge. Suppose I had a rotating plate, and I place a charge at 0 degrees. This plate radiates. Now, I put another charge at 180, and now I no longer have any electric dipole radiation. I do still have quadrupole radiation, so I place charges at 90 and 270. Now the highest surviving moment is electric octopole, which can be zeroed out with charges every 45 degrees. In the limit of a continuous ring of charge, there is no radiation: radiation from any point on the ring is canceled by radiation from other points on the ring.
This has nothing to do with superconductivity. It's true if you simply glued charges to a ring. It's all in Chapter 9 of Jackson.
Now, you might ask, "yes, but we know charges aren't continuous: they're discrete - doesn't that wreck your argument?" Yes, but they are really, really close to discrete: you have millions of electrons, so it all cancels except for the million-pole terms, and those radiate very, very little power. Possibly the age of the universe per photon.
Vanadium 50 said:I've got to disagree with a lot of people here.
What is the radiation from a purely classical charged ring spinning around its axis? I maintain that it is zero.
Radiation requires a changing multipole moment. This system has exactly one non-zero moment, the magnetic dipole moment, and it's static.
The argument that a single accelerating charge radiates neglects a very important fact: this is not a single accelerating charge. Suppose I had a rotating plate, and I place a charge at 0 degrees. This plate radiates. Now, I put another charge at 180, and now I no longer have any electric dipole radiation. I do still have quadrupole radiation, so I place charges at 90 and 270. Now the highest surviving moment is electric octopole, which can be zeroed out with charges every 45 degrees. In the limit of a continuous ring of charge, there is no radiation: radiation from any point on the ring is canceled by radiation from other points on the ring.
This has nothing to do with superconductivity. It's true if you simply glued charges to a ring. It's all in Chapter 9 of Jackson.
Now, you might ask, "yes, but we know charges aren't continuous: they're discrete - doesn't that wreck your argument?" Yes, but they are really, really close to discrete: you have millions of electrons, so it all cancels except for the million-pole terms, and those radiate very, very little power. Possibly the age of the universe per photon.
To be specific, you don't need to know any properties of superconductors to understand why the loop does not radiate. Even a loop of resistive current will not radiate (other than black body, of course).Vanadium 50 said:Calculate the multipole moments. Do they change with time? Then you have radiation. Simple as that.
Again, note that this is a purely classical result. You don't need to know any properties of superconductors.
DaleSpam said:To be specific, you don't need to know any properties of superconductors to understand why the loop does not radiate. Even a loop of resistive current will not radiate (other than black body, of course).
DaleSpam said:You still need to know the QM properties of superconductors to explain superconductivity itself.
True. An ideal dc current will not radiate. Except dc currents are comprised of real quantized electrons, and dc currents therefore have shot (Schottky) noise due to the randomicity in the time interval between them. So a dc loop can radiate some power at all harmonics of the revolution frequency. Furthermore, in some circumstances loops of circulating electrons will self-bunch and radiate more, such as in the presence of passive structures, like the vanes of magnetrons.DaleSpam said:To be specific, you don't need to know any properties of superconductors to understand why the loop does not radiate. Even a loop of resistive current will not radiate (other than black body, of course).