How can current remain in a super-conducting loop without any applied emf? Okay, let's say we have a circular wire loop in which there are no positive nuclei for the electrons to collide to. There are only electrons, no force between electrons, neither can they colide with each other, because there is always a fixed separation between them. Clearly, this wire has zero resistance.
Now, we apply an initial emf which sets a finite current in the wire initially, then we remove the emf source. Now, people explain it by saying that the current can remain forever because of Newton's first law. They can move with a constant velocity forever. But that would be true only for a striaght wire. I have a loop here. So, technically, all the electrons are going in cricles, which is only possible if there is an external emf even if the wire has zero resistance. But my book clearly says that a superconducting loop can have current forever without any applied emf. How?
 A: Superconductivity is a quantum effect. It requires that electrons can travel without interacting with the superconductor. This implies delocalized wavefunctions, in other words that the position of a given electron is unknown and it appears to jump from point A to point B. The classical reasoning that the path from A to B has curves, and so each particle must have been pushed at each curve apex, doesn't apply.
A: Thought experiment:
Imagine the inside of the wire covered in layer of electrons. The flowing electrons that make up the current will at any point have such an electron "wall" beside them. But they don't care - such "wall" will only push (repel) them sideways, so there is no influence on the forward speed.


*

*At every instant, the flowing electrons are pushed just slightly sideways. No change in forward speed, but a lighte sideways motion is added. This turns them slightly.

*At the next instant, when the electron has moved a tiny distance forward, it is again pushed slightly sideways. And turns slightly again.

*etc.


Such continuous sideways push works as it does on a biker in a Wall of Death driving horizontally on the walls. It's the basic principle of circular motion, just in this case with another force pushing against the centre.
The actual reality as to what is pushing and how might be more complicated than a "wall of electrons", but this explains the principle and that is can indeed possible.
A: The angular momentum of a supercurrent in loop is quantized and conserved. An existing angular momentum (once established) can decrease/increase by a quant, and only an external force (EMF) can modify it. So without the external force the supercurrent (as angular momentum) flows forever.
