I’ve been reading about superconductivity - it really is fascinating. And I think this is the clearest page for the beginner: Home | CERN Teacher Programmes
So I think I understand why/how superconductors can allow an electrical current to flow through them without resistance (I’m sure it’s more complicated than stated though). But there are some things I’m not clear on.
Firstly, how do scientists know that two electrons follow each other in the explanation given? Has it been deduced or has it been directly observed via experimentation?
Secondly, I still don’t know why or how a superconductor produces a Meissner Effect - the strong magnetic field that surrounds it. Can anyone explain?
There are a host of superconductor properties that are explained by the Bardeen-Cooper-Smith theory (BCS theory), the model that you describe. Among other things, it correctly predicts the fact that the transition temperature depends on the mass of the nuclei in the material, the properties of the specific heat near the superconducting transition, and the existence of the Meissner effect.
That said, the Meissner effect isn’t quite what you’ve described it as. It’s usually stated along the lines of “a superconductor expels magnetic fields”: when you cool a metal in a magnetic field, turning it into a superconductor, you invariably find that the metal ends up with currents circulating in it that exactly cancel out the effects of the external magnetic field. I’m not quite sure how this relates to your statement about a “strong magnetic field surrounding it”.
Thanks Mike, I’m just trying to get my head around these concepts so I may have made some inaccurate assumptions. Thanks for correcting. I still struggle to understand this though. So the superconductor expels the external field (of the magnet?) And how does this cause it to lock with a magnet and do the floating thing? Do those “currents” or other elements of the superconductor become a part of the nearby magnet’s em field? Apologies of this is really obvious to some of you and for the repeated questions.
Here’s my handwavy explanation of superconductors and magnetic fields (IIRC). First, a superconductor has no resistance (duh). Because it has no resistance it CANNOT have a magnetic field inside it. Well, you have the external magnetic field from a magnet penetrating it. How do you “get rid of that” so to speak? You “make” an internal field of the exact same strength and opposite polarity. Now, your magnetic field inside the superconductor is zero and everyone is happy again. But because the field associated with the superconductor is opposite that of the external field, if the field is strong enough in relation to the weight of the superconductor it will levitate above the external magnet.
Thats the simple (and I am sure not quite right) explanation, but at least its simple to understand and avoids some rather non-trivial math and such. Could be totally offbase here also.
Oof, you want to know about the locking thing. I’m by no means an expert in this, but here’s the understanding I have of it:
There are actually two types of superconductors that we know about, creatively called “Type I” and “Type II”. Type-I superconductors are actually the only ones that, properly speaking, exhibit the Meissner effect; all superconducting metals that are single elements (aluminum, lead, mercury, zinc) are Type I. These require extremely low temperatures (liquid helium) to become superconducting, and because of this you can’t actually handle them at anywhere room temperature. (The BCS theory, properly speaking, only explains Type-I superconductivity, not Type-II.)
Type-II superconductors are different in that they create currents that drive out the magnetic field almost everywhere. However, the currents can also form vortices (whirlpools, essentially) that have a certain specified number of magnetic field lines threaded through them. (The number is specified by quantum mechanics, via a mechanism that I don’t understand myself.) These vortices can move through the material, and left to their own devices they would eventually migrate out to the edges of the superconductor and expel the field. However, if the superconductor isn’t a perfectly regular lattice of atoms, these vortices can get “stuck” on the irregularities. In this event, the superconductor “locks” in place relative to the magnetic field; the vortices freeze a certain number of field lines in certain positions and orientations in the superconductor. All “high-temperature” superconductors (i.e., those that only require liquid nitrogen, and so can be manipulated at room temperature) are Type-II superconductors, so that’s why the guy in those “quantum locking” videos that have been going around YouTube can manipulate the superconductor directly for minutes at a time.
As I said above, I’m really not an expert in this stuff, so I’m open to corrections from anyone who knows more about it than I do.