# Superconductors ?

1. Mar 27, 2004

### drag

Greetings !

- Would a superconducting wire be affected by
the lorentz force just like a normal current carrying wire
or would the Meissner effect somehow prevent this due
to the surface currents or something ?

- What's the general scale of voltage that needs to be
applied for the high currents that could pass through them -
10^4, 10^5 A/cm^2 and higher ?

Thanks a lot !

Live long and prosper.

2. Mar 28, 2004

### turin

The Lorentz force acts directly on the current density, so it doesn't matter that it is all on the surface. If you have ever heard an MRI making that loud banging noise, well, that's the main coil torquing as the gradient is alternated.

I don't understand. Going back to the MRI, the main coil is energizes so that the $miles (I forget exactly how long the wire is) carries about 80 A without a source. Liquid He is used to keep it cool, and liquid N is used to keep the liquid He cool. I don't know much about the process of energizing the coil, but it is very expensive. 3. Mar 28, 2004 ### drag Thanks Turin. Hmm... Well, I don't quite understand this either then, how's the current in a superconductor's produced then ? (And how do you control its magnitude ?) Peace and long life. 4. May 4, 2004 ### drag O.K. I read some stuff and I understand how the current flows in a superconductor. However, wouldn't the exclusion of magnetix flux prevent an external magnetic field from affecting the current inside a superconductor ? Thus, only acting on the current at the surface - leading to a decreased force or something like that ? Thanks. 5. May 4, 2004 ### turin I thought that there was no current inside the superconductor (i.e. it's all on the surface)? Imagine a cylindrical conductor carrying some surface current in the axial direction. Then, there is an external magnetic field applied perpendicular to the axis. The current will be affected so that the charge will be deflected to an extreme axial line on the surface of the cylinder (like a seem line). The charge still carries the current, and the current has no more room to deflect, so it then pulls on the wire itself. There is probably some characteristic time for this process, but I don't know how to determine it. 6. May 5, 2004 ### drag I looked it up in a couple of books and asked my Physics Prof. - A superconductor excludes all external magnetic flux (magnetic fields lines go around a superconducting wire), however near Tc - the critical temp. the penetration depth is on the order of a few hundred Angstroms for weak fields(fractuations or a few Gauss). (High magnetic fields destroy superconductivity - order of tenths of Tesla and higher.) In short, there goes the Lorentz force... (The MRI coils are electromagnets with a conducting core, I assume that the core recoils in a changing field gradient.) Thanks a bunch anyway, Turin. 7. May 6, 2004 ### turin Well, now I'm confused. Magnetic field lines go around any kind of conducting wire that carries current, so I don't understand how that makes superconductors special. I don't understand what "excludes all external magnetic flux" means. Are you saying that MRI coils are not coils of superconducting wire? I never read a book on this stuff - I am just regurgitating what some of the grad students had told me in the MRI lab years ago. It seemed to make sense at the time Last edited: May 6, 2004 8. May 6, 2004 ### drag An external magnetic field should panetrate a normal conductor, if I'm not mistaken. Thus the Lorentz force acts on all the current throughout the volume of the wire. About MRI, all I'm saying is that electromagnets have conducting cores that could repel/attract each other when you energize the coils , but the force is between the cores not the superconducting wires. Peace and long life. 9. May 6, 2004 ### turin But this is still current flowing through a superconducting wire (on its surface), correct? Is this my misundertanding? Is the current fundamentally a different mechanism? This is the part that confuses me. My experience (which is somewhat limited) involves a 10 T MRI system. I was informed that the field was generated by a$ mile long hair thin superconducting wire coiled around the inner sample tube. Inside the tube was the immense 10 T B-field (if you walked in the room with a credit card it would be erased). There was no core, as I understood it. Frankly, I don't have the slightest idea of how MRI would work if the electromagnet required a core. Where would the patient go? Are you saying that the walls of the inner sample tube act like a core?

10. May 6, 2004

### drag

Greetings Turin !
http://en.wikipedia.org/wiki/Meissner_effect
Basicly, as I understood it, when there's no resistance then
any magnetic field that would enitialy appear or exist when the
material became superconducting, after cooling, will induce a current
in the superconductor that will oppose that field and cancel it -
since there's no resistance that current will be constant.
Fortunetly, I guess , I've not had any experience
with an MRI scanner. If it has no core then what you describe
could be caused by the magnetic field gradient acting
on the surface currents, I suppose. My physics prof. explained
that as the field bends around the wire there are forces
produced in the up/down directions (the enitial axis of the external
magnetic field before it bends around the wire) acting due to the
surface currents and that they will try to brake the wire, but
I didn't quite follow him there, I'm afraid. Alternatively, if the wire
is really thin and is near Tc than maybe there is a Lorentz force,
but I can't quite understand how such a force would act in
any single direction on a coil.

Live long and prosper.

11. May 6, 2004

### turin

I followed several links around from the one that you gave me.

Here is what I have learned/verified:
- The "electron gas" in a superconductor is bosonic. This character allows the electrons to flow around without interaction, as long as the lattice phonons are below a critical coupling energy.
- The Meissner effect is not the same as perfect diamagnetism.
- The electrons still flow from one side of the conductor to the other in order to constitute conduction. This flow induces a magnetic field.

There are still two issues that I do not have quite clear, but they seem crucial to the understanding: 1) How can phonons cause attraction? 2) It is not at all obvious to me that the London equation minimizes the electromagnetic free energy, which is what the site offered as an explanation of the Meissner effect.

12. May 7, 2004

### drag

13. May 7, 2004

### turin

I liked the first link better. That last one seems much more qualitative. One thing I did notice was:

http://www.americanmagnetics.com/tutorial/magnetp.html

Unfortunately, that site does not give any further detail. It does not talk at all about the Meissner effect (at least, not that I saw).

14. May 8, 2004

### ZapperZ

Staff Emeritus
Honestly, questions like these should have been asked in the PHYSICS section of this forum. Not only people who have the knowledge in this area will get to see the questions and answer them, there are probably already answers to them posted there (see the section that covers condensed matter). I just happened to get bored and decided to stick my nose in an area I never visited and found all these superconductivity questions.

1) phonon mechanism can cause the formation of Cooper Pairs. There are already several discussions on this in the section I've mentioned.

2) the London equation is phenomenological. You need to look at the BCS Theory to understand the microscopic detail of this.

Zz.

15. May 9, 2004

### turin

ZapperZ,
Do you have any input re drag's original questions? They are "to the point" engineering type questions in my regard. I forgot in which board this thread was and got carried away with understanding the physical process; I appologize.

16. May 9, 2004

### chroot

Staff Emeritus
Should I move this to the Gen. Phys. forum?

- Warren

17. May 10, 2004

### ZapperZ

Staff Emeritus
Turin, there's nothing to apologize about, you didn't do anything wrong. I just want to make sure that if one really is seeking answers, then maybe figuring out the appropriate area of this forum might be something one should consider.

Now, as far as the original question in this thread, I'd say they're appropriate for either section. However, unless there's someone here who is an expert in the field of superconductivity (either a physicist or an engineer) and who also happens to regularly read this section of PF, then I'd say that this question would get more responses in the Physics section. We already know that there are several people reading the Physics section regularly who can and able to tackle these types of problems. I hate to think that question such as this remains unanswered not because no one could, but because people who can do not read this section.

Having said that, I'll try and tackle the original question:

It would. That is how we also get the hall effect measurements on superconductors. For Type II superconductors, magnetic fields can penetrate the material without destroying the bulk superconducting state.

The issue here isn't how high a voltage to apply, but what kind of supercurrent density a superconductor can carry. At some point, one reach a saturation point in the current - this is why the high Tc superconductors, while having a rather high critical temperature, is still not that suitable for many applications due to its low current density.

Zz.

18. May 10, 2004

### turin

Do you know whether MRI magnets use Type I or II superconductors?

19. May 10, 2004

### ZapperZ

Staff Emeritus
Unless my info is outdated, not all MRI use superconducting magnets. I think most that do use Nb or Nb compounds, which are Type II. However, the type here doesn't matter in MRI applications. It is just that generally, Type II superconductors tend to have higher Tc than Type I.

Zz.

20. May 10, 2004

### turin

ZapperZ,
The way I attempted to address the original question was with the example of the recoil (torque) of an MRI coil to an applied alternating gradient. If the MRI coil is made from type II material, then that seems to neglect the context of type I superconductors with which I have no experience and cannot comment.

Are you saying that even type I superconducting magnets can be torqued by an applied B-field? The original question includes the Meissner effect. I did not think this occured in type II superconductors.