Questions about Superconductors | Greetings

[drag]

Greetings !

I'd like to ask a couple of Qs about superconductors:

- 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.

 

[turin]

- 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 ?

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.

- 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 ?

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.

 

[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.

 

[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.

 

[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.

 

[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.

 

[turin]

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), ...

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

 

[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.

 

[turin]

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.

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?

About MRI, all I'm saying is that electromagnets have conducting
cores that could repel/attract each other when you energize
the coils , ...

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?

 

[drag]

Greetings 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?

Here's a little something to start with:
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.

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?

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.

 

[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.

 

[drag]

Found a good link:
http://www.americanmagnetics.com/tutorial/basics.html

 

[turin]

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

One source of heating is wire motion caused by the Lorentz force on the conductor in the magnet.

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).

 

[ZapperZ]

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.

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.

 

[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.

 

[chroot]

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

- Warren

 

[ZapperZ]

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.

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:

- 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 ?

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.

- 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 ?.

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.

 

[turin]

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

 

[ZapperZ]

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

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.

 

[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 occurred in type II superconductors.

 

[ZapperZ]

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 occurred in type II superconductors.

We are then now getting into the specifics of the technical workings of an MRI, of which I have no direct experience with. I can neither concur nor contradict your claim of "torquing" in the coils and that being the source of the loud sound one hears. You will have to wait for someone with that kind of knowledge to answer that question.

The main point in my reply to you was that since all these "coils" are for is to generate magnetic fields, it really doesn't matter if it is from a Type I or Type II superconductors. If you're making a magnetic levation system, then there are ample physical arguments for wanting specifically a Type II (stability).

In any case, I think using the example of MRI might be a tad too complicated to answer the original question. I'm guessing that there are more undergraduates who are more familiar with the simple classical Hall effect than there are those who have the same degree of knowledge with MRI. One is bound to see a Hall probe at least once (one would hope) in an undergraduate physics lab.

Zz.

 

[turin]

I did undergrad engineering. I have never seen, much less had the privelage to use, a Hall probe, but I have read about them. I didn't think they had anything to do with necessarily superconductivity.

At any rate, this is my crux. I believe that I have a sufficient understanding of the Hall effect in the context of regular conductors. Basically, it is just caused by magnetic deflection, the same reason that a current carrying wire, as a whole, experiences a Lorentz force due to a magnetic field. This new issue of the Meissner effect has cast doubt on my consideration of superconductors of type I. The Meissner effect seems like it would have a nontrivial influence on the Hall effect. If this is so, it stands to reason in my mind that the same could be said regarding the Lorentz force on a superconductor that exhibits the Meissner effect.

 

[drag]

Greetings!

If Zappperz thinks there are more people to discuss this
in one of the Physics sections, than I have no problem with
moving this thread.

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.

Could you elaborate on that a bit, please:
Is it just near Tc ? What kind of fields (magnitude) - low fields,
for example, from fractions of a Gauss and up to a few tens
of Gauss ? And what are the approximate penetration depths ?
(I assume we're talking about superconducting wires, not
thin films or something with smaller cross-sections/deimensions.)

Thanks.

Live long and prosper.

 

[ZapperZ]

Greetings!

If Zappperz thinks there are more people to discuss this
in one of the Physics sections, than I have no problem with
moving this thread.

Could you elaborate on that a bit, please:
Is it just near Tc ? What kind of fields (magnitude) - low fields,
for example, from fractions of a Gauss and up to a few tens
of Gauss ? And what are the approximate penetration depths ?
(I assume we're talking about superconducting wires, not
thin films or something with smaller cross-sections/deimensions.)

Thanks.

Live long and prosper.

OK.. This is another example of the fact that I don't read this section of PF regularly and FORGOT that I posted something that may need followup. Or maybe this is another example that I am becoming senile? :)

No, the Hall effect measurements were done over a large range of temperatures. In YBCO, for example, it was done from 10 to 200 K (Tc for this compound is around 90K). Fields up to 6 T was used. And it WAS done on a thin film, but this is more for geometric simplicity and not requirement based on physics.[1]

BTW, to follow up to an earlier question, the hall effect isn't restricted to just superconductors. It can can occur in any system with "free" charge carriers. This includes conductors and semiconductors. Most Hall probes (at least the good ones that we use) are made of semiconductors.

Zz.

[1] S. Spielman et al., PRL v.73, p.1537 (1994).

 

[drag]

Greetings !

O.K. let me ask something a bit more specific,
like Turin said, I'm more interested in the practical
aspect here (which is why I chose this section).

Let's say I got a type II superconductor near and
below Tc with cross section 1 (cm^2), current
100 Amps and external magnetic field of 1 Gauss.
(I'm not throwing in numbers for any quantative answers,
but just to clarify my question.) Would the superconductor
be affected the same way as a standard conductor ?
Would there be some resistance/other changes in it ?

Thanks.

Peace and long life.

 

[ZapperZ]

Greetings !

O.K. let me ask something a bit more specific,
like Turin said, I'm more interested in the practical
aspect here (which is why I chose this section).

Let's say I got a type II superconductor near and
below Tc with cross section 1 (cm^2), current
100 Amps and external magnetic field of 1 Gauss.
(I'm not throwing in numbers for any quantative answers,
but just to clarify my question.) Would the superconductor
be affected the same way as a standard conductor ?
Would there be some resistance/other changes in it ?

Thanks.

Peace and long life.

You will need to supply a lot more info than that. For example, what is the Hc1 and Hc2 of the superconductor. By how much are you below Tc? (A H-T phase diagram of the Type II superconductor will be helpful). And you can't just throw out a current value because there are many superconductors that have very low current density saturation.

I'm also not sure what you mean by a superconductor be "affected" the same way as a standard conductor. What particular properties are you looking for? If it is just "resistance", then if it is still in its superconducting state, then you will still get zero resistance, by definition, no matter what you do to it. As long as what you are doing doesn't destroy the bulk superconductivity, you will read zero resistance. If you have crossed above Tc or Hc2, then you have a normal piece of metal (assuming we're dealing with a conventional superconductor).

Zz.