Superconducting Levitation

[INRM]

Okay I understand that a superconductor is a substance that has either no resistance to electricity or so little as to be regarded as nonexistant.

But how does that equate to levitating a magnet? Does the superconductor have a charge, or is it electrified?


INRM

[Olowkow]

My understanding is that magnetic field flux lines must flow around a superconductor, not through it. This is part of the definition of superconductivity. So, when you put a superconducting block in liquid nitrogen, it becomes a superconductor, and then you put a magnet above it, the magnet will levitate as the flux lines surround the block.
I guess you can say this property is not directly related to electrical conductivity, just an interesting side effect.

[krazyKemist]

As somebody who uses an NMR spectrometer as an everyday tool, I feel somewhat qualified to answer, but a phyicist might maybe explain better.

There are a few ways of creating a magnetic field. Surely you've made once in science class an electromagnet with a nail, copper wire and a battery. So, to increase the power of that magnet, you'd have to crank up the current. The more you crank up the current, the more powerful the magnet.

But, there comes a point where you can't do that anymore, because the wire will heat up because of electrical resistance. Even worse, as it becomes hotter, the wire will become even more resistant to current, up to the point where the wire melts.

However, if you were to use a supraconducting wire, by definition you have no resistance, and therefore no heating. You might then crank up the current a lot more and obtain a really, really powerful magnet.

Even better, you don't even have to keep giving it current for it to keep working. You charge it up, and if you can make sure the wire stays supraconductive (generally that means keeping its temperature very low - about -200C), you have a permanent electromagnet without any current input.

So these magnets are very powerful, orders of magnitude beyond any permanent steel magnet or normal electromagnet. So powerful they can overcome gravity and levitate.

[Subduction Zone]

What youare describing is called the Meissner effect. Here is a wiki article (I know not the best source) on it:

http://en.wikipedia.org/wiki/Meissner_effect

[Ziggurat]

So these magnets are very powerful, orders of magnitude beyond any permanent steel magnet or normal electromagnet. So powerful they can overcome gravity and levitate.

It's not the power of superconductors that allow them to levitate. The fact that you can lift magnets with other magnets proves that ordinary magnets are strong enough (think of those jun-yard electromagnets which lift cars). But with two ordinary magnets, you can't get a repulsive configuration that is stable: any perturbation and the floating magnet will flip around so that the interaction becomes attractive. And you need a repulsive interaction, not an attractive one, in order to levitate. With a superconductor, perfect diamagnetism means that the magnet and superconductor will repel each other regardless of their orientation.

[INRM]

What's a magnetic field flux line?

[Rob Lister]

What's a magnetic field flux line?

$6.95 a yard, but discounts are available for lengths in excess of 100.

[Rob Lister]

What's a magnetic field flux line?

but on a serious note...

[Rob Lister]

$6.95 a yard, but discounts are available for lengths in excess of 100.

Or more betterestly...

http://www.coilgun.info/images/flux_anim.gif

[MRC_Hans]

A simple explanation:

Whenever you move a piece of conducting material closer to a magnet, it is repulsed. This is because the magnet induces an eddy-current into the conductor.

For a normal conductor, however, the eddy-current will get lost due to resistance, so the repulsion wears off. Therefore, of you drop the conductor on the magnet, its fall will be slowed somewhat, but it will end up falling on the magnet.

In a superconductor, there is no loss due to resistance, to the eddy-current remains, and is even reinforced as the conductor moved closer to the magnet. At some point, the repulsion equals the weight of the conductor, and it hovers.

In principle you can do the same with two permanent magnets, but the hovering magnet has an opposite pole in the other end, and of you don't keep it aligned in some way, it will flip and get attracted instead.

In contrast to a permanet magnet, the magnet field of the superconductive material is generated by the above-mentioned eddy current, and as this eddy current is caused by the fixed magnet, it stays aligned with it, even if the material is flipped.

Therefore it can hover.

Hans

[GreyICE]

Magnetic fields induce a resistance in a wire.

Superconductors have zero resistance.

As a consequence of this, they obviously must generate an equal and opposite field that exactly cancels the effect of any magnetic field applied to them (for if they were to have a field applied, they would have a positive resistance).

This is a 'what' explanation. The 'why' is a little more complicated. We lack a truly 'good' explanation with predictive powers.

[Ziggurat]

Magnetic fields induce a resistance in a wire.

I presume you mean a current (or an electric field) in the wire.

This is a 'what' explanation. The 'why' is a little more complicated. We lack a truly 'good' explanation with predictive powers.

No, we have a very good theory for superconductivity (the BCS theory), which has considerable predictive power. The only problem is that it only really works for conventional superconductors, not high-temperature superconductors.

[GreyICE]

I presume you mean a current (or an electric field) in the wire.



No, we have a very good theory for superconductivity (the BCS theory), which has considerable predictive power. The only problem is that it only really works for conventional superconductors, not high-temperature superconductors.

Ziggernaut, the theory is a failure. It was a very good theory for its time. It made testable predictions unlike a large number of theories involving quantum mechanics.

One of those predictions was that no superconductor could exceed 30 K. That prediction failed.

When we discovered the erratic orbit of Mercury, we didn't try to say Newton's Laws only applied to certain planets. Newton's Laws flat out broke. They were replaced with a much better framework.

BCS is flat-out broken. It's going to get replaced. I have no doubt that like the Theory of Relativity, the replacement will contain and incorporate much of BCS, but the theory doesn't work in its current form.

[Ziggurat]

Ziggernaut, the theory is a failure. It was a very good theory for its time. It made testable predictions unlike a large number of theories involving quantum mechanics.

One of those predictions was that no superconductor could exceed 30 K. That prediction failed.

You apparently misunderstand the nature of that prediction. BCS says you can't get superconductivity from purely phonon-mediated interactions above about 30K. That does not preclude the possibility of additional attractive electron-electron interactions (such as magnetic interactions). BCS can't explain such cases, but that DOES NOT mean that it is wrong in cases where phonon-mediated attraction is the only relevant interaction. In short, BCS does work for conventional superconductors.

When we discovered the erratic orbit of Mercury, we didn't try to say Newton's Laws only applied to certain planets. Newton's Laws flat out broke. They were replaced with a much better framework.

Wrong analogy. It's more akin to saying Coulomb's law is wrong because it can't predict magnetic induction. Well, it's not wrong. That it doesn't cover every case doesn't make it wrong.

BCS is flat-out broken. It's going to get replaced.

No, it is not broken, and no, it will not get replaced, because there's no reason to replace it. The only reason we would ever have to replace it is if we found something in conventional superconductors which contradicted it, but that has not happened. Let me stress that again: we have found no evidence that BCS theory is wrong for conventional superconductors.

It's true that we need a new theory of superconductivity, but it will be in addition to BCS, not instead of it.

[Zeuzzz]

What's a magnetic field flux line?


Nothing. Its like saying whats a vector. Its a conceptual tool, not a real physical thing. Its what we add in to model the behaviour of a field based on our observations. Its very similar to a contour line on a map. Every magnetic field is a continuum, i.e., a vector field. Each of the infinite and uncountable points in this continuum has a magnitude and a direction that is associated with it. A lot of people find this conceptually confusing, as this continuum is not made of (does not contain) a set of discrete lines. Lines can be drawn on paper to describe the magnetic fields direction and magnitude but the field itself is not made of these lines.

When people choose to give the lines real physical properties, like magnetic knots or 'tangled up' magnetic field lines, then it can lead to confusing dilemas, as they have reified an abstract concept. In the area of superconductors, this has not really been an issue as they often consider the electric current component instead which is based on more definitive particle interactions and definitions, and it can be shown that an electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.

[Ziggurat]

Nothing. Its like saying whats a vector. Its a conceptual tool, not a real physical thing.

That's like saying a number is not a real physical thing. Sure, it's true, but it's not exactly profound. Real physical things can be characterized by numbers, and by vectors. The number is not the thing, but that's the nature of representation. The word "pipe" is not the same thing as an actual pipe. Welcome to abstraction 101.

Its what we add in to model the behaviour of a field based on our observations.

No, actually, it isn't: it's what we use to visualize our models.

Its very similar to a contour line on a map.

This is correct.

Lines can be drawn on paper to describe the magnetic fields direction and magnitude but the field itself is not made of these lines.

This is true. But flux lines are particularly handy for type II superconductors because of vortices, which have quantized magnetic flux (which is a real, physical, measurable thing), and which can therefore be represented by discrete numbers of flux lines (each flux line representing one flux quanta). Yes, they're an abstraction, but they're a useful abstraction. And no, they don't lead to confusion unless you don't know what you're doing.

[GreyICE]

You apparently misunderstand the nature of that prediction. BCS says you can't get superconductivity from purely phonon-mediated interactions above about 30K. That does not preclude the possibility of additional attractive electron-electron interactions (such as magnetic interactions). BCS can't explain such cases, but that DOES NOT mean that it is wrong in cases where phonon-mediated attraction is the only relevant interaction. In short, BCS does work for conventional superconductors.
Just like Newton's laws work just peachy for most slow-moving objects in gravity fields.

No, it is not broken, and no, it will not get replaced, because there's no reason to replace it. The only reason we would ever have to replace it is if we found something in conventional superconductors which contradicted it, but that has not happened. Let me stress that again: we have found no evidence that BCS theory is wrong for conventional superconductors.

It's true that we need a new theory of superconductivity, but it will be in addition to BCS, not instead of it. Or rather it will encompass BCS, and also go beyond it. The concept that there's two categories of superconductors arbitrarily divided by a line that does not exist is simply too silly for words. I'd have to see a lot stronger evidence to conclude that there were two types of exactly identical phenomena (and some odd behavior at temperatures extremely close to the limit does not count).

[Ziggurat]

The concept that there's two categories of superconductors arbitrarily divided by a line that does not exist is simply too silly for words.

There are indeed different categories of superconductors, and the dividing lines are not arbitrary. Even within conventional superconductors, there are Type I and Type II, for example (I'll let you figure out the difference). But the distinction between conventional and high-Tc superconductors most certainly exists (who told you otherwise?), and it is not arbitrary. One of the distinguishing features between conventional and high-Tc superconductors is the fact that the former are s-wave, while the latter are d-wave superconductors. Another, of course, is that the former are metals, while the latter are doped Mott insulators (if you don't know what that term means, you really aren't qualified to dismiss BCS theory).

I'd have to see a lot stronger evidence to conclude that there were two types of exactly identical phenomena (and some odd behavior at temperatures extremely close to the limit does not count).

And all you really need to form superconductivity is an attractive interaction between conduction electrons (or holes). Given any such attraction, Bose condensation of Cooper pairs can happen, and that leads directly to superconductivity. BCS is a theory of how you get such an attraction via phonon mediation. But any other attractive interaction could lead to superconductivity as well if it's strong enough, and nothing about BCS precludes the existence of other interactions, or even limits their strength.

And superconductivity is itself not exactly unique. It's just an electronic equivalent of superfluidity (zero resistance versus zero viscosity, magnetic flux vortices versus rotational vortices, etc), where you're just getting bose condensation. And in He3, you even need an attractive potential for pair formation to do it (since He3 is a fermion, while He4 is a boson).

[sol invictus]

Magnetic fields induce a resistance in a wire.

Not sure what that's supposed to say - what you wrote is nonsense. Perhaps you meant, "changing magnetic fields induce a current in a wire"? Simply putting a magnetic field in a wire does not change its resistance - for example, if the field is parallel to the wire it has zero effect on electrons drifting down the wire.

As a consequence of this, they obviously must generate an equal and opposite field that exactly cancels the effect of any magnetic field applied to them (for if they were to have a field applied, they would have a positive resistance).

That type of explanation fails. Start with a superconductor above its critical temperature, and put it in a constant magnetic field. Now cool it down. No current is induced during that process, and yet the field is still repelled.

And Zig is right - the BCS theory is the correct explanation for low T_c superconductors, as far as anyone knows.

[Dr H]

Not sure what that's supposed to say - what you wrote is nonsense. Perhaps you meant, "changing magnetic fields induce a current in a wire"?

Perhaps he was thinking of inductive reactance?

[Toke]

I saw a real cool pen at deuche Museum´s shop a few weeks ago. It was howering in a magnetic field above its stand.

Superconduction would get you some much cooler/colder magnets.
The cooling requirements might make it a bit impratical for everyday use.

I read of energy storage by rotary energy. A flyweel suspended on magnetich bearing in a vacum tank. The flywheel form one half of a electrical motor.
Guess you would still get iron loss, and if you buildt them too large there would be problems in earthquake zones.

[GreyICE]

Not sure what that's supposed to say - what you wrote is nonsense. Perhaps you meant, "changing magnetic fields induce a current in a wire"? Simply putting a magnetic field in a wire does not change its resistance - for example, if the field is parallel to the wire it has zero effect on electrons drifting down the wire.
Not really. Flowing current interacts with magnetic fields. Magnet does not have to be undergoing movement in the wire's frame of reference if current is flowing, and current flowing through a magnetic field suffers from resistance caused by that very field.


That type of explanation fails. Start with a superconductor above its critical temperature, and put it in a constant magnetic field. Now cool it down. No current is induced during that process, and yet the field is still repelled.

And Zig is right - the BCS theory is the correct explanation for low T_c superconductors, as far as anyone knows. Certainly it's not a perfect Faraday Conductor. But it's a decent understanding of the situation.

As for BCS, I agree with you - it's the rightest theory we currently have. That still doesn't make it perfect or complete.

[sol invictus]

Not really. Flowing current interacts with magnetic fields. Magnet does not have to be undergoing movement in the wire's frame of reference if current is flowing, and current flowing through a magnetic field suffers from resistance caused by that very field.

"Interacts" does not equal "resists". Magnetic fields do not generally create resistance in wires. There are some situations in which that can happen for various reasons, but it requires something special.

Since you disagree, why not give us a formula for the simplest case: the resistance induced in a long straight wire by a constant magnetic field B?

[GreyICE]

"Interacts" does not equal "resists". Magnetic fields do not generally create resistance in wires. There are some situations in which that can happen for various reasons, but it requires something special.

Since you disagree, why not give us a formula for the simplest case: the resistance induced in a long straight wire by a constant magnetic field B?

http://en.wikipedia.org/wiki/Inductance

Negligible != nonexistent (like the effects of relativity on the mass of your car when it accelerates to 60 mph - just because we choose to neglect it in all calculations doesn't mean it's not there).

[Ziggurat]

http://en.wikipedia.org/wiki/Inductance


Inductance (or, as Dr. H said, inductive reactance) is not the same thing as resistance.

Negligible != nonexistent

The effects Sol was referring to are things like magnetoresistance. Not only are they typically quite small (though there are important exceptions), they're not general. They don't apply to any wire in any magnetic field - in many cases, they are nonexistent.

[INRM]

The conducting material which is passed by the magnet and is repulsed. Is this conducting currant carrying current through it, or is it simply capable of conducting? Is it magnetic?

[Ziggurat]

As for BCS, I agree with you - it's the rightest theory we currently have. That still doesn't make it perfect or complete.

We can never be certain any theory is perfect or complete. But as already mentioned, there are no observations in conventional superconductors that conflict with BCS. So it is as complete as we could hope it to be for those materials. But it will not get replaced even if it becomes a subset of some larger theory (which by necessity would be more complicated and therefore more cumbersome), unless we find experimental evidence that it does not accurately describe conventional superconductors. And once again, no such evidence exists at the present time.

[INRM]

Nobody?

[Ziggurat]

Nobody?

Nobody what?

[Dr H]

Nobody what?

Apparently nobody knows what. ;)

[INRM]

I was wondering if anybody had an answer to my question

"The conducting material which is passed by the magnet and is repulsed. Is this conducting currant carrying current through it, or is it simply capable of conducting? Is it magnetic?"


INRM

[Ziggurat]

"The conducting material which is passed by the magnet and is repulsed. Is this conducting currant carrying current through it, or is it simply capable of conducting? Is it magnetic?"

You might want to consider editing your question before reposting it. For example, what do you mean by asking if a current can carry current? (presuming that this was a typo, and you weren't talking about electric fruit (http://www.merriam-webster.com/dictionary/currant)).

[INRM]

I meant conducting material. Not conducting fruit

[Ziggurat]

When a conducting material is passed through a region with a varying magnetic field, that field will induce a current in that material. That current produces its own magnetic field. This is not, however, what is generally meant by a material being "magnetic" (which lay people use to indicate ferromagnetism, but more technically may cover antiferromagnetism, ferrimagnetism, etc - basically phenomena linked to electron (spin or orbital) moments, not macroscopic currents).

[INRM]

Ziggurat,

So you're talking about the same principle behind a dynamo?

[Reality Check]

Ziggurat,

So you're talking about the same principle behind a dynamo?
Ziggurat is talking about the same principle as a dynamo.
A dynamo usually has a rotating magnetic field from a ferromagnetic material generating an electric current.

[Cuddles]

Firstly, Sol and Ziggurat are correct. I'd just like to clarify this though:

Okay I understand that a superconductor is a substance that has either no resistance to electricity or so little as to be regarded as nonexistant.

A superconductor has zero electrical resistance. Not just very small, exactly zero. Of course, that's according to theory. In practice, it's not possible to ever confirm that it is truly zero, but it's as close as we've been able to measure so far. Type II superconductors can show some resistance at temperatures close to the critical temperature, but even that disappears at lower temperatures.

[INRM]

RealityCheck,

Okay... that I understand. The magnet interacting with the superconducting material produces a dynamo effect which produces electricity which the superconducting material doesn't have any resistance for which produces an electro-magnet effect that repels the magnet.


Cuddles,

Of course, that's according to theory. In practice, it's not possible to ever confirm that it is truly zero, but it's as close as we've been able to measure so far.

Which is why I wrote "a substance that either has no resistance to electricity or so little as to be regarded as nonexistant".

If there is zero resistance, it's zero though.


INRM