Superconductivity at room temperature??

[johanfprins]

You can start off a persistent current by:

a) turning on a magnet field through a warm conductor,
b) cooling the conductor below Tc
c) turning off the applied magnetic field.

Again---whatever gave you the opposite idea?

You left out a crucial step. Let us look again at this argument in the correct way:

a) turning on a magnetic field through a warm superconducting ring
b) waiting for the current caused by the induced electric field to dissipate to zero. Which it must do since the ring is not superconducting.
c) Cooling the ring to become superconducting.
d) the same super-current appears around the ring, even though there is now no induced electric-field.

Again--- whatever gave you the idea that the current is caused by an electric-field?

 

[johanfprins]

This is also incorrect. A bent wire (normal or superconducting) does not emit radiation. A single electron might so emit but a current does not.

(Why not? Several different explanations, but the same physics:

a) There are many electrons in a current, undergoing their accelerations at different times.

This is correct for a normal current, since the charge-carriers are regularly scattered; but it cannot be correct for a superconductor in which the charge-carriers are not scattered; and in addition, according to you, are "accelerated".

 

[W.D.Clinger]

I must confess that circular vector fields always confused me; since when you integrate them by using Stoke's theorem, you find that the total current and the resultant magnetic flux do not occupy the same regions in space. It will help me if you could explain why this is so? This seems to be also so for Ampere's law and the law of Biot and Savart.

If I understand you correctly, you're asking why the magnetic field induced by current flowing within a wire is not confined to the wire.

That fact can be derived from charge invariance, the electrostatic field, and the special theory of relativity. Before we show you the derivation, please confirm my understanding of your question.

 

[johanfprins]

If I understand you correctly, you're asking why the magnetic field induced by current flowing within a wire is not confined to the wire.
That fact can be derived from charge invariance, the electrostatic field, and the special theory of relativity. Before we show you the derivation, please confirm my understanding of your question.

What I want to know is whether any part of the magnetic-field generated by a constant current flowing through the wire can be within the wire: i.e. occupying the same region through which the current is flowing. Does your derivation answer this? Even if it does not, I would very much like to see your derivation since it has been a problem about which I have wondered often.

 

[sol invictus]

A very good argument Sol; and you probably proved me wrong. Obviously, your mathematics is better than mine.

I must confess that circular vector fields always confused me; since when you integrate them by using Stoke's theorem, you find that the total current and the resultant magnetic flux do not occupy the same regions in space. It will help me if you could explain why this is so? This seems to be also so for Ampere's law and the law of Biot and Savart.

Are you asking why B is non-zero in regions where J is zero? Mathematically that's easy to explain. Consider a very simple equation in 1 dimension: dB/dx=J. That's similar to Ampere's law in that it says (derivative of B)=J.

Now, clearly if J=/=0, neither is B (except possibly at one point). For example if J is constant, B=Jx.

But even if J=0 in some region, B can still be non-zero and constant.

So J=/=0 implies B=/=0 (except possibly at one point), but J=0 does not imply B=0.

Physically it happens because electromagnetic forces are long-range. If the field was zero outside charges and currents, they wouldn't exert any force on each other.

You left out a crucial step. Let us look again at this argument in the correct way:

a) turning on a magnetic field through a warm superconducting ring
b) waiting for the current caused by the induced electric field to dissipate to zero. Which it must do since the ring is not superconducting.
c) Cooling the ring to become superconducting.
d) the same super-current appears around the ring, even though there is now no induced electric-field.

Again--- whatever gave you the idea that the current is caused by an electric-field?

That sounds unlikely. But even if its true it doesn't support your point - when you cool the ring to become superconducting it will expel the magnetic field. As it is doing that there is a changing B field and therefore an E field induced.

This is correct for a normal current, since the charge-carriers are regularly scattered; but it cannot be correct for a superconductor in which the charge-carriers are not scattered; and in addition, according to you, are "accelerated".

That's not true. If you compute the EM radiation from a perfect ring of current, no scattering, everything smooth and uniform, you get.... zero.

Think about it - what would the frequency of the radiation be? In order for the ring to radiate the current has to change with time, and it's that change that causes the field outside to "wiggle" and send rippled of radiated power out. If the current is constant, it can't radiate.

 

[W.D.Clinger]

What I want to know is whether any part of the magnetic-field generated by a constant current flowing through the wire can be within the wire: i.e. occupying the same region through which the current is flowing. Does your derivation answer this?

Yes. Maxwell's equations answer that question, as sol invictus has already explained. Maxwell's equations can be derived in turn from charge conservation, the electrostatic field, and relativity.

Even if it does not, I would very much like to see your derivation since it has been a problem about which I have wondered often.

I'm still confused concerning which derivation you want to see:

• the derivation (from Maxwell's equations) that shows the magnetic field is nonzero throughout most of a current-carrying wire, or
• the derivation (from special or general relativity) of Maxwell's equations

 

[sol invictus]

I must confess that circular vector fields always confused me; since when you integrate them by using Stoke's theorem, you find that the total current and the resultant magnetic flux do not occupy the same regions in space. It will help me if you could explain why this is so? This seems to be also so for Ampere's law and the law of Biot and Savart.

Sorry, I misread your question (because it is based on a false premise). When you integrate the equation I wrote, you obtain an integral form of the same equation:

[latex]\int_{O} \vec B \cdot d\vec l = \int_{area} \vec J \cdot d\vec \sigma = I[/latex]

where I is the total current flowing through whatever area you choose, and first integral is over the closed loop that is the edge of that area.

Roughly speaking this tells you that B field lines circle around current. It does not tell you that the B field is zero inside the current - quite the contrary. Consider a cylindrical tube of constant current density. Choose your surface to lie anywhere inside that tube. Then the above integral equation tells you immediately that B is non-zero inside the tube (the surface encloses some current, therefore I is non-zero).

As i explained above, these equations tell you that B may be non-zero even when J is zero, but they also tell you that B is definitely non-zero in any region where you have (non-singular) current density.

ETA - weird latex/forum problem: if I used "loop" as the subscript on the first integral, it gave a "forbidden tag" error.

 

[johanfprins]

Are you asking why B is non-zero in regions where J is zero?

No I am not asking this, this is easy to understand from Ampere's law. I am asking whether the magnetic field around a wire, carrying a current (Ampere's experiment) penetrates the wire; and how deep.[

That sounds unlikely.

Try it yourself.

But even if its true it doesn't support your point - when you cool the ring to become superconducting it will expel the magnetic field.

.
We have lightning, I will thus return later.

 

[sol invictus]

No I am not asking this, this is easy to understand from Ampere's law. I am asking whether the magnetic field around a wire, carrying a current (Ampere's experiment) penetrates the wire; and how deep.

It penetrates to whatever depth there is current. If the current is confined to a thick skin at the edge of the wire, there might be zero field in the part of the wire beneath the skin. But inside the skin itself there is guaranteed to be field.

Try it yourself.

Why should I? Essentially nothing you've said so far has been correct, so I have little faith in this claim either. If there's an experiment, link to it.

So you are saying that a circular current will now be induced to run opposite to the way the current will run when you apply the magnetic field only after cooling?

Expelling the lines from the superconductor is not the time-reverse process of turning on the field in the first place. So there is no reason to think the net current induced around the ring - if there is one at all, which is not obvious - will run in the opposite direction.

 

[W.D.Clinger]

Are you asking why B is non-zero in regions where J is zero?

No I am not asking this, this is easy to understand from Ampere's law. I am asking whether the magnetic field around a wire, carrying a current (Ampere's experiment) penetrates the wire; and how deep.

Yes, it does. If the wire is carrying a radially symmetric current, then the magnetic field is nonzero except at the very center of the wire (a set of measure zero).

Derivation from Maxwell's equations:

[latex]
\begin{align}
\int_{\partial S} \hbox{{\bf B}} \cdot \hbox{d{\bf l}}
&= \int_{S} ( \nabla \times \hbox{{\bf B}} ) \cdot d \hbox{{\bf A}} \\
&= \int_{S} \hbox{{\bf J}} \cdot d \hbox{{\bf A}} \\
&= I_S
\end{align}
[/latex]

Line (1) is Stokes's theorem.
Line (2) is Ampère's Law (omitting a unit-dependent constant of proportionality, and omitting Maxwell's correction because the current is assumed constant).
Line (3) is the definition of I_S.

Those equations hold for any compact oriented surface S with boundary ∂S. For any flat surface S that's perpendicular to and intersects enough of the wire for I_S to be nonzero, the equations above imply the integral of B around the boundary of S is nonzero; hence B must be nonzero on at least part of that boundary.

You can take S to be contained entirely within the wire. By the proof above, B cannot be zero on all of S. Hence B is nonzero somewhere within the wire.

As sol invictus has explained, B is nonzero everywhere within the wire except at its center (a set of measure zero). That too is an easy consequence of Maxwell's equations.

You now have two different proofs (sol's and mine) that the magnetic field is nonzero within the wire. (ETA: My proof is really the same as sol's, but giving the same proof in different words might help.)

 

[Skwinty]

If you try and describe a quantum effect manifest itself macroscopically, then your description must conform to the established science and mathematics from both sides of the great divide.

If you can do that, then you are surely unifying the very big and the very small.

Daunting task.

 

[sol invictus]

If you try and describe a quantum effect manifest itself macroscopically, then your description must conform to the established science and mathematics from both sides of the great divide.

If you can do that, then you are surely unifying the very big and the very small.

Daunting task.

Done decades ago with great success for superconductivity (at least of the low T variety); Nobel prizes duly awarded.

What's remarkable about superconductivity is how many of its bizarre phenomena can be explained from a very simple principle - the spontaneous breaking of gauge invariance, of the same type that particle physicists believe is the origin of all mass.

Without knowing anything at all about the microscopic structure of superconductors - knowing only that the symmetry is broken - you can derive the Meissner effect, zero resistivity, the DC and AC Josephson effects, the fact that there are Type 1 and Type II superconductors, and the consequent behavior of magnetic flux tubes within them.

It's amazing, and truly beautiful.

 

[Skwinty]

It's amazing, and truly beautiful.

My big regret in life is not having studied mathematics.

Words fail in so many ways.

 

[Skwinty]

I found this paper by Mourachkine.
Very interesting as it seems to cover most of what has been said in this thread.

http://arxiv.org/abs/0704.2188

Here is his web site.

http://www.mourachkine.org.uk/index.htm

ETA: Here is an excerpt.

In the literature, one can find more than 20 papers reporting evidence of superconductivity
near or above room temperature [2]. Most researchers in superconductivity
do not accept the validity of these results because they cannot be reproduced
by others. The main problem with most of these results is that superconductivity
is observed in samples containing many different phases, and the superconducting
fraction (if such exists at all) of these samples is usually very small. Thus, superconductivity
may exist in these complex materials, but nobody knows what phase
is responsible for its occurrence. In a few cases, however, the phase is known but
superconductivity was observed exclusively on the surface. For any substance, the
surface conditions differ from those inside the bulk, and the degree of this difference
depends on many parameters, and some of them are extrinsic.

 

[johanfprins]

I am sorry I had to leave in a hurry. But I have lost so many modems that I had to. In the rush I also posted a sentence which I had not yet vetted and immediately, notwithstanding the weather, had to remove. As I expected Sol immediately pounced on it with glee! So, let me proceed from where I had to leave:

That sounds unlikely.

As I have posted above, do the experiment yourself: You do not have to question my integrity as you have immediately done. It should be easy to do the experiment and verify whether what I claim is correct or not.

But even if its true it doesn't support your point - when you cool the ring to become superconducting it will expel the magnetic field. As it is doing that there is a changing B field and therefore an E field induced.

It sounds oh so logical: The current is generated in "both cases" to cancel the applied magnetic field energy. Very "logical", but I think you are now violating energy conservation: And if it is so, man will a enjoy it.

Now why is an electric-field induced and a current generated by an increasing magnetic field? To oppose the applied magnetic field energy (This effect is called Lenz's law if Sol does not know). The resultant current generates an opposite magnetic field which cancels the applied magnetic-field, but does not achieve this in a normal conductor, since this extra magnetic energy to cancel the applied energy dissipates soon after the applied magnetic field reaches equilibrium. In a superconductor this added energy does not dissipate and it cancels the applied magnetic field as one expects for a superconductor.

But now you first apply the magnetic field before cooling the superconductor, and then allow this extra magnetic energy which cancels the applied magnetic energy to dissipate. You now cool the superconductor to become superconducting and the same current appears to cancel the applied magnetic field energy. Where does the extra magnetic field energy now comes from? It just does not add up, Solly!

That's not true. If you compute the EM radiation from a perfect ring of current, no scattering, everything smooth and uniform, you get.... zero.

Sure when you have a current with an average drift speed this will happen: However, when you accelerate the charge-carriers around the ring as you claim that is happening within a superconductor, you have a mini-cyclotron; and then you must radiate EM. Now where does the average drift speed comes from when you accelerate the charge carriers and they do not scatter?

Think about it - what would the frequency of the radiation be? In order for the ring to radiate the current has to change with time, and it's that change that causes the field outside to "wiggle" and send rippled of radiated power out. If the current is constant, it can't radiate.

This will be the case if the charge carriers are accelerated around the ring like you and Ziggy adamantly claim is happening. I have no problem with a current with an average drift speed which manifests when the acclerated charge cariers are also continuously scattered.

 

[sol invictus]

As I have posted above, do the experiment yourself: You do not have to question my integrity as you have immediately done.

It's not your integrity that I question, it's your understanding.

It sounds oh so logical: The current is generated in "both cases" to cancel the applied magnetic field energy. Very "logical", but I think you are now violating energy conservation:

That's not what I said.

Where does the extra magnetic field energy now comes from? It just does not add up, Solly!

I have no idea what you're talking about.

Sure when you have a current with an average drift speed this will happen: However, when you accelerate the charge-carriers around the ring as you claim that is happening within a superconductor, you have a mini-cyclotron; and then you must radiate EM.

As usual, you're wrong - and countless experiments, textbooks, solutions to Maxwell's equations, and radio towers prove it.

I'm afraid this has gotten rather boring. I suggest you go play with your grandchildren, Johan, and leave physics to physicists. If nothing else, this conversation should convince you that you are not equipped to convince anyone knowledgeable in physics that you are correct. You don't understand or accept basic high school and undergraduate physics - and so you can hardly expect anyone to pay attention to your pronouncements on something like high Tc superconductivity.

In one of your first posts you said "it has now been ten years and it has not yet happened no matter how hard I have tried to bring my message across. It has tired me out, nearly bankrupted me and has affected my personal relations within my family."

I advise you to learn from that. Let this go.

 

[johanfprins]

Sorry, I misread your question (because it is based on a false premise). When you integrate the equation I wrote, you obtain an integral form of the same equation:

[latex]\int_{O} \vec B \cdot d\vec l = \int_{area} \vec J \cdot d\vec \sigma = I[/latex]

I am very happy with this since this is the proof I wanted: i.e. that when a normal current flows there is a magnetic field present. But this magnetic field does not accelerate the charge-carriers: The applied voltage does.

Now comes the problem. It is claimed in the main stream literature, which you are defending with a vengeance, that within a superconductor a magnetic field must be cancelled. Thus the magnetic-field produced by the current through a superconductor must, in contrast to a normal conductor, also be cancelled within the superconductor. But then it is immediately, illogically, claimed in the main stream literature that there must be such a magnetic field within a superconductor to "accelerate" the charge-carriers, even though the same magnetic field with a normal conductor does not do this.

Firstly it is claimed that a field "that must be expelled" is present, and that this static field, which does not accelerate charge-carriers in a normal conductor is now "magically" capable of doing this in a superconductor! Come on become serious!

 

[sol invictus]

Now comes the problem. It is claimed in the main stream literature, which you are defending with a vengeance, that within a superconductor a magnetic field must be cancelled.

No such claim exists in the "main stream" literature - you are wrong, as usual. Magnetic fields attenuate exponentially within a superconductor, but they do not need to be zero.

Thus the magnetic-field produced by the current through a superconductor must, in contrast to a normal conductor, also be cancelled within the superconductor.

That's completely wrong, as any text on superconductivity will tell you within the first few pages.

 

[johanfprins]

That's not what I said.

That is exactly what you said

I have no idea what you're talking about.

What you are really saying is that you do not know how to answer the question and are thus retreating

As usual, you're wrong - and countless experiments, textbooks, solutions to Maxwell's equations, and radio towers prove it.

Unfortunately notwithstanding all this "evidence" on your side, you feign an inability to "understand" what I am saying.

I'm afraid this has gotten rather boring. I suggest you go play with your grandchildren, Johan, and leave physics to physicists. If nothing else, this conversation should convince you that you are not equipped to convince anyone knowledgeable in physics that you are correct. You don't understand or accept basic high school and undergraduate physics - and so you can hardly expect anyone to pay attention to your pronouncements on something like high Tc superconductivity.

DITTO TO YOU!! Although I hope that you do not have grandchildren. No grandchild will hold you in high regard ever.

 

[johanfprins]

No such claim exists in the "main stream" literature - you are wrong, as usual. Magnetic fields attenuate exponentially within a superconductor, but they do not need to be zero.
That's completely wrong, as any text on superconductivity will tell you within the first few pages.

So why must a magnetic field be cancelled within a superconductor for a superconducting current to flow? If you think that a superconducting current flows through a surface region in which there is still a net magnetic field that accelerates the charge carriers, you are demented. I know that this is the main stream way of fudging what is really happening during superconduction; but if this is possible, a magnetic field will assist superconduction instead of quenching it.

Goodbye!