# Superconductors



## qubit (Jul 12, 2015)

Aquinus said:


> We could have an entire discussion on this topic alone.  What you describe is a little unusual in a DC circuit though because even if voltage were to go to zero, the current induced by an inductor releasing its energy will result in a change in voltage since resistance is most likely going to remain constant. Since inductors resist change in current, the current created by the inductor will actually cause voltage as a result of the resistance in the circuit. Also zero resistance doesn't mean infinite current, it means zero resistance, as in zero energy lost as current travels through it. You're still limited by the number of electrons available to be moved (which is where Dave's usage of coulombs, which is the actual measurement of electric charge (not electric potential, volts,) comes into play.) Usually the case of current and no voltage is in AC circuits where inductive loads cause the current to go out of phase from the provided AC voltage and this is referred to as reactive power. This can cause a current draw when the AC circuit voltage is zero but, it can also be the case that a non-zero voltage can have zero current.
> 
> I just felt a little more information on the topic was in order as there seemed to be a bit of confusion. Not to say that what anyone said was particularly incorrect, just incomplete.
> 
> ...


Right, here's that entire discussion on this topic alone. 

Ok, but the material is a superconductor ie no resistance at all, so I don't see how there can be any voltage across it, inductance or no inductance? If there's some unusual physics at play that somehow does allow voltage in this situation then I'm happy to learn. It sounds like you're saying that with the limited number of electrons that can flow, that would still allow a voltage across the superconductor, then? Well, maybe, but I'm doubtful. As I've said in the other thread, I'm hardly an expert on superconductors, so do you have a link to an article that explains this situation?

Any other normal conductor with even a tiddly bit of resistance then of course I'd completely agree with you and obviously voltage without current is extremely common: it's just an open circuit.

And just to clarify, I know current can't go to infinity as I've already said a couple of times in the other thread.

Shame about CircuitLab, sounds like it's a very useful tool.


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## Aquinus (Jul 12, 2015)

qubit said:


> Ok, but the material is a superconductor ie no resistance at all, so I don't see how there can be any voltage across it, inductance or no inductance?


You're confusing inductance and resistance. The two terms are not equitable. I quote from the inductance acticle from Wikipedia might be in order:


> When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor, according to Faraday’s law of electromagnetic induction, which opposes the change in current that created it. As a result, inductors always oppose a change in current, in the same way that a flywheel oppose a change in rotational velocity. Care should be taken not to confuse this with the resistance provided by a resistor.



An inductor could have zero resistance but, could still resist chance to current which would generate something similar to resistance (except somewhat different, think of two magnets of the same polarity repel each other, but it would be based on the interactions in the magnetic field in the iron core of an inductor.

A super conducting inductor would still resist chance to current because of how magnetic fields and electric currents interact. So what you would probably have is when current changes (in a superconductor,) that the inductor would produce temporary voltages until the magnetic field adjusts to the new current. This resistance in current would show up as, as I said before, a temporary voltage when current changes. That is all.

In fact, this effect where inductors voltage gets boosted when abruptly stopping current going into an inductor is the basis for DC step-up converts as it's this that provides the higher-than-input, output voltage.

Side note: If a wire, inductor, and capacitor were perfect super conductors, you could create a permanent resonant circuit since no energy is lost when current reverses on each cycle. See LC circuit.

Lastly: Thank you for making a discussion outside of the other thread.


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## cadaveca (Jul 12, 2015)

qubit said:


> Right, here's that entire discussion on this topic alone.
> 
> Ok, but the material is a superconductor ie no resistance at all, so I don't see how there can be any voltage across it, inductance or no inductance? If there's some unusual physics at play that somehow does allow voltage in this situation then I'm happy to learn. It sounds like you're saying that with the limited number of electrons that can flow, that would still allow a voltage across the superconductor, then? Well, maybe, but I'm doubtful. As I've said in the other thread, I'm hardly an expert on superconductors, so do you have a link to an article that explains this situation?
> 
> ...


Electricity is more about magnetic fields, and although we consider that electrons flow down a wire, that analogy in and of itself is not quite accurate since really all that travels is the spin of an electron. The only time electrons move is in the case of chemical reactions, but the majority of power we use is magnetic, not chemical, since chemical energy generation is destructive.

Inductance is just that... a magnetic field affecting another field, and causing a physical reaction, akin to you flowing on a fan and making it move, a magnetic field affects the spin of the electrons in the conductor and causes a difference in the energy level contained within the object.


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## BiggieShady (Jul 12, 2015)

cadaveca said:


> Electricity is more about magnetic fields, and although we consider that electrons flow down a wire, that analogy in and of itself is not quite accurate since really all that travels is the spin of an electron. The only time electrons move is in the case of chemical reactions, but the majority of power we use is magnetic, not chemical, since chemical energy generation is destructive.


This reminds me of a question from an exam back when I was in university:

Suppose that we have electric circuit around the world on the equator with electric source and the switch in Brasil and a bulb in Indonesia. How much time will pass between the switch flips and the bulb lights up?

The trick is that disturbance of electromagnetic field is propagated through conductor at the speed of light. So to travel half a world away its around 6,68 hundredths of a second.
But electrons do travel through the circuit but super slowly as a resulting summary vector of Brownian motion. That Brownian motion of free electrons in the metal's crystalline lattice happens instantly ("speed of light" instantly) in the whole circuit, each electron almost vibrates in it's place but slowly the resulting flow of all electrons is steadily in the same direction.


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## Aquinus (Jul 12, 2015)

cadaveca said:


> Electricity is more about magnetic fields, and although we consider that electrons flow down a wire, that analogy in and of itself is not quite accurate since really all that travels is the spin of an electron.


Actually, electrons do move, however it's actually a lot slower than people think but, electrons do move. It's called drift velocity which describes electron mobility. Electrons create a magnetic feild because of their spin but that's not how they "power" devices. The movement of electrons themselves actually does represent electric power. Remember, current is coulombs per second and coulombs are the real charge carrier.

See numerical example of drift velocity on wikipedia to see how this works.

Side note: I'm glad that after 4-5 years, I still remember a lot of university physics in college. (When I say university physics, I mean physics that includes calculus.)


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## cadaveca (Jul 12, 2015)

...and we're off!


LoL.


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## MrGenius (Jul 12, 2015)

Careful with the "voltage without current" jargon. That is provable to be a false statement. Voltage without current cannot be measured/observed. Yes, voltage _might_ exist(hypothetically). But you can't prove voltage is present without a measurement/observation of current through a circuit. That's impossible. Voltage requires a current to be detectable.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/movcoil.html#c1


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## Aquinus (Jul 12, 2015)

MrGenius said:


> Careful with the "voltage without current" jargon. That is provable to be a false statement. Voltage without current cannot be measured/observed. Yes, voltage _might_ exist(hypothetically). But you can't prove voltage is present without a measurement/observation of current through a circuit. That's impossible. Voltage requires a current to be detectable.
> 
> http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/movcoil.html#c1


Your link describes how meters work. It says absolutely nothing about a voltage not being able to exist without a current which is a false claim. There is a physics.stackexchange thread about this very idea that you propose and the accepted answer was this:


> What flows is not the voltage but the charge, and that flow is called current. Voltage can be without a current, if you have a single charge, that charge induces a voltage in all space, even if it's empty. Voltage, in the most physical way, is a scalar field that determines the potential energy per unit charge at every point in space.
> 
> Now, you can't have currents without voltages because if there's a current there's a charge moving, and every charge produces a voltage, but you can have currents without voltage differences in space. For example, if you have a charged sphere, and you make it rotate, the charge will be on the surface and by rotating the sphere you will have a current on the surface, but the voltage is the same in all surface. Also magnetization of materials can induce currents by the same way.



Voltage simply implies electric potential, much like gravitation potential energy and other forms of energy. It does not describe movement of charge, it describes a difference in charge, really. Also meters use current because that is how they determine voltage. It's a tuned circuit that operates with certain components remaining constant. If you consider a digital voltmeter, you could have resistances as high as 10gΩ which would have an insanely small current.

A great example of how you can see voltage is when you rub your feet (wearing socks or shoes,) on a dry day on a rug. Your hair stands on end because the electrical potential difference between you and the surrounding space. Since you're insulated, there is no current. Current exists when you get a shock from touching a door knob but, that doesn't mean it wasn't there until you touched the door knob. That's simply nonsense.

So no, @MrGenius , you can have voltage without current and we can measure that from electrostatic forces when an object is charged. In fact, all of this can be described without even referring to voltage or current with Coulomb's law. Your statement at a lower level makes absolutely no sense and it's a false equivalency. Correlation doesn't imply causation, friend.

How do you measure this, you ask? It's called an *electroscope* and was one of the earliest means of detecting electric charge.

Side note: Sources don't mean squat if you don't understand the underlying principles.


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## MrGenius (Jul 12, 2015)

That's nice. But it doesn't explain the fact that a Volt is a unit of measurement. And that you need a voltmeter to measure it. Which is in simpler terms a current detector. I carefully worded my statement. It's as correct as it could possibly be. You can call it whatever you wish. But if you haven't measured it with a voltmeter, it's not a Volt. I'm talking about Volts, period. What you're talking about is almost everything but. Not that it's totally irrelevant. It certainly is relevant. It just lacks specificity.

Not trying to argue here. Just advising caution when using certain terminology. It can be very misleading.


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## Aquinus (Jul 13, 2015)

MrGenius said:


> That's nice. But it doesn't explain the fact that a Volt is a unit of measurement. And that you need a voltmeter to measure it. Which is in simpler terms a current detector. I carefully worded my statement. It's as correct as it could possibly be. You can call it whatever you wish. But if you haven't measured it with a voltmeter, it's not a Volt. I'm talking about Volts, period. What you're talking about is almost everything but. Not that it's totally irrelevant. It certainly is relevant. It just lacks specificity.
> 
> Not trying to argue here. Just advising caution when using certain terminology. It can be very misleading.


What's misleading is how you're saying "I'm talking about a volt and nothing else," because physics doesn't work that way. Do you know what a volt is? A volt is joules per coulomb; amount of work (energy) per electric charge and you say that coulombs are irrelevant? Since when is the charge carrier not important when talking about electricity? You can most definitely measure voltage without a voltmeter because volts is a derived value. You know voltage if you know the amount of charge at the two points and the the force that interacts between them. Such a thing can be measure with a...


Aquinus said:


> It's called an *electroscope* and was one of the earliest means of detecting electric charge.


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## qubit (Jul 13, 2015)

Some seriously cool posts on here, guys. 

@Aquinus @cadaveca Reading through these posts, I've realized that I've made a simple, but glaring error when explaining my argument. I was thinking of a totally steady DC voltage, not an AC voltage, but forgot to specify it. This would naturally confuse the discussion, so my bad and I'm sorry!

With AC, I would expect there to be transient effects, so I'm sure there would be an oscillating voltage across that superconductor due to inductance. As soon as it settles down however, I reckon it would disappear. I'm gonna have to start reading up on this to check if I'm right. The issue of course, is that a superconductor can only carry so much current (albeit a huge one) before it stops superconducting, even if the temperature remains the same. Hence, does this allow a DC voltage to exist across it? Worth finding out, I think.

Regarding our hypothetical round the world wire, I've been taught that electricity propagates through a wire at about a third to half the speed of light, rather than at the speed of light, with different materials having different speeds. I'd want to check on this though before saying this authoritatively though.

I remember about the electron drift, too. You'd hardly think it with multigahertz circuits, wouldn't you? But it's true.

Good point about the lossless LC circuit, Aquinus.


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## erocker (Jul 13, 2015)

To the two who's posts I just put in the garbage, either learn to address each other in a civil, relevant and on-topic manner or avoid each other. I don't care either way.

Thank you.


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## silkstone (Jul 13, 2015)

qubit said:


> Right, here's that entire discussion on this topic alone.
> 
> Ok, but the material is a superconductor ie no resistance at all, so I don't see how there can be any voltage across it, inductance or no inductance? If there's some unusual physics at play that somehow does allow voltage in this situation then I'm happy to learn. It sounds like you're saying that with the limited number of electrons that can flow, that would still allow a voltage across the superconductor, then? Well, maybe, but I'm doubtful. As I've said in the other thread, I'm hardly an expert on superconductors, so do you have a link to an article that explains this situation?
> 
> ...



I'm wondering what happens when you have a superconducting filament which is extremely thin. If it is so thin that it can physically only let a limited amount of charge to flow, it can still have zero resistance as the charge that is able to flow does not lose energy. If you have an abundance of charge/energy density at one end and a deficit on the other, this is a potential difference. Also, charge also takes time to move and so, by definition, there is always a potential difference when charge moves.


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## Aquinus (Jul 13, 2015)

qubit said:


> Regarding our hypothetical round the world wire, I've been taught that electricity propagates through a wire at about a third to half the speed of light, rather than at the speed of light, with different materials having different speeds. I'd want to check on this though before saying this authoritatively though.


Nah, you're right. It depends the material. EM waves such as radio waves travel through a vacuum at the speed of light but, may be 40-90% of the speed of light while traveling through a copper medium. IIRC, it depends on the insulator around the conductor it's traveling through but my memory on this in particular is a bit hazy so I don't want to say much about it. I'm not exactly sure why this is the case either.



qubit said:


> I remember about the electron drift, too. You'd hardly think it with multigahertz circuits, wouldn't you? But it's true.


Yeah, it's a little strange. It's a little less strange thinking about it in AC because it only represents the direction the electrons tend to be going but, in reality through every cycle, it's more like the electrons are vibrating as opposed to simply moving one direction. Should also be careful as to not underestimate how many electrons are in even a very small space. A lot more may have moved than one may think even if it's only a tiny distance.



silkstone said:


> I'm wondering what happens when you have a superconducting filament which is extremely thin. If it is so thin that it can physically only let a limited amount of charge to flow, it can still have zero resistance as the charge that is able to flow does not lose energy. If you have an abundance of charge/energy density at one end and a deficit on the other, this is a potential difference. Also, charge also takes time to move and so, by definition, there is always a potential difference when charge moves.


Well, if it's super conducting wouldn't it not be prohibited from going any full steam ahead? Without resistance, the only thing to slow current down would be with magnetic fields, I would imagine.


erocker said:


> To the two who's posts I just put in the garbage, either learn to address each other in a civil, relevant and on-topic manner or avoid each other. I don't care either way.
> 
> Thank you.


I updated my posts to be a little less douche-like, in the spirit of not hurting people's feelings.


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## silkstone (Jul 13, 2015)

silkstone said:


> I'm wondering what happens when you have a superconducting filament which is extremely thin. If it is so thin that it can physically only let a limited amount of charge to flow, it can still have zero resistance as the charge that is able to flow does not lose energy. If you have an abundance of charge/energy density at one end and a deficit on the other, this is a potential difference. Also, charge also takes time to move and so, by definition, there is always a potential difference when charge moves.



Hmm.. Found an answer from a quick search:

"Note that superconductivity doesn't eliminate impedance. Even if you had an ideal voltage source to connect across a piece of superconductor, the current would not be infinite. It would be limited by inductance (which would allow the current to gradually rise without bound). To properly model the circuit, it would have to be drawn as an ideal voltage source connected to an ideal inductor. Such a thing is mathematically possible and analyzable (and in fact probably occurs in numerous elementary textbooks as an example).

Ohm's Law is an idealization based on ideal resistance, which has no parasitic inductance or capacitance. As such, it breaks down long before we reach zero resistance. So the singularity at R=0 is purely academic. At R=0, we have a piece of wire which may be superconductive, but it exhibits capacitance and inductance.

Note, by the way, that in superconductors, current can flow without voltage. But this fits in with all ordinary laws that we apply in analyzing simple circuits. If draw the schematic of a ciruit which consists of a loop of ideal wire, then some finite current can flow in that loop forever without any potential differences anywhere in that loop. We can divide the loop in half, and each half can "think" that there is a current source in the other half."


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## qubit (Jul 13, 2015)

@silkstone Thanks man, it's great to be vindicated! It looks like that still holds true for really skinny wires too, like you asked in the previous post.


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## cadaveca (Jul 13, 2015)

qubit said:


> @silkstone Thanks man, it's great to be vindicated! It looks like that still holds true for really skinny wires too, like you asked in the previous post.


It's an ideal though, not practical reality. That's where you have to be careful. The LHC basically uses this principal to make itself work (LHC uses superconducting magnets), but to even send just light down a particle accelerator requires conditions that cost millions to replicate. The highest temp IIRC for a superconductor is well below -100c. As in a past thread I related that CPU current handling ability is dependent on doping, and the same applies to superconductors. Also note the author was careful to say "if we draw", since it's dealing with an ideal, not a practical application, since the current would be so small it would be practically unmeasurable, and again, "As such, it breaks down long before we reach zero resistance" means you weren't actually vindicated at all, because, as I said before... inductance. Or as Silkstone's quote says, capacitance. These two things are present everywhere, since things like gamma rays and whatever else in that realm of physics easily passes through any containment field.


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## silkstone (Jul 13, 2015)

cadaveca said:


> It's an ideal though, not practical reality. That's where you have to be careful. The LHC basically uses this principal to make itself work (LHC uses superconducting magnets), but to even send just light down a particle accelerator requires conditions that cost millions to replicate. The highest temp IIRC for a superconductor is well below -100c. As in a past thread I related that CPU current handling ability is dependent on doping, and the same applies to superconductors. Also note the author was careful to say "if we draw", since it's dealing with an ideal, not a practical application, since the current would be so small it would be practically unmeasurable, and again, "As such, it breaks down long before we reach zero resistance" means you weren't actually vindicated at all, because, as I said before... inductance. Or as Silkstone's quote says, capacitance. These two things are present everywhere, since things like gamma rays and whatever else in that realm of physics easily passes through any containment field.



I think I missed the original argument/point  Got a link to the thread?

I'm not sure if this is relevent, but I believe that there are superconductors in existence that can allow charge to flow almost indefinitely or at least longer than the life of the universe. The superconductors in MRIs are pretty damned advanced.

(Linky: http://www.supraconductivite.fr/en/index.php?p=supra-resistance-supra)


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## cadaveca (Jul 13, 2015)

silkstone said:


> I think I missed the original argument/point  Got a link to the thread?
> 
> I'm not sure if this is relevent, but I believe that there are superconductors in existence that can allow charge to flow almost indefinitely or at least longer than the life of the universe. The superconductors in MRIs are pretty damned advanced.


Yes, but they (superconductors) require constant cooling, which requires a power draw, which requires...


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## silkstone (Jul 13, 2015)

cadaveca said:


> Yes, but they (superconductors) require constant cooling, which requires a power draw, which requires...



Again, I'm not sure of the original point. But yes they must be cooled (unless you want to keep them on the dark-side of the moon).

New discoveries are being made into superconductivity though. I'm not sure how legit this is, but apparently superconductivity has been achieved at room temperature. So it's physically possible to do.
http://www.sciencealert.com/physicists-achieve-superconductivity-at-room-temperature


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## cadaveca (Jul 13, 2015)

silkstone said:


> Again, I'm not sure of the original point.


That's a mutual feeling. LoL. Maybe just "Yea, Superconductors!", which is cool. Pun intended.


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## BiggieShady (Jul 13, 2015)

qubit said:


> Regarding our hypothetical round the world wire, I've been taught that electricity propagates through a wire at about a third to half the speed of light, rather than at the speed of light, with different materials having different speeds. I'd want to check on this though before saying this authoritatively though.





Aquinus said:


> EM waves such as radio waves travel through a vacuum at the speed of light but, may be 40-90% of the speed of light while traveling through a copper medium.


In copper medium speed of light is 95.1% of speed o light in vacuum and plastic isolation layers shouldn't affect it at all IMO edit: https://en.wikipedia.org/wiki/Velocity_factor


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## silkstone (Jul 13, 2015)

BiggieShady said:


> In copper medium speed of light is 95.1% of speed o light in vacuum and plastic isolation layers shouldn't affect it at all IMO



Light travels through copper? Over copper, the propagation delay is .59c to.77c. But the term propagation delay means different things in different fields. The presence of material around a wire does affect the speed at which a field propagates, and so plastic layers can affect the speed.

In response to Quibit's post you quoted, electrons only propagate at their drift velocity, but EM field changes propagate a little under the speed of light (thus the energy).
If you picture electricity as a sound wave, you will be closer to reality than picturing it as flowing water. I'm not sure how clear people are on their conceptual understanding of what 'electricity' actually is without using more scientific terminology.

A little reading on the confusion generated by the term 'electricity': http://amasci.com/miscon/whatis.html


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## BiggieShady (Jul 13, 2015)

silkstone said:


> Light travels through copper?


It's not the visible light of course, it's still called light because any wave (propagation of disturbance) in the EM field is considered light


silkstone said:


> The presence of material around a wire does affect the speed at which a field propagates, and so plastic layers can affect the speed.


Found it ... it's called velocity factor and you can go up to 95% with right wire ... and true, insulation can bring it down to 66%

https://en.wikipedia.org/wiki/Velocity_factor


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## silkstone (Jul 13, 2015)

BiggieShady said:


> It's not the visible light of course, it's still called light because any wave (propagation of disturbance) in the EM field is considered light



Light is just the name we give to the visible portion of the EM spectrum. While light is a propagation of variations in the EM field, not all propagation are considered light. I understand the confusion though as radio, infra-red, etc. are the same oscillations, just with a different frequency.


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## BiggieShady (Jul 14, 2015)

silkstone said:


> Light is just the name we give to the visible portion of the EM spectrum. While light is a propagation of variations in the EM field, not all propagation are considered light. I understand the confusion though as radio, infra-red, etc. are the same oscillations, just with a different frequency.


Confusion or semantics? Infrared light or ultraviolet light are not visible and yet those portions of spectrum are still called light. Who will set the arbitrary wavelength after which photons stop being light and become radio?
Since discussions about semantics are usually futile and obviously we understand each other very well, I say we let photons decide for themselves what they are.


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