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Show ALL Forums  > Science/philosophy  > Quick question about current in a superconductor      Home login  
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 Ubiquitous.
Joined: 11/7/2009
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Quick question about current in a superconductorPage 1 of 1    
Simple question.

Say I have a large, super conductive ring in which I induce a current. Does the current spread throughout the whole ring evenly as it travels (like paint in a spinning can)? Or does it travel around the ring, mostly grouped together (like a train around a track)?

I'm thinking the former, but hoping for the latter. If it is the latter, then I might have stumbled upon something veddddy interesting.
 Paul K
Joined: 3/10/2006
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Quick question about current in a superconductor
Posted: 9/16/2010 5:09:14 PM
Are you assuming that the super conductor will allow the current to keep flowing even after whatever it was that caused it in the first place ceases to "push" it?

Here's why..... Lets say your SCring is X in circumference...... pick a BIG number, and you induce current for the time period of 1/4th of the time that it takes to make one revolution, I would say that as soon as you stopped inducing current, your "train" would stop dead in its tracks, so to say.

As long as you have an inductive force "forcing" the current along, you will have the current flowing. It makes sense that once that force ceases, so does the flow.

A super conductive coil reduces drag to very finite numbers, allowing incredible speeds, but it doesn't eliminate drag completely, so the current will stop.

Thats what I remember from college physics.

Paul K
 Kelso
Joined: 3/11/2007
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Quick question about current in a superconductor
Posted: 9/16/2010 5:23:30 PM
Great question OP, I've been laying awake at nights wondering that myself... Okay, I lied, but I'll give it my best shot anyway, with the caveat that I'm a Mech Eng, not an Electrical Eng (so any Electrical's can correct me). I don't understand your “paint in a spinning can” analogy, so I may not be answering the question you've asked.


Say I have a large...
Whether large, medium, or small, it shouldn't matter.


...super conductive...
but not likely operating at absolute zero (temperature), correct? If not, then it's not perfectly “super conductive,” which means there is still some friction, albeit small. That's important, which I'll get to in a minute.


Does the current spread throughout the whole ring evenly as it travels (like paint in a spinning can)?
Yes and no. The current (the moving electrons) will spread ( “are” spread) throughout the whole ring, but the speed of those electrons will not be even across a cross section of the ring. Because the electrons close to the outer edge of the ring will experience more friction, they will move slower than the electrons near the centre (of the cross section) of the ring. Current within a wire operates the same way, as does water in a hose. There's less friction near the centre, so things move faster there than things towards the edge.


Or does it travel around the ring, mostly grouped together (like a train around a track)?
Unless there is a force “forcing” the electrons into a line (like a train on a track) then, no, there is no reason for them to do so. They'll fill, and use, the space in which they are allowed to travel.
 stargazer1000
Joined: 1/16/2008
Msg: 4
Quick question about current in a superconductor
Posted: 9/16/2010 6:18:28 PM
What do you think you've "stumbled" upon?
 Ubiquitous.
Joined: 11/7/2009
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Quick question about current in a superconductor
Posted: 9/16/2010 6:30:44 PM
I guess the question really is, would there be an orbiting magnetic field induced by the electrons or would the magnetic field be static?

Is there any way to get the electrons in a superconductor to produce an "orbiting" magnetic field?

We all know what a picture's worth, so here's an illustration:

(Replace "force" with "field")

http://i56.photobucket.com/albums/g176/ijust1/POF.png
 AppleGeek
Joined: 9/26/2006
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Quick question about current in a superconductor
Posted: 9/17/2010 4:55:18 PM
The electrons will be spread out around the superconductor depending on the shape of its surface. The fact that its a superconductor doesn't impact this, it happens with any conductor. You won't see any pulses or waves of electrons unless you look in incredibly small time intervals since the electrons are moving at the speed of light.
 Paul K
Joined: 3/10/2006
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Quick question about current in a superconductor
Posted: 9/18/2010 3:36:52 PM
One of the fun things about having an honest to goodness rocket scientist in the family is asking them questions, and then having them trying to show you how much they know..................... He didn't do that great in English class......... This was his answer:




"as far as i know the difference between a superconductor and a regular conductor is that the internal resistance, or how much power is lost in the conductor itself, is much less.



if you’re thinking of speed, you can look at electricity in a couple of ways.

either how fast the signal is moving, or how fast the actual electrons are moving.

the signal moves at the speed of light, while the actual electrons (on average) are traveling across the conductor much much MUCH slower than that. in all actuality any conductive material has electrons floating from one atom to another. electrons are moving pretty near the speed of light around an atom anyway. in fact, you cannot pinpoint one without moving it when you try and observe it, for this reason physicists typically think of electrons in terms of clouds or probable locations. in otherwords ‘theres x number of electrons floating around at the speed of light somewhere in a certain cloud, or zone’. for the atoms in a conductor, the electrons are shared across the whole piece of material. so, for a conductor, a single electron could possibly be floating all over the entire thing at the speed of light. to get useful electricity, however, you have to get an appreciably larger number of electrons out one side and in the other side of a conductor.



to talk about the speed of electricity you really gotta nail down what exactly you are talking about. for sake of illustration say you have 1 billion electrons floating happily on a wire. put a battery to it now and you have started adding some electrons to the left end, and taking some off the right end. when we talk about current, we are talking essentially about how many electrons are being added to one end and taken off the other. voltage is the potential to push electrons at a certain rate. think of a water pump pushing water through a pipe and back to itself, thats like current flowing. if you plug the pipe (ie break a wire) no water flows (no electrical current flows), but the pump is still putting alot of pressure on the plugged ends of the pipes. this pressure is analogous to the voltage.



now think about this, if we have a looong pipe full of water, do i have to send the atom of water at my end of the pipe all the way to your end before you realize any water is moving? no, you notice the instant water starts moving, because if i add a galon to my end, a gallon has to come out your end right away, even if the whole pipe can hold 100 gallons. actually, the time it would take you to notice the push of water has to do with the speed of sound inside the water. think about it...if i just bump the water really fast, itll send a sound wave throught the pipe that you wont notice until it gets to you. in electricity its the same way...except you notice even faster because it has to do with the speed of light ... much faster than the speed of sound.



When the speed of light becomes important to electricity is when you have electrical vibrations with frequencies on the order of GHz (gigahertz), giga meaning billion, and hertz meaning cycles...or billions of ‘plus-minus’ voltage cycles per second. with high frequencies like that, the electrons an inch or so away from the source of the vibration have only just noticed the ‘push’ by the time the source of the vibration has started to ‘pull’



so to use the train analogy, yes, they are grouped together like a train, but the train is compressible such that a compression wave propagates along the train at the speed of light. if you push the caboose, the engine will feel it after a time equal to the length of the train divided by the speed of light (roughly 300,000 km per second, or 186,000 mi/sec, or 11 million miles per hour)



It takes a signal 1/186,000 seconds (.0000053 seconds or 5.3 microseconds) to get across a mile long wire.



now we’re getting into the physics of radio antennas. the length of a radio antenna has to do with how long a standing wave of electricity of that specific frequency is.



if you could completely empty any piece of metal of all electricity, you would have a hell of a voltage on that thing! every electron around it would want to stick to it and it would probably trigger some sparks to get them there."


Definitely a fun person to talk to.......

Paul K
 Appreciative9809
Joined: 9/8/2009
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Quick question about current in a superconductor
Posted: 9/20/2010 2:51:28 PM
Ubiquitous:


I guess the question really is, would there be an orbiting magnetic field induced by the electrons or would the magnetic field be static?

Is there any way to get the electrons in a superconductor to produce an "orbiting" magnetic field?


Electrons moving in unchanging circular paths about a common center generate a static magnetic field.

Induction motors generate and use a rotating magnetic field.

AppleGeek:

electrons are moving at the speed of light.


As Paul K. pointed out, the elections don't move at the speed of light.
 .dej
Joined: 11/6/2007
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Quick question about current in a superconductor
Posted: 9/20/2010 4:06:37 PM

Say I have a large, super conductive ring in which I induce a current. Does the current spread throughout the whole ring evenly as it travels (like paint in a spinning can)? Or does it travel around the ring, mostly grouped together (like a train around a track)?

Current is the flow of all electrons in the conductor. It cannot group. It is merely the rate of change of charge. You can sample the current at any point, and on a continuous conductor, it will be the same everywhere (Khirkoff's current law).


A super conductive coil reduces drag to very finite numbers, allowing incredible speeds, but it doesn't eliminate drag completely, so the current will stop.

Not quite. A superconductor reduces resistance to zero. A coil will cause induced resistance due to electromagnetics. Current will flow due to voltage differences. Once there is no voltage difference, current will settle to zero.


the electrons are moving at the speed of light.

No massive particle may move at the speed of light.


now think about this, if we have a looong pipe full of water, do i have to send the atom of water at my end of the pipe all the way to your end before you realize any water is moving? no, you notice the instant water starts moving, because if i add a galon to my end, a gallon has to come out your end right away, even if the whole pipe can hold 100 gallons. actually, the time it would take you to notice the push of water has to do with the speed of sound inside the water. think about it...if i just bump the water really fast, itll send a sound wave throught the pipe that you wont notice until it gets to you. in electricity its the same way...except you notice even faster because it has to do with the speed of light ... much faster than the speed of sound.

^This is a pretty good explanation. Except the velocity of propagation (how fast that wave moves) is different in each substance. In copper, it's about 7/10ths the speed of light.
 CoolBreezez
Joined: 8/20/2006
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Quick question about current in a superconductor
Posted: 9/20/2010 4:40:09 PM

I guess the question really is, would there be an orbiting magnetic field induced by the electrons or would the magnetic field be static?

Is there any way to get the electrons in a superconductor to produce an "orbiting" magnetic field?


Usually in a conductor a magnet field is created around the exterior, such as in a copper wire, but it could be different in nature in the case of a superconductor.

Look it this - interesting piece- it may answer your questions.

http://skepticsplay.blogspot.com/2010/08/superconductors.html
 abelian
Joined: 1/12/2008
Msg: 11
Quick question about current in a superconductor
Posted: 9/20/2010 5:11:41 PM
The former. The electrons pair up to form a condensate of cooper pairs which then flow through the superconductor as a frictionless superfluid.
 RATHLINLIGHTHOUSE
Joined: 2/10/2009
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Quick question about current in a superconductor
Posted: 9/21/2010 5:32:34 AM
Asssuming the cross section of the ring is a circle .

If there was any mass in the movement of the electrons around the ring then the electrons would flow to the outside of the ring due to centrifugal forces.

This would result in an increasingly smaller cross section carrying the electrons and would therefore eventually increase the resistance and cause heat on the outermost part of the circle. ie the current would be flowing in only the equator of the ring. Similar to a congested highway.

Since there is no mass the electrons can move at the speed of light throughout the total cross section of the ring. Therfore no localised electron highway and no increase in resistance.
 RATHLINLIGHTHOUSE
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Quick question about current in a superconductor
Posted: 9/21/2010 5:37:48 AM
Since there is no resistance the current will continue to flow.
Flowing current will form a static magnetic field.

As soon as the static magnetic field is interrupted then there will be interruption of the current flow in the hypothetical ring such that it opposes the flow of electrons.
 Appreciative9809
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Quick question about current in a superconductor
Posted: 9/21/2010 9:16:14 AM
About whether the magnetic field will be static:

An unchanging electrical current will generate an unchanging magnetic field.
 Paul K
Joined: 3/10/2006
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Quick question about current in a superconductor
Posted: 9/21/2010 10:26:44 AM
One question about an electrical field........ You wrote:

An unchanging electrical current will generate an unchanging magnetic field.

That makes sense. However, if you were to pass a copper wire through the magnetic field, thereby creating a current through the copper wire you just passed through the unchanging magnetic field, would that in any way disturb, or change, or place resistance on the original unchanging electrical current?

Paul K
 .dej
Joined: 11/6/2007
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Quick question about current in a superconductor
Posted: 9/21/2010 11:56:56 AM
If there was any mass in the movement of the electrons around the ring then the electrons would flow to the outside of the ring due to centrifugal forces.

This is not true. Electrons have more scattered movement than that. They do not flow like a rigid rope.


This would result in an increasingly smaller cross section carrying the electrons and would therefore eventually increase the resistance and cause heat on the outermost part of the circle. ie the current would be flowing in only the equator of the ring. Similar to a congested highway.

Also not true. They do not flow in the way a highway flows. This is not a classically modeled fluid flow. Electrons do not behave that way.


Since there is no mass the electrons can move at the speed of light throughout the total cross section of the ring. Therfore no localised electron highway and no increase in resistance.

This is also not true. First of all, electrons DO have mass. The electron is an elementary massive particle. Second of all they do NOT move at the speed of light. Third of all, statistical electron drift is VERY slow (meaning the center of mass of some given group of electrons moves very slowly, while the electrons themselves move very fast).

If you don't know what you're talking about, why do you try and explain things like this? Literally every single thing you said was incorrect.
 abelian
Joined: 1/12/2008
Msg: 17
Quick question about current in a superconductor
Posted: 9/21/2010 1:48:49 PM
First, a drift velocity cannot be defined for a superconductor in the same way that it can in an ordinary conductor. In an ordinart conductor, the resitance is finite and there is a potential difference between any two points along the wire because the resistance is finite. Electrons are accelerated until they collide with other electrons and transfer some of their momentum. The collisions account for the resistance and the drift velocity may be determined from the mean free path of the electron between collisions and the velocity it gains when accelerated over that distance by the microscopic electric field.

For a superconductor, there is no resistance, so the potential difference between any two points in the superconductor must also be zero and hence the electric field is zero. The supercurrent is due to the cooper pairs, not individual electrons and since the superconductivity is due to the fact that the cooper pairs cannot transfer energy through collisions, a drift velocity in the sense above is meaningless. A sort-of local drift velocity can be obtained from the quantum mechanical description, however. It is related to the coherence length of the cooper pairs and the magnitude of the energy gap that seperates the superconducting phase from the ordinary phase. I don't have the time to obtain an expression for that right now, but I'll derive one and post it. Anyone who is interested can look up the London equation.
 hyoid
Joined: 5/12/2009
Msg: 18
Quick question about current in a superconductor
Posted: 9/21/2010 5:16:53 PM
One question about an electrical field........ You wrote:

An unchanging electrical current will generate an unchanging magnetic field.

That makes sense. However, if you were to pass a copper wire CIRCUIT through the magnetic field, thereby creating a current through the copper wire you just passed through the unchanging magnetic field, would that in any way disturb, or change, or place resistance on the original unchanging electrical current?


From the copper wires perspective, the magnetic field is changing. The change induces a current in the wire, which in turn generates a magnetic filed whose polarity opposes the polarity of the static field. There is a force generated between the opposing magnetic fields which would cause a rotation of either the stator or circuit to a position of least flux interaction between the two.
If you hold the stator steady,you have the beginnings of an induction motor/generator.

To answer your question: Yes. The opposing magnetic field acts on the stator field causing a counter EMF proportional to the effort you take to move the circuit.
 Appreciative9809
Joined: 9/8/2009
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Quick question about current in a superconductor
Posted: 9/21/2010 5:44:41 PM
[
One question about an electrical field........ You wrote:

An unchanging electrical current will generate an unchanging magnetic field.

That makes sense. However, if you were to pass a copper wire through the magnetic field, thereby creating a current through the copper wire you just passed through the unchanging magnetic field, would that in any way disturb, or change, or place resistance on the original unchanging electrical current?

Paul K. :

When you pass the wire sideways through the magnetic field, a current is induced in the wire, as you said. That current creates a magnetic field around the wire. As that magnetic field comes into existence, it subjects the original current-carrying wire to an induced voltage, due to this new changing (increasing) magnetic field. I'd expect that induced voltage to opppose or augment the voltage driving the original current in the original current-carrying wire, changing the original current temporarily as the new wire's new magnetic field comes into being. But only temporarily, as the new wire's magnetic field is established. Then, when the moving wire stops moving or leaves the vicinity, its magnetic field will decrease, and the original wire will again experience a changing magnetic field--decreasing this time--inducing a voltage in it, opposite to the previous induced voltage. Then, when the moving wire is again sitting stationary, or has moved far away, the original wire won't be experiencing any induced voltage, and its current will again be driven by, and only affected by, the original battery-voltage that was driving it before, and its current will return to its original value.

A one-phase squirrel-cage induction motor makes use of a both-ways interaction between a moving wire and the magnetic field in which it moves. Changes in the field magnets' magnetic field, due to the AC current passing through them, induce current in the squirrel-cage. But the changing currents in the spinning squirrel-cage generate their own changing magnetic field, perpendicular to that generated by the field magnets. These two perpendicular and varying magnetic fields amount to a rotating magnetic field.

That rotating magnetic field, sweeping past the squirrel cage, continues to induce the current needed to give to it the magnetic field that interacts with the field magnets to produce the motor's output torque.

1-phase induction motors, unlike 3-phase motors, need some special electrical provison to make the initial rotating magnetic field to get the motor started. So we hear of "capacitor-start", "split-phase", or "shaded-pole" induction motors. They're commonly used in fans and other appliances.

The squirrel-cage never rotates at quite the rate of the rotating magnetic field.
 hyoid
Joined: 5/12/2009
Msg: 20
Quick question about current in a superconductor
Posted: 9/21/2010 6:04:37 PM

Is there any way to get the electrons in a superconductor to produce an "orbiting" magnetic field?


Actually, that's the only kind of magnetic field possible in a superconductor. One of the definitions of a material that is superconducting is that it expels all magnetic fields from its interior. Not absolutely true, since it admits enough magnetic flux to generate local supercurrents whose own field exactly cancels the admitted flux. Place a superconducting wire in a strong enough magnetic field and it quenches (goes normal). I think because the local supercurrents exceed the current density of the material.

In a single conductor, the magnetic field builds as a torus around the conductor. North pole and south pole kind of lose their meaning but the field does have an orientation that changes from clockwise to counterclockwise with opposite current flow.

I think you're hoping for field variations that would propagate around the circumference of the circle. To achieve that you'd have to have currents flowing in opposite directions at different points around the ring. You might devise a very high frequency oscillator but getting energy out would be the trick.
 Appreciative9809
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Quick question about current in a superconductor
Posted: 9/22/2010 12:45:20 PM
When answering about the moving wire in the magnetic field, I was looking at the result of the increasing and decreasing of the moving wire's magnetic field, in inducing a voltage in the stationary wire.

So I said that when the moving wire's magnetic field was established, voltage would no longer be induced in the stationary wire. I forgot that the motion of the moving wire, with its magnetic field would, itself, induce voltage in the stationary wire.

When the moving wire's magnetic field is increasing, staying the same, or decreasing, it will induce voltage in the stationary wire--until the moving wire stops, and loses its magnetic field, or moves too far away to significantly affect the stationary wire.

Of course we're assuming that the moving wire is part of a circuit.
 RATHLINLIGHTHOUSE
Joined: 2/10/2009
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Quick question about current in a superconductor
Posted: 9/22/2010 2:48:51 PM
dei

The beginning of my suggestion is "IF".

If there was mass circulating at any speed then it would move to outside of ring .
Perhaps you know of some form of physics in which mass does not try to travel in a straight line except when acted upon by a force.
 .dej
Joined: 11/6/2007
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Quick question about current in a superconductor
Posted: 9/22/2010 11:10:32 PM
If there was mass circulating at any speed then it would move to outside of ring .
Perhaps you know of some form of physics in which mass does not try to travel in a straight line except when acted upon by a force.

These are not golf balls. They are electrons. You cannot apply classical mechanics to this situation. Electric current is not just a bunch of electrons all moving down a pipe, and the electrons are not just flowing parallel to the wire direction tangent. The "form of physics" you seek can be introduced here:

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

Best of luck.
 RATHLINLIGHTHOUSE
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Quick question about current in a superconductor
Posted: 9/23/2010 7:35:43 AM
dei

Since the current continues to flow with no resistance it can not be confined to a "highway" running round the equator of the ring we are discussing.

Therefore the electrons must move through "All" of the cross section of the ring.

Immediately this provides large quantities of metal atoms to interact with (I assume it is a metal).
Therefore there are large quantities of routes by which the electrons can be moved from one atom to the next.

My understanding is even this is not what happens in the circulating current.

I stand by what i said. IF the circulating current was moving in a material with zero resistance then there would be no reason to limit the speed of the circulation of the electrons.
Even if the electrons were of almost neglible mass the rotation at high speed would still move them to the outside of the ring.

However this apparently is not what happens.
 .dej
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Quick question about current in a superconductor
Posted: 9/23/2010 9:58:37 PM
No, it's not. Electrons move very fast, but not in the direction of current. They move randomly. When there is current flow, their movement is simply slightly more likely to be in the direction of the current than in the other direction.
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