How many Ampere in a superconducting spire?

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    Ampere Superconducting
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Discussion Overview

The discussion revolves around the properties of superconductors, specifically focusing on the current that can be induced in a superconducting spire or ring, the relationship between voltage and current in superconductors, and the behavior of electrons within these materials. The scope includes theoretical considerations, practical implications, and some experimental aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a current in a superconducting ring can flow indefinitely due to the lack of friction against the lattice, suggesting that electrons could be accelerated indefinitely under ideal conditions.
  • Another participant clarifies that the critical current density (Jc) of a superconductor varies based on multiple factors, including the type of superconductor, temperature, and material structure, indicating a wide range of possible current densities.
  • A participant inquires about the voltage required to induce a specific current in a superconductor, noting that Ohm's law may not apply.
  • One response states that no voltage can be applied across a superconductor, emphasizing the need for a current source instead.
  • Another participant reiterates the need for a current source or a voltage source with a resistor for inducing current in a superconducting ring.
  • Questions arise regarding the maximum achievable current and the speed of electrons in superconductors, with one participant mentioning that electron speed is a complex concept due to the nature of Cooper pairs.
  • Another participant provides an estimate of the maximum current density achievable in certain superconductors at low temperatures, while noting that this may not be realized in large-scale structures.
  • There is speculation about the behavior of electrons in superconductors, comparing their movement to that of "small balls" rather than waves, which prompts a suggestion for further learning about superconductivity.
  • One participant expresses a desire for resources to better understand the physics terminology used in the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the nature of current and voltage in superconductors, with some asserting that voltage cannot be applied while others discuss methods for inducing current. There is no consensus on the maximum current achievable or the interpretation of electron behavior in superconductors.

Contextual Notes

Limitations include the dependence of critical current density on various factors and the unresolved nature of electron behavior in superconductors, particularly regarding the distinction between average speed and drift velocity.

jumpjack
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As far as I can understand, after inducing a current inside a superconducting ring or spire, the current keeps flowing into the ring "for ever". Is this correct?

Since electrons have no "friction" against the superconducting lattice, does this mean that I can virtually accelerate them "indefinitely", thus inducing an "indefinitely" high current?

I mean, if current is caused by electric force applied to electrons, and resistance is due to electrons friction against lattice, maybe they accelerate indefinitely as long as I apply the force?

"for ever" and "indefinitely" of cours refer to ideal conditions; so what does it actually happen in real world? How much a current can I induce into a superconducting ring?
 
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The "model" you are using to describe what is happening in a superconductor is not quite correct.

However, the answer to the actual question is: it depends:cool:
The critical current density Jc of a real superconductor depends on many different factors: the type of superconductor (aluminium is very different from say YBCO), the temperature (lower T gives higher Jc), the structure of the material (mainly the amount and type of grain boundaries), the frequency (superconductors are only lossless at DC), the magnetic field etc.

Hence, the answer can be anything from say 100 A/cm^2 to 10^8 A/cm^2.

(note the units: a cable with cross-sectional area of 1 cm^2 and a a Jc of 100 A/cm^2 can carry 100 A of current)
 
But how much voltage would I need to induce 100 A, being I=V/R law no more valid?
Can I apply multiple low-voltage pulses to get an high current, considering that current wouldn't stop flowing between a pulse and the other? Or do I need a constant high voltage?
 
0 volts. You cannot apply a voltage across a superconductor.
 
Dalespam is of course correct.
You can not "inject voltage" in a superconducting ring, you need a current source (or a voltage source with a suitable resitor in series).
 
apart from the method, which is the max current achievable and which is the electrons speed?
 
jumpjack said:
apart from the method, which is the max current achievable and which is the electrons speed?

Electron speed is a strange concept here. The cooper pairs have long-range coherence, which is the definition of superconductivity.

Maximum current depends on the superconductor. Look up "superconducting critical current density".

Zz.
 
jumpjack said:
apart from the method, which is the max current?QUOTE]

I gave you that answer above, I'd say about 10^8 A/cm^2 (Ca doped YBCO at temperatures below 4K). However, I don't think anyone has ever achived that in a large scale structure.
 
ZapperZ said:
Electron speed is a strange concept here. The cooper pairs have long-range coherence, which is the definition of superconductivity.
But this is like talking about lightspeed in a conductor, although electrons move at a few cm/sec in a conductor.

Apart from the EM propagation into a SC, I wonder if electrons in SC do behave more like "small balls" than like waves, having no obstacles to their movements around.
 
Last edited:
  • #11
jumpjack said:
But this is like talking about lightspeed in a conductor, although electrons move at a few cm/sec in a conductor.

Apart from the EM propagation into a SC, I wonder if electrons in SC do behave more like "small balls" than like waves, having no obstacles to their movements around.

I strongly suggest you learn a bit more about superconductivity before making such silly speculation.

Furthermore, you should also look up the average speed of electrons in a regular conductor. Do not confuse that with the drift velocity!

Zz.
 
  • #12
ZapperZ said:
I strongly suggest you learn a bit more about superconductivity before making such silly speculation.

Furthermore, you should also look up the average speed of electrons in a regular conductor. Do not confuse that with the drift velocity!

Zz.

any link? I'm not comfortable with English physics jargon.
 

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