# How many Ampere in a superconducting spire?

1. Jul 5, 2011

### jumpjack

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?

2. Jul 5, 2011

### f95toli

The "model" you are using to describe what is happening in a superconductor is not quite correct.

However, the answer to the actual quesiton is: it depends
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)

3. Jul 5, 2011

### jumpjack

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?

4. Jul 5, 2011

### Staff: Mentor

0 volts. You cannot apply a voltage across a superconductor.

5. Jul 5, 2011

### f95toli

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).

6. Jul 5, 2011

### jumpjack

apart from the method, which is the max current achievable and which is the electrons speed?

7. Jul 5, 2011

### ZapperZ

Staff Emeritus
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.

8. Jul 6, 2011

9. Jul 6, 2011

### jumpjack

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: Jul 6, 2011
10. Jul 6, 2011

### jumpjack

Last edited: Jul 6, 2011
11. Jul 6, 2011

### ZapperZ

Staff Emeritus
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. Jul 7, 2011

### jumpjack

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