Superconductivity: Zero Resistance & Its Effects

[abdullahbameh]

is it really in super coundactivity the resistance is ZERO and how it become ZERO is it because of law temperature or the applied voltage and does that mean if we have a current in circuit of super conductor it will go for ever or not and why?
and why we can not have super conductivity from noble metals like silver or gold?

 

[f95toli]

Yes, in a perfect superconductor the dc resistance is zero*
And yes, a current would circulate forever.
There is no "simple" reason why the gold, silver and copper do not become superconducting, it just happens to be the case that they have a crystal structure that means they are not BCS superconductors even under pressure (and there are also other elements that do not become superconducting). Note that there is no simple correlation between the conductivity of a material and its superconducting transition temperature


*In real superconductors there are always mechanisms (such as flux flow, hopping etc) that cause some loss of energy so the dc resistance is not actually zero, just very,very low. The decay time for the supercurrent can be of the order of years.

 

[aegeansea]

the reply above is good. I think he is major in physics

 

[malawi_glenn]

he is post doc...

 

[Gigacore]

...and if gold and silver had super conducting property, then it would've been the most expensive super condutors ever!

 

[aegeansea]

i have a question like this: when a superconductor is on the meissner state, how does it interact with a magnet near it ? i think it maybe interact through the supercurrent, but how does the current circulate?

 

[FortranMan]

it interacts with a magnetic field by repelling it, no matter which pole of the magnet faces the superconductor. It does so be using surface currents that generate an equal but opposite field to the applied field from the magnet. Also, are these surface currents superconducting or supercurrents in that they are flowing without resistance? Could one say they result from Lenz's law?

 

[genneth]

it interacts with a magnetic field by repelling it, no matter which pole of the magnet faces the superconductor. It does so be using surface currents that generate an equal but opposite field to the applied field from the magnet. Also, are these surface currents superconducting or supercurrents in that they are flowing without resistance? Could one say they result from Lenz's law?

They are supercurrents, but it would not be accurate to say that they arise due to Lenz's law. If they did, you would not observe the Meissner effect. People seem to mistake the Meissner effect with perfect diamagnetism. The former is more than the latter. Specifically, when a superconductor is put into a magnetic field whilst above its Tc, and *then* cooled all the while in the field, it will spontaneously exclude the magnetic field at Tc. Thus you see that it is not a consequence of Lenz's law, since there is no change in the magnetic field. A perfect diamagnet would create currents which exactly oppose any magnetic field brought near it, but it would not necessary do this spontaneous-expulsion.

 

[BANG!]

Could one say they result from Lenz's law?

Expulsion of flux is a consequence of the cooper pairs maintaining a zero ground state momentum, ie: this is the only assumption needed to derive the London Equations. Genenth also brings up a good point.