# Ideal conductor vs superconductor

Raze2dust
What is the difference, apart from the Meissner effect?

What are the practical consequences of Meissner effect? How can I, by experiment, deduce whether a material is an ideal conductor or superconductor?

superconductor has zero resistivity. So what happens when we apply a potential difference at the ends of a superconductor? Surely infinite current does not flow, since anyway if current density > critical current density then it would no longer be a superconductor.

How do we induce current in a superconductor?

Homework Helper
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Do you know any material that is an ideal conductor that is not a superconductor?

A Superconductor has many properties, among them is that is no resistance. So when you apply a current, it will never "die" - even if you remove the source.

Gold Member
The most "obvious" difference between a superconductor and an ideal conductor is that the former expels all magnetic fields when it goes through the superconducting transition. A perfect conductor would just "freeze" the field, but in an ideal superconductor (well, in a type I at least) the field is always zero.

But there are many other differences as well. Even in simplified "semiconductor type" models of superconductors you will find that the presence of a gap has many practical consequences that are easy to measure (e.g. how it react to electromagnetic fields).

cesiumfrog
I've heard that superconductors are ideal diamagnets (implying zero DC resistance plus Meissner effect), not ideal conductors (but also there may be a semantics issue in concept idealisation).

Is there some difference in the responses to alternating current?

Staff Emeritus
Gold Member
What is the difference, apart from the Meissner effect?

How can I, by experiment, deduce whether a material is an ideal conductor or superconductor?
It would help to start with a definition for an "ideal conductor".

What are the practical consequences of Meissner effect?
Levitation is one, if that ever that may become commercially usaful.

superconductor has zero resistivity. So what happens when we apply a potential difference at the ends of a superconductor? Surely infinite current does not flow, since anyway if current density > critical current density then it would no longer be a superconductor.

How do we induce current in a superconductor?
It takes an infinite amount of energy to set up a non-zero voltage drop across a SC. So SCs are driven by current sources rather than voltage sources.

ironhill
An 'ideal conductor' is a conductor where the resistivity drops away following Matthiessen's law all the way to 0K.

The practical consequences of the Meissner effect? Well for one it means that the suerconducting state is a Thermodynamic state. The state of an ideal conductor depends on B and T and its history. The state of a superconductor just depends on B and T independant of how it got there. So you can do thermodynamics on superconductors because of it.

Current along a superconductor depends on the inductance.

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Staff Emeritus
Gold Member
An 'ideal conductor' is a conductor where the resistivity drops away following Matthiessen's law all the way to 0K.
But even at 0K, an ideal conductor will have a nonzero resistivity from Umklapp scattering.

ironhill
But even at 0K, an ideal conductor will have a nonzero resistivity from Umklapp scattering.

Of course, but since it's ideal we ignore residual resistivity from impurities and lattice distortions and so on. That is in fact how supercondctivity was discovered. Osanger wanted to see if the resistivity of a metal (mercury) would keep following that smooth curve all the way to 4K. The reason he used mercury was because he could distil it and get it extremely pure so it would closely approximate an ideal conductor.

JimChampion
superconductor has zero resistivity. So what happens when we apply a potential difference at the ends of a superconductor? Surely infinite current does not flow

Zero resistance in this case means that a finite current can flow with zero potential difference across it (from this definition of resistance: V=IR). Also a current can persist in a superconducting ring as the potential difference is (necessarily) zero if you go all the way around the ring.