Why do superconductors act the way they do?

[RyanJ]

Why do superconductors act the way they do? Why do they have 0 resistance when everything else has atleast some how ever small?

 

Also, Does this make the transfer of energy through superconducting cables 100% efficient?

 

 

 

I would also like to know how can liquid helium act to flow against the pull of gravity - I find this facinating

 

Cheers,

 

Ryan Jones

[5614]

Super conductors and super fluids... great things!

 

Superconductors:

 

The theory explaining these is called the BCS Theory (developed by Bardeen, Cooper, and Schrieffer).

 

The main properties of a superconductor are that (as well known) they have 0 resistance and also the fact that expel magnetic fields, so the magnetic field cannot go inside the wire, only around the outside, this is known as the Meissner effect.

 

BCS Theory basically says that electrons (with opposite spin) can pair up (into Cooper pairs). These Cooper pairs behave in a way which allows the superconductor to have 0 resistance.

 

Superfluids:

 

Topics related to superfluids are collectively known as quantum hydrodynamics.

 

There are two types of superfluids, well, I would divide them as two things anyway.

 

1) These are Bose-Einstein condensates. An example would he helium-4. These work because all of the particles (which are bosons) can all collapse into the same (minimum energy) quantum state. If they're all in the same quantum state they can all act as one.

 

2) The other type, well, I don't know if there's really name for it. It is basically a superfluid made from fermions. Fermions cannot all collapse to the same state, in fact no fermion can be in the same quantum state as another (so obviously in #1 they all collapse into the same state (lowest) cannot be the case here). These superfluids (an example being helium-3) are explained using the BCS Theory (and Cooper pairs).

 

Superfluids can "defy gravity" because they have soo little energy (the particles are in their lowest possible energy state) they superfluid cannot give any energy to anything. Friction is the transfer of energy between the moving body and the surface. As a superfluid can supply no energy, it cannot have friction.

 

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There is a lot to say on the matter of superconductors and superfluids, they are fascinating (IMO!). I've given a lot of the keywords above, so if you do some searching around there's a lot (and a lot!) to read.

 

Try these:

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

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

(search (ctrl+f) for 'super')

http://scienceworld.wolfram.com/physics/

And of course SFN!