Re: How does BCS theory explain superconductivity?

There is an aspect of resistance that is due to the nuclei of the material getting in the way or pulling on the conduction electrons, but a bigger contribution is the impedance of electrons with each other.  

Conduction electrons are also called free electrons because we think of them as not bound to any one nuclei.  They are free to move about the metal or semiconductor but this ability to move (conductivity) depends properties of the material, cross-section, and temperature.  In metals, resistance is pretty much proportional to temperature because conductivity (1/resistivity) falls as temperature rises because thermal motion of the free electrons interferes with the flow of current (http://www.encyclo pedia.com/html/c1/conducton.asp).  Collisions with the metal ions are less of a factor at room temperature.

Superconductors have no (none, nil, nada, zip - engineers will argue otherwise about infinitesimal) resistance, and thus, no losses or heat generation.  Unfortunately, superconducting materials must be kept cold to stay superconductive.  The cold temperature seems necessary to keep the system ordered (in the lower energy states).  

The short answer to superconductivity is that electrons working in pairs.  These pairs are called Cooper pairs, named after Nobel Laureate Leon Copper one of the developers of BCS theory.  The pairing creates a sort of "conductive superfluid of bosons" [my term].  The bosons do not get in each others way like the unpaired single electrons (fermions).  And the pairs of electrons can move about the lattice of the system without having their motion disrupted by collisions with the lattice.   (online reference: http://www.encyclop edia.com/html/s1/supercon.asp).  And to you your wonderful visual analogy, the organization and quantum mechanics allows these cars to travel rapidly through the forest without crashing into the trees.  It is like the trees are planted in predictable lines, and the cars can zip right through without incident.

A wonderful explanation of how electrons could be paired is given in a previous MadSci response:

(BCS theory and electron pairs) htt p://www.madsci.org/posts/archives/dec2000/976031589.Ph.r.html

Other related or interesting answers and websites:

(High temperature superconductivity) htt p://www.madsci.org/posts/archives/dec2000/976815728.Ph.r.html

(Bose-Einstein Condensation) htt p://www.madsci.org/posts/archives/dec2000/976722357.Ph.r.html

(What is a superconductor) http: //www.madsci.org/posts/archives/mar97/856891135.Eg.r.html

(Free electrons in metal) ht tp://www.madsci.org/posts/archives/nov2001/1005426899.Ph.r.html

I hope this helps answer your question or leads you to further research.

Sincerely,

Tom "BCS Champion" Cull