This can be calculated in a manner very similar to the electrical conductivity. For the ratio we find which is independent of the collision time and should therefore be the same for all materials. The Lorenz number is equal to . The phenomenon of superconductivity is also very interesting. It was discovered that at very low temperatures many materials loose their resistivity and become perfect conductors.
Electrons have a negative charge and free electrons repel eachother.
Electrons in a material, however, can attract eachother by the following mechanism. One electron is close to an atom and slightly polarizes this atom by moving it. This actually creates a lattice vibration or phonon. The other electron is then attracted by this polarized atom. Add all effects together and one ends up with a very small attractive potential between some electrons.