Thermal conduction is the transmission of heat from places of higher temperature to places of lower temperature in a substance, by the interaction of atoms, molecules, or ions, or electrons possessing greater kinetic energy with those possessing less. In gases the heat energy is transmitted by collision of the gaseous molecules, those possessing the greater kinetic energy imparting, on collision, some of their energy to molecules having less. Conduction in liquids is mainly due to the same process. In solid electrical conductors, the chief contribution to thermal conduction arises from a similar process taking place between the free electrons present. The interaction of the molecules responsible for thermal conduction in solid electrical insulators arises from the elastic binding forces between the molecules, which are effectively fixed in space.
The rate of heat transfer by conduction is determined by the temperature difference (ΔT), length (l), cross-sectional area (A), and thermal conductivity of the material (k) according to the following formula:
Conductivities are usually given in units of W/m K. They are highest in metallic solids, lower in nonmetallic solids, very low in liquids, and extremely low in gases. The best ordinary metallic conductors are silver (429 W/m K), copper (401), gold (317), aluminum (237), and tungsten (174). Among nonmetallic solids, diamond is best (895); most silicate minerals have low values (< 2) The material with the greatest thermal conductivity (105!) is the superfluid helium II, which only exists at temperatures below 2.17 K.