Thermal conduction is a very important and a major topic in
the study of heat transfer. Conduction is the transfer of
energy from energetic particles of a substance to the adjacent
less energetic ones as a result of interactions between the
particles.
Conduction can take place in solids, liquids, or gases. In gases
and liquids, conduction is due to the collision and diffusion of
the molecules during their random motion. The rate of heat
conduction is proportional to the area and the temperature
difference, and inversely proportional to the thickness of the
material. The constant of proportionality is the thermal
conductivity.
Thus, the thermal conductivity, k, of a material is defined as
the rate of heat transfer through a unit thickness of the material
per unit area per unit temperature difference. Therefore, it is a
measure of how fast heat will flow in the material. A large
value for thermal conductivity indicates that the material is a
good conductor, while a low value indicates that the material is
a poor conductor or a good insulator.
The thermal conductivities of materials vary with temperature.
This variation, for some materials over certain temperature
ranges, is small enough to be neglected. However, in many
cases, such as liquids and gases, the variation of the thermal
conductivity with temperature is significant.