Conduction

Conduction is the transfer of energy or charge within a material or between materials that are in direct contact. In common usage, the term is most often applied to thermal conduction—the transfer of heat from regions of higher temperature to regions of lower temperature within a material or between touching materials. It is also used for electrical conduction, the flow of electric charges through a material under the influence of an electric field.

Thermal conduction occurs mainly through two mechanisms: the movement of free electrons and the transfer of energy by lattice vibrations (phonons) in solids. In metals, free electrons rapidly transport energy, while phonons also contribute. In non-metals, gases, liquids, and insulators, energy transfer is primarily through phonons and molecular interactions. Fourier's law describes the relation between heat flux and the temperature gradient: q = -k ∇T, where q is the heat flux, k is the material’s thermal conductivity, and ∇T is the temperature gradient. Materials with high k are good conductors of heat; those with low k are insulators. At interfaces between materials, interfacial or boundary resistances can reduce heat transfer, a phenomenon sometimes called Kapitza resistance.

Electrical conduction is the movement of charged particles in response to an electric field. In metals, electrons act as charge carriers; Ohm's law relates current density J to the electric field E via J = σE, with σ the electrical conductivity and ρ = 1/σ the resistivity. In electrolytes and ionic solutions, currents are carried by ions; in semiconductors, by electrons and holes, with conductivities dependent on carrier concentration, temperature, and impurities. Temperature can have opposite effects on conductivity depending on the material, increasing carrier density in semiconductors while often increasing scattering in metals. Applications include heating and cooling systems, electrical wiring, and electronic devices.