Thermal conductivity and resistivity

Thermal conductivity
Common symbols	κ
SI unit	watt per meter-kelvin (W/(m⋅K))
In SI base units	kg⋅m⋅s−3⋅K-1
Dimension	${\displaystyle {\mathsf {M}}{\mathsf {L}}{\mathsf {T}}^{-3}{\mathsf {\Theta }}^{-1}}$

Thermal resistivity
Common symbols	ρ
SI unit	kelvin-meter per watt (K⋅m/W)
In SI base units	kg-1⋅m-1⋅s3⋅K
Dimension	${\displaystyle {\mathsf {M}}^{-1}{\mathsf {L}}^{-1}{\mathsf {T}}^{3}{\mathsf {\Theta }}}$

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by ${\displaystyle k}$, ${\displaystyle \lambda }$, or ${\displaystyle \kappa }$ and is measured in W·m⁻¹·K⁻¹.

Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials such as mineral wool or Styrofoam. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications, and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity.

The defining equation for thermal conductivity is ${\displaystyle \mathbf {q} =-k\nabla T}$, where ${\displaystyle \mathbf {q} }$ is the heat flux, ${\displaystyle k}$ is the thermal conductivity, and ${\displaystyle \nabla T}$ is the temperature gradient. This is known as Fourier's law for heat conduction. Although commonly expressed as a scalar, the most general form of thermal conductivity is a second-rank tensor. However, the tensorial description only becomes necessary in materials which are anisotropic.