Thermal conductivity is a physical property of a material that measures its ability to conduct heat. It quantifies how well heat energy can transfer through a material when there is a temperature gradient. Materials with high thermal conductivity can quickly transfer heat, while those with low thermal conductivity are better insulators.
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Thermal conductivity is the material response coefficient in Fourier's law. For an isotropic material it is a scalar , with units ; larger positive means a larger conductive heat flux for the same temperature gradient. It can depend on temperature and composition. Direction-dependent response in an anisotropic material is described by a thermal conductivity tensor. The two worked examples use stipulated positive constants, rather than measured values for named materials.
Thermal conductivity describes material response, whereas thermal resistance also includes geometry: for the steady bar, . Doubling at fixed length, cross-sectional area and end temperatures halves thermal resistance and doubles the rate of heat transfer. Doubling length changes thermal resistance without changing the material's thermal conductivity. This distinction lets a designer improve cooling either by choosing a higher thermal conductivity or by shortening and widening the path. It also explains why the less conductive layer in thermal resistance takes the greater temperature drop.
Thermal diffusivity answers a different question: how rapidly a temperature disturbance spreads. Its usual model also involves mass density and specific heat capacity , so equal thermal conductivities need not imply equal rates of warming.