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The thermal conductivity of a material is a measure of its ability to conduct heat.It is commonly denoted by , , or 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.
Plot of the Wiedemann–Franz law for copper. Left axis: specific electric resistance ρ in 10 −10 Ω m, red line and specific thermal conductivity λ in W/(K m), green line. Right axis: ρ times λ in 100 U 2 /K, blue line and Lorenz number ρ λ / K in U 2 /K 2, pink line. Lorenz number is more or less constant.
is the thermal conductivity (W/(K·m)) of the sample; is the thermal resistivity (K·m/W) of the sample; is the cross-sectional area (m 2) perpendicular to the path of heat flow. In terms of the temperature gradient across the sample and heat flux through the sample, the relationship is:
Chapman–Enskog theory also predicts a simple relation between thermal conductivity, , and viscosity, , in the form =, where is the specific heat at constant volume and is a purely numerical factor. For spherically symmetric molecules, its value is predicted to be very close to 2.5 {\displaystyle 2.5} in a slightly model-dependent way.
Thermal conduction rate, thermal current, thermal/heat flux, thermal power transfer P = / W ML 2 T −3: Thermal intensity I = / W⋅m −2: MT −3: Thermal/heat flux density (vector analogue of thermal intensity above) q
where is the thermal conductivity of the material, = (,) is the temperature, and = (,) is a vector field that represents the magnitude and direction of the heat flow at the point of space and time . If the medium is a thin rod of uniform section and material, the position x is a single coordinate and the heat flow q = q ( t , x ) {\displaystyle ...
In thermal conductivity, k is defined as "the quantity of heat, Q, transmitted in time (t) through a thickness (L), in a direction normal to a surface of area (A), due to a temperature difference (ΔT) [...]". Thermal conductivity is a material property that is primarily dependent on the medium's phase, temperature, density, and molecular bonding.
The thermal contact conductance coefficient, , is a property indicating the thermal conductivity, or ability to conduct heat, between two bodies in contact. The inverse of this property is termed thermal contact resistance .