# Thermal effusivity

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In Thermodynamics, the thermal effusivity or thermal responsivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.

$e={(\lambda \rho c_{p})}^{1/2}$ Here, $\lambda$ is the thermal conductivity, $\rho$ is the density and $c_{p}$ is the specific heat capacity. The product of $\rho$ and $c_{p}$ is known as the volumetric heat capacity.

A material's thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings.

If two semi-infinite bodies initially at temperatures T1 and T2 are brought in perfect thermal contact, the temperature at the contact surface Tm will be given by their relative effusivities.

$T_{m}=T_{1}+(T_{2}-T_{1}){e_{2} \over (e_{2}+e_{1})}$ This expression is valid for all times for semi-infinite bodies in perfect thermal contact. It is also a good first guess for the initial contact temperature for finite bodies.

Direct measurement of thermal effusivity may be performed using specialty sensors, as pictured to the right.