# Thermal effusivity

In Thermodynamics, the thermal effusivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.[1]

$e = {(k\rho c_p)}^{1/2}$

Here, k 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. [2]

$T_m = T_1 + (T_2-T_1){e_2 \over (e_2+e_1)}$

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.