# Thermal effusivity

In thermodynamics, the thermal effusivity, thermal inertia 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.[1][2]

${\displaystyle e={(\lambda \rho c_{p})}^{1/2}}$
Thermal effusivity sensor typically used in the direct measurement of materials.

Here, ${\displaystyle \lambda }$ is the thermal conductivity, ${\displaystyle \rho }$ is the density and ${\displaystyle c_{p}}$ is the specific heat capacity. The product of ${\displaystyle \rho }$ and ${\displaystyle c_{p}}$ 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[i] bodies initially at temperatures ${\displaystyle T_{1}}$ and ${\displaystyle T_{2}}$ are brought in perfect thermal contact, the temperature at the contact surface ${\displaystyle T_{m}}$ will be given by their relative effusivities.[3]

${\displaystyle T_{m}=T_{1}+(T_{2}-T_{1}){e_{2} \over (e_{2}+e_{1})}}$

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.