# 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]}

Here, is the thermal conductivity, is the density and is the specific heat capacity. The product of and 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 and are brought in perfect thermal contact, the temperature at the contact surface will be given by their relative effusivities.^{[3]}

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.

## See alsoEdit

## ReferencesEdit

**^**i.e. their thermal capacity is sufficiently large that their temperatures will not change measurably owing to this heat transfer

**^**A reference defining various thermal properties^{[dead link]}**^**Williams, F. A. (2009). "Simplified theory for ignition times of hypergolic gelled propellants".*J. Propulsion and Power*.**25**(6): 1354–1357. doi:10.2514/1.46531.**^**Baehr, H.D.; Stephan, K. (2004).*Wärme- und Stoffübertragung 4. Auflage*. Springer. p. 172. doi:10.1007/978-3-662-10833-8. ISBN 978-3-662-10834-5.

## External linksEdit

- "Thermal heat transfer".
*HyperPhysics*.