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In thermodynamics, a material's thermal effusivity, thermal inertia or thermal responsivity is a measure of its ability to exchange thermal energy with its surroundings. It is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.
Here, λ {\displaystyle \lambda } is the thermal conductivity, ρ {\displaystyle \rho } is the density and c p {\displaystyle c_{p}} is the specific heat capacity. The product of ρ {\displaystyle \rho } and c p {\displaystyle c_{p}} is known as the volumetric heat capacity.
Hence the SI units for thermal effusivity are W s / {\displaystyle {\rm {W}}{\sqrt {\rm {s}}}/} , or, equivalently, J / {\displaystyle {\rm {J}}/}.
If two semi-infinite bodies initially at temperatures T 1 {\displaystyle T_{1}} and T 2 {\displaystyle T_{2}} are brought in perfect thermal contact, the temperature at the contact surface T m {\displaystyle T_{m}} will be given by their relative effusivities.