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| 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]].<ref>http://www.evitherm.org/default.asp?ID=277</ref>
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| :<math>e = {(k\rho c_p)}^{1/2}</math>
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| Here, '''k''' is the thermal conductivity, '''<math>\rho</math>''' is the [[density]] and '''<math>c_p</math>''' is the [[specific heat capacity]]. The product of '''<math>\rho</math>''' and '''<math>c_p</math>''' is known as the '''volumetric heat capacity'''.
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| A material's thermal effusivity is a measure of its ability to exchange [[thermal energy]] with its surroundings.
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| If two semi-infinite bodies initially at temperatures '''T<sub>1</sub>''' and '''T<sub>2</sub>''' are brought in perfect thermal contact, the temperature at the contact surface '''T<sub>m</sub>''' will be given by their relative effusivities. <ref>H. D. Baehr and K. Stephan, ''Wärme- und Stoffübertragung 4. Auflage'', Springer, (2004), p 172, ISBN 3-540-40130-X </ref>
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| :<math>T_m = T_1 + (T_2-T_1){e_2 \over (e_2+e_1)}</math>
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| 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.
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| ==See also ==
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| * [[heat capacity]]
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| * [[heat equation]]
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| * [[Laser Flash Analysis|laser flash analysis]]
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| * [[specific heat capacity]]
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| * [[thermal conductivity]]
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| * [[thermal diffusivity]]
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| * [[thermal inertia]]
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| ==References==
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| <references />
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| ==External links==
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| * [http://www.evitherm.org/default.asp?ID=277 A reference defining various thermal properties]
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| * [http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html Thermal heat transfer on HyperPhysics site]
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| [[Category:Thermodynamics]]
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| [[Category:Physical quantities]]
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| [[Category:Heat conduction]]
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