Chapter3_5 - e is the emissivity of the material 0 ≤ e...

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PHY3063 R. D. Field Department of Physics Chapter3_5.doc University of Florida Emission and Absorption from a Surface Thermal Radiation: Electromagnetic radiation occurs as a result of the thermal motion of charged particles near the surface of the material. The whole spectrum of wavelengths is emitted and one would expect that the intensity of the radiation would increase with temperature ( i.e. more violent random motion). Emission: In 1879 measurements by Stefan yield the following empirical equation: 4 1 ) ( eT dt dU A T I σ = = , where I(T) is the intensity ( energy per unit time per unit area ) at temperature T integrated over all wavelengths and
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Unformatted text preview: e is the emissivity of the material ( 0 ≤ e ≤ 1 ) and 4 2 8 10 672 . 5 − − − × = K Wm . Absorption: Here electromagnetic radiation is absorbed by a surface and the absorptivity, a , of a material is the ratio of the thermal energy absorbed by the surface to the total thermal energy incident upon it . One can prove (using thermodynamic arguments) that a e = . The emissivity of a material is equal to its absorptivity (also verified experimentally). Random Motion EM Radiation Stefan-Boltzmann Constant Kirchhoff’s Theorem Intensity of radiation as a function of T!...
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