laws of radiation

laws of radiation

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  PROCESS HEAT TRANSFER  LAWSOF RADIATION By: PARAS H BORICHA (130280105004)
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  WHAT IS RADIATION???
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  • Radiation isthe transfer of energy by rapid oscillations of electromagnetic fields. • The most important general characteristic is its wavelength . • Radiation travels through space at the speed of light (3 x 108 m s-1) or 670,616,630 MPH.
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  LAWS OF RADIATION
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  A black body, by definition, absorbs radiation at all wavelengths completely. Real objects are never entirely “black” – the cannot absorb all wavelengths completely, but show a wavelength-dependent absorptivity ε(λ) (which is < 1). According to Kirchhoff’s law (Gustav Kirchhoff, 1859) the Emission of a body, Eλ (in thermodynamic equilibrium) is: For a given wavelength and temperature, the ratio of the Emission and the absorptivity equals the black body emission. This shows also, that objects emit radiation in the same parts of the spectrum in which they absorb radiation. ),( )(ε ),( T T      B E  Atmo II 84 Kirchhoff’s Law
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  We rearrange Kirchhoff’slaw and see: At a given temperature, real objects emit less radiation than a black body (since ε < 1). Therefore we can regard ε(λ) also as emissivity. Quite often you will thus find Kirchhoff’s law in the form: Emissivity = Absorptivity Important: it applies wavelength-dependent. )()()( TBTE ,ε,    Kirchhoff’s Law Atmo II 85 In the infrared all naturally occurring surfaces are – in very good approximation – “black” – even snow! (which is – usually – not black at all in the visible part of the spectrum). For the Earth as a whole (in the IR): ε = 0.95 („gray body“)
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  Wien’s law andthe Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars • A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths • Stars closely approximate the behavior of blackbodies, as do other hot, dense objects • The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown by a blackbody curve
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  Wien’s Law Wien’s lawstates that the dominant wavelength at which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the object
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  Stefan-Boltzmann Law • TheStefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux E directly proportional to the fourth power of the Kelvin temperature T of the object: E = T4
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  Planck’s Law According toPlanck’s Law (Max Planck, 1900) the energy emitted by a black body (un-polarized radiation) per time, area, solid angle and wave length λ equals: c0 = Speed of light (in vacuum) = 299 792 458 m s–1 h = Planck constant = 6.626 069 57·10–34 Js kB = Boltzmann constant = 1.380 6488·10–23 J K-1 According to our last slides this has to be – right: Spectral radiance with respect to wavelength [Wm–2 sr–1 m–1] 1exp 12 5 2           Tk hc hc TB B 0 0 ),(
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  Planck’s Law (lastslide) refers to un-polarized radiation per solid angle. In case of linear polarization we would just get half of it. If you should miss a factor π – this comes be integrating over the half space. Planck‘s law often comes in frequency formulation:       ),( ),( TB TB 1exp 12 2 3        Tk hc h TB B    0 ),( 0c B d dBB 2           0c Planck’s Law