Thermal radiation, Interchange of energy

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Summary

Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. When the temperature of the body is greater than absolute zero, interatomic collisions cause the kinetic energy of the atoms or molecules to change. This results in charge-acceleration and/or dipole oscillation which produces electromagnetic radiation, and the wide spectrum of radiation reflects the wide spectrum of energies and accelerations that occur even at a single temperature.

Details

Thermal radiation is one of the principal mechanisms of heat transfer. It entails the emission of a spectrum of electromagnetic radiation due to an object's temperature. Other mechanisms are convection and conduction. The interplay of energy exchange by thermal radiation is characterized by the following equation:

\alpha+\rho+\tau=1. \,

Here, \alpha \, represents the spectral absorption component, \rho \, spectral reflection component and \tau \, the spectral transmission component. These elements are a function of the wavelength (\lambda\,) of the electromagnetic radiation. The spectral absorption is equal to the emissivity \epsilon \,; this relation is known as Kirchhoff's law of thermal radiation. An object is called a black body if, for all frequencies, the following formula applies:

\alpha = \epsilon =1.\,

In a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation. However, the energy lost by emitting infrared light is partially regained by absorbing the heat flow due to conduction from surrounding objects, and the remainder resulting from generated heat through metabolism. Human skin has an emissivity of very close to 1.0 . Using the formulas below shows a human, having roughly in surface area, and a temperature of about 307 K, continuously radiates approximately However, if people are indoors, surrounded by surfaces at they receive back about from the wall, ceiling, and other surroundings, so the net loss is only about These heat transfer estimates are highly dependent on extrinsic variables, such as wearing clothes, i.e. decreasing total thermal circuit conductivity, therefore reducing total output heat flux. Only truly gray systems (relative equivalent emissivity/absorptivity and no directional transmissivity dependence in all control volume bodies considered) can achieve reasonable steady-state heat flux estimates through the Stefan-Boltzmann law. Encountering this "ideally calculable" situation is virtually impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "gray" approximations will get you close to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most STP lab controlled environments).

If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared. Most household radiators are painted white but this is sensible given that they are not hot enough to radiate any significant amount of heat, and are not designed as thermal radiators at all - they are actually convectors, and painting them matt black would make virtually no difference to their efficacy. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature (meaning the term "black body" does not always correspond to the visually perceived color of an object). These mater

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ials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence.

Calculation of radiative heat transfer between groups of object, including a 'cavity' or 'surroundings' requires solution of a set of simultaneous equations using the Radiosity method. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called view factors, which give the proportion of radiation leaving any given surface that hits another specific surface. These calculations are important in the fields of solar thermal energy, boiler and furnace design and raytraced computer graphics.

A selective surface can be used when energy is being extracted from the sun. For instance, when a green house is made, most of the roof and walls are made out of glass. Glass is transparent in the visible (approximately 0.4 µm

Using the reciprocity rule, A__= A__, this simplifies to:

\dot= \sigma A__T_1^4-T_2^4) \!

where \sigma is the Stefan–Boltzmann constant and F_lt;/math> is the view factor from surface 1 to surface 2.

  • For a grey body with only two surfaces the heat transfer is equal to:
\dot \dfrac\dfracA_1\epsilon_1}+ \dfracA_1F_+ \dfracA_2\epsilon_2}}

where \epsilon are the respective emissivities of each surface. However, this value can easily change for different circumstances and different equations should be used on a case per case basis.

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External Links

  • WikipediaBlack Body Emission CalculatorHeat TransferThermal RadiationAtmospheric RadiationInfrared Temperature Calibration 101

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