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Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planck’s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefan-Boltzmann law gives the heat intensity.
Properties There are three main properties that characterize thermal radiation: Interchange of energy Thermal radiation is an important concept in thermodynamics as it is partially responsible for heat exchange between objects, as warmer bodies radiate more heat than colder ones. (Other factors are convection and conduction.) The interplay of energy exchange is characterized by the following equation: ho+ au=1 , Here, represents spectral absorption factor, spectral reflection factor and spectral transmission factor. All these elements depend also on the frequency . The spectral absorption factor is equal to the emissivity ; this relation is known as Kirchhoff's law of thermal radiation. An object is called a black body if, for all frequencies, the following fomula applies: In a practical situation and room-temperature setting, objects lose considerable energy due to thermal radiation. However, the energy lost by emitting infrared heat is regained by absorbing the heat of surrounding objects. For example, a human being, roughly 1 square meter in area, and about 310 K in temperature, continuously radiates about 500 watts. However, if the person is indoors, in a room of 293 degrees K, they receive back about 400 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts. Clothes (which are at an intermediate temperature in equilibrium) reduce this loss still further. If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared; e. g. most household radiators are painted white despite the fact that they have to be good thermal radiators. Formula Thermal radiation power of a black body per unit of area, unit of solid angle and unit of frequency is given by u,T)= raccdot rac1 Integrating the above equation over obtains the power output given by the Stefan-Boltzmann law, as: Further, the wavelength , for which the emission intensity is highest, is given by Wien's Law as: For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity correction factor . This correction factor has to be multiplied with the radiation spectrum formula before integration. The resulting formula for the power output can be written in a way that contains a temperature dependent correction factor which is (somewhat confusingly) often called as well: Constants Definitions of constants used in the above equations: | ||||||||
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