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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Kirchhoff's law of thermal radiation ： ウィキペディア英語版 Kirchhoff's law of thermal radiation :''See also Kirchhoff's laws (disambiguation) for other laws named after Kirchhoff''. In thermodynamics, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium. A body at temperature radiates electromagnetic energy. A perfect black body in thermodynamic equilibrium absorbs all light that strikes it, and radiates energy according to a unique law of radiative emissive power for temperature , universal for all perfect black bodies. Kirchhoff's law states that: :''For a body of any arbitrary material, emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature. That universal function describes the perfect black-body emissive power.'' Here, the dimensionless coefficient of absorption (or the absorptivity) is the fraction of incident light (power) that is absorbed by the body when it is radiating and absorbing in thermodynamic equilibrium. In slightly different terms, the emissive power of an arbitrary opaque body of fixed size and shape at a definite temperature can be described by a dimensionless ratio, sometimes called the emissivity, the ratio of the emissive power of the body to the emissive power of a black body of the same size and shape at the same fixed temperature. With this definition, a corollary of Kirchhoff's law is that for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity. In some cases, emissive power and absorptivity may be defined to depend on angle, as described below. The condition of thermodynamic equilibrium is necessary in the statement, because the equality of emissivity and absorptivity often does not hold when the material of the body is not in thermodynamic equilibrium. Kirchhoff's law has another corollary: the emissivity cannot exceed one (because the absorptivity cannot, by conservation of energy), so it is not possible to thermally radiate more energy than a black body, at equilibrium. In negative luminescence the angle and wavelength integrated absorption exceeds the material's emission, however, such systems are powered by an external source and are therefore not in thermodynamic equilibrium. Before Kirchhoff's law was recognized, it had been experimentally established that a good absorber is a good emitter, and a poor absorber is a poor emitter. Naturally, a good reflector must be a poor absorber. This is why, for example, lightweight emergency thermal blankets are based on reflective metallic coatings: they lose little heat by radiation. Kirchhoff's great insight was to recognize the universality and uniqueness of the function that describes the black body emissive power. But he did not precisely know the form or character of that universal function. It was eventually found precisely, in mathematical terms, by Planck in 1900.〔Kangro, H. (1970/1976).〕 == Theory == In a blackbody enclosure that contains electromagnetic radiation with a certain amount of energy at thermodynamic equilibrium, this "photon gas" will have a Planck distribution of energies.〔Rybicki & Lightman, pp. 15–20.〕 One may suppose a second system, a cavity with walls that are opaque, rigid, and not perfectly reflective to any wavelength, to be brought into connection, through an optical filter, with the blackbody enclosure, both at the same temperature. Radiation can pass from one system to the other. For example, suppose in the second system, the density of photons at narrow frequency band around wavelength $\backslash lambda$ were higher than that of the first system. If the optical filter passed only that frequency band, then there would be a net transfer of photons, and their energy, from the second system to the first. This is in violation of the second law of thermodynamics, which requires that there can be no net transfer of heat between two bodies at the same temperature. In the second system, therefore, at each frequency, the walls must absorb and emit energy in such a way as to maintain the black body distribution.〔 〕 For the condition of thermal equilibrium, the absorptivity $\backslash alpha\_\backslash lambda$ is the ratio of the energy absorbed by the wall to the energy incident on the wall, for a particular wavelength. Thus the absorbed energy is $\backslash alpha\_\backslash lambda\; E\_(\backslash lambda,T)$ where $E\_(\backslash lambda,T)$ is the intensity of black body radiation at wavelength $\backslash lambda$ and temperature $T$. Independent of the condition of thermal equilibrium, the emissivity of the wall is defined as the ratio of emitted energy to the amount that would be radiated if the wall were a perfect black body. The emitted energy is thus $\backslash epsilon\_\backslash lambda\; E\_(\backslash lambda,T)$ where $\backslash epsilon\_\backslash lambda$ is the emissivity at wavelength $\backslash lambda$. For the maintenance of thermal equilibrium, these two quantities must be equal, or else the distribution of photon energies in the cavity will deviate from that of a black body. This yields Kirchhoff's law: :$\backslash alpha\_\backslash lambda=\backslash epsilon\_\backslash lambda$ By a similar, but more complicated argument, it can be shown that, since black body radiation is equal in every direction (isotropic), the emissivity and the absorptivity, if they happen to be dependent on direction, must again be equal for any given direction.〔 Average and overall absorptivity and emissivity data are often given for materials with values which ''differ'' from each other. For example, white paint is quoted as having an absorptivity of 0.16, while having an emissivity of 0.93.〔http://www.solarmirror.com/fom/fom-serve/cache/43.html〕 This is because the absorptivity is averaged with weighting for the solar spectrum, while the emissivity is weighted for the emission of the paint itself at normal ambient temperatures. The absorptivity quoted in such cases is being calculated by: :$\backslash alpha\_\}\; (\backslash lambda)\backslash ,d\backslash lambda\}$ while the average emissivity is given by: :$\backslash epsilon\_(\backslash lambda,T)\backslash ,d\backslash lambda\}$ Where $I\_(\backslash lambda,T)$ is the emission spectrum of the paint. Although, by Kirchhoff's law, $\backslash epsilon\_\backslash lambda=\backslash alpha\_\backslash lambda$ in the above equations, the above ''averages'' $\backslash alpha\_\}$ are not generally equal to each other. The white paint will serve as a very good insulator against solar radiation, because it is very reflective of the solar radiation, and although it therefore emits poorly in the solar band, its temperature will be around room temperature, and it will emit whatever radiation it has absorbed in the infrared, where its emission coefficient is high. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Kirchhoff's law of thermal radiation」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.