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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 Intensity (physics) ： ウィキペディア英語版 Intensity (physics) In physics, intensity is the power transferred per unit area, where the area is an imagined surface that is perpendicular to the direction of propagation of the energy.〔(【引用サイトリンク】title=intensity )〕 In the SI system, it has units watts per square metre (W/m2). It is used most frequently with waves (e.g. sound or light), in which case the ''average'' power transfer over one period of the wave is used. ''Intensity'' can be applied to other circumstances where energy is transferred. For example, one could calculate the intensity of the kinetic energy carried by drops of water from a garden sprinkler. The word "intensity" as used here is not synonymous with "strength", "amplitude", "magnitude", or "level", as it sometimes is in colloquial speech. Intensity can be found by taking the energy density (energy per unit volume) at a point in space and multiplying it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area. ==Mathematical description== If a point source is radiating energy in all directions (producing a spherical wave), and no energy is absorbed or scattered by the medium, then the intensity decreases in proportion to distance from the object squared. This is an example of the inverse-square law. Applying the law of conservation of energy, if the net power emanating is constant, :$P\; =\; \backslash int\; \backslash bold\; I\backslash ,\; \backslash cdot\; \backslash mathrm\backslash bold\; A$, where ''P'' is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. If one integrates over a surface of uniform intensity ''I'', for instance over a sphere centered around the point source, the equation becomes :$P\; =\; |I|\; \backslash cdot\; A\_\backslash mathrm\; =\; |I|\; \backslash cdot\; 4\backslash pi\; r^2\; \backslash ,$, where ''I'' is the intensity at the surface of the sphere, and ''r'' is the radius of the sphere. ($A\_\backslash mathrm\; =\; 4\backslash pi\; r^2$ is the expression for the surface area of a sphere). Solving for ''I'' gives :$|I|\; =\; \backslash frac$. If the medium is damped, then the intensity drops off more quickly than the above equation suggests. Anything that can transmit energy can have an intensity associated with it. For a monochromatic propagating wave, such as a plane wave or a Gaussian beam, if ''E'' is the complex amplitude of the electric field via, then the time-averaged energy density of the wave is given by: :$\backslash left\backslash langle\; U\; \backslash right\; \backslash rangle\; =\; \backslash frac\; |E|^2$, and the local intensity is obtained by multiplying this expression by the wave velocity, c/''n'': :$I\; =\; \backslash frac\; |E|^2$, where ''n'' is the refractive index, c is the speed of light in vacuum and $\backslash epsilon\_0$ is the vacuum permittivity. For non-monochromatic waves, the intensity contributions of different spectral components can simply be added. The treatment above does not hold for arbitrary electromagnetic fields. For example, an evanescent wave may have a finite electrical amplitude while not transferring any power. The intensity should then be defined as the magnitude of the Poynting vector. 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「Intensity (physics)」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.