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In physics, the Wiedemann–Franz law is the ratio of the electronic contribution of the thermal conductivity (''κ'') to the electrical conductivity (''σ'') of a metal, and is proportional to the temperature (''T''). : Theoretically, the proportionality constant ''L'', known as the Lorenz number, is equal to : : This would lead, however, to an infinite velocity. The further assumption therefore is that the electrons bump into obstacles (like defects or phonons) once in a while which limits their free flight. This establishes an average or drift velocity ''V''d. The drift velocity is related to the average scattering time as becomes evident from the following relations. : ==Temperature dependence== The value ''L0'' = 2.44×10−8 W Ω K−2 results from the fact that at low temperatures ( K) the heat and charge currents are carried by the same quasi-particles: electrons or holes. At finite temperatures two mechanisms produce a deviation of the ratio from the theoretical Lorenz value ''L0'': (i) other thermal carriers such as phonon or magnons, (ii) inelastic scattering. In the 0 temperature limit inelastic scattering is weak and promotes large q scattering values is favored (trajectory a in the figure). For each electron transported a thermal excitation is also carried and the Lorenz number is reached ''L=L0''. Note that in a perfect metal, inelastic scattering would be completely absent in the limit K and the thermal conductivity would vanish . At finite temperature small q scattering values are possible (trajectory b in the figure) and electron can be transported without the transport of an thermal excitation ''L(T) In every system at higher temperature the contribution of phonon to thermal transport is important. This can lead to ''L(T)>L0''. Above the Debye temperature the phonon contribution to thermal transport is constant and the ratio ''L(T)'' is again found constant. For references see: 〔Thermal conductivity: theory, properties, and applications, edited by Terry Tritt, Kluwer Academic / Plenum Publishers, New York (2004), ISBN 978-0-387-26017-4〕 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「Wiedemann–Franz law」の詳細全文を読む スポンサード リンク
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