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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 Force-free magnetic field ： ウィキペディア英語版 Force-free magnetic field A force-free magnetic field is a magnetic field that arises when the plasma pressure is so small, relative to the magnetic pressure, that the plasma pressure may be ignored, and so only the magnetic pressure is considered. For a force free field, the electric current density is either zero or parallel to the magnetic field. The name "force-free" comes from being able to neglect the force from the plasma. ==Basic Equations == Neglecting the effects of gravity, the Navier-Stokes equation for a plasma, in steady state, reads $0=-\backslash nabla\; p+\backslash mathbf\backslash times\backslash mathbf,$ where $p$ is the thermal pressure, $\backslash mathbf$ is the magnetic field and $\backslash mathbf$ is the electric current. Assuming that the gas pressure $p$ is small compared to the magnetic pressure, i.e., $p\backslash ll\; B^2/2\backslash mu$ then the pressure term can be neglected. Here $\backslash mu$ is the magnetic permeability of the plasma. Therefore $\backslash mathbf\backslash times\backslash mathbf\; =\; 0$. This equation implies that: $\backslash mu\_\backslash mathbf=\backslash alpha\backslash mathbf$. e.g. the current density is either zero or parallel to the magnetic field, and where $\backslash alpha$ is a spatial-varying function to be determined. Combining this equation with Maxwell's equations: $\backslash nabla\backslash times\backslash mathbf=\backslash mu\backslash mathbf$ $\backslash nabla\backslash cdot\backslash mathbf=0$ and the vector identity: $\backslash nabla\backslash cdot(\backslash nabla\backslash times\backslash mathbf)=0$ leads to a pair of equations for $\backslash alpha$ and $\backslash mathbf$: $\backslash mathbf\backslash cdot\backslash nabla\backslash alpha=0,$ $\backslash nabla\backslash times\backslash mathbf=\backslash alpha\backslash mathbf.$ 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「Force-free magnetic field」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.