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In electromagnetism, current density is the electric current per unit area of cross section. It is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space (i.e. it is a vector field). In SI units, the electric current density is measured in amperes per square metre.〔Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3〕 == Definition == Electric current density ''J'' is simply the electric current ''I'' (SI unit: A) per unit area ''A'' (SI unit: m2). Its magnitude is given by the limit:〔Essential Principles of Physics, P.M. Whelan, M.J. Hodgeson, 2nd Edition, 1978, John Murray, ISBN 0-7195-3382-1〕 : For current density as a vector J, the surface integral over a surface ''S'', followed by an integral over the time duration ''t''1 to ''t''2, gives the total amount of charge flowing through the surface in that time (''t''2 − ''t''1): : The area required to calculate the flux is real or imaginary, flat or curved, either as a cross-sectional area or a surface. For example, for charge carriers passing through an electrical conductor, the area is the cross-section of the conductor, at the section considered. The vector area is a combination of the magnitude of the area through which the mass passes through, ''A'', and a unit vector normal to the area, , then : where · is the dot product of the unit vectors. This is, the component of current density passing through the surface (i.e. normal to it) is ''J'' cos θ, while the component of current density passing tangential to the area is ''J'' sin θ, but there is ''no'' current density actually passing ''through'' the area in the tangential direction. The ''only'' component of current density passing normal to the area is the cosine component. 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「current density」の詳細全文を読む スポンサード リンク
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