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The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). For a transverse-electric-magnetic (TEM) plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the impedance of free space. The symbol ''Z'' is used to represent it and it is expressed in units of ohms. The symbol η (eta) may be used instead of ''Z'' for wave impedance to avoid confusion with electrical impedance. The wave impedance is given by : where is the electric field and is the magnetic field, in phasor representation. The impedance is in general a complex number. In terms of the parameters of an electromagnetic wave and the medium it travels through, the wave impedance is given by : where μ is the magnetic permeability, ε is the electric permittivity and σ is the electrical conductivity of the material the wave is travelling through. In the equation, ''j'' is the imaginary unit, and ω is the angular frequency of the wave. In the case of a dielectric (where the conductivity is zero), the equation reduces to the real number : As usual for any electrical impedance, the ratio is defined only for the frequency domain and never in the time domain. ==Wave impedance in free space== (詳細はfree space the wave impedance of plane waves is: :: and: :: 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「wave impedance」の詳細全文を読む スポンサード リンク
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