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Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle. It is necessary to introduce the concept of impedance in AC circuits because there are two additional impeding mechanisms to be taken into account besides the normal resistance of DC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part. The symbol for impedance is usually and it may be represented by writing its magnitude and phase in the form . However, cartesian complex number representation is often more powerful for circuit analysis purposes. The term ''impedance'' was coined by Oliver Heaviside in July 1886.〔''Science'', p. 18, 1888〕〔Oliver Heaviside, ''The Electrician'', p. 212, 23 July 1886, reprinted as ''Electrical Papers'', p 64, AMS Bookstore, ISBN 0-8218-3465-7〕 Arthur Kennelly was the first to represent impedance with complex numbers in 1893.〔(Kennelly, Arthur. ''Impedance'' (AIEE, 1893) )〕 Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency . In general, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm (). For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular: * The magnitude of the complex impedance is the ratio of the voltage amplitude to the current amplitude. * The phase of the complex impedance is the phase shift by which the current lags the voltage. The reciprocal of impedance is admittance (i.e., admittance is the current-to-voltage ratio, and it conventionally carries units of siemens, formerly called mhos). ==Complex impedance== Impedance is represented as a complex quantity and the term ''complex impedance'' may be used interchangeably. The polar form conveniently captures both magnitude and phase characteristics as : where the magnitude represents the ratio of the voltage difference amplitude to the current amplitude, while the argument (commonly given the symbol ) gives the phase difference between voltage and current. is the imaginary unit, and is used instead of in this context to avoid confusion with the symbol for electric current. In Cartesian form, impedance is defined as : where the real part of impedance is the resistance and the imaginary part is the reactance . Where it is needed to add or subtract impedances, the cartesian form is more convenient; but when quantities are multiplied or divided, the calculation becomes simpler if the polar form is used. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers. 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「Electrical impedance」の詳細全文を読む スポンサード リンク
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