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In electrical and electronic systems, reactance is the opposition of a circuit element to a ''change'' in current or voltage, due to that element's inductance or capacitance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current. The notion of reactance is similar to electrical resistance, but it differs in several respects. In phasor analysis, reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element. It is denoted by the symbol . An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance – that is, respond to current only by reactance. The magnitude of the reactance of an inductor rises in proportion to a rise in frequency, while the magnitude of the reactance of a capacitor decreases in proportion to a rise in frequency (or increases in proportion to wavelength). As frequency goes up, inductive reactance goes up and capacitive reactance goes down. == Capacitive reactance == (詳細はconductors separated by an insulator, also known as a dielectric. ''Capacitive reactance'' is an opposition to the change of voltage across an element. Capacitive reactance is inversely proportional to the signal frequency (or angular frequency ω) and the capacitance .〔Irwin, D. (2002). ''Basic Engineering Circuit Analysis'', page 274. New York: John Wiley & Sons, Inc.〕 There are two choices in the literature for defining reactance for a capacitor. One is to use a uniform notion of reactance as the imaginary part of impedance, in which case the reactance of a capacitor is a negative number:〔〔Hayt, W.H., Kimmerly J.E. (2007). ''Engineering Circuit Analysis'', 7th ed., McGraw-Hill, p. 388〕〔Glisson, T.H. (2011). ''Introduction to Circuit Analysis and Design'', Springer, p. 408〕 : Another choice is to define capacitive reactance as a positive number,〔Horowitz P., Hill W. (2015). ''The Art of Electronics'', 3rd ed., p. 42〕〔Hughes E., Hiley J., Brown K., Smith I.McK., (2012). ''Hughes Electrical and Electronic Technology'', 11th edition, Pearson, pp. 237-241〕〔Robbins, A.H., Miller W. (2012). ''Circuit Analysis: Theory and Practice'', 5th ed., Cengage Learning, pp. 554-558〕 : In this case however one needs to remember to add a negative sign for the impedance of a capacitor, i.e. . This article uses the former definition throughout. At low frequencies a capacitor is an open circuit so no current flows in the dielectric. A DC voltage applied across a capacitor causes positive charge to accumulate on one side and negative charge to accumulate on the other side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero. Driven by an AC supply, a capacitor will only accumulate a limited amount of charge before the potential difference changes polarity and the charge dissipates. The higher the frequency, the less charge will accumulate and the smaller the opposition to the current. 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 reactance」の詳細全文を読む スポンサード リンク
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