Reactance is the opposition of a circuit element to a change of current, caused by the build-up of electric or magnetic fields in the element. Those fields act to produce counter-emf that is proportional to either the rate of change (time derivative), or accumulation (time integral), of the current. An ideal resistor has zero reactance, while ideal inductors and capacitors consist entirely of reactance, with neither series resistance nor parallel conductance.
In vector analysis, reactance is the imaginary part of electrical impedance, used to compute amplitude and phase changes of sinusoidal alternating current going through the circuit element. It is denoted by the symbol . The SI unit of reactance is the same as that of resistance: the ohm.
Both reactance and resistance are required to calculate the impedance . In some circuits one of these may dominate, but an approximate knowledge of the minor component is useful to determine if it may be neglected.
Both the magnitude and the phase of the impedance depend on both the resistance and the reactance.
The magnitude is the ratio of the voltage and current amplitudes, while the phase is the voltage–current phase difference.
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