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In the analysis of an alternating-current electrical circuit (for example a RLC series circuit), reactance is the imaginary part of impedance, and is caused by the presence of inductors or capacitors in the circuit. Reactance is denoted by the symbol X and is measured in ohms. If X > 0 the reactance is said to be inductive, and if X < 0 it is said to be capacitive. If X = 0, then the circuit is purely resistive, i.e. it has no reactance.

Inductive reactance (symbol XL) is caused by the fact that a current is accompanied by a magnetic field; therefore a varying current is accompanied by a varying magnetic field; the latter gives an electromotive force that resists the changes in current. The more the current changes, the more an inductor resists it: the reactance is proportional with the frequency (hence zero for DC). There is also a phase difference between the current and the applied voltage.

Inductive reactance has the formula

<math>X_L=2\pi fL</math>

where f is the frequency (in hertz) and L is the inductance (in henry).

Capacitive reactance (symbol XC) reflects the fact that electrons can not pass, yet effectively alternating current (AC) can: the higher the frequency the better. There is also a phase difference between the alternating current flowing through a capacitor and the potential difference across the capacitor's electrodes.

Capacitive reactance has the formula

<math>X_C=1/(2\pi fC)</math>

where f is the frequency (in hertz) and C is the capacitance (in farad).

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