Reactance

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. The reciprocal of reactance is susceptance.

The relationship between impedance, resistance, and reactance is given by the equation:

<math>

Z = R + j X <math>

Often it is enough to know the magnitude of the impedance:

<math>

Z = \sqrt {R^2 + X^2} <math>

Z is impedance, R is resistance, and X is reactance.

Inductive reactance (symbol X_L) 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 X_C) 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).

Opposition to current

• Impedance
• Resistance
• Reactance
• Admittance
• Conductance
• Susceptance