What is reactance in an electrical circuit?

Reactance refers specifically to the opposition encountered by alternating current (AC) due to inductance and capacitance within a circuit. Unlike resistance, which is a measure of the opposition to direct current (DC) that dissipates energy as heat, reactance is associated with the ability of inductors and capacitors to store and release energy.

In inductors, this opposition arises because they resist changes in current; they generate an opposing voltage when the current through them changes. In capacitors, reactance occurs as they resist changes in voltage; they charge and discharge with the AC signal, affecting how current flows through the circuit. This behavior is fundamentally important in AC circuits, where inductive and capacitive components can cause the current and voltage to be out of phase, leading to complex interactions in circuit behavior.

Understanding reactance is crucial in designing and analyzing circuits that operate with AC power, as it affects the overall impedance— the total opposition to current flow—including both resistance and reactance. This concept is foundational in electrical engineering and helps explain phenomena such as resonance in circuits containing both inductors and capacitors.