Capacitive Reactance

Formula

Description

Capacitive reactance is the frequency-dependent opposition that a capacitor presents to alternating current. Unlike resistance, reactance does not dissipate energy as heat but instead stores and releases it in the electric field between the capacitor plates. At low frequencies the capacitor has time to fully charge and block current flow, resulting in high reactance. At high frequencies the capacitor barely charges before the polarity reverses, presenting very low opposition. At DC (0 Hz), a capacitor is an open circuit with infinite reactance.

Variables

• Xc — Capacitive reactance in ohms (Ω)
• f — Frequency in hertz (Hz)
• C — Capacitance in farads (F)

Practical Notes

Capacitive reactance causes current to lead voltage by 90 degrees. This phase relationship is essential in filter design, power factor correction, and coupling/decoupling applications. In bypass capacitor selection, choose a capacitor whose reactance is less than one-tenth of the circuit impedance at the frequency of interest.