a quantity characterizing the opposition presented to an alternating current by the capacitance of a circuit or of part of a circuit. Capacitive reactance is measured in ohms.
In the case of a sinusoidal current, the capacitive reactance xcc can be expressed as the ratio 1/ωC, where ω is the angular frequency of the current and C is the capacitance of the circuit. Capacitive reactance is equal to the ratio of the amplitude of the voltage applied to the terminals of a circuit and the amplitude of the current through the circuit if the circuit has a capacitive character (that is, has little inductance and resistance); such a circuit can be regarded as equivalent to a capacitor. If ω 4 0, the variation in the voltage across the capacitor causes a variation in the charge on its plates. As a result, a charging (discharge) current flows continuously in the circuit of the capacitor. During the charging and discharging of the capacitor, electrical energy is periodically transferred from the current source to the electric field of the capacitor and then back to the current source; the average power during a period is equal to zero.