The reactance of a capacitor for a sine wave signal is given by:
where C is the capacitance, in farads and f is frequency, in hertz. Reactance is measured in units of ohms and is defined by the ratio:
˜V/˜I or v/i using the convention noted below,
where ˜V is the AC voltage across the capacitor and ˜I is the AC current through the circuit containing the capacitor. Unlike resistance, reactance is not a constant but, for a capacitor, varies inversely with frequency (Figure). In addition, the sine wave of current is ¼ cycle (90◦) ahead of the sine wave of voltage across the capacitor plates.