where Z = impedance, W; R = resistance, W; L = inductance, H; XL = inductive reactance, W; XC = capacitive reactance, W; and q = phase angle, degrees, by which current leads voltage in a capacitive circuit or lags voltage in an inductive circuit (0° indicates an in-phase condition). Resonant Frequency When an inductor and capacitor are connected in series or parallel, they form a resonant circuit. The resonant frequency can be determined from the equation