reactances

Reactances are the components of electrical impedance that oppose alternating current through the storage of energy in magnetic and electric fields. In AC circuits, the impedance Z has an imaginary part, called reactance and denoted by X. Positive reactance (inductive) and negative reactance (capacitive) determine how current and voltage are phase-shifted. The net reactance of a network is X = X_L − X_C, and the full impedance is Z = R + jX.

Inductive reactance X_L arises from an inductor and increases with frequency: X_L = ωL = 2πfL, where ω is angular frequency and f is frequency. An inductor causes current to lag voltage by up to 90 degrees in a pure case. Capacitive reactance X_C originates from a capacitor and decreases with frequency: X_C = 1/(ωC) = 1/(2πfC). A capacitor causes current to lead voltage by up to 90 degrees in a pure case.

When analyzing networks, the total reactance X determines the phase relationship between current and voltage and, together with resistance R, sets the magnitude and phase of the impedance Z. By adjusting L and C, designers create resonant conditions where X_L equals X_C, giving a purely resistive impedance at the resonant frequency.

Reactances play a central role in impedance matching, filters, tuners, and many RF and audio applications. Real components also include parasitic resistances and losses that modify ideal reactance; thus practical designs use the concept of reactance alongside resistance to predict circuit behavior across frequencies.