In the realm of AC circuits, passive circuit elements like resistors, inductors, and capacitors take on a different character when characterized by phasor voltage and current. Their behavior is expressed through impedance, a vital concept in AC circuit analysis.

Impedance is a measure of resistance to sinusoidal current flow in an AC circuit. Unlike their behavior in DC circuits, where inductors appear as short circuits and capacitors as open circuits, the behavior of these components in AC circuits is frequency-dependent. At high frequencies, inductors act as open circuits, while capacitors become short circuits.

Impedance is a complex quantity with a real part denoting resistance and an imaginary part representing reactance. Reactance can be either positive or negative, indicating inductive impedance when current lags behind voltage and capacitive impedance when current leads voltage. Impedance can also be represented in polar form, highlighting its magnitude and phase angle.

The reciprocal of impedance is admittance, which is measured in Siemens (S). Admittance represents the ease with which current flows through a circuit. It comprises conductance (real part) and susceptance (imaginary part). Admittance, like impedance, is a valuable tool in AC circuit analysis, enabling engineers to understand and manipulate electrical circuits operating under sinusoidal conditions.