Impedances and Admittance

In AC circuits, the passive elements - the resistor, the inductor, and the capacitor - are characterized by the phasor voltage and current.

Impedance is the ratio of the phasor voltage to the current and represents the resistance to the sinusoidal current flow in an AC circuit.

For DC sources with zero frequency, the inductor operates like a short circuit with zero impedance, while the capacitor functions as an open circuit with infinite impedance.

However, at high frequencies, the inductor acts as an open circuit, and the capacitor becomes a short circuit.

Impedance is a complex quantity that can be expressed in rectangular form, where the real part denotes the resistance and the imaginary part represents the reactance.

Reactance can be positive or negative. A positive reactance implies inductive impedance, where the current lags behind the voltage. Conversely, a negative reactance signifies capacitive impedance, where the current leads the voltage.

In polar form, impedance can be characterized by its magnitude and phase angle.

Admittance, the reciprocal of impedance, can also be expressed in rectangular form, where the real and imaginary components represent the conductance and the susceptance, respectively.