Impedance

For alternating current, impedance (Symbol Z) refers to a resistance, where electrical current and voltage are not in phase. Inductivities have an impedance of jωL, while capacitors have an impedance of 1/jωC, whereas j is an imaginary unit and ω is the angular velocity of the signal. The impedances of these components can be summarized with complex analysis.

 

Often, the imaginary unit of the impedance is described as reactance. In general it applies Z = R + jX, where X is the reactance and Z is the impedance. For a signal with increasing frequency, the capacitive reactance XC declines, while the inductive reactance XL rises.

This leads to changes of the total impedance as a function of frequency. The impedance of a simple ohmic resistance is independent of the frequency. Being a real quantity, the reactance can be positive (inductive behavior) or negative (capacitive behavior).