Impedance (symbol Z ) is used in AC technology as a resistor in the current and voltage that are not in phase. Inductors have an impedance of jωL and capacitors have an impedance of 1/jωC , where j is the imaginary part and ω is the angular frequency of the signal.

The impedances of these components can be summarised by complex number arithmetic. The imaginary part of impedance is called reactance. In general, Z = R + jX , where X is the reactance , and Z is the impedance.

When signal frequency increases, the capacitive reactance XC decreases, while the inductive reactance XL increases. This leads to changes in the total impedance as a function of frequency. The impedance of a purely ohmic resistance is independent of frequency. The reactance itself is a real quantity, and can be positive (inductive behavior) or negative (capacitive behavior).

With an impedance analyzer, one can measure the impedance across multiple frequencies.

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