**Device Examples**

The impedance of an ideal resistor is purely real and is referred to as a *resistive impedance*:

In this case, the voltage and current waveforms are proportional and in phase.

Ideal inductors and capacitors have a purely imaginary *reactive impedance*:

the impedance of inductors increases as frequency increases;

the impedance of capacitors decreases as frequency increases;

In both cases, for an applied sinusoidal voltage, the resulting current is also sinusoidal, but in quadrature, 90 degrees out of phase with the voltage. However, the phases have opposite signs: in an inductor, the current is *lagging*; in a capacitor the current is *leading*.

Note the following identities for the imaginary unit and its reciprocal:

Thus the inductor and capacitor impedance equations can be rewritten in polar form:

The magnitude gives the change in voltage amplitude for a given current amplitude through the impedance, while the exponential factors give the phase relationship.

Read more about this topic: Electrical Impedance

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