Impedance essentially can be viewed as frequency-dependent resistance. While resistance of a circuit is the instantaneous ratio between voltage and current, impedance of a circuit is the ratio between voltage and current for steady-state sinusoidal signals, which can vary with of frequency. As the later parts of this section will show, the voltage and current caused by applying a steady-state sinusoidal signal to any combination of resistors, capacitors, and inductors, are related by a constant factor and a phase shift. Therefore, impedance can be expressed by a complex constant using an extended version Ohm’s law.
The real part and imaginary part of impedance are interpreted as a resistive part that dissipates energy and a reactive part that stores energy. Resistors can only dissipate energy and therefore their impedances have only a real part. Capacitors and inductors can only store energy and therefore their impedances have only an imaginary part. When resistors, capacitors, and inductors are combined, the overall impedance may have both real and imaginary parts. It is important to note that the definition of impedance preserves the definition of resistance.
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