What is a circuit's 'impedance'?

Impedance is a crucial concept in electrical engineering, particularly when dealing with alternating current (AC) circuits. It refers to the total opposition that a circuit offers to the flow of alternating current. This total opposition is not just due to resistance, which is the pure opposition to current flow; it also includes reactance, which is the opposition caused by inductors and capacitors in the circuit.

In AC circuits, the current is constantly changing due to the alternating nature of the voltage supply, and so the effects of both resistance and reactance must be taken into account. Reactance is frequency-dependent and arises from the inductive and capacitive elements in the circuit. Impedance is therefore expressed as a complex number, combining both resistance (the real part) and reactance (the imaginary part), and is typically measured in ohms.

Understanding impedance is vital for analyzing and designing AC circuits, as it influences how circuits behave, including their current and voltage relationships. This knowledge allows engineers to predict how circuits will respond to different signals and helps in ensuring that various components within a circuit function together effectively.