In AC circuits, how are current, voltage, and impedance related, and what does Z comprise?

• A. I = V / Z; Z combines resistance and reactance; phase angle indicates power factor and safety.
• B. V = I / Z; Z equals resistance only.
• C. I = Z / V; Z equals reactance only.
• D. V = I R; Z does not include reactance.

Answer: (A)

In AC circuits, impedance is the link between voltage and current, and it’s a complex quantity that includes both resistance and reactance. The current equals the voltage divided by impedance, I = V / Z, and equivalently the voltage equals the current times impedance, V = I Z. The impedance itself is Z = R + jX, where R is resistance and X is reactance produced by inductors and capacitors. The magnitude of impedance relates to how large the current is for a given voltage, |I| = |V| / |Z|, and the phase difference between voltage and current is the angle phi = arctan(X/R). That phase angle is the power factor, cos(phi), which tells how effectively real power is being used and relates to how reactive power can affect heating and safety in a system. So the best description is that current and voltage are connected through an impedance that combines resistance and reactance, with the phase angle indicating the power factor. The other statements fall short because they either misstate the fundamental equations or treat impedance as only resistance or only reactance, or restrict it to V = IR, which only holds for purely resistive circuits.