The Z of an AC circuit is the ______ of the circuit.

• A. Resistance
• B. Impedance
• C. Reactance
• D. Voltage

Answer: (B)

In AC circuits, Z represents impedance, the total opposition to current that includes both resistive losses and energy storage effects. Impedance is a complex quantity that combines resistance (the real part, which is the same in both AC and DC for a pure resistor) and reactance (the imaginary part, due to inductors and capacitors). You can think of it as Z = R + jX, where X is the net reactance from inductors and capacitors (X = X_L − X_C). This means impedance captures not just how much the circuit resists current, but also how much the current and voltage are shifted in time relative to each other.

The magnitude of impedance, |Z|, tells you how much the current is reduced in amplitude for a given voltage, while the phase angle φ = arctan(X/R) indicates the phase difference between voltage and current. Ohm’s law for AC is V = IZ, which reduces to V and I being in phase for a purely resistive circuit (X = 0) and becoming out of phase when reactive elements are present. So impedance is the right term because it encapsulates both resistance and reactance, whereas resistance alone or reactance alone wouldn’t fully describe how an AC circuit opposes and shifts the current. Voltage is the driving force, not the opposition.