Impedance Z is the total opposition to current flow in an AC circuit. It is the vector sum of resistance and reactance. Which statement best completes this definition?

• A. The measure of energy stored in a magnetic field
• B. The rate of energy transfer
• C. The unit of impedance is henry
• D. The total opposition to current flow in an AC circuit. It is the vector sum of resistance and reactance

Answer: (D)

Impedance in AC circuits is a complex quantity that describes how much the circuit opposes current, combining two parts: resistance, which acts in phase with voltage, and reactance, which shifts the current in time due to inductors or capacitors. Treating impedance as the vector sum of these components captures both the amount of opposition and the phase relationship, which is why it’s written as Z = R + jX and has a magnitude |Z| = sqrt(R^2 + X^2). The best statement is the one that says impedance is the total opposition to current flow in an AC circuit and that it’s the vector sum of resistance and reactance, because it faithfully mirrors both the real and reactive aspects. The other options describe related ideas that aren’t impedance: energy stored in a magnetic field relates to inductance, rate of energy transfer is power, and the unit of impedance is the ohm (not the henry).