Which statement best describes impedance in an AC circuit?

• A. The combination of resistance and reactance in a circuit
• B. Only resistance
• C. Only reactance
• D. The rate of energy consumption

Answer: (A)

Impedance in an AC circuit is the combination of resistance and reactance. You can think of it as a complex quantity with a real part that dissipates energy (resistance) and an imaginary part that stores and releases energy in magnetic or electric fields (reactance). This blend is what determines how much current flows for a given voltage and how the voltage and current relate in time.

Mathematically, impedance is written as Z = R + jX, where R is the resistance and X is the reactance. The overall opposition to current has a magnitude |Z| = sqrt(R^2 + X^2) and a phase angle φ = arctan(X/R). The current is I = V / Z, so both the size of the opposition and the phase shift between voltage and current come from the combination of these two parts. Inductive reactance makes current lag behind voltage, capacitive reactance makes it lead, and resistance continually dissipates power.

The rate of energy consumption is not impedance itself; it’s real power, P = VI cosφ. So impedance isn’t just resistance, nor just reactance, nor the rate at which energy is used. The correct view is that impedance is the combination of resistance and reactance in an AC circuit.