The impedance of an AC circuit is composed of which components?

• A. Resistance and reactance
• B. Voltage and current
• C. Capacitance and inductance
• D. Power and energy

Answer: (A)

In AC circuits, impedance is the total opposition to current formed by two parts: resistance and reactance. Resistance dissipates energy as heat and is in phase with current, while reactance stores energy in electric or magnetic fields (from capacitors and inductors) and causes a phase shift between voltage and current. This is written as Z = R + jX, where X is the net reactance (X = X_L − X_C, with X_L = ωL and X_C = 1/(ωC)). The impedance’s magnitude is |Z| = √(R^2 + X^2). Voltage and current relate through V = ZI, illustrating that impedance combines how much the circuit opposes current, not the actual voltage or current themselves. Capacitance and inductance determine the reactive part, but the two components of impedance are resistance and reactance, not voltage/current or power/energy.