What is impedance in AC circuits?

• A. The total opposition to the flow of alternating current, equal to the vector sum of resistance, capacitive reactance, and inductive reactance.
• B. The product of voltage and current
• C. The rate at which energy is consumed by a circuit
• D. The direct opposition to current in a DC circuit

Answer: (A)

Impedance is the total opposition an AC circuit presents to current, with both the resistive losses and the energy-storage effects of inductors and capacitors included. In AC, voltage and current can be out of phase, so you don’t just have a single resistance value—the circuit’s opposition combines R with the reactive parts from inductors and capacitors as a vector (complex) sum: Z = R + j(X_L − X_C). The magnitude |Z| determines how much the current is reduced for a given voltage, and the angle tells how far the current lags or leads. Capacitive reactance decreases with frequency and inductive reactance increases with frequency, shaping the total impedance at that frequency. So impedance is the total, frequency-dependent opposition to AC current, arising from resistance and reactance together.