Certified Irrigation Designer Practice Exam

Impedance is the total opposition to alternating current, combining resistance and reactance. In AC, resistance dissipates energy like in DC, but reactance from inductors and capacitors stores and releases energy and depends on frequency. Impedance, often written as Z = R + jX, brings these effects together: the magnitude |Z| tells how much current a given voltage will drive, and the phase angle shows how far the current lags or leads the voltage (φ = arctan(X/R)).

That’s why it’s the best answer. The total DC resistance describes only resistance at zero frequency and misses the frequency-dependent reactance. The current describes the circuit’s response, not the opposition itself. The voltage across the load is the driving quantity, not the opposition to current.