What is meant by reactive power in an AC circuit?

• A. The energy dissipated as heat.
• B. The power consumed in the inductive and capacitive reactances in an AC circuit.
• C. The average power.
• D. The power delivered to a resistor.

Answer: (B)

Reactive power is the portion of power in an AC circuit that comes from energy being stored and released by inductors and capacitors each cycle. It doesn’t do net work and doesn’t heat resistive elements; instead, it shuttles energy between the source and the reactive elements as the magnetic and electric fields build up and collapse. This energy exchange is why it’s measured in volt-amperes reactive (VAR) and is described by Q = V_rms I_rms sin(phi), with phi the phase angle between voltage and current. Inductive loads cause the current to lag voltage and contribute positive reactive power, while capacitive loads cause the current to lead and contribute negative reactive power. Real power, which actually does work or dissipates as heat, is P = V_rms I_rms cos(phi) and is measured in watts. The difference between apparent power (S = V_rms I_rms) and real power is the reactive component; the power factor is cos(phi). In a purely resistive circuit, phi is zero and reactive power is zero.