Answer: Vrms x Irms x cos(phi).
- A Vrms x Irms
- B Vrms x Irms x sin(phi)
- C Vrms x Irms x cos(phi)
- D V<sub>0</sub> x I<sub>0</sub>/2
Correct answer: C. Vrms x Irms x cos(phi)
Explanation: Average power P = Vrms x Irms x cos(phi). cos(phi) is the power factor. For pure reactance: cos(phi) = 0, P = 0.
In a pure inductor the current lags the voltage by 90° (energy is briefly stored in the magnetic field before current responds); in a pure capacitor the current leads the voltage by 90° (current must flow to build up charge before voltage rises) - the mnemonic "ELI the ICE man" keeps these straight.
Concept context
AC circuits, impedance, LCR series circuit, resonance, transformers, and power factor.