Answer: R/Z.
- A R/Z
- B Z/R
- C XL/R
- D R x Z
Correct answer: A. R/Z
Explanation: Power factor cos(phi) = R/Z. Average power P = Vrms x Irms x cos(phi).
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.