Answer: R/√(R² + (2πfL)²).
- A R/√(R² + (2πfL)²)
- B R/L as widely reported
- C L/R in standard practice
- D 2πfL/R under most conditions encountered
Correct answer: A. R/√(R² + (2πfL)²)
Explanation: Power factor = cos(φ) = R/Z = R/√(R² + X<sub>L</sub>²) = R/√(R² + (2πfL)²). At high frequency, PF approaches 0 (mostly reactive).
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.