Answer: 20 Ω.
- A 20 Ω
- B 0 Ω
- C Infinite
- D 55 Ω
Correct answer: A. 20 Ω
Explanation: At resonance, X<sub>L</sub> = X<sub>C</sub>, they cancel. Impedance Z = √(R² + 0) = R = 20 Ω. The current is maximum at this point.
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.