WAEC Physics 2014 Paper

Concept:At resonance in a series RLC circuit, the inductive and capacitive reactances cancel, making the circuit purely resistive.Explanation:At resonance, $X_L = X_C$, so the net reactance is zero.Therefore, impedance $Z = \sqrt{R^2 + (X_L - X_C)^2} = R$.Thus, condition i is correct: impedance equals resistance.Since impedance is minimum, the current is maximum for a given voltage.Thus, condition ii is correct: current is maximum.Condition iii is wrong because net reactance is minimum, not maximum.Condition iv is wrong because at resonance, current and voltage are in phase, i.e. phase difference is $0^\circ$, not $90^\circ$.Hence, only i and ii occur at resonance.Answer:A. i and ii only