What happens to current and impedance at resonance in a series RLC circuit?

• A. Impedance equals the resistive component; current reaches a maximum; voltage across L and C can be large and opposite in phase.
• B. Impedance becomes infinite; current is zero; voltages cancel out.
• C. Impedance equals the capacitive reactance only; current is minimal; voltage across L and C is zero.
• D. Impedance equals the inductive Reactance only; current is maximum; voltage across L and C are in phase.

Answer: (A)

At resonance in a series RLC circuit, the inductive reactance and capacitive reactance cancel each other out exactly. This makes the total impedance purely real and equal to the resistance, Z = R. With a given source, the current is then I = V/R, which is the maximum possible for that circuit because the impedance is at its lowest value (the reactive part has vanished). The voltages across the inductor and capacitor can become quite large because they are driven by the same current and XL equals Xc, so V_L = I·XL and V_C = I·Xc have large magnitudes but are 90 degrees out of phase with the current and opposite in phase with each other. Their phasor sum cancels, leaving only the resistive voltage drop across R. The other descriptions imply an infinite impedance, zero current, or a remaining reactive part, which isn’t the case at resonance.