Resonance in a series RLC — total current behavior: If the given circuit operates exactly at series resonance, is the total current drawn from the source at its maximum value (for a fixed source voltage)?

Difficulty: Easy

Correct Answer: Correct

Explanation:


Introduction / Context:
Series resonance is a cornerstone concept in AC circuit theory and filter/tank design. It occurs when the inductive reactance equals the capacitive reactance in magnitude (X_L = X_C), causing reactances to cancel so the impedance is purely resistive and minimal.


Given Data / Assumptions:

  • Series RLC circuit driven by a fixed RMS voltage.
  • Ideal L and C, with a finite series resistance R.
  • Frequency is adjustable and can be set to the resonant frequency.


Concept / Approach:
Impedance of a series RLC is Z = R + j(X_L − X_C). At resonance, X_L = X_C → Z = R (minimum magnitude). Ohm’s law shows I = V/Z is largest when |Z| is smallest (for fixed V), hence current peaks at resonance. Large reactive voltages may appear across L and C individually, but the net reactive effect in the loop is zero at resonance.


Step-by-Step Solution:

Write Z(f) = √(R^2 + (X_L − X_C)^2).At f = f_0, X_L = X_C → |Z| = R (minimum possible for that circuit).Thus I(f_0) = V/R is the maximum attainable loop current for that applied V.


Verification / Alternative check:
Plot I versus frequency (a resonance curve). The peak occurs at f_0. Bandwidth relates to quality factor Q = X_L/R at resonance.


Why Other Options Are Wrong:

“Incorrect”: contradicts Z minimization at resonance.“Only if R = 0”: even with finite R, current is maximal at resonance for that V.“Parallel resonance”: refers to a different topology where current dips in the feed branch.“XL = 2XC”: not the resonance condition.


Common Pitfalls:
Confusing series and parallel resonance characteristics; focusing on large VL or VC and missing that total impedance is minimized, not maximized.


Final Answer:
Correct

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