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For an LCR a.c. series circuit, L = 25 mH, R = 3 Ω and C = 62.5 μF. What is the frequency of the source at which resonance occurs?

Asked in GUJCET 2021 · Resonance condition and tuning

Answer: (1) 127.39 Hz

Step-by-step solution

Given: L = 25×10⁻³ H, C = 62.5×10⁻⁶ F (R does not affect the resonant frequency).

Idea: resonance occurs when X_L = X_C, i.e. at f₀ = 1/(2π√LC).

LC = 25×10⁻³×62.5×10⁻⁶ = 1.5625×10⁻⁶ s², so √LC = 1.25×10⁻³ s.

f₀ = 1/(2×3.14×1.25×10⁻³) = 1/(7.85×10⁻³) ≈ 127.39 Hz (with π = 3.14).

Why the other options are wrong

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