CBSE Class 12 Physics Alternating Current PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage Breakdown: 1-2 MCQs/assertion-reason (1 mark), 1-2 short answers (2 marks), 1-2 numericals/derivations on LCR circuit & transformer (4-6 marks).
Repeated Topics: AC voltage & current phasor diagrams, impedance & phase in R, L, C circuits, LCR series circuit (resonance, Q-factor, power factor), power in AC circuits (average power, wattless current), transformer (principle, efficiency, losses), LC oscillations (qualitative), power transmission (high voltage advantage).
Difficulty: Medium; phasor diagrams, resonance numericals & transformer efficiency are most repeated & scoring.
Changes: More numericals on power factor improvement & resonance frequency/voltage in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Derive expression for impedance, current & phase in LCR series circuit. Explain resonance condition & its applications.	Derivation (4 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; Z = √(R² + (XL - XC)²), tanφ = (XL - XC)/R; Resonance: XL = XC → ω = 1/√(LC), Z min = R, power max.
Find average power in AC circuit. Show that power factor cosφ = R/Z. Also explain wattless current in pure L/C circuit.	Derivation/Short Answer (3 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; P_avg = V_rms I_rms cosφ; cosφ = R/Z; Pure inductor/capacitor: φ = ±90°, cosφ = 0 → P_avg = 0.
A transformer steps up/down voltage. Derive relation between turns ratio & voltage/current. Explain energy losses & efficiency.	Derivation (4 marks)	2022, 2023, 2024, 2025	Repeated 4x; V_s / V_p = N_s / N_p = I_p / I_s (ideal); Losses: copper, iron (hysteresis + eddy), efficiency η = output/input = V_s I_s / (V_p I_p).
Assertion: In a pure resistive AC circuit, current & voltage are in phase. Reason: Power factor is 1 in resistive circuit.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
Numerical: In LCR series circuit, R=10Ω, L=0.1H, C=100μF, V=220V, 50Hz. Find impedance, current, power factor & resonance frequency.	Numerical (3-4 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; XL = 2πfL, XC = 1/(2πfC), Z = √(R² + (XL-XC)²), I = V/Z, cosφ = R/Z, f₀ = 1/(2π√(LC)).
Explain quality factor (Q-factor) of LCR circuit. How does it affect sharpness of resonance?	Short Answer (2-3 marks)	2022, 2024	Repeated 2x; Q = ω₀ L / R = 1/(ω₀ C R) = ω₀ / Δω; Higher Q → sharper resonance peak.
MCQ: In an AC circuit containing only inductor, the power factor is: (a) 1 (b) 0 (c) 0.5 (d) ∞	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (b) 0 (cosφ = 0).
Why high voltage is used for long-distance power transmission? Explain role of step-up & step-down transformers.	Short Answer (3 marks)	2023, 2025	Repeated 2x; P = I²R loss → high V → low I → low loss; Step-up at generating station, step-down at distribution.
Case-based: Given LCR circuit, find resonance condition, power factor at resonance, current at resonance.	Case-Based (4 marks)	2023, 2025	Repeated 2x; At resonance: Z = R, I max = V/R, power factor = 1.
Find reactance of inductor/capacitor & impedance in series LCR at frequency f.	Short Answer (2 marks)	2021 Term 2, 2024	Repeated 2x; XL = 2πfL, XC = 1/(2πfC), Z = √(R² + (XL - XC)²).

Best Format for Preparation and Answering

Study Format: Memorize phasor diagrams for R, L, C, LCR. Practice resonance numericals & power factor calculations daily. Draw clear phasor diagrams in exam.
Answer Format: Derivations: Use phasor → impedance Z → current I = V/Z → phase angle φ. Numericals: Calculate XL, XC → Z → I → cosφ → power. Always box final answer with units.
Tips: LCR series resonance (frequency, current, power factor) & transformer (voltage/current ratio & losses) are highest repeat & marks. Always draw phasor diagram for phase. Time: 10–12 mins for 4–6 mark questions. Very scoring if phasors are accurate.

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