CBSE Class 12 Physics: Alternating Current Previous Year Questions (2019–2025)

CBSE Class 12 Physics Alternating Current Previous Year Questions (2019–2025) | PhysicsQandA

Updated for CBSE 2025 exams — This curated collection includes all official Alternating Current (Chapter 7) questions asked in CBSE Class 12 Physics board exams from 2019 to 2025. Includes both theoretical derivations and numerical problems on LCR circuits, transformers, AC generators, resonance, RMS value, and more — ideal for NEET, JEE, and board preparation.

CBSE Class 12 Physics – Alternating Current Questions: 2025

Theoretical Questions

• The alternating current I in an inductor is observed to vary with time t as shown in the graph for a cycle. Which one of the following graphs is the correct representation of the waveform of voltage V with time t?
• Differentiate between the peak value and root mean square value of an alternating current. Derive the expression for the RMS value of alternating current in terms of its peak value.
• An AC source of voltage is connected to a series LCR circuit. Draw the phasor diagram and use it to obtain an expression for the impedance and the phase difference between voltage and current.
• AC voltage of frequency ω is applied across a series LCR circuit. Draw the phasor diagram and obtain the impedance of the circuit.
• Discuss ‘resonance’ in a series LCR circuit and write the expression for resonant frequency.

Numerical Questions

• The turns ratio of an ideal transformer is 20:1. If 240 V AC is applied to the primary and a 6.0 W resistor is connected across the secondary, the current drawn from the source is:
  (A) 4.0 A (B) 3.8 A (C) 0.97 A (D) 0.10 A
• A transformer connected to a 200 V AC source supplies 3000 V to a device. If the primary has 450 turns, the number of secondary turns is:
  (A) 30 (B) 450 (C) 4500 (D) 6750
• An input voltage of 20 V is applied between terminals A and B of a step-up transformer. Between which terminals is the output 120 V?
  (A) P and Q (B) Q and S (C) P and R (D) P and S

2024 CBSE Class 12 Physics – Alternating Current Questions

Theoretical Questions

• Mention the factors affecting the resonant frequency of a series LCR circuit. Plot a graph showing the variation of impedance with frequency.
• Explain the working of a step-up transformer using a suitable diagram.
• Write two causes of energy loss in a real transformer.
• Explain the construction and working of an AC generator with a neat diagram.
• Establish the expression for impedance when elements X, Y, and Z are connected in series to an AC source. Show how current varies with frequency.
• Obtain the condition for minimum impedance in a series LCR circuit.
• Under what condition does wattless current flow in a series LCR circuit?
• Describe the construction and working of a transformer. Derive the relation .
• Discuss four main causes of energy loss in real transformers.

2023 CBSE Class 12 Physics – Alternating Current Questions

Theoretical Questions

• Define coefficient of self-induction. Derive the expression for self-inductance of a long solenoid of length , area , and turns per unit length.
• Describe the principle and working of an AC generator using a labelled diagram. Derive the expression for instantaneous emf.

Numerical Questions

• Calculate the self-inductance of a coil using AC (frequency Hz) and DC test data (voltage and current values provided).
• An AC generator coil has 100 turns, each of area 0.5 m², resistance 100 Ω, rotating at 60 rad/s in a 0.8 T magnetic field. Find the maximum emf and power dissipated.

2022 CBSE Class 12 Physics – Alternating Current Questions

Theoretical Questions

• Define coefficient of self-induction. Derive self-inductance of a long solenoid (area , length , turns/unit length).
• Explain the principle and working of an AC generator with a labelled diagram. Derive instantaneous emf expression.

Numerical Questions

• A 50 Ω resistor,  µF capacitor, and  H inductor are in series with . Find:
  (i) net reactance, (ii) impedance, (iii) effective current.
• Determine self-inductance using given AC (frequency  Hz) and DC voltage-current data.
• Same as 2023 numerical: 100-turn coil, 0.5 m² area, 100 Ω, 0.8 T, 60 rad/s → max emf and power.

Important Notes

• No CBSE Class 12 Physics board exam was conducted in 2021 due to the COVID-19 pandemic.
• Verified 2020 question papers were not released by CBSE; hence, data for that year is excluded.

Use these questions for: CBSE board exam revision, NEET Physics practice, JEE Main preparation, or classroom teaching resources.

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