LEARN REVISE HUB · CBSE STUDY RESOURCE
Alternating Current Class 12 Physics Assertion and Reason Questions 2026-27 | Chapter 7
Class 12 Physics • Chapter 7 • CBSE 2026–27
Alternating Current — Assertion and Reason Questions
35 original Assertion–Reason questions with answers • RMS • Reactance • LCR • Resonance • Power Factor • Generator • Transformer
Practise statement-based reasoning, not just formula recall. Each question uses the four-option CBSE Assertion–Reason format.
35 QuestionsAnswers + ExplanationsCBSE 2026–27Competency Practice
Current CBSE format: The official 2026–27 Physics Sample Question Paper has four Assertion–Reason questions in Section A, each carrying 1 mark. The same four-option logic is used throughout this practice page. This is original practice material, not an official CBSE question bank. Official Physics SQP 2026–27.
Syllabus boundary: Chapter 7 covers alternating currents, peak/RMS values, reactance and impedance, series LCR circuit with phasors, resonance, AC power, power factor, wattless current, AC generator and transformer. The current CBSE curriculum does not require Q-factor as a core Chapter 7 topic, so it is not used as a prerequisite here. Official Physics Curriculum 2026–27.
How to Solve Assertion–Reason Questions
Step 1: Judge Assertion independently.
Step 2: Judge Reason independently.
Step 3: Only if both are true, ask whether the Reason actually explains the Assertion.
Step 4: Check formulas, limiting cases and phase relationships before selecting the option.
CBSE Option Map
Section A — Core Assertion–Reason Practice
Q1.
Assertion (A): At resonance in a series LCR circuit, the impedance is equal to the resistance.
Reason (R): At resonance, inductive and capacitive reactances are equal and opposite, so the net reactance is zero.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q2.
Assertion (A): In a pure resistive AC circuit, current and voltage are in phase.
Reason (R): A resistor does not introduce a phase difference between voltage and current.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q3.
Assertion (A): In a pure inductive AC circuit, current lags voltage by 90°.
Reason (R): For an ideal inductor, v = L di/dt, giving a quarter-cycle phase difference.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q4.
Assertion (A): In a pure capacitive AC circuit, current leads voltage by 90°.
Reason (R): For an ideal capacitor, i = C dv/dt, so current leads voltage by a quarter cycle.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q5.
Assertion (A): The inductive reactance of a coil increases when the AC frequency increases.
Reason (R): X_L = 2πfL.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q6.
Assertion (A): The capacitive reactance of a capacitor increases when the AC frequency increases.
Reason (R): X_C = 1/(2πfC), so it decreases as frequency increases.
Answer: C
The Assertion is false, but the Reason is true. Capacitive reactance decreases as frequency increases.
The Assertion is false, but the Reason is true. Capacitive reactance decreases as frequency increases.
Q7.
Assertion (A): At series resonance, the current is maximum for a fixed applied RMS voltage.
Reason (R): At resonance, impedance is minimum and equals R.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q8.
Assertion (A): At series resonance, the power factor of an ideal series LCR circuit is unity.
Reason (R): At resonance, the phase angle is zero, so cos φ = 1.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q9.
Assertion (A): A purely inductive AC circuit consumes zero average power.
Reason (R): The current and voltage differ in phase by 90°, so cos φ = 0.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q10.
Assertion (A): A purely capacitive AC circuit consumes zero average power.
Reason (R): The current leads voltage by 90°, giving zero average power for an ideal capacitor.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q11.
Assertion (A): A capacitor blocks steady-state DC but permits AC current.
Reason (R): For DC, f = 0 and X_C tends to infinity; for AC, finite frequency gives finite X_C.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q12.
Assertion (A): A transformer cannot operate from a steady DC supply.
Reason (R): A transformer requires changing magnetic flux to induce secondary EMF.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q13.
Assertion (A): A step-up transformer increases voltage and decreases current ideally.
Reason (R): For an ideal transformer, input power equals output power.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q14.
Assertion (A): At resonance, the voltages across L and C can be individually non-zero even though the net reactive voltage is zero.
Reason (R): The inductive and capacitive voltage phasors are equal in magnitude and opposite in phase at resonance.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q15.
Assertion (A): Below the resonant frequency, a series LCR circuit is capacitive in nature.
Reason (R): Below resonance, X_C is greater than X_L, so the net reactance is capacitive.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q16.
Assertion (A): Above the resonant frequency, a series LCR circuit is inductive in nature.
Reason (R): Above resonance, X_L is greater than X_C.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q17.
Assertion (A): The average value of a sinusoidal AC over one complete cycle is zero.
Reason (R): The positive and negative half-cycles cancel over a complete cycle.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q18.
Assertion (A): The RMS value of sinusoidal current I = I₀ sin ωt is I₀/√2.
Reason (R): RMS value is the square root of the mean of I² over a complete cycle.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q19.
Assertion (A): For a fixed peak voltage, increasing frequency increases the RMS voltage of an ideal sinusoidal source.
Reason (R): VRMS = V₀/√2 and does not depend on frequency when V₀ is fixed.
Answer: D
The Assertion is false, while the Reason is true. With fixed peak voltage, VRMS = V₀/√2 is independent of frequency.
The Assertion is false, while the Reason is true. With fixed peak voltage, VRMS = V₀/√2 is independent of frequency.
Q20.
Assertion (A): For a fixed L and C, increasing frequency above resonance increases the net inductive character of a series LCR circuit.
Reason (R): X_L increases with f while X_C decreases with f.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q21.
Assertion (A): In a series LCR circuit, current leads the applied voltage when the circuit is inductive.
Reason (R): In an inductive circuit, current lags the applied voltage.
Answer: D
Both statements are false. In an inductive circuit, current lags voltage.
Both statements are false. In an inductive circuit, current lags voltage.
Q22.
Assertion (A): In a series LCR circuit, current leads the applied voltage when the circuit is capacitive.
Reason (R): In a capacitive circuit, current leads voltage.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q23.
Assertion (A): At resonance, the power factor of a series LCR circuit is unity.
Reason (R): At resonance, the current is maximum for a fixed applied voltage.
Answer: B
Both statements are true, but the Reason does not directly establish why the power factor is unity.
Both statements are true, but the Reason does not directly establish why the power factor is unity.
Q24.
Assertion (A): At resonance, the net reactance of a series LCR circuit is zero.
Reason (R): X_L = X_C at resonance.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q25.
Assertion (A): The resonant frequency of an ideal series LCR circuit is 1/(2π√LC).
Reason (R): Resonance occurs when X_L = X_C, giving 2πfL = 1/(2πfC).
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q26.
Assertion (A): The average power in an AC circuit is P = V_RMS I_RMS cosφ.
Reason (R): Only the component of current in phase with voltage contributes to average power.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q27.
Assertion (A): A circuit with power factor 0.6 has a phase angle of 53.1° approximately.
Reason (R): cos φ = 0.6, so φ = cos⁻¹(0.6) ≈ 53.1°.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q28.
Assertion (A): A choke coil can limit AC current with relatively small power loss compared with a resistor.
Reason (R): An ideal inductor has zero average power consumption.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q29.
Assertion (A): In an ideal transformer, the frequency of the secondary voltage is different from the primary frequency.
Reason (R): The transformer changes voltage/current magnitude and also changes the supply frequency.
Answer: D
Both the Assertion and the Reason are false.
Both the Assertion and the Reason are false.
Q30.
Assertion (A): The turns ratio of an ideal step-up transformer is greater than one when N_s > N_p.
Reason (R): For an ideal transformer, V_s/V_p = N_s/N_p.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q31.
Assertion (A): Increasing the magnetic field amplitude in an AC generator increases its peak induced EMF if other factors remain constant.
Reason (R): The peak EMF is proportional to B, as ε₀ = NABω for a coil of N turns and area A.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q32.
Assertion (A): An AC generator converts mechanical energy into electrical energy.
Reason (R): Electromagnetic induction produces the electrical output from mechanical rotation.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q33.
Assertion (A): In an ideal purely reactive circuit, current may flow even though average power is zero.
Reason (R): Energy is alternately stored and returned by the inductor or capacitor.
Answer: A
Both statements are true, and the Reason directly establishes the Assertion.
Both statements are true, and the Reason directly establishes the Assertion.
Q34.
Assertion (A): At resonance in a series LCR circuit, current and source voltage are out of phase by 90°.
Reason (R): At resonance, the net reactance is zero and current is in phase with source voltage.
Answer: D
Both the Assertion and the Reason are false.
Both the Assertion and the Reason are false.
Q35.
Assertion (A): A transformer works equally well on DC and AC once the turns ratio is fixed.
Reason (R): A steady DC supply provides continuously changing magnetic flux needed for sustained secondary EMF.
Answer: D
Both the Assertion and the Reason are false.
Both the Assertion and the Reason are false.
High-Yield Concepts Tested
| Area | What you should be able to reason about |
|---|---|
| RMS & AC basics | Peak-to-RMS relation, complete-cycle average, fixed-amplitude changes. |
| Pure R, L and C | Phase difference, reactance and average power. |
| Series LCR | Impedance, phase angle, inductive/capacitive character and resonance. |
| Power factor | P = VRMSIRMScosφ and wattless current. |
| Frequency effects | How XL and XC change with frequency. |
| Generator | Induction principle and factors affecting peak EMF. |
| Transformer | Turns ratio, voltage/current relation, frequency and AC requirement. |
Common Assertion–Reason Traps
Trap 1: Equal XL and XC does not mean there is no voltage across L or C individually.
Trap 2: Zero average power does not mean zero current in a purely reactive circuit.
Trap 3: At resonance, current is maximum and in phase with source voltage for a series LCR circuit.
Trap 4: A transformer changes voltage/current levels; it does not change frequency.
Trap 5: Always test Assertion and Reason separately before deciding whether the Reason explains the Assertion.
Exam reasoning rule: A familiar formula is not enough. In Assertion–Reason questions, test the truth of A and R separately, then test causality. A true statement plus a related true statement can still be option B if R does not explain A.
Frequently Asked Questions
What is the CBSE Assertion–Reason format for Class 12 Physics 2026–27?
The official 2026–27 Physics SQP places four Assertion–Reasoning questions in Section A, each worth 1 mark, using four answer choices.
What are the most important Alternating Current Assertion–Reason topics?
Focus on RMS values, phase relationships in R/L/C circuits, reactance, series LCR resonance, power factor, wattless current, AC generator and transformer.
Is Q-factor required for these questions?
Q-factor is not listed in the core Chapter 7 scope of the current CBSE 2026–27 Physics curriculum, so it is not required to solve this practice set.
Are these official CBSE questions?
No. These are original Learn Revise Hub practice questions designed around the current syllabus and assessment format. Use official CBSE documents as the final authority.
Quick Revision Checklist
✓ I can distinguish peak, RMS and average values of sinusoidal AC.
✓ I know the phase relation for pure R, L and C circuits.
✓ I can determine whether a series LCR circuit is inductive, capacitive or resonant.
✓ I can explain why current is maximum at series resonance.
✓ I can calculate and interpret power factor.
✓ I understand wattless current and zero average power.
✓ I can explain the AC generator principle.
✓ I can explain the turns-ratio relation and why a transformer requires AC.
Continue Chapter 7 Preparation
Official references: CBSE Physics Curriculum 2026–27 · CBSE Class XII SQP & Marking Schemes 2026–27
Source discipline: The questions are original practice content. Official CBSE documents are used for syllabus and assessment-format boundaries; external educational resources were used only to identify recurring search language, misconception patterns and practice-demand areas; their questions are not reproduced here. No third-party question is presented as an official CBSE question.
Comments
Post a Comment