Alternating Current — Class 12 MCQs with Answers
Class 12 CBSE Physics · Chapter 7
90 practice questions · 30 Easy · 30 Medium · 30 Hard · Updated
Practise the most important Class 12 CBSE Physics questions from Chapter 7, "Alternating Current". You get 9 timed quizzes made from 90 NCERT-based MCQs, with answers and explanations. The questions are split into 30 Easy, 30 Medium and 30 Hard. Warm up on the basics, then move on to the exam-level questions that set top scorers in CBSE & Maharashtra HSC Board exams, JEE Main, MHT-CET, JEE Advanced and NEET UG apart.
To score well in "Alternating Current", focus on numericals, derivations and applying concepts. Each MCQ here is timed and uses exam-style marking (+4 correct, −1 wrong, 0 skipped). This trains you to stay accurate under time pressure, as real papers need. Every question has a short explanation, so a wrong answer becomes a quick lesson. It is the fastest way to fix gaps before a test.
Use this chapter for focused revision. Start with the Easy set to check your basics on Alternating Current, then move to Medium and Hard to practise applying them. Your accuracy, streaks and XP save automatically. This chapter also adds to your overall Class 12 Physics mastery score. 14 sample questions are solved in full below, with the answer and a worked explanation. Sign in free to start practising.
Key concepts: Alternating Current (Class 12 Physics)
Circuits driven by sinusoidal sources: RMS and average values, the behaviour of R, L and C individually with their phase relations, the series LCR circuit with impedance and phase angle, resonance and the quality factor, power and the power factor, and the transformer with its energy losses.
- Alternating current
- Current whose magnitude and direction vary sinusoidally with time. Indian mains supply is 220 V at 50 Hz.
- RMS value
- The steady direct current that would produce the same heating, equal to the peak value divided by √2. Every AC meter reads RMS, not peak.
- Average value
- Zero over a full cycle, since the positive and negative halves cancel exactly. Over a half cycle it is 2I₀/π.
- AC through a resistor
- Voltage and current stay exactly in phase, so a resistor behaves in AC just as it does in DC and dissipates full power.
Alternating Current — important questions & MCQs with answers (Class 12 Physics)
14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.
- Q1Easy
Vrms = Vpeak ÷
A.2B.√2✓ CorrectC.πD.3Answer: B. √2
Explanation: For a sinusoid, V_rms = V₀/√2 ≈ 0.707V₀, since the mean of sin²θ over a cycle is ½. Dividing by 2 (a common slip) would underestimate rms by nearly 30%.
- Q2Easy
Capacitive reactance:
A.C/ωB.ωCC.ωLD.1/(ωC)✓ CorrectAnswer: D. 1/(ωC)
Explanation: Capacitive reactance X_C = 1/(ωC) — it falls as frequency rises, opposite to an inductor's X_L = ωL. Swapping the two formulas is the classic mix-up here.
- Q3Easy
Indian household AC frequency:
A.100 HzB.60 HzC.50 Hz✓ CorrectD.1 HzAnswer: C. 50 Hz
Explanation: Indian domestic supply is standardized at 50 Hz, 230 V rms — unlike the 60 Hz used in the US. Mixing up the two is the usual mistake on this fact-recall question.
- Q4Easy
Inductive reactance:
A.ωL✓ CorrectB.1/(ωL)C.L/ωD.ωCAnswer: A. ωL
Explanation: Inductive reactance X_L = ωL grows with frequency, since a faster-changing current induces a larger back-EMF (ε = −L di/dt). This is the opposite trend to X_C = 1/(ωC).
- Q5Easy
Average value of sinusoidal AC over a full cycle:
A.I₀B.Zero✓ CorrectC.I₀/√2D.2I₀/πAnswer: B. Zero
Explanation: Over one full cycle a sinusoid spends equal time above and below zero, so ∫sin ωt dt = 0 — the positive and negative halves cancel exactly, giving zero average.
- Q6Easy
In a pure inductor, current:
A.Lags by 45°B.Leads by 90°C.In phaseD.Lags voltage by 90°✓ CorrectAnswer: D. Lags voltage by 90°
Explanation: In a pure inductor v_L = L di/dt, so voltage peaks where current is changing fastest — a quarter cycle before current peaks. Hence current lags voltage by 90°, not 45°.
- Q7Medium
Transformer: N_p = 1000, N_s = 100, V_p = 220 V. V_s =
A.22 V✓ CorrectB.2200 VC.110 VD.440 VAnswer: A. 22 V
Explanation: V_s = (N_s/N_p)V_p = (100/1000)(220) = 22 V — inverting the turns ratio (N_p/N_s) is what produces the tempting 2200 V distractor.
- Q8Medium
Impedance of pure inductor:
A.L/ωB.1/(ωC)C.RD.ωL✓ CorrectAnswer: D. ωL
Explanation: An ideal inductor's opposition to AC is purely reactive: Z = X_L = ωL, which rises with frequency — unlike a resistor's constant R that ignores frequency entirely.
- Q9Medium
An ideal transformer steps 220 V to 22 V, primary current 0.5 A. Secondary current:
A.50 AB.0.05 AC.0.5 AD.5 A✓ CorrectAnswer: D. 5 A
Explanation: Power is conserved in an ideal transformer: V_pI_p = V_sI_s, so I_s = (220×0.5)/22 = 5 A — the secondary's lower voltage means a proportionally higher current.
- Q10Medium
At resonance in series LCR:
A.X_L = X_C, current is maximum✓ CorrectB.X_L > X_CC.Current zeroD.V = 0Answer: A. X_L = X_C, current is maximum
Explanation: At resonance X_L = X_C, so the reactive parts cancel and impedance drops to its minimum Z = R, driving current to its maximum for a given source voltage.
- Q11Medium
In LC oscillation, total energy oscillates between:
A.Heat in RB.E in C and B in L✓ CorrectC.IncreasesD.DecreasesAnswer: B. E in C and B in L
Explanation: In an LC circuit, energy shuttles between the capacitor's electric field (½CV²) and the inductor's magnetic field (½LI²) — the AC analogue of a spring-mass SHM.
- Q12Hard
Transformer works on principle of:
A.Self-inductionB.Mutual induction✓ CorrectC.ResistanceD.CapacitanceAnswer: B. Mutual induction
Explanation: A transformer transfers energy between two separate, magnetically-linked coils via a changing flux — this is mutual induction, distinct from a single coil's self-induction.
- Q13Hard
Average power in AC: P =
A.V·IB.V_peak · I_peakC.V_rms · I_rms · cosφ✓ CorrectD.V²/RAnswer: C. V_rms · I_rms · cosφ
Explanation: Average AC power must weight V_rms I_rms by the power factor: P = V_rms I_rms cos φ, since only the in-phase component of current does real work — cos φ = 1 only when V and I align.
- Q14Hard
Transformer with 200 W output and 250 W input has efficiency:
A.80%✓ CorrectB.50%C.100%D.20%Answer: A. 80%
Explanation: Efficiency η = P_output/P_input = 200/250 = 0.8 = 80% — the missing 20% is lost as heat to core hysteresis, eddy currents, and winding resistance.
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Start this chapter free →Alternating Current — FAQs
What are the key concepts in Class 12 Physics Alternating Current?+
Circuits driven by sinusoidal sources: RMS and average values, the behaviour of R, L and C individually with their phase relations, the series LCR circuit with impedance and phase angle, resonance and the quality factor, power and the power factor, and the transformer with its energy losses. Key ideas include Alternating current, RMS value, Average value, AC through a resistor.
What does Class 12 Physics Chapter 7 (Alternating Current) cover on XamBaaz?+
It has 90 NCERT-based MCQs on "Alternating Current": 30 Easy, 30 Medium and 30 Hard. Together they make 9 timed quizzes, and you never get the same set twice. Every question has an instant explanation. They help you prepare for CBSE & Maharashtra HSC Board exams, JEE Main, MHT-CET, JEE Advanced and NEET UG.
Are these "Alternating Current" questions free to practise?+
Yes. Sign in with Google to practise "Alternating Current" free. Full unlimited access is ₹999/year on a launch offer until 1 December 2026. No chapter is charged separately.
How should I revise "Alternating Current" for the exam?+
Start with the Easy quiz to check your basics, then try Medium and Hard to practise applying them. There are 9 timed quizzes on this chapter, so you can come back for a fresh set instead of one you have seen. Read each explanation, retry the questions you miss, and track your accuracy until it stays high.
Are these "Alternating Current" MCQs available with answers?+
Yes. 14 sample questions are shown here in full, each with the correct option and a step-by-step "Why" explanation. Sign in free with Google to start practising, with instant scoring.
Is there negative marking in the "Alternating Current" quizzes?+
Yes. The timed quizzes use exam-style marking: +4 for a right answer, −1 for a wrong one and 0 for a skip, the same negative marking as JEE Main, JEE Advanced and NEET UG. MHT-CET and CBSE board papers have no negative marking. Our mocks for those are scored their way.
What are the important questions from Alternating Current (Class 12 Physics)?+
The questions that matter most test Alternating current, RMS value, Average value, AC through a resistor. This page shows 14 solved important MCQs with answers and explanations; all 90 questions on the chapter are available as timed quizzes once you sign in.
Is there an online quiz for Alternating Current?+
Yes — Class 12 Physics Alternating Current has timed online quizzes at Easy, Medium and Hard levels, with instant scoring and a worked explanation on every question. The first quiz on the chapter is free.