Alternating Current Class 12 Physics PYQs 2026-27 | Chapter 7
Prioritise RMS and average AC values, pure R/L/C phase relations, inductive and capacitive reactance, series LCR impedance, resonance, power factor and average power, wattless current, AC generator and transformer. Recent board-paper records also show direct numerical and competency-style questions on LCR voltage relations, power factor, RMS/average value and transformer reasoning.
1. What the PYQs show about Chapter 7
Peak/RMS conversion, instantaneous AC and average power remain important objective-level ideas.
Phase relations and pure-element circuits repeatedly test whether you understand, not just memorise, AC behaviour.
Impedance, phase, resonant condition and frequency dependence form a major numerical/concept cluster.
Average power, cosφ and wattless current appear in conceptual and numerical forms.
Principle, construction, working and instantaneous emf are recurring long-answer themes.
Turns ratio, voltage/current relations, losses and ideal/real transformer reasoning recur across years.
2. Quick PYQ Map
| Cluster | Typical PYQ demand | What to revise |
|---|---|---|
| AC fundamentals | 1-mark concept / formula | v = V₀sinωt, i = I₀sin(ωt ± φ), RMS, average value |
| Pure R, L, C | 1–3 marks | phase relation, reactance, current-voltage behaviour |
| Series LCR | 1–5 marks | Z = √[R² + (XL−XC)²], phase, phasors |
| Resonance | 1–5 marks | XL = XC, ω₀ = 1/√LC, minimum Z, maximum I |
| Power | 1–3 marks | P = VrmsIrmscosφ, wattless current |
| Generator | 2–5 marks | principle, construction, working, emf equation |
| Transformer | 1–5 marks | turns ratio, power/current relation, losses, efficiency |
3. 1-Mark & Objective PYQs
For v = V₀sinωt and i = I₀sin(ωt + φ), identify the expression for average power consumed over a complete cycle.
An ideal transformer has Np:Ns = 20:1. A 240 V AC source is applied to the primary and a 6 Ω load is connected to the secondary. Find the current drawn from the source.
In a series LCR circuit, what happens to impedance when the source frequency is increased from a low value through resonance and beyond resonance?
Why is a transformer core laminated?
For a pure inductor connected to an AC source, state the phase relation between current and voltage.
Find the ratio XL/XC for an AC circuit.
State the condition for series resonance in an LCR circuit.
What is the average power consumed by an ideal purely inductive or purely capacitive circuit over one complete cycle?
4. 2-Mark & Short-Answer PYQs
A series CR circuit is connected to an AC source. Determine its impedance, phase angle and voltage across the resistor when R, C, source voltage and frequency are specified.
Three circuit elements are separately connected to an AC source. For one, V and I are in phase; for another V leads I; for the third I leads V. Identify the elements.
Write the expression for impedance of a series LCR circuit.
For a series LCR circuit, state the condition under which current and source voltage are in phase.
Under what condition does wattless current flow in a series LCR circuit?
State what changes and what does not change when an ideal transformer steps AC voltage up or down.
Give causes of energy loss in a real transformer.
5. 3-Mark & Numerical PYQs
A resistor, capacitor and inductor are connected in series to an AC source. Given R, C, L and frequency, find the net reactance, impedance and effective current.
A series circuit contains a resistor, an inductor and an unknown capacitor. At a specified frequency the source voltage and current are in phase. Determine the capacitance.
An LCR circuit is connected to a variable-frequency AC source. The current first increases and then decreases as frequency is raised. Identify the circuit behaviour responsible.
Voltages across R, L and C are separately given in a series AC circuit. Find the resultant source voltage.
A phase difference φ is specified between current and source voltage. Determine which type of circuit element combination can produce that phase relationship.
An AC generator coil has a specified number of turns, area, magnetic field and angular speed. Find the maximum induced emf.
A series LCR circuit has R = 3 Ω, XL = 4 Ω and XC = 8 Ω. Find its power factor.
An AC circuit has known voltage, current and phase difference. Find the average power consumed.
A transformer receives a specified primary voltage and must supply a much higher secondary voltage. Given primary turns, determine secondary turns.
Obtain the condition under which the impedance of a series LCR circuit is minimum.
6. 4-Mark & 5-Mark Long-Answer PYQs
State the basic principle of an AC generator, describe its construction and working, and obtain the expression for instantaneous induced emf.
Describe the construction and working of a transformer and derive the relation between primary and secondary voltages and turns.
Discuss major causes of energy loss in a practical transformer.
Establish the impedance of a series LCR circuit, describe how current varies with frequency, and state the conditions for minimum impedance and wattless current.
Explain the construction and working of a transformer and then discuss four principal causes of energy loss in a real transformer.
6A. Recent 2026 Board-Paper Records
Recent 2026 board-paper compilations show that Chapter 7 continues to test direct application of RMS/average values, LCR voltage relations, power factor and transformer concepts. The records below are paraphrased so they are not presented as verbatim paper text.
In a series LCR circuit, the voltages across R, L and C are each 10 V. If the capacitor is short-circuited, determine the voltage across the inductor.
A resistor and an ideal inductor are connected in series to a 20 V AC source. If the voltage across the resistor is 12 V, find the voltage across the inductor.
An AC voltage is v = 14 sin(314t) V. State its average value over a complete cycle and its RMS value.
A series LCR circuit has R = 3 Ω, XL = 4 Ω and XC = 8 Ω. Determine the power factor.
A recent board-paper record asks students to reason about a step-up transformer, including whether it can operate as a step-down transformer and whether stepping up voltage violates conservation of energy.
7. Repeated PYQ Patterns You Should Master
If f increases, XL increases and XC decreases. This drives the circuit toward or away from resonance depending on its starting condition.
Remember the full chain: XL = XC → Z = R → I maximum → φ = 0 → cosφ = 1.
R: V and I in phase. L: current lags by 90°. C: current leads by 90°.
P = VIcosφ. Pure L/C has zero average power but can still carry current.
E₀ = NBAω is the high-yield maximum-emf relation.
Vs/Vp = Ns/Np; for an ideal transformer, VpIp = VsIs.
8. PYQ-Based Topic Priority Map
| Topic | Practice target | Must be able to do |
|---|---|---|
| RMS / peak / average | MCQ + 1–2 marks | Convert values and distinguish full-cycle average from RMS. |
| R, L, C phase | MCQ + short answer | Identify phase and reactance immediately. |
| LCR impedance | Numerical + derivation | Calculate Z, I and phase angle. |
| Resonance | MCQ + numerical + long answer | Find f₀ and explain minimum Z/max current. |
| Power factor | MCQ + numerical | Use P = VIcosφ and interpret φ. |
| Wattless current | Conceptual | Explain zero average power correctly. |
| Generator | 3–5 marks | Draw/describe construction and derive emf expression. |
| Transformer | 1–5 marks | Use turns/current ratios and explain losses. |
9. How to Solve a Chapter 7 PYQ in the Board Exam
Step 1 — Identify the circuit: R, L, C or series LCR?
Step 2 — Identify the frequency role: Does the question require XL, XC, resonance or phase?
Step 3 — Choose RMS or peak correctly: Never mix V₀/I₀ with Vrms/Irms.
Step 4 — Keep phasor quantities separate: VL and VC do not add as ordinary scalars.
Step 5 — Check units: X and Z are in Ω; L in H; C in F; f in Hz; ω in rad s⁻¹.
Step 6 — Sanity-check: At resonance, Z should reduce to R and power factor should become 1.
10. Common PYQ Traps
11. Frequently Asked Questions
Are these exact CBSE questions?
No. The page intentionally uses paraphrased PYQ records and PYQ-derived summaries. The official CBSE archive is the authority for original question papers. Published board-question compilations were used only to identify recurring records and concepts; the wording on this page is paraphrased rather than reproduced verbatim.
Which years are represented?
The collection includes recent 2026, 2025, 2024 and 2023 records, plus selected 2022 and 2021–22 records and recurring board concepts. The 2026 section is kept clearly labelled as paraphrased board-paper records.
What are the most repeated Alternating Current PYQ areas?
Across the reviewed board-question compilations, recurring areas include LCR impedance and resonance, R/L/C phase behaviour, power factor and average power, AC generator, transformer turns ratio and transformer losses.
Is Q-factor required for this page?
Q-factor is not listed in the core Chapter 7 scope of the current CBSE 2026–27 Physics curriculum, so it is not treated as a required topic here.
Where can I get the original CBSE papers?
Use the official CBSE Previous Years' Question Papers archive.
12. Continue the Chapter 7 Resource Chain
Research/source note: Current CBSE curriculum and official archive establish syllabus and source boundaries. External educational compilations were used for question discovery and recurring-pattern research. This page does not present third-party paraphrases as verbatim CBSE wording.
13. Final PYQ Readiness Check
✓ I can convert peak AC values to RMS values.
✓ I know the phase relation for R, L and C.
✓ I can calculate XL, XC, Z and current.
✓ I can identify and solve series resonance questions.
✓ I can calculate average AC power and power factor.
✓ I understand wattless current.
✓ I can derive/use the AC-generator emf relation.
✓ I can solve transformer turns, voltage and current questions.
✓ I can write structured 3–5 mark answers for generator and transformer questions.
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