PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 12 Physics sample paper 2026-27: question 33(A)

From the official CBSE sample question paper for 2026-27, section E, question 33(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(A) · 5 marks · Long answerAlternating Current

A certain electrical device is used to convert an alternating voltage of smaller magnitude into a much larger alternating voltage without violating the law of conservation of energy.

I. With the help of a neat labelled diagram, describe the principle on which this device operates and explain how it increases the voltage.
II. State the basic working mechanism of the device and mention the condition required in its input signal for proper functioning.
III. Give one reason why this device does not operate with 100% efficiency in practical situations.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Which device changes the size of an a.c. voltage using two coils on an iron core?

Hint 2

Think about how the emf per turn compares in both coils when the same flux links them.

Hint 3

For efficiency, think of where energy is wasted as heat or lost flux.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

I. Device: step-up transformer.
Diagram (in words): A laminated soft-iron core carries two separate windings.
- The primary coil $P$ has fewer turns $N_{p}$ and is connected to the a.c. input $V_{p}$ .
- The secondary coil $S$ has more turns $N_{s}$ and is connected to the output/load $V_{s}$ .
- Label the core, primary, secondary, input a.c. and output.
1 mark
Principle: The transformer works on mutual induction. A changing current in the primary sets up a changing magnetic flux in the core. This flux links the secondary and induces an emf in it (Faraday's law).
½ mark
How the voltage increases: The same flux links every turn, so for an ideal transformer $\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}}$ . With $N_{s} > N_{p}$ , we get $V_{s} > V_{p}$ .
Energy is conserved because $V_{p} I_{p} = V_{s} I_{s}$ : as the voltage is stepped up, the current falls in the same ratio.
½ mark
II. Working mechanism:
- The alternating voltage across the primary drives an alternating current in it.
- This current sets up an alternating magnetic flux in the iron core.
- The changing flux linked with the secondary induces an alternating emf in it.
- Energy thus passes from the primary to the secondary through the magnetic field, with no electrical contact between the windings.
1 mark
Condition on the input: The input must be alternating (a.c.). A steady d.c. produces constant flux, so $d ϕ / d t = 0$ and no emf is induced in the secondary.
1 mark
III. Why the efficiency is below 100% (any one):
- Copper ( $I^{2} R$ ) loss in the windings
- Eddy-current loss in the core
- Hysteresis loss in the core
- Flux leakage
Because of these losses the output power is less than the input power.
1 mark

Answer: Step-up transformer: works on mutual induction; $V_{s} / V_{p} = N_{s} / N_{p}$ with $N_{s} > N_{p}$ ; the input must be a.c.; efficiency is below 100% due to copper, eddy-current or hysteresis losses, or flux leakage.

Where the marks are

Labelled diagram of a transformer with the core, primary (fewer turns) and secondary (more turns) (1 mark)
States the principle of mutual induction (½ mark)
$V_{s} / V_{p} = N_{s} / N_{p}$ with $N_{s} > N_{p}$ , and $V_{p} I_{p} = V_{s} I_{s}$ for energy conservation (½ mark)
Describes the working: changing flux in the core induces emf in the secondary (1 mark)
Input must be a.c., because d.c. gives no change of flux (1 mark)
Any one loss, e.g. eddy currents, hysteresis, copper loss or flux leakage (1 mark)

Revise the chapter

Chapter: Alternating Current (notes, formulas, where marks are lost and every past question)
Topic: Transformer

Practise this chapterBoard-style questions from Alternating Current, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.

CBSE Class 12 Physics sample paper 2026-27: question 33(A)

Hints

Step-by-step solution

Revise the chapter

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