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I. A bar magnet falls… (Class 12 Physics SQP 2026-27, Q28)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2026-27: question 28

From the official CBSE sample question paper for 2026-27, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerElectromagnetic Induction

I. A bar magnet falls from a height through a metal ring. Its acceleration will be less than ‘g’. Explain.
II. A coil of copper wire is wound uniformly on a wooden equilateral triangular frame. If each side of the triangle is increased to 5 times its original length, while keeping the number of turns per unit length unchanged, how does the self-inductance of the coil change?

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

For I, what does Lenz's law say about the force on the moving magnet?

Hint 2

For II, write L in terms of N, A and l, and see how each scales.

Hint 3

Area scales as the square of linear size.

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.

  1. I. As the magnet falls towards the ring, the flux linked with the ring changes, inducing a current. By Lenz's law this current opposes the cause (the magnet's motion), exerting an upward force on the magnet; hence its acceleration is less than .

    1 mark

  2. II. Self-inductance of the coil: (equivalently ). With turns per unit length fixed, making each side 5 times gives and .

    ½ mark

  3. Area of an equilateral triangle ∝ (side), so .

    ½ mark

  4. ½ mark

  5. , so — the self-inductance becomes 125 times.

    ½ mark

Answer: I. Induced current opposes fall (Lenz's law); II. L′ = 125 L

Where the marks are

  • Lenz's law: induced current opposes motion of magnet (1 mark)
  • L ∝ N²A/l with N → 5N, l → 5l (½ mark)
  • A → 25A (½ mark)
  • Correct ratio expression (½ mark)
  • L′ = 125L (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Electromagnetic Induction, 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.