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I. An ammeter has an… (Class 12 Physics SQP 2026-27, Q27A)

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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 27(A)

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

Q27(A) · 3 marks · Short answerMoving Charges and Magnetism

I. An ammeter has an internal resistance of 13 Ω and is originally designed to measure currents only up to 100 A. To extend its range for high-current measurements, a shunt resistor is connected in parallel with it. After adding the shunt, the same meter can now read currents as high as 750 A without damage. Using the principle of current division in parallel circuits, determine the value of the shunt resistance required to increase the measuring range of the ammeter.
II. The coil of a moving coil galvanometer is wound over a metal frame. Why?

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

Meter and shunt are in parallel, so they have the same potential difference.

Hint 2

The shunt carries the extra current I − Ig.

Hint 3

For II, think what is induced in a conducting frame moving in a magnetic field.

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. Treat the ammeter (G = 13 Ω, = 100 A) as a galvanometer. In parallel, p.d. is the same: .

    1 mark

  2. ½ mark

  3. (connected in parallel).

    ½ mark

  4. II. When the coil oscillates, eddy currents are induced in the metal frame which oppose the motion (Lenz's law). This provides electromagnetic damping, so the coil comes to rest quickly at its deflected position.

    1 mark

Answer: I. S = 2 Ω; II. for electromagnetic damping (dead-beat galvanometer).

Where the marks are

  • Uses equal p.d. across meter and shunt: S = IgG/(I − Ig) (1 mark)
  • Correct substitution (½ mark)
  • S = 2 Ω (½ mark)
  • Metal frame gives eddy-current (electromagnetic) damping (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Moving Charges and Magnetism, 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.