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A charge q is placed… (Class 12 Physics SQP 2025-26, Q19)

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PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 2 marks · Very short answer

CBSE Class 12 Physics sample paper 2025-26: question 19

From the official CBSE sample question paper for 2025-26, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerElectric Charges and Fields

A charge q is placed inside a sphere of radius ‘a’ filled with water and another charge 2q is placed inside cube of side ‘2a’ which is vacuumed inside. Find the ratio of the flux linked with the sphere to that linked with the cube. (Take relative permittivity of water as 80)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Flux depends only on the enclosed charge and the medium, not on the size of the surface.

Hint 2

In a medium, replace by .

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. By Gauss's law in a medium, (independent of the size or shape of the closed surface).

  2. For the water-filled sphere:

    ½ mark

  3. For the vacuum cube:

    ½ mark

  4. , i.e. ratio .

    1 mark

Answer: 1 : 160

Where the marks are

  • Flux through sphere (½ mark)
  • Flux through cube (½ mark)
  • Correct ratio 1:160 (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Electric Charges and Fields, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: 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.