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I. An isolated… (Class 12 Physics SQP 2026-27, Q31A)

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

CBSE Class 12 Physics sample paper 2026-27: question 31(A)

From the official CBSE sample question paper for 2026-27, section E, question 31(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(A) · 5 marks · Long answerElectrostatic Potential and Capacitance

I. An isolated conductor cannot have a large capacitance. Why?
II. How would you connect 6 μF, 9 μF and 18 μF capacitors to obtain-
i. the minimum capacitance
ii. the maximum capacitance
III. The capacitance of a parallel plate capacitor is increased as the area of the plates increases. What are the other two factors on which the capacitance depends?

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

Write the capacitance of an isolated sphere and see what it depends on.

Hint 2

Series combination gives a value smaller than the smallest; parallel adds up.

Hint 3

Look at C = ε₀KA/d for the remaining factors.

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. Capacitance of an isolated spherical conductor is ; since is fixed, C depends only on size.

    ½ mark

  2. Even for the Earth ( m), F, so a large capacitance would need an impractically huge conductor; hence an isolated conductor has small capacitance.

    ½ mark

  3. II(i). Minimum capacitance: connect all three in series.

    ½ mark

  4. ½ mark

  5. F

    ½ mark

  6. II(ii). Maximum capacitance: connect all three in parallel.

    ½ mark

  7. ½ mark

  8. F

    ½ mark

  9. III. From : (i) separation between the plates, ; (ii) permittivity (dielectric constant) of the medium between the plates, .

    1 mark

Answer: I. C = 4πε₀R is tiny for practical sizes; II. series: 3 μF (min), parallel: 33 μF (max); III. plate separation and permittivity of medium.

Where the marks are

  • C = 4πε₀R depends only on size (½ mark)
  • Large C needs impractically large conductor (½ mark)
  • Series for minimum (½ mark), correct reciprocal sum (½ mark), 3 μF (½ mark)
  • Parallel for maximum (½ mark), sum (½ mark), 33 μF (½ mark)
  • Distance between plates and permittivity of medium (½ × 2 marks)

Revise the chapter

Practise this chapterBoard-style questions from Electrostatic Potential and Capacitance, 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.