Skip to content

A parallel plate capacitor of… (Class 12 Physics 2026, Q23)

questionbuddy.in/q/cbse-12-physics-2026-55-1-1-q23-parallel-plate-capacitor-capacitance-dielectric

PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 3 marks · Short answer

CBSE Class 12 Physics 2026: question 23

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerElectrostatic Potential and Capacitance

A parallel plate capacitor of capacitance C has a dielectric slab between its plates. It is charged to a potential difference V by connecting it across a battery. The battery is then disconnected. If the dielectric slab is now withdrawn from the capacitor, how will the following be affected ?
(a) Capacitance of the capacitor,
(b) Energy stored in the capacitor, and
(c) The potential difference between the plates of the capacitor.
Justify your answer in each case.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

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

Hint 1

Once the battery is removed, which quantity cannot change: charge or voltage?

Hint 2

Use , then express U and V in terms of the constant Q.

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. After the battery is disconnected the charge on the plates stays constant, because it has no path to flow away.

  2. (a) Capacitance: with the slab . Without it, . The capacitance decreases by a factor .

    1 mark

  3. (b) Energy: with constant, so . The energy increases times. The extra energy comes from the work done in pulling the slab out against the attractive force.

    1 mark

  4. (c) Potential difference: . The p.d. increases times.

    1 mark

Answer: (a) Decreases to C/K; (b) increases to K times; (c) increases to KV

Where the marks are

  • Capacitance decreases to C/K, with reason (1 mark)
  • Energy increases K times, since U = Q²/2C at constant Q (1 mark)
  • Potential difference increases to KV, since V = Q/C (1 mark)

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 paper2026 board paper, set 55/1/1: 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.