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(i) Write any two features… (Class 12 Physics 2026, Q26a)

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PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 3 marks · Short answer

CBSE Class 12 Physics 2026: question 26(a)

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, section C, question 26(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26(a) · 3 marks · Short answerNuclei

(i) Write any two features of nuclear forces.
(ii) If both the number of protons and the neutrons are conserved in each nuclear reaction, in what way is mass converted into energy (or vice versa) in a nuclear reaction ? Explain.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/5/1.

Hints

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

Hint 1

Compare nuclear force with gravitational and electrostatic forces in strength and range.

Hint 2

Is the mass of a nucleus equal to the sum of its nucleon masses? Think of binding energy.

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) Features of nuclear forces (any two):
    • They are very strong — much stronger than Coulomb force between protons.
    • They are short-range — negligible beyond a few femtometres (attractive for separation > ~0.8 fm, strongly repulsive below it).
    • They are charge independent — n–n, p–p and n–p forces are about the same.

    1 mark

  2. (ii) Although the total numbers of protons and neutrons are unchanged, the binding energies of the nuclei before and after the reaction are different. The mass of a nucleus is less than the sum of the masses of its nucleons by the mass defect .

    1 mark

  3. Hence the total rest mass of the reactant nuclei differs from that of the product nuclei. If the products are more tightly bound, the difference in mass appears as energy (released); if less tightly bound, energy is absorbed and converted into mass. Thus mass–energy conversion takes place through the difference in binding energies.

    1 mark

Answer: (i) Strong, short-range, charge-independent (any two); (ii) Difference in binding energy (mass defect) of initial and final nuclei appears as energy Q = Δm c²

Where the marks are

  • Any two correct features of nuclear force (½ + ½ = 1 mark)
  • Identifies that binding energies/mass defects of nuclei differ before and after reaction (1 mark)
  • Explains mass difference Δm converted to energy Q = Δmc² (or vice versa) (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Nuclei, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/5/1: 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.