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Briefly explain how energy is… (Class 12 Physics 2025, Q21)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2025: question 21

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerNuclei

Briefly explain how energy is produced in stars, giving two examples of the nuclear reactions involved.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 55/7/1.

Hints

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

Hint 1

Is it fission or fusion that powers the Sun?

Hint 2

Think of the proton–proton cycle that converts hydrogen into helium.

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. Energy in stars comes from thermonuclear fusion. The core temperature is about K, so light nuclei (mainly hydrogen) have enough kinetic energy to overcome Coulomb repulsion and fuse.

    ½ mark

  2. The heavier nucleus formed has a higher binding energy per nucleon, so its mass is less than the total mass of the reacting nuclei. This mass defect is released as energy .

    ½ mark

  3. Example 1 (proton–proton cycle): MeV.

    ½ mark

  4. Example 2: MeV. Further, MeV. Overall, four protons combine to form one He nucleus and about 26.7 MeV is released.

    ½ mark

Answer: By nuclear fusion of hydrogen into helium (proton–proton cycle), e.g. ¹H + ¹H → ²H + e⁺ + ν, and ³He + ³He → ⁴He + 2¹H.

Where the marks are

  • Identifies thermonuclear fusion at very high temperature (½ mark)
  • Explains the release of energy through mass defect / higher binding energy per nucleon (½ mark)
  • Any two correct fusion reactions (½ + ½ mark)

Revise the chapter

  • Chapter: Nuclei (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Nuclei, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 55/7/1: 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.