Explain why nuclear fusion releases energy and describe the conditions needed for nuclear fusion to occur.

AQA GCSE Physics (8463) — 4.4.4 Nuclear fission and fusion (Phys only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

When the nuclei combine, the mass of the product is less than the sum of the reactants. The missing mass is released as energy according to E=mc².

Fusion can only occur at very high temperatures.

These temperatures give the nuclei enough kinetic energy to overcome the electrostatic repulsion and get close enough for the strong nuclear force to bind them together.

Examiner tips

  • Use the term ‘light nuclei’ and ‘heavier nucleus’ to show understanding of the reaction. Explain the mass defect and E=mc² to justify energy release. Mention high temperature as a condition and link it to overcoming Coulomb repulsion. Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing fusion with fission or describing only the products. Failing to mention the mass defect or E=mc². Omitting the need for high temperature or the reason for it. Using vague terms like ‘big’ or ‘small’ instead of ‘light’ and ‘heavy’.

Mark scheme (4 marks)

  1. Nuclear fusion involves two small/light nuclei joining to form a larger/heavier nucleus
  2. Energy is released during the process of fusion
  3. Very high temperatures are needed for fusion to occur
  4. The high temperatures are needed so that nuclei have sufficient energy/speed to overcome the electrostatic/repulsive force between them and get close enough to fuse

Key terms in this question

nuclear fusion · energy

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