Nuclear fusion releases large amounts of energy and is the process that powers stars. Explain why nuclear fusion is difficult to achieve on Earth, and state what happens to the mass of the nuclei during a fusion reaction.

Pearson Edexcel International GCSE Physics (4PH1) — 7.3 Fission and fusion · Explain · 4 marks · View as Markdown

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

Scientists have been attempting to harness nuclear fusion as a clean energy source for decades. In fusion, two small, light nuclei are brought together to form a larger, heavier nucleus.

Model answer (4 marks)

At low temperatures and pressures the electrostatic repulsion between the positively charged protons is too great for the nuclei to approach each other closely enough for the strong nuclear force to bind them together.

To overcome this repulsion, very high temperatures (to give the nuclei enough kinetic energy) and very high pressures (to force the nuclei together) are required.

These extreme conditions are extremely difficult to produce and maintain in a controlled laboratory setting, which is why achieving sustained nuclear fusion on Earth is challenging.

During a fusion reaction a small amount of the mass of the reacting nuclei is lost; this mass defect is converted into energy according to E=mc².

Examiner tips

  • Use the word "electrostatic repulsion" and mention both high temperature and high pressure as the required conditions.
  • Explain that the conditions are hard to sustain on Earth, linking to the difficulty of controlled fusion.
  • State that a small loss of mass occurs and is converted to energy, citing mass‑energy equivalence.

Common mistakes

  • Failing to mention both temperature and pressure as required conditions.
  • Confusing fusion with fission or omitting the mass loss/energy conversion.
  • Using vague terms like "very hot" without specifying the need for high temperature and pressure.

Mark scheme (4 marks)

  1. At low temperatures/pressures the electrostatic repulsion between protons is too great (to allow fusion to occur)
  2. Very high temperatures AND very high pressures are required to overcome the electrostatic repulsion
  3. These extreme conditions are very difficult to produce and sustain on Earth (making controlled fusion challenging)
  4. There is a (very small) loss of mass during fusion, which is converted into energy (mass-energy equivalence)

Key terms in this question

nuclear fusion

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