A scientist is investigating the potential of nuclear fusion as a future energy source. Explain why nuclear fusion releases energy, and why it is difficult to achieve on Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear fusion is the joining of two small, light nuclei to form a larger, heavier nucleus. Scientists have been trying for decades to harness fusion as a clean energy source, but it has proved extremely challenging to sustain the reaction under controlled conditions.
Model answer (5 marks)
In fusion two light nuclei combine to form a heavier nucleus. The mass of the product nucleus is less than the sum of the reactants. The ‘missing’ mass is released as energy (E=mc²) and appears as radiation. To force the nuclei together the electrostatic repulsion between their positive charges must be overcome, which requires extremely high temperatures and pressures. These extreme conditions are very difficult to maintain or achieve on Earth, making controlled fusion challenging.
Examiner tips
- Use the term ‘mass defect’ and E=mc² to explain energy release
- Explain that high temperature/pressure is needed to overcome Coulomb barrier
- Mention that maintaining such conditions is the main difficulty
Common mistakes
- Confusing fusion with fission or saying energy comes from kinetic energy
- Omitting the need for high temperature/pressure
- Using vague phrases like ‘very hot’ without specifying the extreme conditions
Mark scheme (5 marks)
- In fusion, two small/light nuclei join to form a larger/heavier nucleus
- The mass of the heavier nucleus produced is less than the total mass of the two original nuclei
- The 'missing' mass is converted into energy, released as radiation
- High temperatures and pressures are needed to overcome the electrostatic repulsion between the positively charged nuclei
- These extreme conditions (very high temperatures/pressures) cannot easily be maintained/achieved on Earth
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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