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A2 Daily A Level Physics question

2026-07-11 OCR A Nuclear physics (Module 6): mass–energy and binding energy; fission/fusion (qualitative) 6.4.4(d) 6.4.4(e)

In a simplified reactor model, a U-235 nucleus fissions into two medium-mass fragments. Before fission, the average binding energy per nucleon is about 7.6 MeV; for the fragments it is about 8.5 MeV. If the two fragments together contain roughly 200 nucleons (the remainder appear as emitted neutrons and gamma), which statement must be true?

  1. A The products have a smaller total mass than the original by an amount corresponding to about 180 MeV of released energy. (correct)
  2. B The total binding energy falls by about 180 MeV, so energy is released as the nucleus becomes less tightly bound.
  3. C Because nucleon number is almost unchanged, the mass and energy change is negligible (at most a few MeV).
  4. D The higher binding energy per nucleon in the products means the products have more mass, so about 180 MeV must be supplied to make fission occur.

Answer

The correct answer is A.

Correct: A — The products have a smaller total mass than the original by an amount corresponding to about 180 MeV of released energy. A 0.9 MeV increase per nucleon across ~200 nucleons gives ~180 MeV more total binding, so mass decreases and this energy is released. B is wrong because the total binding energy increases (not decreases) when moving to more tightly bound fragments; that increase appears as released energy. C is wrong because even though nucleon number is nearly conserved, the increase in binding energy is large: 0.9 MeV × 200 ≈ 180 MeV, not just a few MeV. D is wrong because higher binding energy per nucleon means lower mass (greater mass defect) and energy release, not an energy input.

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