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A2 Daily A Level Physics question

2026-05-31 OCR A Nuclear II: mass–energy; binding energy per nucleon; fission/fusion contexts (qual.) OCR-A 6.4.2 Nuclear energy: mass–energy equivalence; binding energy per nucleon; fission and fusion (qualitative)

Which estimate is closest to the energy released per event when a U-235 nucleus undergoes fission into two medium-mass fragments? A lab chart shows the average binding energy per nucleon rises from about 7.6 MeV in U-235 to about 8.5 MeV in the fragments. Assume about 235 nucleons end up bound in the fragments and estimate the energy release from the change in binding.

  1. A Approximately 0.9 MeV
  2. B Approximately 8.5 MeV
  3. C Approximately 200 MeV (correct)
  4. D Approximately 2000 MeV

Answer

The correct answer is C.

Correct: C — Approximately 200 MeV. A uses only the per-nucleon increase (8.5 − 7.6 ≈ 0.9 MeV) and forgets to scale by the ~235 nucleons. B wrongly takes the final binding energy per nucleon (8.5 MeV) as the total energy change, ignoring the number of nucleons. C multiplies the per-nucleon gain by the nucleon count: 0.9 MeV × 235 ≈ 2.1 × 10^2 MeV, so about 200 MeV. D overestimates by roughly a factor of 10, as if double-counting fragments or mis-scaling the per-nucleon change.

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