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A2 Daily A Level Physics question

2026-03-18 OCR A Nuclear II: binding energy per nucleon (qualitative trends) OCR A Module 6.2.2: Mass–energy and nuclear binding energy; binding energy per nucleon curve (qualitative) OCR A Module 6.2.3: Energy considerations from the BE per nucleon trend (qualitative)

In a teaching lab discussion, a student proposes two nucleon-conserving rearrangements: X: combine two identical nuclei each with A ≈ 60 into a single nucleus with A ≈ 120; Y: split one very heavy nucleus with A ≈ 240 into two identical nuclei each with A ≈ 120. Using only the qualitative trend of binding energy per nucleon versus mass number, which statement must be true about energy transfer with the surroundings?

  1. A X and Y both release energy, because putting nucleons into fewer nuclei always increases total binding energy.
  2. B Neither X nor Y releases energy, because the total number of nucleons is unchanged so binding energy is unchanged.
  3. C X releases energy but Y requires energy, because combining nuclei increases binding whereas splitting reduces it.
  4. D X requires energy but Y releases energy, because BE per nucleon is near a maximum around A ≈ 60 and lower at A ≈ 120, whereas very heavy nuclei have even lower BE per nucleon than A ≈ 120. (correct)

Answer

The correct answer is D.

Correct: D — X requires energy but Y releases energy, because BE per nucleon is near a maximum around A ≈ 60 and lower at A ≈ 120, whereas very heavy nuclei have even lower BE per nucleon than A ≈ 120. A conflates having fewer nuclei with greater stability and ignores that BE per nucleon falls when moving from A ≈ 60 to A ≈ 120. B wrongly assumes conserving nucleon number conserves binding energy; energy change depends on how BE per nucleon changes, not on nucleon count alone. C reverses the correct direction: near the peak (A ≈ 60), combining to A ≈ 120 lowers BE per nucleon (so needs energy), while moving from very heavy to mid-mass raises BE per nucleon (so releases energy). D correctly applies the qualitative BE per nucleon curve: total binding increases only when the final BE per nucleon is higher, given nucleon number is conserved in each case.

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