OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(e) binding energy per nucleon against nucleon number curve; energy…
Full specification wording
Nuclear fission and fusion: (e) binding energy per nucleon against nucleon number curve; energy changes in reactions
A graph of binding energy per nucleon against nucleon number rises steeply for light nuclei, peaks near iron-56 at about 8.8 MeV per nucleon, then falls slowly for heavy nuclei.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 18
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A graph of binding energy per nucleon against nucleon number rises steeply for light nuclei, peaks near iron-56 at about 8.8 MeV per nucleon, then falls slowly for heavy nuclei. Nuclei near the peak are the most stable. In fission a heavy nucleus splits into medium-mass fragments with higher binding energy per nucleon, so the products are more tightly bound and energy is released. In fusion light nuclei combine to form a heavier nucleus, again moving towards the peak and releasing energy. The energy released equals the increase in total binding energy between reactants and products.
What earns the marks
- Describe the shape of the curve: a steep rise for light nuclei, a broad maximum near iron-56, and a gradual fall for heavy nuclei.
- State that the peak of the curve corresponds to the most stable nuclei, with the highest binding energy per nucleon.
- Explain that fission of heavy nuclei releases energy because the fragments have higher binding energy per nucleon.
- Explain that fusion of light nuclei releases energy because the product has higher binding energy per nucleon.
- Calculate the energy released in a reaction from the increase in total binding energy, or equivalently from the mass defect using E = Δmc².
Common mistakes and how to put them right
Thinking the curve peaks at uranium or another heavy nucleus.
Put it right: The maximum is near iron-56.
Believing fission releases energy because the products have lower binding energy per nucleon.
Put it right: The products have higher binding energy per nucleon.
Assuming any fusion releases energy.
Put it right: Fusion releases energy only for nuclei lighter than the peak region, while heavier nuclei would absorb energy.
Exam tips
- Sketch the axes with binding energy per nucleon on the vertical axis and nucleon number on the horizontal axis, and mark the iron-56 peak.
- For energy-change questions, compare the binding energy per nucleon of reactants and products before calculating.
- Use the mass defect method as a check on any binding-energy calculation.
Try the lesson
The start of the Nuclear fission and fusion: (e) binding energy per nucleon against nucleon number curve; energy changes in reactions lesson, made from checked teaching notes for this specification point.
Listen
A graph of binding energy per nucleon against nucleon number rises steeply for light nuclei, peaks near iron-56 at about 8.8 MeV per nucleon, then falls slowly for heavy nuclei. Nuclei near the peak are the most stable. In fission a heavy nucleus splits into medium-mass fragments with higher binding energy per nucleon, so the products are more tightly bound and energy is released. In fusion light nuclei combine to form a heavier nucleus, again moving towards the peak and releasing energy. The energy released equals the increase in total binding energy between reactants and products. A strong answer earns marks for these points. Describe the shape of the curve: a steep rise for light nuclei, a broad maximum near iron-56, and a gradual fall for heavy nuclei. State that the peak of the curve corresponds to the most stable nuclei, with the highest binding energy per nucleon. Explain that fission of heavy nuclei releases energy because the fragments have higher binding energy per nucleon. Explain that fusion of light nuclei releases energy because the product has higher binding energy per nucleon. A common slip: Thinking the curve peaks at uranium or another heavy nucleus. Instead, the maximum is near iron-56. Exam tip: Sketch the axes with binding energy per nucleon on the vertical axis and nucleon number on the horizontal axis, and mark the iron-56 peak.
Key words
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Binding energy per nucleon against nucleon number curve
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Nuclear stability
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Energy release in fission and fusion
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See every definition
- Binding energy per nucleon against nucleon number curve
- A graph that rises steeply for light nuclei, peaks near iron-56 at about 8.8 MeV per nucleon, then falls slowly for heavy nuclei.
- Nuclear stability
- Nuclei near the peak of the curve are the most stable, with the highest binding energy per nucleon.
- Energy release in fission and fusion
- Fission and fusion release energy because the products have higher binding energy per nucleon, so they are more tightly bound.
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