AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Students may be expected to identify, on the plot, the regions where…
Full specification wording
Mass and energy (A-level only): Students may be expected to identify, on the plot, the regions where nuclei will release energy when undergoing fission/fusion.
Energy is released when the products have a higher average binding energy per nucleon than the reactants, because the increase in binding energy per nucleon corresponds to a decrease in total…
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 6 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy is released when the products have a higher average binding energy per nucleon than the reactants, because the increase in binding energy per nucleon corresponds to a decrease in total mass, converted by E = mc². On the graph, fusion of light nuclei moves them up the steep left-hand side towards the peak, so energy is released. Fission of heavy nuclei splits them into mid-mass fragments nearer the peak, also releasing energy. The region around the maximum near iron-56 is where little or no energy is released by either process. Reading the graph means comparing the starting and finishing heights on the vertical axis.
What earns the marks
- Energy is released when the average binding energy per nucleon increases from reactants to products, because the total mass decreases.
- Fusion releases energy for light nuclei on the rising left-hand region of the curve, where products lie closer to the peak.
- Fission releases energy for heavy nuclei on the falling right-hand region, where mid-mass fragments lie closer to the peak.
- The change in binding energy per nucleon multiplied by the number of nucleons gives the total energy released, consistent with E = mc².
- Nuclei near the maximum around iron-56 are the most stable, so reactions producing them release little further energy.
Common mistakes and how to put them right
Claiming fission releases energy because heavy nuclei are unstable in a general sense.
Put it right by comparing binding energy per nucleon of the heavy nucleus with that of its fragments.
Stating that fusion always releases energy for any two nuclei.
Put it right by restricting energy-releasing fusion to light nuclei whose products move towards the peak.
Using total binding energy instead of binding energy per nucleon when comparing reactants and products.
Put it right by comparing values per nucleon before multiplying by nucleon number.
Exam tips
- Mark arrows on the graph showing light nuclei moving up towards the peak for fusion and heavy nuclei moving towards the peak for fission.
- State the direction of change in average binding energy per nucleon explicitly, then link it to the mass decrease and energy release.
- Use the phrase per nucleon when comparing values, and avoid quoting a single fixed energy release for all reactions.
Try the lesson
The start of the Mass and energy (A-level only): Students may be expected to identify, on the plot, the regions where nuclei will release energy when undergoing fission/fusion. lesson, made from checked teaching notes for this specification point.
Have a go first
On the graph of average binding energy per nucleon against nucleon number, in which region do nuclei release energy by fusion?
Listen
Energy is released when the products have a higher average binding energy per nucleon than the reactants, because the increase in binding energy per nucleon corresponds to a decrease in total mass, converted by E = mc². On the graph, fusion of light nuclei moves them up the steep left-hand side towards the peak, so energy is released. Fission of heavy nuclei splits them into mid-mass fragments nearer the peak, also releasing energy. The region around the maximum near iron-56 is where little or no energy is released by either process. Reading the graph means comparing the starting and finishing heights on the vertical axis. A strong answer earns marks for these points. Energy is released when the average binding energy per nucleon increases from reactants to products, because the total mass decreases. Fusion releases energy for light nuclei on the rising left-hand region of the curve, where products lie closer to the peak. Fission releases energy for heavy nuclei on the falling right-hand region, where mid-mass fragments lie closer to the peak. The change in binding energy per nucleon multiplied by the number of nucleons gives the total energy released, consistent with E = mc². A common slip: Claiming fission releases energy because heavy nuclei are unstable in a general sense. Instead, comparing binding energy per nucleon of the heavy nucleus with that of its fragments. Exam tip: Mark arrows on the graph showing light nuclei moving up towards the peak for fusion and heavy nuclei moving towards the peak for fission.
Key words
Say what each word means before you turn the card.
Fusion and fission
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Binding energy per nucleon comparisons
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Mass–energy conservation
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Energy release in nuclear reactions
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See every definition
- Fusion and fission
- Fusion of light nuclei moves them up the steep left-hand side towards the peak, and fission of heavy nuclei splits them into mid-mass fragments nearer the peak, both releasing energy.
- Binding energy per nucleon comparisons
- Reading the graph means comparing the starting and finishing heights on the vertical axis, since energy is released when the products have a higher average binding energy per nucleon than the reactants.
- Mass–energy conservation
- The increase in binding energy per nucleon corresponds to a decrease in total mass, converted by E = mc².
- Energy release in nuclear reactions
- Energy is released when the average binding energy per nucleon increases from reactants to products, because the total mass decreases.
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