AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Mass and energy (A-level only): Fission and fusion processes.
Fission splits a heavy nucleus (e.g., uranium-235) into two smaller nuclei after absorbing a neutron, releasing neutrons and energy.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fission splits a heavy nucleus (e.g., uranium-235) into two smaller nuclei after absorbing a neutron, releasing neutrons and energy. Fusion joins two light nuclei (e.g., deuterium and tritium) to form a heavier nucleus, releasing energy. In both processes, the total product mass is slightly less than the reactant mass; this mass defect appears as energy via ΔE = Δmc². The energy arises because the product nuclei sit higher on the binding-energy-per-nucleon curve, closer to the iron-56 peak, meaning they have a higher binding energy per nucleon than the reactants. Fission is used in nuclear reactors to generate power, while fusion is the process that powers stars.
What earns the marks
- Fission: a heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing two or three neutrons and energy.
- Fusion: two light nuclei combine to form a heavier nucleus, releasing energy.
- In both processes the total mass after the reaction is less than the total mass before, and the mass difference Δm corresponds to energy released by ΔE = Δmc².
- The energy release is linked to an increase in binding energy per nucleon of the products compared with the reactants, placing them higher on the curve.
Common mistakes and how to put them right
Thinking that mass is destroyed in fission and fusion.
Put it right: That mass is converted into energy, so total mass-energy is conserved.
Believing that fission releases energy because the nucleus is unstable, without linking the release to the increase in binding energy per nucleon of the products.
Confusing the two processes, for example describing fusion as the splitting of a heavy nucleus.
Put it right: State that fusion joins light nuclei while fission splits a heavy nucleus.
Exam tips
- When asked to describe fission or fusion, name the nuclei involved and state what happens to them before discussing energy.
- Use the binding-energy-per-nucleon curve to justify why energy is released: products must have higher binding energy per nucleon than reactants.
Try the lesson
The start of the Mass and energy (A-level only): Fission and fusion processes. lesson, made from checked teaching notes for this specification point.
Have a go first
Which process releases energy by joining two light nuclei to form a heavier nucleus?
Listen
Fission splits a heavy nucleus (e.g., uranium-235) into two smaller nuclei after absorbing a neutron, releasing neutrons and energy. Fusion joins two light nuclei (e.g., deuterium and tritium) to form a heavier nucleus, releasing energy. In both processes, the total product mass is slightly less than the reactant mass; this mass defect appears as energy via ΔE = Δmc². The energy arises because the product nuclei sit higher on the binding-energy-per-nucleon curve, closer to the iron-56 peak, meaning they have a higher binding energy per nucleon than the reactants. Fission is used in nuclear reactors to generate power, while fusion is the process that powers stars. A strong answer earns marks for these points. Fission: a heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing two or three neutrons and energy. Fusion: two light nuclei combine to form a heavier nucleus, releasing energy. In both processes the total mass after the reaction is less than the total mass before, and the mass difference Δm corresponds to energy released by ΔE = Δmc². The energy release is linked to an increase in binding energy per nucleon of the products compared with the reactants, placing them higher on the curve. A common slip: Thinking that mass is destroyed in fission and fusion. Instead, that mass is converted into energy, so total mass-energy is conserved. Exam tip: When asked to describe fission or fusion, name the nuclei involved and state what happens to them before discussing energy.
See it
Key words
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Fission
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Fusion
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Mass defect
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See every definition
- Fission
- A heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing two or three neutrons and energy.
- Fusion
- Two light nuclei combine to form a heavier nucleus, releasing energy.
- Mass defect
- The total mass after the reaction is less than the total mass before, and this mass difference corresponds to energy released by ΔE = Δmc².
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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