Explain fission, fusion and nuclear energy release
Use the binding-energy curve and mass–energy equivalence to explain why fission or fusion can release energy.
Section 23 · Nuclear physics
A Level · Cambridge International Physics 9702, 2025–2027 syllabus. The Topic 23 outcomes were also checked against Cambridge’s 2028–2030 syllabus, which reports no significant teaching changes. Online practice is self-checked and cannot replace supervised practical work, full timed assessment or human subject-teacher review.
Before you begin
- Use scientific notation, SI units, proportional reasoning and the particle numbers in nuclide notation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is nuclear fusion?
What you will learn
- Explain nuclear fission and fusion
- Relate fusion of light nuclei and fission of heavy nuclei to higher binding energy per nucleon
- Calculate the energy released from a supplied mass decrease
Fusion joins light nuclei; fission splits a heavy nucleus
Nuclear fusion is the joining of two lighter nuclei to form a heavier nucleus. Nuclear fission is the splitting of a heavy nucleus into two or more lighter nuclei, often after neutron absorption.
The syllabus asks you to explain these processes using binding energy per nucleon. Avoid describing every fission event as identical: the exact products depend on the reaction.
A rise in total binding energy can release energy
Light nuclei can release energy by fusion when the products are more tightly bound per nucleon. Heavy nuclei can release energy by fission when the products move toward the higher-binding region of the curve.
Calculate released energy from the mass decrease using E = c²Δm, with Δm defined as initial total mass minus final total mass when that decrease is positive. State the mass and energy units and explain the binding-energy change.
Pause and explain
Why can fission of a heavy nucleus release energy?
Worked example
A fission reaction has a mass decrease of 3.0 × 10⁻²⁸ kg. Estimate the energy released using c = 3.0 × 10⁸ m s⁻¹.
Show the worked solution
- Use E = c²Δm.
- c² = 9.0 × 10¹⁶ m² s⁻².
- E = 9.0 × 10¹⁶ × 3.0 × 10⁻²⁸ = 2.7 × 10⁻¹¹ J.
Answer 2.7 × 10⁻¹¹ J
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A reaction’s total mass decreases by 4.0 × 10⁻²⁹ kg. Calculate the energy released. Use c = 3.0 × 10⁸ m s⁻¹.
Give me a hint
Square c, then multiply by the positive mass decrease.
Compare my reasoning
- c² = 9.0 × 10¹⁶ m² s⁻².
- E = c²Δm = 9.0 × 10¹⁶ × 4.0 × 10⁻²⁹.
- E = 3.6 × 10⁻¹² J.
3.6 × 10⁻¹² J.
Look for these in your work
- Uses a positive mass decrease
- Squares c
- Gives joules
Explain why fusion of suitable light nuclei can release energy.
Give me a hint
Connect the product’s position on the binding-energy curve with total binding energy.
Compare my reasoning
- The light reactants lie on the rising part of the curve.
- Fusion can produce nuclei with greater binding energy per nucleon.
- If total binding energy increases, the difference is released as energy.
Suitable fusion products are more tightly bound; the increase in total binding energy can be released as energy.
Look for these in your work
- Identifies light nuclei
- Uses the rising curve
- Connects greater total binding with released energy
A student computes a negative mass change as final mass minus initial mass and calls the result negative energy released. Correct the method.
Give me a hint
Define the mass decrease in the direction that is positive for a release.
Compare my reasoning
- For released energy, calculate initial total mass − final total mass.
- If this mass decrease is positive, use E = c²Δm.
- A positive released-energy magnitude is reported; a negative sign would describe the system’s energy change under a different sign convention.
Use Δm = initial mass − final mass for the released-energy magnitude, then E = c²Δm.
Look for these in your work
- Defines the direction of subtraction
- Uses the mass decrease
- Distinguishes energy released from system energy change
Common mistakes
- Calling fusion the splitting of a nucleus
- Claiming fission and fusion release energy for every possible pair of nuclei
- Using Δm with the final mass minus initial mass when asked for energy released
What can you explain now?
What is nuclear fusion?
Compare with the explanation
Joining lighter nuclei to form a heavier nucleus
Fusion combines nuclei.
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Make it stick
How does the binding-energy curve explain energy release in fission and fusion?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.