Fission and fusion Edexcel International A Level Physics revision
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In plain words
There are two opposite ways to get energy out of nuclei: split a very heavy one in two, or squeeze two very light ones together. Both work for the same reason. The products are more tightly bound than what you started with, and the difference comes out as energy.
4 things to know
- A graph of binding energy per nucleon against nucleon number rises steeply for light nuclei, peaks near iron (nucleon number about 56) and then falls slowly for heavy nuclei.
- Fission: a heavy nucleus splits into two smaller nuclei, usually with a few neutrons. Fusion: two light nuclei join to make a heavier one.
- In both, the products have a higher binding energy per nucleon than the starting nuclei, so energy is released. The energy released is c² × (the mass before − the mass after).
- Fusion needs a very high temperature, so that the nuclei move fast enough to overcome the electrostatic repulsion between their positive charges and get close enough to fuse. It also needs a very high density, so that collisions happen often enough. The cores of stars have both.
Worked example
A deuterium nucleus (2.0136 u) fuses with a tritium nucleus (3.0155 u) to give a helium-4 nucleus (4.0015 u) and a neutron (1.0087 u). Find the energy released.
- Mass before = 2.0136 + 3.0155 = 5.0291 u. Mass after = 4.0015 + 1.0087 = 5.0102 u.
- Decrease in mass = 0.0189 u = 0.0189 × 1.66 × 10⁻²⁷ = 3.14 × 10⁻²⁹ kg.
- Energy released = c²Δm = (3.00 × 10⁸)² × 3.14 × 10⁻²⁹ = 2.8 × 10⁻¹² J, about 18 MeV.
Tips and tricks
- Energy is released by moving towards the peak of the curve: light nuclei by fusing, heavy nuclei by splitting.
- When you explain either process, say that the binding energy per nucleon of the products is greater.
It lands in your notebook with its questions as flashcards.
Fission and fusion: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is nuclear fusion?
It is what powers the stars.
Why does fusion need a very high temperature?
Both nuclei are positively charged.
Where does the energy released in fission or fusion come from?
ΔE = c²Δm.
Near which element does the binding energy per nucleon curve peak?
Nuclei near iron are the most stable.
Which process releases energy from very heavy nuclei?
Splitting them moves the products towards the peak of the curve.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 10 marks. Write your answers on paper, then check them.
Fission and fusion
Edexcel International A Level Physics WPH · 10 marks · papermunch.org
Name ______________________________ Date ______________
State the difference between nuclear fission and nuclear fusion.[2]
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Fission is the splitting of a heavy nucleus into two lighter nuclei. Fusion is the joining of two light nuclei to make a heavier nucleus.
Explain why fusion needs a very high temperature and a very high density.[3]
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Nuclei are positively charged and repel each other. At a very high temperature they have enough kinetic energy to get close enough to fuse. A very high density is needed so that enough collisions happen each second to keep the reaction going.
The fission of one uranium-235 nucleus releases about 200 MeV. Find the energy released by the fission of all the nuclei in 1.0 kg of uranium-235, which contains 2.56 × 10²⁴ nuclei.[3]
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8.2 × 10¹³ J. 2.56 × 10²⁴ × 200 × 1.60 × 10⁻¹³.
Use the binding energy per nucleon curve to explain why energy is released when a uranium nucleus splits.[2]
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The two smaller nuclei are nearer the peak of the curve, so they have a higher binding energy per nucleon than uranium. The total binding energy increases, and the difference is released.
Answers: Fission and fusion
- 1. Fission is the splitting of a heavy nucleus into two lighter nuclei. Fusion is the joining of two light nuclei to make a heavier nucleus.
- 2. Nuclei are positively charged and repel each other. At a very high temperature they have enough kinetic energy to get close enough to fuse. A very high density is needed so that enough collisions happen each second to keep the reaction going.
- 3. 8.2 × 10¹³ J. 2.56 × 10²⁴ × 200 × 1.60 × 10⁻¹³.
- 4. The two smaller nuclei are nearer the peak of the curve, so they have a higher binding energy per nucleon than uranium. The total binding energy increases, and the difference is released.