Nuclear fission and fusion Cambridge IGCSE Physics revision
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In plain words
There are two ways to get energy out of nuclei. Fission splits a big nucleus (such as uranium) into two smaller ones: it's what nuclear power stations do. Fusion joins two small nuclei (such as hydrogen) into a bigger one: it's what powers the Sun.
Three things to know
- Fission: a large nucleus (such as uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing energy and two or three more neutrons, which can cause a chain reaction.
- Fusion: two small nuclei (such as hydrogen) join to make a larger one, releasing energy. It powers the Sun and other stars.
- Fusion needs very high temperature and pressure, because the nuclei are positive and repel each other.
Tips and tricks
- Fission is splitting (think "fissure", a crack). Fusion is joining (think "fuse together").
- In a reactor, the control rods absorb neutrons to slow the chain reaction down. The moderator slows the neutrons themselves down. Don't mix them up.
It lands in your notebook with its questions as flashcards.
Nuclear fission and fusion: 6 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 fission?
A heavy nucleus splits into two lighter ones.
What starts the fission of a uranium-235 nucleus?
Neutrons have no charge, so the nucleus does not repel them.
Where does nuclear fusion happen naturally?
Stars are hot and dense enough for hydrogen nuclei to fuse.
In a nuclear reactor, what do the control rods do?Stretch
Absorbing neutrons controls the rate of the chain reaction. (Slowing them is the moderator's job.)
Why is fusion hard to achieve on Earth?
The nuclei need enormous speeds to overcome their repulsion.
What makes a chain reaction possible in fission?
Each fission releases neutrons that can cause more fissions.
Quiz
6 questions
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Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Nuclear fission and fusion
Cambridge IGCSE Physics 0625 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Describe what happens in the fission of a uranium-235 nucleus.[3]
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It absorbs a neutron, becomes unstable and splits into two smaller nuclei, releasing energy and two or three neutrons.
Explain how a chain reaction can happen.[2]
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The neutrons released by one fission go on to split other uranium nuclei, which release more neutrons, and so on.
State where fusion happens naturally, and name the element made from hydrogen there.[2]
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In stars such as the Sun. Helium.
Explain why fusion needs extremely high temperatures.[2]
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Nuclei are positively charged and repel; they must be moving very fast to get close enough to join.
Answers: Nuclear fission and fusion
- 1. It absorbs a neutron, becomes unstable and splits into two smaller nuclei, releasing energy and two or three neutrons.
- 2. The neutrons released by one fission go on to split other uranium nuclei, which release more neutrons, and so on.
- 3. In stars such as the Sun. Helium.
- 4. Nuclei are positively charged and repel; they must be moving very fast to get close enough to join.