AQA GCSE Physics Foundation (8463), Foundation tier · Atomic structure › Nuclear fission and fusion
Practise Nuclear fusion. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, with some of the mass converted into the energy of radiation. You need to describe fusion and compare it with fission. Expect short 'describe' questions and 4 to 6 mark comparisons. GCSE Physics only, not in Combined Science.
Key facts
Fusion: two light nuclei join to form a heavier nucleus
Some of the mass is converted into energy of radiation
Stars: hydrogen nuclei fuse to form helium
Fission splits heavy nuclei; fusion joins light nuclei; both release energy
Fission is used in power stations now; fusion is not yet used to generate electricity
Notes
What fusion is
diagram
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
In this process some of the mass may be converted into the energy of radiation. The new nucleus has slightly less mass than the two nuclei that joined.
Example: two isotopes of hydrogen fuse to form helium: \({}^{2}_{1}\mathrm{H} + {}^{3}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + {}^{1}_{0}\mathrm{n}\)
Fusion of two hydrogen isotopes. The products have slightly less mass: that mass became energy.
Fusion is the energy source of stars. In the core of the Sun, hydrogen nuclei fuse to form helium.
Fission and fusion compared
Fission: a large, heavy nucleus splits into two smaller nuclei. Fusion: two light nuclei join to form a heavier nucleus.
Both release energy.
Fission usually starts when a nucleus absorbs a neutron. Fusion needs very high temperatures and pressures.
Fission fuels are uranium and plutonium; fusion fuels are light nuclei such as isotopes of hydrogen.
Fission is used in nuclear power stations today. Fusion happens naturally in stars.
How to answer each type of question
Describe nuclear fusion
2 marksGrade 4
Say that two light nuclei join.
Say that a heavier nucleus forms and energy is released (some mass is converted to energy).
Example. Describe what happens during nuclear fusion.
Show the model answerHide the model answer
Two light nuclei join together (1) to form a heavier nucleus, and energy is released as some of the mass is converted into energy (1).
Don’t lose marks
Mixing up fission and fusion, the most common slip in this topic.
Saying fusion joins atoms or molecules. It joins nuclei.
Saying energy is 'created'. Some of the mass is converted into energy.
Saying today's nuclear power stations use fusion. They use fission.
More tips
Memory tricks
Fusion = fuse = join together. Fission = split.
Heavy nuclei split (fission); light nuclei join (fusion).
Stars run on fusion; today's nuclear power stations run on fission.
Exam technique
In 'compare' questions, make every point about both processes, using 'whereas'. A list about only one process scores little.
Say 'nuclei', not 'atoms', in your definitions of fission and fusion.
Questions sometimes link fusion to stars in space physics, so be ready to use both topics together.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Define nuclear fusionTwo light nuclei join to form a heavier nucleus.
Grade 5
State where fusion happens naturallyIn the cores of stars, e.g. hydrogen nuclei fuse to form helium in the Sun.
Grade 5
State where the energy comes fromSome of the mass is converted into the energy of radiation.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Where does nuclear fusion happen naturally?
In stars, such as the Sun
The equation shows a fusion reaction that scientists hope to use in future power stations. \({}^{2}_{1}\mathrm{H} + {}^{3}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + {}^{a}_{b}\mathrm{X}\) Determine the values of a and b, and identify particle X.
a = 1, b = 0: X is a neutron
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is nuclear fusion? Tick (✓) one box.[1]
The joining of two light nuclei to form a heavier nucleus
The splitting of a heavy nucleus into two lighter nuclei
The emission of radiation from an unstable nucleus
The absorption of a neutron by a nucleus
(b) Where does nuclear fusion happen naturally?[1]
(c) In a fusion reaction, some of the mass of the nuclei is converted into another form. What is it converted into?[1]
(d) Give one similarity between nuclear fission and nuclear fusion.[1]
Show the answer and mark scheme
(a)Answer: The joining of two light nuclei to form a heavier nucleus
(b)Answer: In stars, such as the Sun
in stars / in the Sun
(c)Answer: Energy (radiation)
energy (of radiation)
(d)Answer: Both release energy.
both release energy
both are nuclear reactions / change the nuclei of atoms
Question 2Medium5 marks
(a) Give two differences between nuclear fission and nuclear fusion.[2]
(b) Explain why very high temperatures are needed for fusion to happen.[3]
Show the answer and mark scheme
(a)Answer: Fission splits a heavy nucleus; fusion joins light nuclei. Fission is started by neutrons; fusion needs extremely high temperatures.
fission splits a large (heavy) nucleus, but fusion joins two small (light) nuclei
fission is usually started by a neutron, but fusion needs very high temperatures (and pressures)
fission is used in power stations now, but fusion power stations are still being developed
fission products are radioactive (long-lived waste), but fusion produces little long-lived radioactive waste
fusion is the energy source of stars, but fission is not
(b)Answer: The positive nuclei repel each other; only at extreme temperatures do they move fast enough to get close enough to fuse.
nuclei are positively charged, so they repel each other
at very high temperatures the nuclei move very fast / have a lot of kinetic energy
so they can get close enough (overcoming the repulsion) to join together