Nuclear FusionOxford AQA IGCSE Physics: Revision notes
Section 1
What is nuclear fusion?
Nuclear fusion is the joining of two light (small) nuclei to form a single heavier nucleus.
Fusion is the opposite process to fission: fission splits a large nucleus apart, while fusion joins small nuclei together.
Do not confuse fusion (joining) with fission (splitting) — examiners frequently test whether students mix these up.
Section 2
Where does the energy in fusion come from?
When two light nuclei fuse, some of the mass of the original nuclei is converted directly into energy.
This is why fusion releases such enormous amounts of energy — the energy does not come from chemical bonds or electron rearrangement, but from mass itself being converted into energy.
Section 3
Why is fusion so hard to achieve?
Both nuclei involved in fusion are positively charged (they contain protons), so they repel each other through electrostatic repulsion.
To overcome this repulsion and get the nuclei close enough to fuse, you need:
- Very high temperatures (so the nuclei move fast enough to overcome repulsion)
- Very high pressures (so nuclei are forced close together)
This combination of extreme temperature and pressure is only naturally found inside stars.
When explaining why fusion needs high temperature/pressure, always mention the electrostatic repulsion between the positive nuclei — this is the key reasoning examiners want.
Section 4
Where does fusion happen naturally?
Nuclear fusion is the process by which energy is released in stars.
Inside a star's core, the temperature and pressure are high enough to force hydrogen nuclei to fuse into helium, releasing the energy that makes stars shine.
Think of fusion like forcing two magnets together north-pole to north-pole — you need a lot of force (here, heat and pressure) to overcome their natural repulsion before they can 'stick'.
Must Know
- Nuclear fusion is the joining of two light nuclei to form a heavier nucleus
- In fusion, some of the mass of the smaller nuclei is converted into energy
- Positively charged nuclei repel each other (electrostatic repulsion), so very high temperatures and pressures are needed to force fusion to happen
- Nuclear fusion is the process that releases energy in stars
- Fusion is the opposite process to fission (joining vs splitting)
That's the notes covered.
Carry on to the next subtopic.