IGCSE Physics / Fission and fusion
Nuclear fusion revision notes
Edexcel International GCSE Physics (4PH1), chapter 25: Fission and fusion, specification point 7.23, 7.24, 7.25. Free lesson.
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Nuclear fusion is when two light nuclei join to make a heavier nucleus. Some mass is lost and released as energy, which is why fusion powers stars.
How fusion joins nuclei
Fusion is the opposite of fission. Instead of splitting a large nucleus, two light nuclei collide at very high speed and join to form one larger nucleus, such as helium. The nuclei involved are in atoms, but fusion itself happens to the nuclei, not the atoms. The starting nuclei can be isotopes of hydrogen. Isotopes are forms of the same element with different numbers of neutrons.
Where fusion's energy comes from
The new nucleus has slightly less mass than the two starting nuclei had altogether. The missing mass is released as energy. A tiny loss of mass can produce a huge amount of energy. The product's mass is less than the total mass of the original nuclei. The lost mass is turned into energy.
How fusion powers stars
Stars, including the Sun, release energy through fusion in their core. Hydrogen nuclei fuse to form helium nuclei. The released energy travels out through the star and leaves its surface as light and heat. To describe this process, name both the nuclei and the location: hydrogen nuclei fuse into helium nuclei in the core of the star. Do not describe this as burning. Fusion is a nuclear process.
How fission and fusion differ
Both processes release energy, and in both the products have less mass than the starting nuclei. Their main difference is what happens to the nuclei: fission splits, while fusion joins. Fission splits one large, heavy nucleus, such as uranium-235, into smaller nuclei. Fusion joins two small, light nuclei, such as hydrogen, into one larger nucleus. Fission needs a neutron to be absorbed. Fusion needs very high temperature and pressure. Fission produces radioactive daughter nuclei as waste. Fusion produces helium, which is not radioactive. Fission is used in nuclear power stations.…
How to state two differences
For a comparison question, give two clear differences. One mark is available for each correct difference, so make each contrast easy to identify.