A level Physics · Edexcel 9PH0 · Topic 11B
Binding energy, fission and fusion
Key points
- The mass of a nucleus is less than the total mass of its separate nucleons: the difference is the mass deficit .
- Binding energy is the energy needed to separate a nucleus into individual nucleons: . Use 1 u ≈ 931.5 MeV.
- Binding energy per nucleon peaks around iron-56, the most stable nuclei.
- Fission: a heavy nucleus (e.g. uranium-235) absorbs a neutron and splits into two smaller nuclei plus more neutrons, which can cause a chain reaction.
- Fusion: light nuclei (e.g. hydrogen isotopes) join; this powers stars.
- Both release energy because the products have a greater binding energy per nucleon than the starting nuclei.
- Fusion needs very high temperature (so nuclei move fast enough to overcome electrostatic repulsion) and high density (so collisions are frequent), which is why it is hard to achieve on Earth.
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A nucleus weighs less than the protons and neutrons it is made from. That missing mass is the key to the energy released in nuclear power stations and in the stars.
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