IGCSE Physics / Fission and fusion
Why fusion needs very high temperatures and pressures
Edexcel International GCSE Physics (4PH1), chapter 25: Fission and fusion, specification point 7.26. Free lesson.
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Fusion happens when two nuclei get close enough to join. Their positive charges make them repel, so very high temperature and pressure are needed to make fusion occur often enough.
Why nuclei repel
Every nucleus contains protons, which have a positive charge. Two nuclei are therefore both positively charged, and like charges repel. This push apart is called electrostatic repulsion. It gets stronger as the nuclei move closer, even though they must get extremely close to fuse. At low temperatures, nuclei do not get close enough to fuse, so fusion does not happen.
How temperature helps nuclei fuse
Temperature measures how fast particles move. At millions of degrees, nuclei move much faster than they do at everyday temperatures. This gives them more kinetic energy, meaning energy due to their motion. Fast-moving nuclei can get close enough to fuse despite the electrostatic repulsion. Low temperature: nuclei move slowly, have little kinetic energy and are pushed apart before they meet. Very high temperature: nuclei move very fast and can get close enough to fuse.
Why high pressure helps
High pressure packs nuclei close together. This makes collisions happen more often, so there are more chances for nuclei to get close enough to fuse.
Why fusion power is difficult on Earth
Fusion needs a temperature of millions of degrees and very high pressure. These conditions are very hard to create and keep up on Earth. Any material that touches the fuel would melt. The conditions needed for fusion exist in stars, but are very hard to make on Earth.
Build a linked explanation
For a full explanation, link the cause of repulsion to what temperature and pressure do. Each point should explain how the conditions help fusion happen.