Nuclear fusion
3 marks · NormalExplain why nuclear fusion reactions are the main source of energy in stars, but not in most power stations on Earth.
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Fusion is the main source of energy in stars because their extremely high core temperatures and pressures allow nuclei to overcome their repulsion and fuse. On Earth, such conditions are very difficult and expensive to achieve, so power stations use fission instead.
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Marking points
B1 · 1 mark · Explains that high temperatures and pressures in stars allow fusion.
B2 · 1 mark · Mentions that fusion is currently difficult to achieve on Earth.
B3 · 1 mark · Recognises that power stations use fission instead of fusion.
Worked solution
Fusion occurs naturally in stars because their cores reach temperatures of millions of degrees and have huge pressures. These extreme conditions allow hydrogen nuclei to overcome their electrostatic repulsion and fuse, releasing energy. On Earth, such conditions are very hard to achieve and maintain, so fusion is not yet used in most power stations. Instead, fission, which can occur at lower temperatures, is used.