Figure 8 shows a helium nucleus - Edexcel - GCSE Physics - Question 5 - 2018 - Paper 1

Fusion reactions involve the combination of two light atomic nuclei to form a heavier nucleus, releasing a significant amount of energy in the process. This typically occurs under extreme temperatures and pressures, like those found in stars. In contrast, fission reactions involve the splitting of a heavy atomic nucleus into two smaller nuclei, along with the release of energy and additional neutrons. Fission is commonly used in nuclear reactors and can occur spontaneously or be induced by neutron absorption.

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