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LearnCambridge97022025–2027Paper 4 · A Level Structured QuestionsNuclear physics

Paper 4 · Module

Nuclear physics

2 subtopics≈ 80 min in total

Start the module: Mass defect and binding energy Open graphing calculator

Mass defect and binding energy
Mass and energy are equivalent: $E=mc^2$ . A nucleus has less mass than its separate protons and neutrons; this mass defect $\Delta m$ corresponds to the binding energy $c^2\Delta m$ , the energy needed to separate the nucleus into its nucleons. Binding energy per nucleon peaks near iron-56. Splitting heavy nuclei (fission) or joining light ones (fusion) moves towards the peak, releasing energy.
≈ 40 min
Radioactive decay
Radioactive decay is random (you can't predict which nucleus will decay next) and spontaneous (unaffected by temperature, pressure or chemistry). The activity is proportional to the number of undecayed nuclei: $A=\lambda N$ , where $\lambda$ is the decay constant. So $N$ , $A$ and the count rate all fall exponentially: $x=x_0e^{-\lambda t}$ . The half-life is $t_{1/2}=\dfrac{\ln2}\lambda=\dfrac{0.693}\lambda$ .
≈ 40 min