Nuclear Physics: Revision Notes (Form 5 SPM Physics)

What this chapter covers

• 6.1 Radioactive Decay
• 6.2 Nuclear Energy

Key ideas explained

• What half-life means, Half-life is the time for half of the radioactive nuclei in a sample to decay. It stays the same no matter how much you start with, because decay is a fixed-chance process for each nucleus.
• Why nuclear reactions release so much energy, In a nuclear reaction the products have slightly less mass than the starting materials. This lost mass is converted into a large amount of energy, following E = mc squared.
• Why radioactive decay is random, You cannot predict which nucleus will decay next or when, and nothing outside can trigger it. Decay is spontaneous and random, yet a huge number of nuclei together follow a steady, predictable rate.

Formulas to memorise (none are given in the exam)

• E = mc², Mass–energy equivalence, E = energy released (J); m = mass defect (kg); c = speed of light (m s⁻¹)
• N = N₀ (½)ⁿ, Radioactive decay (half-life), N = quantity remaining (no unit) (); N₀ = initial quantity (no unit) (); t = elapsed time (s); T½ = half-life (s); n = number of half-lives (= t / T½) ()

Terms you must define precisely

• Gamma ray, A high-frequency electromagnetic wave emitted by an unstable nucleus; it has no charge and very high penetrating power.
• Radioactive decay, The random process by which an unstable nucleus loses energy by emitting radiation and changes into a more stable nucleus.
• Half-life, The time taken for half the radioactive nuclei in a sample to decay.
• Nuclear fission, The splitting of a heavy nucleus into two lighter nuclei, releasing a large amount of energy.
• Nuclear fusion, The joining of two light nuclei to form a heavier nucleus, releasing a large amount of energy, as happens in the Sun.
• Nuclear energy, The energy released from changes in the nucleus of an atom, such as fission or fusion, described by E = mc².
• Mass defect, The difference between the total mass of the separate nucleons and the mass of the nucleus they form, which is released as energy.
• Nucleus, The small, dense, positively charged centre of an atom, made up of protons and neutrons.

A revision order that works

1. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
2. Finish with a timed set of calculations with units on every line.
3. Read the 2 standards above and tick the ones you can already explain aloud without notes.
4. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.
5. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.

More Nuclear Physics resources

• Chapter overview: Nuclear Physics
• Common Mistakes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)