Questions
1. Calculate: A mass defect m = 2.0 × 10⁻³ kg is converted to energy (c = 3.0 × 10⁸ m s⁻¹). Find the unknown using E = mc².
E = mc² = 2.0 × 10⁻³ × (3.0 × 10⁸)² = 1.8 × 10¹⁴ J.
2. Calculate: N₀ = 800 nuclei, half-life T½ = 5 days, after t = 15 days. Find the unknown using N = N₀ (½)ⁿ.
Number of half-lives = t / T½ = 15 / 5 = 3, so N = N₀ (½)³ = 800 × (1/8) = 100.
3. Explain: 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.
4. Explain: 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.
5. Explain: 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.
6. Define isotope.
Atoms of the same element that have the same number of protons but different numbers of neutrons.
7. Define radioactivity.
The spontaneous emission of radiation (alpha, beta or gamma) from the unstable nuclei of certain atoms.
8. Define alpha particle.
A particle made of two protons and two neutrons (a helium nucleus) emitted by some radioactive nuclei; it has a positive charge and low penetrating power.
9. Define beta particle.
A high-speed electron emitted from the nucleus when a neutron changes into a proton; it has a negative charge and moderate penetrating power.
10. Define gamma ray.
A high-frequency electromagnetic wave emitted by an unstable nucleus; it has no charge and very high penetrating power.
11. Define radioactive decay.
The random process by which an unstable nucleus loses energy by emitting radiation and changes into a more stable nucleus.
Marking yourself
- Calculation: formula written, values substituted with units, final answer with unit and sensible significant figures.
- Variables: manipulated = what you change; responding = what you measure; constant = what you keep the same.
- Definition: one sentence, correct physical quantity, correct unit.
Exam tip
More Nuclear Physics resources
- Chapter overview: Nuclear Physics
- Revision Notes
- Common Mistakes
- 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)