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Secondary 4 Pure Physics Modern Physics Quiz
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Questions
Secondary 4 Pure Physics Quiz - Modern Physics
Name: _________________________ Class: _________________________ Date: _________________________ Score: ______ / 40
Duration: 45 minutes Total Marks: 40 Instructions: Answer ALL questions in the spaces provided. Show all working for calculation questions. Use g = 10 m/s² where needed.
Section A: Atomic Structure and Radioactivity (Questions 1–8)
12 marks
1. State the relative charge and relative mass of a proton, a neutron, and an electron.
| Particle | Relative Charge | Relative Mass |
|---|---|---|
| Proton | ||
| Neutron | ||
| Electron |
[3 marks]
2. An atom of uranium-235 has 92 protons and 143 neutrons.
(a) State the nucleon number of this uranium isotope.
____________________________________________________________ [1 mark]
(b) Explain what is meant by the term "isotope".
____________________________________________________________ [2 marks]
3. Complete the nuclear equation for the alpha decay of radium-226 (atomic number 88):
^{226}_{88}\text{Ra} \rightarrow \text{______} + \text{______}
[2 marks]
4. State two properties of alpha particles that make them less penetrating than beta particles.
[2 marks]
5. A radioactive source has a half-life of 8 days. Its initial activity is 1600 counts per minute. Calculate the activity of the source after 24 days.
____________________________________________________________ [2 marks]
Section B: Nuclear Reactions and Applications (Questions 6–12)
14 marks
6. Describe the process of nuclear fission. In your answer, state what happens to the nucleus and what is released.
____________________________________________________________ [3 marks]
7. Explain why nuclear fusion requires extremely high temperatures to occur.
____________________________________________________________ [2 marks]
8. State one advantage and one disadvantage of using nuclear power to generate electricity.
Advantage: ___________________________________________________
Disadvantage: ________________________________________________
____________________________________________________________ [2 marks]
9. A radioactive isotope emits a beta particle.
(a) State what a beta particle is.
____________________________________________________________ [1 mark]
(b) Explain how the emission of a beta particle changes the composition of the nucleus.
____________________________________________________________ [2 marks]
10. A sample of radioactive material is placed near a Geiger-Müller tube connected to a counter. The background count rate is 30 counts per minute. With the source present, the count rate is 510 counts per minute.
Calculate the corrected count rate due to the source alone.
____________________________________________________________ [1 mark]
11. Describe how a radioactive source can be used to monitor the thickness of paper in a factory. Explain why a beta source is suitable for this purpose.
____________________________________________________________ [3 marks]
Section C: Detection, Safety, and Half-Life (Questions 13–20)
14 marks
12. State two safety precautions that should be taken when handling radioactive sources in a school laboratory.
[2 marks]
13. A radioactive isotope has a half-life of 6 hours. A sample initially contains 8.0 × 10¹⁰ radioactive atoms.
(a) Calculate the number of radioactive atoms remaining after 18 hours.
____________________________________________________________ [2 marks]
(b) Explain what is meant by the term "half-life".
____________________________________________________________ [1 mark]
14. Describe how a Geiger-Müller tube detects radiation.
____________________________________________________________ [2 marks]
15. Compare the ionising ability of alpha, beta, and gamma radiation. State which is the most ionising and which is the least ionising.
____________________________________________________________ [2 marks]
16. A radioactive source emits gamma radiation. Explain why gamma radiation is suitable for sterilising medical equipment.
____________________________________________________________ [2 marks]
17. State what happens to the mass number and atomic number of a nucleus when it emits a gamma ray.
Mass number: _________________________________________________
Atomic number: _______________________________________________ [1 mark]
18. Explain why the activity of a radioactive source decreases over time.
____________________________________________________________ [2 marks]
19. A student investigates the penetrating power of three radioactive sources. Source A is stopped by paper. Source B passes through paper but is stopped by 5 mm of aluminium. Source C passes through both paper and aluminium.
Identify the type of radiation emitted by each source.
Source A: ____________________________________________________
Source B: ____________________________________________________
Source C: ____________________________________________________ [2 marks]
20. Describe one medical use of a radioactive isotope. In your answer, state which type of radiation is used and explain why it is suitable.
____________________________________________________________ [3 marks]
END OF QUIZ
Answers
Secondary 4 Pure Physics Quiz - Modern Physics — Answer Key
Total Marks: 40
Section A: Atomic Structure and Radioactivity (Questions 1–5)
1. [3 marks]
| Particle | Relative Charge | Relative Mass |
|---|---|---|
| Proton | +1 [1] | 1 [1] |
| Neutron | 0 [1] | 1 [1] |
| Electron | –1 [1] | 1/1840 (or negligible/≈0) [1] |
Award 1 mark per correct row (both entries correct).
2. (a) Nucleon number = 92 + 143 = 235 [1 mark]
(b) Isotopes are atoms of the same element (same number of protons / same atomic number) [1] but with different numbers of neutrons (different nucleon number / mass number) [1]. [2 marks]
3.
Award 1 mark for correct daughter nucleus (Rn, mass 222, atomic 86) and 1 mark for correct alpha particle (He, mass 4, atomic 2). Accept "α" in place of He. [2 marks]
4. Any two from:
- Alpha particles have a larger mass / are heavier [1]
- Alpha particles have a greater charge (+2e vs –1e) [1]
- Alpha particles have stronger ionising ability so lose energy more quickly [1]
- Alpha particles have a larger size [1]
[2 marks]
5. Number of half-lives = 24 ÷ 8 = 3 [1] Activity after 3 half-lives = 1600 × (½)³ = 1600 × ⅛ = 200 counts per minute [1]
Award full marks for correct answer with working. Accept alternative stepwise method. [2 marks]
Section B: Nuclear Reactions and Applications (Questions 6–11)
6. Nuclear fission is the splitting of a heavy nucleus (e.g., uranium-235) [1] after absorbing a neutron [1], into two smaller nuclei (fission fragments), releasing two or three neutrons and a large amount of energy [1]. [3 marks]
7. Nuclear fusion requires extremely high temperatures because the positively charged nuclei repel each other (electrostatic repulsion) [1]. High temperatures give the nuclei enough kinetic energy to overcome this repulsion and come close enough for the strong nuclear force to bind them together [1]. [2 marks]
8. Advantage (any one):
- Does not produce greenhouse gases / carbon dioxide during operation [1]
- Produces large amounts of energy from small amounts of fuel [1]
- Reliable / can operate continuously (unlike solar/wind) [1]
Disadvantage (any one):
- Produces radioactive waste that is difficult to dispose of safely [1]
- Risk of nuclear accidents / meltdown (e.g., Chernobyl, Fukushima) [1]
- High construction and decommissioning costs [1]
- Non-renewable (uses finite uranium resources) [1]
Award 1 mark for a valid advantage and 1 mark for a valid disadvantage. [2 marks]
9. (a) A beta particle is a fast-moving electron (emitted from the nucleus). [1 mark]
(b) In beta decay, a neutron in the nucleus changes into a proton [1] and an electron (the beta particle) is emitted. The atomic number increases by 1, but the mass number stays the same [1]. [2 marks]
10. Corrected count rate = 510 – 30 = 480 counts per minute [1 mark]
11. A radioactive source is placed on one side of the paper and a detector on the other side [1]. If the paper is too thick, more radiation is absorbed and the count rate drops; if too thin, the count rate rises [1]. A beta source is suitable because beta particles can penetrate paper but are absorbed by small changes in thickness (alpha would be stopped completely; gamma would pass through too easily) [1]. [3 marks]
Section C: Detection, Safety, and Half-Life (Questions 12–20)
12. Any two from:
- Use tongs / forceps to handle sources (never touch with bare hands) [1]
- Keep sources in lead-lined containers when not in use [1]
- Point sources away from people / direct beam away from the body [1]
- Keep exposure time as short as possible [1]
- Wear protective clothing / lab coat [1]
- Do not eat or drink near radioactive sources [1]
[2 marks]
13. (a) Number of half-lives = 18 ÷ 6 = 3 [1] Remaining atoms = 8.0 × 10¹⁰ × (½)³ = 8.0 × 10¹⁰ × ⅛ = 1.0 × 10¹⁰ [1]
(b) Half-life is the time taken for half the nuclei in a radioactive sample to decay / for the activity to fall to half its original value. [1 mark]
14. Radiation enters the Geiger-Müller tube and ionises the gas inside [1]. The ions created allow a pulse of current to flow between the electrodes, which is detected and counted by the counter [1]. [2 marks]
15. Alpha radiation is the most ionising [1]. Gamma radiation is the least ionising [1]. Beta radiation has intermediate ionising ability. [2 marks]
16. Gamma radiation is highly penetrating and can pass through packaging to reach all surfaces of the equipment [1]. It kills bacteria and other microorganisms by damaging their DNA / ionising their cells [1]. The equipment does not become radioactive. [2 marks]
17. Mass number: unchanged / stays the same [½] Atomic number: unchanged / stays the same [½]
[1 mark]
18. The activity of a radioactive source decreases over time because the number of unstable (parent) nuclei in the sample decreases as they decay [1]. Since activity is proportional to the number of undecayed nuclei, the activity also decreases [1]. [2 marks]
19. Source A: alpha [1] Source B: beta [1] Source C: gamma [1]
Award ½ mark each if only two correct. [2 marks]
20. Any valid medical use, e.g.:
- Iodine-131 for treating thyroid cancer: emits beta and gamma radiation [1]. Beta particles destroy cancerous thyroid cells [1]. Iodine is taken up naturally by the thyroid gland, so the radiation targets the right area [1].
- Technetium-99m as a medical tracer: emits gamma radiation [1]. Gamma rays can be detected outside the body to create images [1]. It has a short half-life (about 6 hours) so the patient's radiation exposure is minimised [1].
- Cobalt-60 for radiotherapy: emits gamma radiation [1]. Gamma rays are directed at cancerous tumours to kill cancer cells [1]. The beam is rotated around the patient to minimise damage to healthy tissue [1].
Award 1 mark for naming a valid use/isotope, 1 mark for identifying the radiation type, 1 mark for explaining suitability. Accept other valid examples. [3 marks]
END OF ANSWER KEY