Secondary 4 Pure Physics Modern Physics Quiz

Secondary 4 Pure Physics Quiz - Modern Physics — Answer Key

Total Marks: 40

Section A: Atomic Structure and Radioactivity (Questions 1–5)

1. [3 marks]

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. $^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$

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