Secondary 3 Physics Modern Physics Quiz

Secondary 3 Physics Quiz - Modern Physics: ANSWER KEY

Total Marks: 40

Section A: Atomic Structure and Radioactivity (10 marks)

1. (a) Proton, neutron, electron [1 mark - all three required]

(b) [3 marks - 1 mark per correct row]

(c) Nucleon number: 12 [1 mark] Proton number: 6 [1 mark]

2. (a) Alpha (α) radiation [1 mark]

(b) Beta (β) radiation [1 mark]

(c) Gamma radiation is an electromagnetic wave with no mass and no charge [1 mark]. It interacts weakly with matter, so it can pass through most materials without being absorbed, making it highly penetrating [1 mark].

3. [2 marks - 1 mark per correct row]

4. (a) ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He [2 marks - 1 mark for correct daughter nucleus, 1 mark for correct alpha particle and balanced equation]

(b) Proton number: 90 [0.5 mark] Nucleon number: 234 [0.5 mark]

5. Nuclei are unstable when they have an imbalance of protons and neutrons [1 mark]. The nucleus has too much energy and undergoes radioactive decay to become more stable by emitting radiation [1 mark].

Section B: Radioactive Decay and Half-Life (10 marks)

6. (a) Half-life is the time taken for half the nuclei in a radioactive sample to decay [1 mark], or the time taken for the activity of a radioactive sample to fall to half its initial value [1 mark].

(b) Number of half-lives = 24 ÷ 8 = 3 half-lives [1 mark] After 1 half-life: 800 ÷ 2 = 400 mg After 2 half-lives: 400 ÷ 2 = 200 mg After 3 half-lives: 200 ÷ 2 = 100 mg [1 mark for correct method] Mass remaining: 100 mg [1 mark for correct answer]

(c) Radioactive decay is a random process [1 mark]. As the number of undecayed nuclei decreases, the activity decreases, but there is always a finite probability that some nuclei remain undecayed. The mass approaches zero asymptotically but never reaches exactly zero in a finite time [1 mark].

7. (a) Half-life: 5 days [1 mark - accept value read from graph where activity halves]

(b) After 20 days: 20 ÷ 5 = 4 half-lives [1 mark] Activity = 800 × (½)⁴ = 800 × 1/16 = 50 cpm [1 mark]

8. Use tongs/forceps to handle sources / keep source at arm's length / store in lead-lined container [1 mark]. This minimises exposure time and increases distance from the source, reducing the radiation dose received by the handler [1 mark]. [Accept any valid precaution with correct explanation]

9. Initial activity = 1600 Bq, final activity = 200 Bq 1600 → 800 → 400 → 200 (3 half-lives) [1 mark] Time = 30 minutes, so half-life = 30 ÷ 3 = 10 minutes [1 mark]

10. Contamination occurs when radioactive material gets onto or into an object or person [1 mark]. Irradiation occurs when a person or object is exposed to radiation from a source, but does not become radioactive itself [1 mark].

Section C: Nuclear Reactions and Applications (10 marks)

11. (a) Nuclear fission is the splitting of a large, unstable nucleus [1 mark] into two (or more) smaller nuclei, releasing energy and neutrons in the process [1 mark].

(b) The reaction produces neutrons (3 neutrons in this case) [1 mark]. These neutrons can go on to cause further fission reactions in other U-235 nuclei, which in turn produce more neutrons, creating a self-sustaining chain reaction [1 mark].

(c) Advantage: Produces large amounts of energy from small amounts of fuel / does not produce greenhouse gases during operation / reliable baseload power [1 mark for any valid advantage]. Disadvantage: Produces radioactive waste that must be stored safely for thousands of years / risk of catastrophic accidents (e.g., meltdown) / high decommissioning costs [1 mark for any valid disadvantage].

12. (a) Nuclear fusion is the process in which two light nuclei [1 mark] combine to form a heavier nucleus, releasing energy in the process [1 mark].

(b) Nuclei are positively charged and repel each other due to electrostatic repulsion [1 mark]. Extremely high temperatures give the nuclei sufficient kinetic energy to overcome this repulsion and come close enough for the strong nuclear force to bind them together [1 mark].

13. Similarity: Both release energy / both involve changes in the nucleus [1 mark]. Difference: Fission splits a large nucleus, while fusion combines small nuclei / fission produces radioactive waste, fusion produces less radioactive waste [1 mark for any valid difference].

14. (a) Half-life of carbon-14: 5730 years [1 mark]

(b) After millions of years, the amount of carbon-14 remaining in a sample would be too small to measure accurately [1 mark]. The half-life of carbon-14 is relatively short compared to millions of years, so nearly all the carbon-14 would have decayed [1 mark].

15. Control rods (made of boron or cadmium) are inserted into the reactor core to absorb neutrons [1 mark]. By adjusting the depth of the control rods, the number of neutrons available to cause further fission reactions is controlled, thus controlling the rate of the chain reaction [1 mark].

Section D: Uses and Hazards of Radioactivity (10 marks)

16. (a) Any valid medical use, e.g.: - Iodine-131 for treating thyroid cancer / investigating thyroid function [1 mark] - Technetium-99m for medical imaging/tracing [1 mark] - Explanation of half-life suitability: The half-life is short enough that the patient is not exposed to radiation for an unnecessarily long time, but long enough for the diagnostic procedure or treatment to be completed [1 mark]. [Accept other valid examples with correct half-life reasoning]

(b) Alpha radiation has very low penetrating power and cannot pass through skin [1 mark]. An alpha emitter injected into the body would not be detectable outside the body, making it useless as a tracer for external detection [1 mark].

17. (a) Number of chest X-rays = 10.0 ÷ 0.1 = 100 [1 mark]

(b) At high altitudes, the atmosphere is thinner and provides less shielding from cosmic radiation from space [1 mark]. Airline crew spend many hours at high altitudes, so they receive a higher cumulative dose of cosmic radiation compared to people at ground level [1 mark].

18. Any two valid methods:

1. Stored in sealed containers and buried deep underground in geologically stable areas [1 mark].
2. Vitrification (mixing waste with glass-forming materials to create a stable solid) before storage [1 mark]. [Accept other valid methods such as cooling in storage ponds, reprocessing, etc.]

19. A smoke detector needs to function reliably for many years without replacement [1 mark]. A source with a long half-life will maintain a relatively constant activity over the detector's lifetime, ensuring consistent performance [1 mark].

20. Disagree. While radioactive materials pose risks such as radiation sickness and cancer, their benefits in medicine (diagnosis and treatment), industry (non-destructive testing, smoke detectors), and energy generation are significant [1 mark]. With proper safety precautions, regulations, and waste management, the risks can be minimised, making the benefits outweigh the risks [1 mark]. [Accept any well-reasoned argument]

END OF ANSWER KEY