Secondary 4 Pure Physics Modern Physics Quiz

Secondary 4 Pure Physics Quiz - Modern Physics (Answer Key)

Total Marks: 40
Topic: Modern Physics

Section A

1. [1] The atomic number (proton number) of the daughter nucleus decreases by 2, but a property unchanged could be "the chemical element changes" – better: "number of neutrons decreases by 2" is changed. Acceptable unchanged property: none of composition is unchanged; however, mass number decreases by 4. A correct unchanged property: "the total charge of the universe" is too broad. For Sec 4 recall: state "the element is transformed" is not unchanged. Simplest accepted: "Alpha decay does not change the fact that it is a nucleus" – but exam expects: "The number of protons in the original nucleus is not a property that remains; instead, the property of being radioactive remains." Per syllabus, a valid answer: "It remains a radioactive substance."
Teaching note: Alpha decay emits $^4_2\text{He}$; the parent nucleus loses 2p and 2n. The only thing "unchanged" in a trivial sense is that the material is still matter/nucleus. For strict marking, accept "the substance is still composed of nuclei" or "radioactivity continues in daughter".
Answer: It remains a nucleus / still radioactive. [1]

2. [1] Alpha radiation.
Teaching note: Alpha particles are heavy and highly ionising; paper stops them. Beta needs aluminium; gamma needs lead.

3. [1] Alpha decay.
Teaching note: Emission of helium nucleus ($^4_2\text{He}$) is alpha decay.

4. [1] Sievert (Sv).
Teaching note: Dose equivalent uses Sv; absorbed dose uses Gray (Gy).

5. [1] Fusion.
Teaching note: Sun fuses hydrogen to helium, releasing energy.

Section B

6. (a) [1] Half-life is the time taken for half the radioactive nuclei in a sample to decay.
(b) [2] 12 h ÷ 4 h = 3 half-lives. Remaining = 800 × (1/2)³ = 800 ÷ 8 = 100 nuclei.
Working: $N = N_0 (1/2)^{t/T_{1/2}} = 800 × (1/2)^3 = 100$.
Answer: 100 nuclei.

7. [2] Count rate = total counts ÷ time = 240 ÷ (2×60 s) = 240 ÷ 120 = 2.0 s⁻¹.
Answer: 2.0 counts per second (s⁻¹).

8. [2] Beta particles have much smaller mass (electron mass) and may be negative or positive; alpha are heavy (4 u). Same field exerts same force magnitude per charge, but acceleration = F/m, so beta accelerate far more → larger deflection. Also beta often single charge, alpha double, but mass difference dominates.
Marking: 1 mark for smaller mass, 1 mark for greater acceleration/deflection.

9. [2] Any two: electromagnetic wave; no mass/no charge; highly penetrating; travels at speed of light; stopped by thick lead/concrete.
(2 marks, 1 each)

10. [3] After 3 half-lives, source count = 320 ÷ 2³ = 320 ÷ 8 = 40 s⁻¹. Subtract background: 40 – 20 = 20 s⁻¹.
Answer: 20 s⁻¹.
Marks: 1 for halving thrice, 1 for subtract background, 1 final.

11. [1] 2 protons and 2 neutrons.

12. [2] $^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^4_2\text{He}$.
Check: A: 238 = 234+4 ✓; Z: 92 = 90+2 ✓.

13. [2] Vapour in chamber condenses along ion trails left by radiation; alpha makes short dense tracks, beta thin wandering tracks.
Marks: 1 for ionisation trail, 1 for track difference.

14. [2] 24 days ÷ 8 = 3 half-lives. Remaining = 16 mg × (1/2)³ = 16 ÷ 8 = 2.0 mg.
Answer: 2.0 mg.

15. [1] Use tongs / store in lead container / limit exposure time / wear badge. (any one)

Section C

16. (a) [1] From table, count halves every 2 h → half-life = 2 hours.
(b) [2] Corrected rates: 390, 190, 90, 40 at t=0,2,4,6. Plot on provided grid with axes labelled.
(c) [1] At 3 h, interpolate between (2,190) and (4,90): ~140 s⁻¹.
Image note: Graph must show points and smooth decay curve.

17. [4] Fission: heavy nucleus splits (e.g., U-235 + n → Ba + Kr + 3n); Fusion: light nuclei join (e.g., H + H → He). Differences: fission uses heavy elements, fusion light; fission chain reaction, fusion needs high T/pressure; example each = 1 mark each, difference = 2 marks.
Answer: Fission: split of U-235; Fusion: join of H; contrast mass and conditions.

18. [3] Alpha ionises air in chamber creating current; smoke particles absorb alpha, reducing current, triggering alarm. Suitable because alpha weak range, safe in device.
Marks: 1 alpha ionises, 1 smoke blocks, 1 safety/range.

19. (a) [2] Round1: 500; Round2: 250; Round3: 125 remain.
(b) [2] Each round ~half removed, models constant probability decay and half-life.
Image: Tray shows coins, heads removed.

20. (a) [1] Neutron absorption by U-235.
(b) [2] Moderator slows fast neutrons to thermal speeds to sustain chain.
(c) [1] Control rods absorb neutrons to regulate rate.

Common mistakes flagged:

• Forgetting background subtraction (Q10, Q16).
• Using wrong half-life count (Q6, Q14).
• Confusing alpha/beta penetration (Q2, Q9).