O Level Physics Modern Physics Quiz

O-Level Physics Quiz - Modern Physics (Answer Key)

Total Marks: 40
Topic: Modern Physics (Radioactivity / nuclear model)
Note: Content generated from syllabus-first LLM templates (Stage 4/5). Past-paper evidence for this topic was very weak (0.2% of corpus); this quiz is NOT claimed to be exam-derived.

Section A: Multiple-Choice (1 mark each)

1. C – Gamma ray.
Teaching note: Gamma rays are electromagnetic waves (photons) with no rest mass and no charge. Alpha = 2 protons + 2 neutrons (charge +2). Beta = electron (charge –1). Proton is a nucleon, not a radiation type here.

2. B – half the nuclei present to decay.
Teaching note: Half-life is defined as the time for the activity (or number of undecayed nuclei) to fall to half its initial value. Not all decay at once.

3. B – Beta.
Teaching note: Charged particles deflect in magnetic fields. Beta (electron, low mass, charge –1) deflects more than alpha (heavy, charge +2) and gamma (no charge) does not deflect.

4. B – 146.
Method: Neutrons = mass number – proton number = 238 – 92 = 146.

5. B – Using gamma rays to sterilise medical equipment.
Teaching note: Gamma sterilisation is a safe controlled industrial use. Radium paint and food irradiation for flavour are not safe/approved everyday uses at O-Level.

Section B: Structured Short Answers (2 marks each)

6. [1 mark each]
Alpha: positively charged, heavy, low penetration, high ionising. (any one)
Gamma: no charge/mass, high penetration, low ionising. (any one)

7. Half-life = 4 days; 8 days = 2 half-lives.
After 1st: 800 ÷ 2 = 400 counts/s
After 2nd: 400 ÷ 2 = 200 counts/s
Answer: 200 counts/s [2 marks: 1 for 2 half-lives, 1 for correct value]

8. Missing particle is alpha: $_{2}^{4}\text{He}$ (or $\alpha$).
Check: 222 = 218 + 4; 86 = 84 + 2. [2 marks]

9. Beta particles are electrons emitted when a neutron in the nucleus changes into a proton and an electron:
$n \rightarrow p + e^{-} + \bar{\nu}$. The electron is ejected from the nucleus. [2 marks]

10. Beta stopper: aluminium (few mm). Gamma stopper: thick lead/concrete. [1+1]

11. Mainly beta radiation.
Reason: Aluminium stops alpha but not beta; sharp drop means beta was being blocked. [1+1]
Image note: Source→aluminium→GM tube; with aluminium, count drops → beta.

12. Charge: +2 (or +2e). Mass: 4 (approx 4 × proton). [1+1]

13. After 3 half-lives, fraction remaining = $(1/2)^3 = 1/8$.
Remaining = $1.2 \times 10^{20} \div 8 = 1.5 \times 10^{19}$. [2 marks]

14. Lead absorbs gamma and reduces exposure; protects user from radiation. [2 marks]

15. Natural: cosmic rays / rocks / radon gas. Artificial: medical X-rays / nuclear tests. [1+1]

Section C: Extended Responses (3 marks each)

16. [3 marks]

• Place source at fixed distance from GM tube, record counts over time intervals. (1)
• Subtract background count, plot activity vs time. (1)
• Find time for count to halve repeatedly; average gives half-life. (1)

17. [3 marks]
Alpha: least penetration (paper), most ionising.
Beta: medium (aluminium), medium ionising.
Gamma: most penetration (lead), least ionising. (1 mark each)

18. 6 h = 3 half-lives (2 h each).
Activity = $1200 \times (1/2)^3 = 1200 \div 8 = 150$ Bq. [2 for working, 1 for unit/answer]
Answer: 150 Bq

19. [3 marks]
Smoke detector has Am-241 alpha source ionising air between plates, causing small current. (1) Smoke enters, absorbs alpha, current drops, alarm sounds. (1) Alpha suitable: short range, safe if enclosed, strong ioniser. (1)

20. [3 marks]
U-235 absorbs neutron, splits into two smaller nuclei + 2–3 neutrons + energy. (1) Released neutrons cause further fissions. (1) If each fission releases enough neutrons to sustain next, chain reaction continues. (1)