Secondary 3 Physics Modern Physics Quiz

Secondary 3 Physics Quiz - Modern Physics

Answer Key

Note: This quiz is syllabus-aligned practice content generated to complement the Secondary 3 Physics curriculum. It is not derived directly from past-year exam papers.

Section A: Multiple Choice

1. B

• Radioactivity is the spontaneous emission of radiation from an unstable nucleus. It is not related to electron transitions (which produce light/X-rays) or nuclear collisions (which are induced reactions).
• Common mistake: Choosing A — students sometimes confuse radioactivity with electron emission from electron shells.

[1]

2. B

• An alpha particle is identical to a helium-4 nucleus: 2 protons and 2 neutrons, giving it a charge of +2 and a mass number of 4.
• Common mistake: Choosing A (confusing alpha with beta) or C (confusing alpha with gamma).

[1]

3. C

• Gamma rays are high-energy electromagnetic waves with no charge and very high penetrating power. Alpha particles are the least penetrating (stopped by paper), and beta particles are moderately penetrating (stopped by a few mm of aluminium).

[1]

4. B

• 18 hours ÷ 6 hours = 3 half-lives.
• After 1 half-life: 80 ÷ 2 = 40 g
• After 2 half-lives: 40 ÷ 2 = 20 g
• After 3 half-lives: 20 ÷ 2 = 10 g
• Common mistake: Students may divide 80 by 3 (the number of half-lives) instead of halving three times.

[1]

5. C

• In nuclear fission, the total mass of the products is slightly less than the mass of the original nucleus. This "mass defect" is converted into energy according to Einstein's equation E = mc². This is the principle behind nuclear power.
• Common mistake: Choosing A — students may not appreciate that mass is lost (converted to energy).

[1]

Section B: Short Answer and Structured Response

6. Any two of the following:

• It is a high-energy electron (₋₁⁰e or ₋₁⁰β). [1]
• It has a charge of −1 (or −1.6 × 10⁻¹⁹ C). [1]
• It has negligible mass compared to a proton/neutron (mass number ≈ 0). [1]
• It is emitted from the nucleus when a neutron converts into a proton. [1]
• It is moderately penetrating (stopped by a few mm of aluminium). [1]
• It is deflected by electric and magnetic fields (because it is charged). [1]

[2] — 1 mark per correct property

7. The half-life of a radioactive substance is the time taken for half of the radioactive nuclei in a sample to decay. [1] Equivalently: the time taken for the count rate (or activity) of a sample to fall to half its original value. [1]

[2] — Both marks for a complete definition. Award 1 mark for a partially correct definition (e.g., only mentioning "time for half to decay" without specifying nuclei or count rate).

8. (a) The undeflected beam represents gamma radiation. [1]

• Gamma radiation has no charge, so it is not deflected by a magnetic field.

(b) The deflected beam represents alpha radiation. [1]

• Alpha particles are positively charged (helium nuclei), so they experience a force in a magnetic field and are deflected. [1]
• (By Fleming's left-hand rule, the direction of deflection confirms the positive charge.)

[3] — 1 mark for correct identification in (a); 1 mark for identifying alpha in (b); 1 mark for explaining the deflection due to charge.

9. $^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$

• Mass number: 226 − 4 = 222
• Atomic number: 88 − 2 = 86
• Element with Z = 86 is Rn (radon)

[2] — 1 mark for correct mass number (222); 1 mark for correct atomic number and element symbol (₈₆Rn). Award 1 mark if only one of A or Z is correct.

10. (a) Number of half-lives = 24 ÷ 8.0 = 3 half-lives [1]

• After 1 half-life: 16 ÷ 2 = 8 mg
• After 2 half-lives: 8 ÷ 2 = 4 mg
• After 3 half-lives: 4 ÷ 2 = 2 mg [1]

(b) Any suitable reason, e.g.:

• Radioactive materials emit ionising radiation that can damage living cells/DNA. [1]
• Prolonged exposure can cause radiation sickness or increase cancer risk. [1]

[3] — 2 marks for calculation; 1 mark for explanation.

11. Use: Any one of:

• To trace blood flow / detect blockages in blood vessels. [1]
• To monitor the function of organs (e.g., thyroid, kidneys).
• To detect tumours / cancer.

Explanation: A short half-life means the radioactivity decays quickly, [1] so the patient is exposed to radiation for only a short time, reducing the risk of cell damage / harmful side effects. [1]

[2] — 1 mark for a valid use; 1 mark for explanation linking short half-life to reduced exposure/risk.

12.

[3] — 1 mark per correct row. Award marks independently for each correct cell.

13. (a) From the table:

• At t = 0, count rate = 800 cpm
• At t = 2, count rate = 400 cpm (= ½ × 800) [1]
• The count rate halves every 2 hours, so the half-life is 2 hours. [1]

(b) At t = 8, count rate = 50 cpm. After one more half-life (2 hours), at t = 10:

• Count rate = 50 ÷ 2 = 25 counts per minute [1]

[3] — 2 marks for (a) (1 for showing the halving, 1 for the answer); 1 mark for (b).

14. Gamma radiation has very high penetrating power, [1] so it can pass through packaging and the medical equipment itself to kill bacteria and viruses on all surfaces and inside the equipment. [1] Alpha radiation has very low penetrating power (stopped by paper/skin) and would not be able to penetrate the packaging or reach all surfaces of the equipment. [1]

[2] — Award marks for: (1) gamma has high penetrating power; (2) alpha has low penetrating power / cannot penetrate packaging. Both comparison points needed for full marks.

15. Fraction remaining = 1/8 = (1/2)³, so 3 half-lives have passed. [1]

• Age = 3 × 5730 = 17 190 years (or approximately 17 200 years) [1]

[2] — 1 mark for determining 3 half-lives; 1 mark for the correct calculation. Accept 17 190 or 17 200.

Section C: Application and Extended Response

16. (a) Control rods absorb neutrons [1] to control the rate of the chain reaction (by absorbing excess neutrons to prevent the reaction from becoming too fast / runaway).

(b) Function: The moderator slows down fast neutrons [1] so that they are more likely to cause further fission of uranium-235 nuclei. Material: Graphite or water (heavy water / light water). [1]

(c) Energy transfers:

• Nuclear energy (from fission) is converted into thermal energy (heat) in the reactor core. [1]
• The coolant carries this thermal energy away from the core to produce steam. [1]
• The steam drives the turbine, converting thermal energy into kinetic energy. [1]
• The turbine drives the generator, converting kinetic energy into electrical energy. [1]

[5] — 1 mark for (a); 2 marks for (b) (1 for function, 1 for material); 2 marks for (c) (award 1 mark per correct energy transfer step, maximum 2).

17. (a) Years = 20 mSv ÷ 2.0 mSv/year = 10 years [1]

(b) Any two of:

• Wearing lead aprons / lead shielding. [1]
• Using tongs / remote handling tools to keep distance from sources. [1]
• Wearing personal dosimeters to monitor exposure. [1]
• Limiting time spent near radioactive sources. [1]
• Working behind lead/concrete shielding walls. [1]

[3] — 1 mark for (a); 2 marks for (b) (1 per correct safety measure).

18. (a) Conservation of mass number:

• Left side: 235 + 1 = 236
• Right side: 141 + 92 + X(1) = 233 + X
• 236 = 233 + X
• X = 3 [2]

(b) The fission reaction releases neutrons. [1] These neutrons can go on to cause further fission reactions in other uranium-235 nuclei, which release more neutrons, creating a self-sustaining chain reaction. [1]

[4] — 2 marks for (a) (1 for setting up the equation, 1 for the answer); 2 marks for (b) (1 for mentioning neutrons are released, 1 for explaining the chain reaction).

19. Natural sources (any two):

• Radon gas from the ground / rocks / soil. [1]
• Cosmic rays from outer space. [1]
• Radioactive materials in food and drink (e.g., potassium-40 in bananas). [1]
• Radioactive materials in building materials. [1]

Artificial source (any one):

• Medical X-rays / medical procedures. [1]
• Fallout from nuclear weapons testing.
• Nuclear power plant waste / emissions.

[2] — 1 mark per correct source, maximum 2 marks (need at least 1 natural and 1 artificial for full marks; if only natural sources given, maximum 1 mark).

20. (a) Source P emits alpha and beta radiation. [1]

• Paper reduces the count rate significantly (600 → 150), indicating alpha particles are present (alpha is stopped by paper). [1]
• Aluminium further reduces the count rate (150 → 50), indicating beta particles are also present (beta is stopped by aluminium). [1]
• Lead stops all remaining radiation (50 → 0), confirming no gamma emission.

(b) Source Q emits gamma radiation only. [1]

• Paper, aluminium, and even thick lead do not significantly reduce the count rate (600 → 200 with lead), which is consistent with the high penetrating power of gamma rays. [1]
• Gamma rays are not charged, so they are not completely stopped by moderate shielding.

(c) Gamma radiation has very high penetrating power. [1] Even thick lead can only reduce (attenuate) the gamma radiation but cannot completely absorb it all; some gamma photons pass through.

[5] — (a): 2 marks (1 for identifying alpha + beta, 1 for reasoning); (b): 2 marks (1 for identifying gamma, 1 for reasoning); (c): 1 mark for explanation.

Mark Summary

End of Answer Key