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Secondary 4 Pure Physics Modern Physics Quiz
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Questions
Secondary 4 Pure Physics Quiz - Modern Physics
Name: ___________________________
Class: ___________________________
Date: ___________________________
Score: ________ / 40
Duration: 50 minutes
Total Marks: 40
Instructions:
- Answer ALL questions.
- Show all working clearly for calculation questions. Marks are awarded for correct method even if the final answer is wrong.
- Write your answers in the spaces provided.
- The number of marks for each question is shown in brackets [ ].
- You may use a calculator.
Section A: Multiple Choice Questions (10 marks)
Questions 1–10: Choose the most correct answer. Each question carries 1 mark.
1. Which of the following best describes the nature of radioactive decay?
(a) It can be sped up by increasing temperature
(b) It is a random and spontaneous process
(c) It only occurs in man-made elements
(d) It always emits gamma rays only
Answer: ___________
2. A radioactive source emits radiation that is stopped by a thin sheet of aluminium but passes through paper. The radiation is most likely:
(a) Alpha particles
(b) Beta particles
(c) Gamma rays
(d) Neutrons
Answer: ___________
3. The half-life of a radioactive isotope is 8 hours. What fraction of the original sample remains after 24 hours?
(a) 1/2
(b) 1/4
(c) 1/8
(d) 1/16
Answer: ___________
4. In a nuclear fission reaction, energy is released because:
(a) The total mass of the products is greater than the total mass of the reactants
(b) The binding energy per nucleon of the products is greater than that of the reactants
(c) The number of protons increases during the reaction
(d) Electrons are converted into energy
Answer: ___________
5. Which particle is NOT emitted during standard radioactive decay?
(a) Alpha particle
(b) Beta particle
(c) Proton
(d) Gamma photon
Answer: ___________
6. A detector placed near a radioactive source records 640 counts per minute. After 3 half-lives, the count rate will be approximately:
(a) 320 counts/min
(b) 160 counts/min
(c) 80 counts/min
(d) 40 counts/min
Answer: ___________
7. Nuclear fusion requires extremely high temperatures because:
(a) The strong nuclear force only acts at high temperatures
(b) Nuclei must have enough kinetic energy to overcome electrostatic repulsion
(c) Electrons must be stripped from atoms first
(d) Gamma radiation is only produced at high temperatures
Answer: ___________
8. An alpha particle is identical to:
(a) A hydrogen nucleus
(b) A helium nucleus
(c) Two protons and two electrons
(d) A high-energy electron
Answer: ___________
9. Which of the following equations correctly represents alpha decay of uranium-238?
(a) ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
(b) ²³⁸₉₂U → ²³⁸₉₃Np + ⁰₋₁e
(c) ²³⁸₉₂U → ²³⁴₉₁Pa + ⁴₂He
(d) ²³⁈₉₂U → ²³⁸₉₀Th + ⁰₋₁e
Answer: ___________
10. The SI unit of absorbed radiation dose is:
(a) Becquerel (Bq)
(b) Sievert (Sv)
(c) Gray (Gy)
(d) Curie (Ci)
Answer: ___________
Section B: Short Answer and Structured Questions (20 marks)
Questions 11–16: Answer in the spaces provided.
11. State two properties of beta particles. [2]
(i) _______________________________________________________________
(ii) _______________________________________________________________
12. Define the term half-life of a radioactive substance. [2]
13. A sample of a radioactive isotope has an initial activity of 400 Bq. After 12 days, the activity drops to 50 Bq. Calculate the half-life of the isotope. [3]
Working:
Answer: half-life = ____________ days
14. Complete the following nuclear equation for beta decay: [2]
¹⁴₆C → ________ + ⁰₋₁e + ̅ν
Show the atomic number and mass number of the daughter nucleus.
Answer: ____________
15. Explain why gamma radiation is more penetrating than alpha radiation. [2]
16. Distinguish between nuclear fission and nuclear fusion. Give one example of where each process occurs naturally or is used. [3]
Fission: _______________________________________________________________
Fusion: _______________________________________________________________
Section C: Calculation and Application Questions (10 marks)
Questions 17–20: Show all working clearly.
17. A radioactive source has a half-life of 5 hours. At time t = 0, the sample contains 1.28 × 10²⁰ atoms.
(a) Calculate the number of atoms remaining after 15 hours. [2]
Working:
Answer: ____________ atoms
(b) Calculate the number of atoms that have decayed. [1]
Working:
Answer: ____________ atoms
18. A patient is given a medical tracer containing technetium-99m, which has a half-life of 6 hours. The initial activity of the tracer is 800 MBq.
(a) Calculate the activity of the tracer after 18 hours. [2]
Working:
Answer: ____________ MBq
(b) Explain why a short half-life is desirable for medical tracers. [1]
19. In a nuclear power plant, uranium-235 undergoes fission when struck by a slow neutron. One possible reaction is:
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n
(a) Verify that the equation is balanced by showing that both mass number and atomic number are conserved. [2]
Mass number: _______________________________________________________________
Atomic number: _______________________________________________________________
(b) Explain why this reaction releases energy, making reference to binding energy per nucleon. [2]
20. A detector is placed 2.0 m from a radioactive source. The count rate recorded is 200 counts per second.
(a) The detector is now moved to a distance of 6.0 m from the same source. Assuming the inverse square law applies, calculate the new count rate. [2]
Working:
Answer: ____________ counts/s
(b) State one assumption made when applying the inverse square law to this situation. [1]
Answers
Secondary 4 Pure Physics Quiz - Modern Physics
Answer Key
Section A: Multiple Choice Questions (10 marks)
1. (b) It is a random and spontaneous process
Marking note: Radioactive decay is spontaneous and random; it cannot be influenced by temperature, pressure, or chemical bonding.
2. (b) Beta particles
Marking note: Beta particles penetrate paper but are stopped by a few mm of aluminium. Alpha is stopped by paper; gamma passes through aluminium.
3. (c) 1/8
Working: 24 hours ÷ 8 hours = 3 half-lives. Fraction remaining = (1/2)³ = 1/8.
4. (b) The binding energy per nucleon of the products is greater than that of the reactants
Marking note: Energy is released when the products are more tightly bound (higher binding energy per nucleon). The mass defect is converted to energy via E = mc².
5. (c) Proton
Marking note: Standard radioactive decay emits alpha particles, beta particles, and gamma photons. Proton emission is extremely rare and not considered standard.
6. (c) 80 counts/min
Working: After 1 half-life: 320; after 2 half-lives: 160; after 3 half-lives: 80 counts/min.
7. (b) Nuclei must have enough kinetic energy to overcome electrostatic repulsion
Marking note: At high temperatures, nuclei have sufficient kinetic energy to overcome the Coulomb repulsion between positively charged nuclei so that the strong nuclear force can bind them together.
8. (b) A helium nucleus
Marking note: An alpha particle consists of 2 protons and 2 neutrons, identical to a helium-4 nucleus (⁴₂He).
9. (a) ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Marking note: In alpha decay, mass number decreases by 4 and atomic number decreases by 2. 238 − 4 = 234; 92 − 2 = 90. The daughter nucleus is thorium-234.
10. (c) Gray (Gy)
Marking note: The gray (Gy) is the SI unit of absorbed dose (1 Gy = 1 J/kg). The sievert (Sv) is the unit of dose equivalent. The becquerel (Bq) measures activity.
Section B: Short Answer and Structured Questions (20 marks)
11. [2 marks — 1 mark each]
Any two of the following:
- Beta particles are high-speed electrons.
- They have a charge of −1 (or −1.6 × 10⁻¹⁹ C).
- They have negligible mass compared to alpha particles.
- They are more penetrating than alpha but less penetrating than gamma.
- They can be stopped by a few millimetres of aluminium.
Marking note: Award 1 mark for each correct property, maximum 2 marks.
12. [2 marks]
The half-life of a radioactive substance is the time taken for half of the radioactive nuclei in a sample to decay.
OR
The time taken for the activity of a radioactive sample to decrease to half of its original value.
Marking note: Must convey the idea of "half" and "time." Award 2 marks for a complete definition, 1 mark for a partially correct answer.
13. [3 marks]
Working:
- Initial activity = 400 Bq; final activity = 50 Bq
- 400 → 200 → 100 → 50 (3 half-lives)
- 3 half-lives = 12 days
- Half-life = 12 ÷ 3 = 4 days
Marking note: Award 1 mark for identifying 3 half-lives, 1 mark for correct division, 1 mark for correct answer with unit.
Answer: half-life = 4 days
14. [2 marks]
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ̅ν
Working:
- Mass number: 14 = A + 0 → A = 14
- Atomic number: 6 = Z + (−1) → Z = 7
- Element with Z = 7 is nitrogen (N)
Marking note: Award 1 mark for correct mass number (14), 1 mark for correct atomic number and element symbol (₇N).
15. [2 marks]
Gamma radiation is electromagnetic radiation (photons) with no mass and no charge, so it interacts very weakly with matter and is highly penetrating. Alpha particles are massive, charged helium nuclei that interact strongly with matter through ionisation and are therefore easily stopped.
Marking note: Award 1 mark for stating gamma is uncharged/electromagnetic, 1 mark for stating alpha is charged/massive and interacts strongly. Comparison must be made for full marks.
16. [3 marks]
Fission [1.5 marks]: A heavy nucleus splits into two or more lighter nuclei, releasing energy. Occurs in nuclear power plants / nuclear reactors.
Fusion [1.5 marks]: Two light nuclei combine to form a heavier nucleus, releasing energy. Occurs in the Sun / stars / hydrogen bombs.
Marking note: Award 1 mark for each correct definition and 0.5 mark for each correct example.
Section C: Calculation and Application Questions (10 marks)
17. [3 marks total]
(a) [2 marks]
Working:
- Number of half-lives: 15 ÷ 5 = 3 half-lives
- N = N₀ × (1/2)ⁿ = 1.28 × 10²⁰ × (1/2)³
- N = 1.28 × 10²⁰ × 1/8
- N = 1.6 × 10¹⁹ atoms
Marking note: Award 1 mark for correct number of half-lives (3), 1 mark for correct substitution and answer.
(b) [1 mark]
Working:
- Number decayed = N₀ − N = 1.28 × 10²⁰ − 1.6 × 10¹⁹
- = 12.8 × 10¹⁹ − 1.6 × 10¹⁹ = 1.12 × 10²⁰ atoms
Answer: 1.12 × 10²⁰ atoms
18. [3 marks total]
(a) [2 marks]
Working:
- Number of half-lives: 18 ÷ 6 = 3 half-lives
- A = A₀ × (1/2)ⁿ = 800 × (1/2)³
- A = 800 × 1/8 = 100 MBq
Marking note: Award 1 mark for correct number of half-lives, 1 mark for correct answer with unit.
(b) [1 mark]
A short half-life means the radioactivity decays quickly, minimising the patient's exposure to harmful radiation after the scan/procedure is complete.
Marking note: Must refer to reduced exposure / reduced harm to patient.
19. [4 marks total]
(a) [2 marks]
Mass number conservation:
Left side: 235 + 1 = 236
Right side: 141 + 92 + 3(1) = 141 + 92 + 3 = 236 ✓
Atomic number conservation:
Left side: 92 + 0 = 92
Right side: 56 + 36 + 3(0) = 56 + 36 = 92 ✓
Marking note: Award 1 mark for each correct verification.
(b) [2 marks]
The products (barium-141 and krypton-92) have a higher binding energy per nucleon than the original uranium-235 nucleus. This means the products are more stable. The increase in total binding energy corresponds to a mass defect, and this lost mass is converted into energy according to E = mc².
Marking note: Award 1 mark for reference to higher binding energy per nucleon of products, 1 mark for linking mass defect to energy release (E = mc²).
20. [3 marks total]
(a) [2 marks]
Working:
- Inverse square law: I ∝ 1/d²
- I₂ / I₁ = (d₁ / d₂)²
- I₂ / 200 = (2.0 / 6.0)² = (1/3)² = 1/9
- I₂ = 200 / 9 = 22.2 counts/s (or 22 counts/s to 2 s.f.)
Marking note: Award 1 mark for correct ratio setup, 1 mark for correct answer.
(b) [1 mark]
Any one of:
- The source is a point source.
- There is no absorption or scattering of radiation by the air between the source and detector.
- The detector is small compared to the distance from the source.
Marking note: Award 1 mark for any valid assumption.