Secondary 4 Pure Physics Modern Physics Quiz

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.