Secondary 3 Physics Modern Physics Quiz

Secondary 3 Physics Quiz - Modern Physics

Answer Key

Section A: Multiple Choice Questions (10 marks)

1. (b) The spontaneous and random decay of unstable nuclei, emitting radiation
Marking note: Award 1 mark for correct answer only.

2. (b) Helium
Marking note: An alpha particle consists of 2 protons and 2 neutrons, identical to a helium-4 nucleus.

3. (c) Gamma (γ)
Marking note: Gamma rays are electromagnetic waves with no charge and very high penetrating power.

4. (c) 1/8
Working: Number of half-lives = 24 ÷ 8 = 3. Fraction remaining = (1/2)³ = 1/8.
Marking note: Award 1 mark for correct answer only.

5. (b) Beta particles are high-energy electrons emitted from the nucleus.
Marking note: Beta particles are electrons (charge –1) produced when a neutron converts to a proton within the nucleus.

6. (a) 90
Working: Alpha particle has mass number 4 and atomic number 2. New atomic number = 92 – 2 = 90.
Marking note: Award 1 mark for correct answer only.

7. (c) Sterilisation of medical equipment
Marking note: Gamma rays kill bacteria and are used to sterilise surgical instruments and food.

8. (a) 25 counts per second
Working: After 1 half-life: 100; after 2 half-lives: 50; after 3 half-lives: 25 counts per second.
Marking note: Award 1 mark for correct answer only.

9. (b) Alpha and beta are deflected in opposite directions; gamma is undeflected.
Marking note: Alpha is positively charged, beta is negatively charged, so they deflect in opposite directions. Gamma has no charge and is undeflected.

10. (b) A heavy nucleus splits into two or more lighter nuclei, releasing energy.
Marking note: Award 1 mark for correct answer only.

Section B: Short Answer and Structured Questions (20 marks)

11. [2 marks]
Any two of the following (1 mark each):

• Positively charged (charge of +2)
• Consist of 2 protons and 2 neutrons (identical to a helium nucleus)
• Relatively massive compared to beta and gamma
• Low penetrating power (stopped by paper or a few cm of air)
• Strongly ionising
• Deflected by electric and magnetic fields

Marking note: Award 1 mark per correct property, maximum 2 marks.

12. [3 marks]
Working:

• Number of half-lives elapsed = 15 ÷ 5 = 3 half-lives
• After 1st half-life: 640 ÷ 2 = 320 Bq
• After 2nd half-life: 320 ÷ 2 = 160 Bq
• After 3rd half-life: 160 ÷ 2 = 80 Bq

Or using formula: Activity = 640 × (1/2)³ = 640 × 1/8 = 80 Bq

Answer: 80 Bq

Marking note: Award 1 mark for correct number of half-lives (3), 1 mark for correct method/substitution, 1 mark for correct final answer with unit.

13. [2 marks]
X = ⁰₋₁e (or ⁰₋₁β, a beta particle / electron)

Working:

• Conservation of mass number: 234 = 234 + A → A = 0
• Conservation of atomic number: 90 = 91 + Z → Z = –1
• Therefore X is a beta particle (⁰₋₁e)

Marking note: Award 2 marks for correct answer. Award 1 mark if student shows correct working but writes an incorrect symbol.

14. [2 marks]

• Lead is a dense material with high atomic number that absorbs/blocks radiation effectively. [1]
• This prevents harmful radiation from escaping and protects people from exposure. [1]

Marking note: Award 1 mark for stating lead absorbs/blocks radiation, 1 mark for linking to safety/protection from exposure.

15. [3 marks]
Application (1 mark): Any one of:

• As a tracer in the body (e.g., Iodine-131 to study thyroid function)
• Radiotherapy / treatment of cancer using gamma rays

Explanation (2 marks):

• A short half-life means the radioactivity decreases rapidly. [1]
• This minimises the patient's exposure to harmful radiation and reduces the risk of damage to healthy cells/tissues. [1]

Marking note: Award 1 mark for a valid medical application. Award up to 2 marks for explanation linking short half-life to reduced radiation exposure/risk.

16. (a) [2 marks]
Working:

• At t = 0, count rate = 800 counts/min
• At t = 10 min, count rate = 400 counts/min (half of 800)
• The count rate halves every 10 minutes

Answer: 10 minutes

Marking note: Award 1 mark for identifying that the count rate halves, 1 mark for stating the half-life as 10 minutes. Accept any valid method using the data (e.g., 400→200 also takes 10 min).

(b) [1 mark]
Any one of:

• Cosmic rays from outer space
• Radioactive materials in the ground/rocks (e.g., radon gas)
• Radioactive materials in building materials
• Medical/industrial sources in the environment

Marking note: Award 1 mark for any valid source of background radiation.

Section C: Extended Response and Application (10 marks)

17. (a) [3 marks]
The source emits both beta and gamma radiation. [1]

• The count rate drops only slightly when paper is placed in the way (600 → 580), indicating a penetrating radiation (gamma) passes through. [1]
• The count rate drops significantly with aluminium (to 200), indicating beta particles are being absorbed by the aluminium. [1]
• The count rate drops further with lead (to 40), showing that even gamma rays are partially absorbed by the dense lead.

Marking note: Award 1 mark for identifying both beta and gamma. Award 1 mark each for explaining the evidence from the data for each type of radiation.

(b) [1 mark]
Any one of:

• Use tongs / keep the source at arm's length to minimise exposure
• Do not point the source at people
• Limit the time of exposure
• Wear protective clothing / use a lead shield

Marking note: Award 1 mark for any valid safety precaution.

18. (a) [2 marks]
Nuclear fission is the splitting of a heavy nucleus into two (or more) lighter nuclei, [1] accompanied by the release of a large amount of energy. [1]

Marking note: Award 1 mark for "splitting of a heavy nucleus," 1 mark for "into lighter nuclei + energy released."

(b) [2 marks]

• During fission, a small amount of mass is converted into a very large amount of energy. [1]
• This is described by Einstein's equation E = mc², where even a tiny mass defect (m) produces enormous energy because c² (speed of light squared) is a very large number. [1]

Marking note: Award 1 mark for mentioning mass converted to energy / mass defect, 1 mark for reference to E = mc² or the large value of c².

(c) [2 marks]
Advantage (1 mark): Any one of:

• Produces a large amount of energy from a small amount of fuel
• Does not produce carbon dioxide / greenhouse gases during operation
• Reliable baseload power source

Disadvantage (1 mark): Any one of:

• Produces radioactive waste that is difficult to dispose of safely
• Risk of nuclear accidents (e.g., meltdown)
• High cost of building and decommissioning nuclear plants

Marking note: Award 1 mark each for a valid advantage and disadvantage.

19. (a) [3 marks]
Working:

• Activity remaining = 25% = 1/4 of original
• 1/4 = (1/2)², so 2 half-lives have elapsed
• Age = 2 × 5730 = 11,460 years

Answer: 11,460 years

Marking note: Award 1 mark for determining that 25% = 1/4 = (1/2)², 1 mark for identifying 2 half-lives, 1 mark for correct final answer with unit.

(b) [1 mark]
After about 50,000 years (approximately 8–9 half-lives), the remaining amount of Carbon-14 is so small that it is too difficult to measure accurately / is indistinguishable from background radiation.

Marking note: Award 1 mark for stating that too little Carbon-14 remains to measure accurately.

20. (a) [2 marks]
Isotopes are atoms of the same element (same number of protons / same atomic number) [1] that have different numbers of neutrons (different mass numbers). [1]

Marking note: Award 1 mark for "same element/same number of protons," 1 mark for "different number of neutrons/different mass number."

(b) [2 marks]
Similarity (1 mark): Both have the same number of protons (6) / same atomic number / same chemical properties / same electron configuration.

Difference (1 mark): They have different numbers of neutrons (6 vs. 8) / different mass numbers (12 vs. 14) / Isotope B is radioactive while Isotope A is stable.

Marking note: Award 1 mark each for a valid similarity and difference.

(c) [2 marks]
$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$

Working:

• Conservation of mass number: 14 = 14 + 0 ✓
• Conservation of atomic number: 6 = 7 + (–1) ✓
• A neutron converts to a proton, emitting a beta particle (electron). The resulting element has atomic number 7, which is Nitrogen.

Marking note: Award 1 mark for correct reactant and product nuclei, 1 mark for correct beta particle. Accept ⁰₋₁β as alternative notation.