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O Level Physics Modern Physics Quiz

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Questions

O-Level Physics Quiz - Modern Physics

Name: _________________________ Class: _________________________ Date: _________________________ Score: ______ / 40

Duration: 45 minutes Total Marks: 40

Instructions:

Answer ALL questions in the spaces provided.
Show all working clearly for calculation questions.
State units in all final answers.
Use g = 10 m/s² where needed.
The number of marks is given in brackets [ ] at the end of each question or part question.

Section A: Short Answer (10 marks)

Answer all questions in this section.

1. State what is meant by the term isotope. [1 mark]

2. A nucleus of uranium-238 is represented as $^{238}_{92}\text{U}$ . State the number of protons and the number of neutrons in this nucleus. [2 marks]

Protons: _______________ Neutrons: _______________

3. Alpha particles, beta particles, and gamma rays are three types of nuclear radiation. State which of these three types of radiation: (a) has the greatest ionising effect, [1 mark] (b) is not deflected by electric or magnetic fields. [1 mark]

(a) _______________ (b) _______________

4. A radioactive source has a half-life of 8 days. Initially, the activity of the source is 1600 counts per second. Calculate the activity of the source after 24 days. [2 marks]

5. State one safety precaution that should be taken when handling a radioactive source in a school laboratory. [1 mark]

Section B: Nuclear Equations and Decay (10 marks)

Answer all questions in this section.

6. Complete the following nuclear equation for the alpha decay of radium-226 ( $^{226}_{88}\text{Ra}$ ): [2 marks]

$^{226}_{88}\text{Ra} \rightarrow$ _______________ $+$ $^{4}_{2}\text{He}$

7. A radioactive sample of carbon-14 undergoes beta decay. The nuclear equation is shown below. $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + \text{X}$ Identify particle X. [1 mark]

8. A radioactive source has a half-life of 5 days. Its initial mass is 80 g. Calculate the mass of the source remaining after 15 days. [2 marks]

9. A student measures the activity of a radioactive source over time. The initial activity is 1200 Bq. After 10 minutes, the activity is 150 Bq. Determine the half-life of the source in minutes. Show your working. [3 marks]

10. Explain why the mass of a radioactive sample does not decrease to zero after many half-lives, even though the number of radioactive nuclei decreases. [2 marks]

Section C: Structured Questions (10 marks)

Answer all questions in this section.

11. A teacher demonstrates the penetrating power of different types of radiation using a radioactive source and a Geiger-Müller (GM) tube connected to a counter.

The teacher places different materials between the source and the GM tube and records the count rate.

Absorber placed between source and GM tube	Count rate (counts per minute)
No absorber	840
Sheet of paper	835
Aluminium sheet (3 mm thick)	340
Lead sheet (10 mm thick)	30

(a) The background count rate is 30 counts per minute. Explain why the count rate with the lead sheet is approximately equal to the background count rate. [2 marks]

(b) Using the data in the table, identify which types of radiation are emitted by the source. Explain your reasoning. [3 marks]

12. Iodine-131 is a radioactive isotope used in medical treatment. It has a half-life of 8 days.

A hospital receives a sample of iodine-131 with an initial activity of 3200 Bq.

(a) Define the term half-life. [1 mark]

(b) Calculate the activity of the iodine-131 sample after 32 days. [2 marks]

(c) Explain why iodine-131 with a half-life of 8 days is suitable for medical treatment, whereas a radioactive isotope with a half-life of several thousand years would not be suitable. [2 marks]

13. A smoke detector contains a small radioactive source that emits alpha particles. The alpha particles ionise the air between two electrodes, allowing a small current to flow. When smoke enters the detector, the current decreases and an alarm sounds.

(a) Explain why alpha particles are used in smoke detectors rather than beta particles or gamma rays. [2 marks]

(b) The radioactive source in the smoke detector is americium-241, which has a half-life of 432 years. Suggest why this long half-life is an advantage for a smoke detector. [1 mark]

14. A student investigates the decay of a radioactive substance using a simulation. The simulation produces the following data for the number of undecayed nuclei over time.

Time (hours)	Number of undecayed nuclei
0	8000
2	5656
4	4000
6	2828
8	2000
10	1414
12	1000
14	707
16	500

(a) Using the data, determine the half-life of this radioactive substance. Show your working. [2 marks]

(b) Use the data to estimate the number of undecayed nuclei after 5 hours. [1 mark]

15. The student repeats the simulation with a different radioactive substance that has a half-life of 3 hours and the same initial number of nuclei (8000).

On the axes below, sketch the decay curve for this second substance. The decay curve for the original substance (half-life 4 hours) has already been drawn. Label your curve clearly. [2 marks]

Number of undecayed nuclei
8000 |
     |
     |    * (original substance curve drawn)
     |       *
     |          *
     |             *
     |                *
     |                   *
     |                      *
     |                         *
     |                            *
     +----------------------------------------> Time (hours)
     0

Section D: Data-Based and Application Questions (10 marks)

Answer all questions in this section.

16. Explain why radioactive decay is described as a random process, even though the half-life of a substance is constant. [2 marks]

17. A radioactive source emits gamma rays and is used to sterilise medical equipment. Explain why gamma rays are suitable for this purpose. [2 marks]

18. Carbon-14 dating is used to estimate the age of ancient organic materials. The half-life of carbon-14 is 5730 years. A sample of ancient wood has a count rate of 15 counts per minute. A similar sample of living wood has a count rate of 60 counts per minute. Calculate the age of the ancient wood. [3 marks]

19. A nuclear power station generates electricity using nuclear fission. State one advantage and one disadvantage of using nuclear power compared to burning fossil fuels. [2 marks]

Advantage: ____________________________________________________________ Disadvantage: _________________________________________________________

20. A student reads that the total mass of the products in a nuclear fission reaction is slightly less than the total mass of the reactants. Explain what happens to this "missing" mass. [1 mark]

END OF QUIZ

Check your answers carefully before submitting.

Answers

O-Level Physics Quiz - Modern Physics — Answer Key

Total Marks: 40

Section A: Short Answer (10 marks)

1. State what is meant by the term isotope. [1 mark]

Answer: Isotopes are atoms of the same element (same proton number) that have different numbers of neutrons (different nucleon number). Award [1] for: same proton number AND different nucleon number/neutron number.

2. A nucleus of uranium-238 is represented as $^{238}_{92}\text{U}$ . State the number of protons and the number of neutrons in this nucleus. [2 marks]

Answer: Protons: 92 [1 mark] Neutrons: 238 − 92 = 146 [1 mark] Accept 146 only if working or correct value shown.

Answer: (a) Alpha particles [1 mark] (b) Gamma rays [1 mark]

4. A radioactive source has a half-life of 8 days. Initially, the activity of the source is 1600 counts per second. Calculate the activity of the source after 24 days. [2 marks]

Answer: Number of half-lives = 24 ÷ 8 = 3 half-lives [1 mark] Activity = 1600 × (½)³ = 1600 × ⅛ = 200 counts per second [1 mark] Accept 200 cps or 200 Bq equivalent. Award [1] for correct number of half-lives even if final answer incorrect.

5. State one safety precaution that should be taken when handling a radioactive source in a school laboratory. [1 mark]

Answer: Any ONE of:

Use tongs/forceps to handle the source (keep distance)
Point the source away from the body/people
Store the source in a lead-lined container when not in use
Wash hands after handling
Do not touch the source directly
Keep exposure time to a minimum Award [1] for any valid safety precaution.

Section B: Nuclear Equations and Decay (10 marks)

6. Complete the following nuclear equation for the alpha decay of radium-226 ( $^{226}_{88}\text{Ra}$ ): [2 marks]

$^{226}_{88}\text{Ra} \rightarrow$ _______________ $+$ $^{4}_{2}\text{He}$

Answer: $^{226}_{88}\text{Ra} \rightarrow$ $^{222}_{86}\text{Rn}$ $+$ $^{4}_{2}\text{He}$ Award [1] for correct mass number (222) and [1] for correct atomic number (86) / correct element symbol (Rn). Accept radon-222 or Rn-222.

7. A radioactive sample of carbon-14 undergoes beta decay. The nuclear equation is shown below. $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + \text{X}$ Identify particle X. [1 mark]

Answer: X is a beta particle (or an electron, $^{0}_{-1}\text{e}$ ). Award [1] for beta particle or electron.

8. A radioactive source has a half-life of 5 days. Its initial mass is 80 g. Calculate the mass of the source remaining after 15 days. [2 marks]

Answer: Number of half-lives = 15 ÷ 5 = 3 half-lives [1 mark] Mass remaining = 80 × (½)³ = 80 × ⅛ = 10 g [1 mark] Accept 10 g. Award [1] for correct number of half-lives even if final answer incorrect.

Answer: 1200 → 600 → 300 → 150 (3 half-lives) [1 mark] Number of half-lives = 3 [1 mark] Half-life = 10 minutes ÷ 3 = 3.33 minutes (or 3 minutes 20 seconds, or 10/3 minutes) [1 mark] Accept alternative methods using the formula. Award marks for correct reasoning and final answer.

10. Explain why the mass of a radioactive sample does not decrease to zero after many half-lives, even though the number of radioactive nuclei decreases. [2 marks]

Answer: The radioactive nuclei decay into daughter nuclei which are still present in the sample [1 mark]. The total mass of the sample includes both the remaining radioactive nuclei and the stable daughter nuclei produced, so the mass does not decrease to zero [1 mark]. Accept any explanation referencing the presence of decay products/daughter nuclei.

Section C: Structured Questions (10 marks)

11. (a) The background count rate is 30 counts per minute. Explain why the count rate with the lead sheet is approximately equal to the background count rate. [2 marks]

Answer: The lead sheet absorbs/blocks (nearly) all the radiation from the source [1 mark]. The remaining count rate (30 counts/min) is due to background radiation only, which is always present [1 mark].

11. (b) Using the data in the table, identify which types of radiation are emitted by the source. Explain your reasoning. [3 marks]

Answer:

Alpha radiation is NOT emitted (or is negligible): The paper sheet causes almost no reduction in count rate (840 to 835), showing that alpha particles (which would be stopped by paper) are not present in significant amounts [1 mark].
Beta radiation IS emitted: The aluminium sheet (3 mm) reduces the count rate significantly (from ~835 to 340), indicating that beta particles are present and are absorbed by the aluminium [1 mark].
Gamma radiation IS emitted: The lead sheet (10 mm) reduces the count rate to background level (30), but some radiation penetrates the aluminium (340 counts remain after aluminium). This penetrating radiation is gamma [1 mark]. Accept alternative valid reasoning. Award marks for correct identification with justification.

12. (a) Define the term half-life. [1 mark]

Answer: The half-life of a radioactive substance is the time taken for half the nuclei (in a sample) to decay / the time taken for the activity of a sample to fall to half its initial value. Award [1] for a clear definition referencing time and half the nuclei/activity.

12. (b) Calculate the activity of the iodine-131 sample after 32 days. [2 marks]

Answer: Number of half-lives = 32 ÷ 8 = 4 half-lives [1 mark] Activity = 3200 × (½)⁴ = 3200 × ¹⁄₁₆ = 200 Bq [1 mark]

12. (c) Explain why iodine-131 with a half-life of 8 days is suitable for medical treatment, whereas a radioactive isotope with a half-life of several thousand years would not be suitable. [2 marks]

Answer: Iodine-131 decays relatively quickly (8 days), so the patient is not exposed to radiation for an unnecessarily long time after treatment [1 mark]. An isotope with a half-life of thousands of years would remain radioactive in the patient's body for a very long time, causing prolonged and unnecessary radiation exposure/damage to healthy tissue [1 mark]. Accept any valid comparison referencing exposure time and patient safety.

13. (a) Explain why alpha particles are used in smoke detectors rather than beta particles or gamma rays. [2 marks]

Answer: Alpha particles have the greatest ionising ability (compared to beta and gamma) [1 mark], so they can effectively ionise the air to maintain a current. They also have very low penetrating power, so they are easily stopped by the detector casing and do not pose a radiation hazard outside the detector [1 mark]. Accept: alpha particles are highly ionising AND have short range/are easily absorbed.

13. (b) The radioactive source in the smoke detector is americium-241, which has a half-life of 432 years. Suggest why this long half-life is an advantage for a smoke detector. [1 mark]

Answer: The source will remain radioactive/effective for the lifetime of the smoke detector (it does not need to be replaced frequently). Accept any answer referencing long-lasting activity/not needing replacement.

14. (a) Using the data, determine the half-life of this radioactive substance. Show your working. [2 marks]

Answer: From the data: at t = 0, N = 8000; at t = 4 h, N = 4000 (half of 8000) [1 mark] Half-life = 4 hours [1 mark] Accept: using any two points where N halves, e.g. 4000 to 2000 takes 4 hours (8 − 4 = 4 h).

14. (b) Use the data to estimate the number of undecayed nuclei after 5 hours. [1 mark]

Answer: After 4 hours, N = 4000. After 6 hours, N = 2828. At 5 hours (midway), N ≈ (4000 + 2828) ÷ 2 = 3414, or approximately 3400 nuclei. Accept any reasonable estimate between 3300 and 3500. Award [1] for a sensible interpolation.

15. The student repeats the simulation with a different radioactive substance that has a half-life of 3 hours and the same initial number of nuclei (8000).

On the axes below, sketch the decay curve for this second substance. The decay curve for the original substance (half-life 4 hours) has already been drawn. Label your curve clearly. [2 marks]

Answer:

Curve should start at (0, 8000) [1 mark]
Curve should fall more steeply than the original, passing through approximately (3, 4000), (6, 2000), (9, 1000), (12, 500) [1 mark]
Curve must be labelled (e.g. "half-life = 3 hours" or "second substance") Award [1] for correct initial point and steeper shape, [1] for correct half-life positions and label.

Section D: Data-Based and Application Questions (10 marks)

16. Explain why radioactive decay is described as a random process, even though the half-life of a substance is constant. [2 marks]

Answer: It is impossible to predict which individual nucleus will decay at a given time / the decay of any particular nucleus is spontaneous and independent of other nuclei [1 mark]. However, with a very large number of nuclei, the overall decay rate follows a predictable statistical pattern, giving a constant half-life [1 mark]. Accept any explanation contrasting random individual events with statistical predictability for large numbers.

17. A radioactive source emits gamma rays and is used to sterilise medical equipment. Explain why gamma rays are suitable for this purpose. [2 marks]

Answer: Gamma rays have high penetrating power, so they can pass through packaging and reach all surfaces of the equipment to kill bacteria/microorganisms [1 mark]. They are also highly energetic and can destroy pathogens effectively without making the equipment radioactive [1 mark]. Accept: highly penetrating AND kills bacteria/sterilises effectively.

Answer: Count rate has halved from 60 to 30 (1 half-life), then from 30 to 15 (2 half-lives) [1 mark] Number of half-lives = 2 [1 mark] Age = 2 × 5730 = 11 460 years [1 mark] Accept alternative methods using the decay formula. Award marks for correct reasoning and final answer.

19. A nuclear power station generates electricity using nuclear fission. State one advantage and one disadvantage of using nuclear power compared to burning fossil fuels. [2 marks]

Answer: Advantage: Any ONE of:

Does not produce greenhouse gases / carbon dioxide (during operation)
Produces a large amount of energy from a small mass of fuel
Fuel reserves (uranium) may last longer than fossil fuels Disadvantage: Any ONE of:
Produces radioactive waste that needs long-term safe storage
Risk of catastrophic accidents (e.g. meltdown) releasing radiation
High initial construction and decommissioning costs Award [1] for a valid advantage and [1] for a valid disadvantage.

20. A student reads that the total mass of the products in a nuclear fission reaction is slightly less than the total mass of the reactants. Explain what happens to this "missing" mass. [1 mark]

Answer: The "missing" mass is converted into energy (according to Einstein's equation E = mc²) / released as kinetic energy of the products and radiation. Award [1] for mass converted to energy or equivalent statement.

END OF ANSWER KEY