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Secondary 4 Pure Physics Modern Physics Quiz

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Secondary 4 Pure Physics Quiz - Modern Physics

Name: __________________________
Class: __________________________
Date: __________________________
Score: _______ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions.
Write your answers in the spaces provided.
For calculations, show all working clearly.
Take the acceleration of free fall, $g = 10 \text{ m/s}^2$ , where necessary (though not typically required for this topic).
The use of an approved scientific calculator is expected.

Section A: Atomic Structure and Radioactive Decay (Questions 1–8)

1. An atom of uranium-238 is represented by the symbol $^{238}_{92}\text{U}$ . (a) State the number of protons in the nucleus of this atom. [1]

(b) State the number of neutrons in the nucleus of this atom. [1]

2. Define the term isotope. [2]

3. The diagram below represents the nucleus of a radioactive atom undergoing decay.

$^{A}_{Z}\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + \text{Particle P}$

(a) Identify Particle P. [1]

(b) State two properties of Particle P regarding its ionising ability and penetrating power. [2]

4. A different radioactive source emits beta-minus ( $\beta^-$ ) particles. (a) Describe the change that occurs in the nucleus of the atom when a $\beta^-$ particle is emitted. [2]

(b) Write the general nuclear equation for beta-minus decay using $^{A}_{Z}\text{X}$ as the parent nucleus. [2]

5. Gamma ( $\gamma$ ) radiation is often emitted alongside alpha or beta radiation. (a) State what gamma radiation consists of. [1]

(b) Explain why the emission of gamma radiation does not change the atomic number or mass number of the nucleus. [1]

6. A Geiger-Müller tube is used to detect radiation from a source. The background count rate is 20 counts per minute (cpm).

When the source is placed near the detector, the total count rate is 220 cpm.

Calculate the count rate due to the source alone. [1]

7. State one safety precaution that should be taken when handling radioactive sources in a school laboratory. [1]

8. Complete the table below by ticking ( $\checkmark$ ) the correct box for each property of alpha, beta, and gamma radiation. [3]

Property	Alpha ( $\alpha$ )	Beta ( $\beta$ )	Gamma ( $\gamma$ )
Most ionising	[ ]	[ ]	[ ]
Stopped by thin paper	[ ]	[ ]	[ ]
Deflected by magnetic field	[ ]	[ ]	[ ]

Section B: Half-Life and Calculations (Questions 9–14)

9. Define the term half-life of a radioactive isotope. [2]

10. A sample of a radioactive isotope has an initial activity of 800 Bq. The half-life of the isotope is 6 hours. (a) Calculate the activity of the sample after 18 hours. [2]

(b) Sketch a graph of Activity (Bq) against Time (hours) for the first 24 hours. Label the axes and plot at least 3 points. [2]

11. A radioactive source has a half-life of 15 minutes. (a) How many half-lives have elapsed after 1 hour? [1]

(b) If the initial mass of the radioactive isotope was 40 g, calculate the mass of the original isotope remaining after 1 hour. [2]

12. Carbon-14 is a radioactive isotope used in carbon dating. It decays into Nitrogen-14.

$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + \beta^-$

A fossilised bone is found to have 25% of the Carbon-14 activity compared to a living bone.

Given that the half-life of Carbon-14 is 5730 years, estimate the age of the fossil. [2]

13. Explain why it is impossible to predict exactly when a specific individual nucleus will decay. [1]

14. A student measures the count rate of a source every 10 minutes. The results are corrected for background radiation.

Time (min)	0	10	20	30	40
Count Rate (cpm)	100	70	50	35	25

Using the data, estimate the half-life of the source. [2]

Section C: Nuclear Energy and Applications (Questions 15–20)

15. Distinguish between nuclear fission and nuclear fusion. [2]

16. In a nuclear power station, Uranium-235 nuclei undergo fission. (a) State what is meant by a chain reaction. [2]

(b) Explain the role of control rods in a nuclear reactor. [2]

17. Nuclear fusion occurs in the Sun. (a) State the conditions required for nuclear fusion to occur. [2]

(b) Explain why nuclear fusion is not yet used as a commercial source of energy on Earth. [1]

18. Radioactive isotopes have many medical applications. (a) State one property required for a radioactive tracer used in medical diagnosis. [1]

(b) Explain why gamma emitters are preferred over alpha emitters for medical tracers inside the body. [2]

19. Irradiation is used to sterilise medical equipment. (a) Explain why gamma radiation is suitable for sterilising sealed surgical instruments. [2]

(b) State one advantage of using irradiation over boiling water for sterilisation. [1]

20. A smoke detector contains a small source of Americium-241, which emits alpha particles. (a) Explain how the smoke detector works under normal conditions (no smoke). [2]

(b) Explain why an alpha source is used instead of a beta or gamma source in a smoke detector. [2]

End of Quiz

Answers

Secondary 4 Pure Physics Quiz - Modern Physics (Answer Key)

Total Marks: 40

Section A: Atomic Structure and Radioactive Decay

1. (a) 92 [1] (b) $238 - 92 = 146$ [1]

2. Atoms of the same element (same number of protons/atomic number) [1] with different numbers of neutrons (different mass numbers). [1]

3. (a) Alpha particle ( $\alpha$ ) / Helium nucleus ( $^4_2\text{He}$ ) [1] (b)

High ionising ability [1]
Low penetrating power (stopped by paper/skin) [1]

4. (a) A neutron changes into a proton and an electron (beta particle). [1] The proton stays in the nucleus, and the electron is emitted. [1] (b) $^{A}_{Z}\text{X} \rightarrow ^{A}_{Z+1}\text{Y} + ^{0}_{-1}\beta$ (or $e$ ) [2] (1 for correct daughter nucleus Z+1, 1 for correct beta symbol)

5. (a) High-energy electromagnetic waves / photons. [1] (b) Gamma emission involves the release of excess energy from the nucleus, not particles. Therefore, the number of protons and neutrons remains unchanged. [1]

6. $220 - 20 = 200$ cpm [1]

7. Any one of:

Use tongs/forceps to handle sources. [1]
Keep source in a lead-lined box when not in use. [1]
Point source away from people/body. [1]
Minimise time of exposure. [1]

Most ionising: Alpha [1]
Stopped by thin paper: Alpha [1]
Deflected by magnetic field: Beta [1] (Note: Alpha is also deflected, but in the opposite direction. Gamma is not. If the student ticks Alpha for deflection, it is also correct, but usually Beta is the distinct answer for "deflected" in simple comparisons if only one tick is allowed per row. However, strictly, both Alpha and Beta are deflected. Accept Alpha or Beta for the last row, but Gamma must be empty. Correction for marking: The question asks to tick the correct box. Alpha and Beta are both deflected. If only one tick is permitted per row, this question is flawed. Standard expectation: Alpha (High Ionising), Alpha (Stopped by paper), Beta/Alpha (Deflected). Let's accept ticks for Alpha and Beta in the last row. If student ticks only one, accept either Alpha or Beta.) Refined Marking for Q8:
Row 1: Alpha [1]
Row 2: Alpha [1]
Row 3: Alpha OR Beta [1] (Gamma is neutral, so not deflected).

Section B: Half-Life and Calculations

9. The time taken [1] for the activity (or count rate/number of undecayed nuclei) of a radioactive source to decrease by half. [1]

10. (a)

18 hours / 6 hours = 3 half-lives. [1]
$800 \rightarrow 400 \rightarrow 200 \rightarrow 100$ Bq. [1] (b)
Axes labelled: Activity (Bq) on y-axis, Time (hours) on x-axis. [1]
Curve starting at 800, passing through (6, 400), (12, 200), (18, 100), (24, 50). Smooth decay curve. [1]

11. (a) 1 hour = 60 mins. $60 / 15 = 4$ half-lives. [1] (b)

After 4 half-lives: $40 \text{ g} \times (1/2)^4$ [1]
$40 / 16 = 2.5$ g. [1]

12.

100% $\rightarrow$ 50% $\rightarrow$ 25%. This is 2 half-lives. [1]
Age = $2 \times 5730 = 11,460$ years. [1]

13. Radioactive decay is a random / spontaneous process. [1]

14.

Look for time taken for count rate to halve.
100 to 50 cpm takes 20 minutes. [1]
70 to 35 cpm takes 20 minutes (10 to 30). [1]
Half-life $\approx 20$ minutes.

Section C: Nuclear Energy and Applications

15.

Fission: Splitting of a heavy nucleus into lighter nuclei. [1]
Fusion: Joining of light nuclei to form a heavier nucleus. [1]

16. (a) Neutrons released from one fission event cause further fission events in other nuclei, sustaining the reaction. [2] (b) Control rods (made of boron/cadmium) absorb neutrons. [1] By inserting/withdrawing them, the rate of fission (and thus power output) is controlled/prevented from running away. [1]

17. (a) High temperature and high pressure. [2] (1 for each) (b) It is difficult to contain the plasma at the required temperatures/pressures; net energy output is currently difficult to sustain. [1]

18. (a) Short half-life. [1] (b) Gamma rays have high penetrating power and can escape the body to be detected externally. [1] Alpha particles are highly ionising and would damage tissue/cells inside the body, and cannot escape the body to be detected. [1]

19. (a) Gamma rays have high penetrating power, so they can penetrate the packaging and the instruments to kill bacteria. [2] (b) It can be used on heat-sensitive materials (e.g., plastics) that would melt in boiling water. [1]

20. (a) Alpha particles ionise the air molecules between two electrodes, creating a current. [2] (1 for ionisation, 1 for current flow) (b) Alpha particles have low penetrating power and are easily stopped by smoke particles. [1] This causes a significant drop in current, triggering the alarm. Beta/Gamma would pass through smoke without significant interaction. [1]