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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.
Write your answers in the spaces provided.
Show all working for calculation questions.
Use $g = 10 \text{ m/s}^2$ where applicable (though not typically required for this topic).
The use of an approved scientific calculator is expected.

Section A: Structure of the Atom and Nuclide Notation (Questions 1–5)

1. The nucleus of a neutral atom of carbon-14 is represented by the symbol ${}^{14}_{6}\text{C}$ . (a) State the number of protons in this nucleus.
_________________________________________________________________________ [1]

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

(c) State the number of electrons in a neutral atom of carbon-14.
_________________________________________________________________________ [1]

2. Define the term isotope.

_________________________________________________________________________ [2]

3. An atom of uranium-238 ( ${}^{238}_{92}\text{U}$ ) undergoes a series of decays. Consider a different isotope, uranium-235 ( ${}^{235}_{92}\text{U}$ ). (a) Explain why uranium-235 and uranium-238 are isotopes of the same element.

_________________________________________________________________________ [2]

(b) Calculate the difference in the number of neutrons between a nucleus of uranium-238 and a nucleus of uranium-235.
_________________________________________________________________________ [1]

4. The diagram below represents the simple nuclear model of an atom.

Image pending generation for this question.

(Imagine a central nucleus with orbiting electrons)

(a) Name the two particles found inside the nucleus.
_________________________________________________________________________ [1]

(b) Which particle in the atom has a negligible mass compared to the others?
_________________________________________________________________________ [1]

5. A specific nuclide is written as ${}^{A}_{Z}\text{X}$ . (a) What does the symbol $Z$ represent?
_________________________________________________________________________ [1]

(b) What does the symbol $A$ represent?
_________________________________________________________________________ [1]

Section B: Radioactive Decay and Properties (Questions 6–12)

6. Complete the table below by ticking (✓) the correct box for each property of the three types of radioactive emission.

Property	Alpha ( $\alpha$ )	Beta ( $\beta$ )	Gamma ( $\gamma$ )
Consists of 2 protons and 2 neutrons
Is a high-speed electron
Is an electromagnetic wave
Has the highest ionising power
Has the greatest penetrating power

[5]

7. A radioactive source emits beta-minus ( $\beta^-$ ) particles. (a) Describe the change that occurs in the nucleus of the atom when it emits a beta particle.

_________________________________________________________________________ [2]

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

8. A student investigates the penetrating power of radiation. She places different absorbers between a radioactive source and a Geiger-Müller tube.

Absorber	Count Rate (counts/min)
None	850
Paper (0.1 mm)	840
Aluminium (3 mm)	120
Lead (5 mm)	15

(a) Suggest which type(s) of radiation are emitted by the source. Explain your answer.

_________________________________________________________________________ [3]

(b) The count rate does not drop to zero even with thick lead shielding. Suggest a reason for this.
_________________________________________________________________________ [1]

9. Alpha particles are deflected by an electric field. (a) State the charge of an alpha particle.
_________________________________________________________________________ [1]

(b) If an alpha particle beam passes between two charged plates (positive plate on top, negative plate on bottom), towards which plate will the beam deflect?
_________________________________________________________________________ [1]

10. Gamma rays are often emitted after alpha or beta decay. (a) Explain why the emission of a gamma ray does not change the proton number or nucleon number of the nucleus.

_________________________________________________________________________ [2]

11. A nucleus of Polonium-210 ( ${}^{210}_{84}\text{Po}$ ) decays by emitting an alpha particle to form Lead (Pb). (a) Write the complete nuclear equation for this decay.
_________________________________________________________________________ [2]

(b) Verify that both nucleon number and proton number are conserved in your equation.
_________________________________________________________________________ [1]

12. State one similarity and one difference between the nature of an alpha particle and a helium nucleus. Similarity: __________________________________________________________________
Difference: __________________________________________________________________ [2]

Section C: Half-Life and Applications (Questions 13–17)

13. Define the term half-life.

_________________________________________________________________________ [2]

14. A sample of a radioactive isotope has an initial activity of 8000 Bq. The half-life of the isotope is 12 hours. (a) Calculate the activity of the sample after 24 hours.

_________________________________________________________________________ [2]

(b) Calculate the activity of the sample after 36 hours.
_________________________________________________________________________ [2]

15. The graph below shows the decay curve of a radioactive isotope.

Image pending generation for this question.

(a) Determine the half-life of the isotope from the graph.
_________________________________________________________________________ [1]

(b) Estimate the count rate after 12 days.
_________________________________________________________________________ [2]

16. Radioactive isotopes are used in medicine and industry. (a) Technetium-99m is used as a tracer in medical diagnostics. It emits gamma radiation and has a half-life of 6 hours. Explain why it is suitable for this purpose, referring to both its radiation type and half-life.
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________ [3]

(b) Americium-241 is used in smoke detectors. It emits alpha particles and has a half-life of 432 years. Explain why an alpha emitter is preferred over a gamma emitter for this application.
_________________________________________________________________________
_________________________________________________________________________ [2]

17. Carbon-14 dating is used to estimate the age of organic materials. (a) Explain the basic principle behind carbon-14 dating.

_________________________________________________________________________ [2]

(b) Why is carbon-14 dating not suitable for determining the age of a rock that is millions of years old?
_________________________________________________________________________ [1]

Section D: Nuclear Energy and Safety (Questions 18–20)

18. Distinguish between nuclear fission and nuclear fusion.

_________________________________________________________________________ [2]

19. In a nuclear power station, uranium-235 undergoes fission. (a) What particle is typically absorbed by the uranium-235 nucleus to initiate fission?
_________________________________________________________________________ [1]

(b) State one form of energy released during fission that is used to generate electricity.
_________________________________________________________________________ [1]

20. Discuss the safety precautions necessary when handling radioactive sources in a school laboratory. Mention at least two precautions and the reason for each. Precaution 1: __________________________________________________________________
Reason: ______________________________________________________________________

Precaution 2: __________________________________________________________________
Reason: ______________________________________________________________________ [4]

End of Quiz

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Answers

O-Level Physics Quiz - Modern Physics (Answer Key)

Total Marks: 40

Section A: Structure of the Atom and Nuclide Notation

1. (a) 6 [1] (b) $14 - 6 = 8$ [1] (c) 6 [1]

2. Isotopes are atoms of the same element (same proton number) [1] with different numbers of neutrons (different nucleon numbers) [1].

3. (a) They have the same proton number (92) [1] but different nucleon numbers (235 vs 238) / different number of neutrons [1]. (b) $238 - 92 = 146$ neutrons in U-238. $235 - 92 = 143$ neutrons in U-235. Difference = $146 - 143 = 3$ [1].

4. (a) Protons and Neutrons [1] (b) Electron [1]

5. (a) Proton number (or atomic number) [1] (b) Nucleon number (or mass number) [1]

Section B: Radioactive Decay and Properties

6. [1 mark per correct row, max 5]

Consists of 2 protons and 2 neutrons: Alpha
Is a high-speed electron: Beta
Is an electromagnetic wave: Gamma
Has the highest ionising power: Alpha
Has the greatest penetrating power: Gamma

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

8. (a) Beta and Gamma [1].

Paper reduces count slightly (or not at all within error), ruling out Alpha (which would be stopped by paper) [1].
Aluminium reduces count significantly (stops Beta), but Lead reduces it further (stops Gamma), indicating presence of Beta and Gamma [1]. (Note: If student says Beta only, award 1 mark if reasoning for Aluminium is correct, but the Lead drop suggests Gamma too. If student says Alpha/Beta/Gamma, 0 marks because Paper didn't stop Alpha.) Refined Answer: The drop from 850 to 840 with paper suggests Alpha is absent or negligible. The large drop with Aluminium suggests Beta is present. The remaining count with Lead suggests Gamma is present. So, Beta and Gamma. (b) Background radiation [1].

9. (a) Positive (+2e) [1] (b) Negative plate [1] (Opposite charges attract).

10. Gamma rays are electromagnetic energy (photons) [1]. They have no mass and no charge, so emitting them does not change the number of protons or neutrons in the nucleus [1].

11. (a) ${}^{210}_{84}\text{Po} \rightarrow {}^{206}_{82}\text{Pb} + {}^{4}_{2}\alpha$ [2] (1 for correct Pb notation, 1 for Alpha notation). (b) Nucleon: $210 = 206 + 4$ [0.5]. Proton: $84 = 82 + 2$ [0.5]. Conserved [1].

12. Similarity: Both have 2 protons and 2 neutrons (or same mass/charge) [1]. Difference: An alpha particle is emitted from a nucleus during decay (high kinetic energy), whereas a helium nucleus is part of a stable helium atom (usually with electrons, or low energy) [1]. Accept: Alpha particle has no electrons, Helium atom has electrons.

Section C: Half-Life and Applications

13. Half-life is the time taken [1] for the activity (or number of nuclei) of a radioactive sample to decrease to half its original value [1].

14. (a) 24 hours is 2 half-lives ( $24/12 = 2$ ). $8000 \rightarrow 4000 \rightarrow 2000$ Bq. Answer: 2000 Bq [2]. (b) 36 hours is 3 half-lives ( $36/12 = 3$ ). $2000 \rightarrow 1000$ Bq. Answer: 1000 Bq [2].

15. (a) 4 days [1] (Time for activity to drop from 1000 to 500). (b) 12 days is 3 half-lives ( $12/4 = 3$ ). $1000 \rightarrow 500 \rightarrow 250 \rightarrow 125$ counts/s. Answer: 125 counts/s [2].

16. (a) Gamma radiation can penetrate the body to be detected externally [1]. Short half-life (6 hours) means it decays quickly, minimizing radiation dose to the patient [1] and reducing long-term waste issues [1]. (b) Alpha particles are strongly ionising but weakly penetrating [1]. They are blocked by the plastic casing of the detector, posing no risk to users outside, but can ionize air inside the chamber effectively [1]. Gamma would escape the device and pose a health risk.

17. (a) Living organisms absorb Carbon-14 from the atmosphere. When they die, absorption stops, and the C-14 decays [1]. By measuring the remaining C-14 activity compared to living matter, the age can be calculated [1]. (b) The half-life of C-14 is approx 5700 years. After millions of years, the remaining amount of C-14 would be too small to measure accurately [1].

Section D: Nuclear Energy and Safety

18. Fission is the splitting of a heavy nucleus into lighter nuclei [1]. Fusion is the joining of light nuclei to form a heavier nucleus [1].

19. (a) Neutron [1] (b) Thermal (heat) energy / Kinetic energy of fragments [1]

20. Any two valid pairs [4]:

Use tongs/forceps to handle sources [1]. Reason: To increase distance from the body, reducing radiation exposure (inverse square law) [1].
Store sources in lead-lined boxes [1]. Reason: Lead absorbs radiation, preventing exposure when not in use [1].
Point source away from people/body [1]. Reason: To minimize direct exposure to sensitive organs [1].
Wash hands after handling [1]. Reason: To remove any potential radioactive contamination [1].