Secondary 4 Pure Physics Modern Physics Quiz

Secondary 4 Pure Physics Quiz - Modern Physics — Answer Key

Total Marks: 40

Section A: Atomic Structure and Radioactivity (10 marks)

1. State the relative charge and relative mass of a proton, a neutron, and an electron.

[3 marks — 0.5 marks per correct entry]

Charge:

• Proton: +1
• Neutron: 0
• Electron: −1

Mass:

• Proton: 1
• Neutron: 1
• Electron: 1/1840 (or negligible / approximately 0)

Marking note: Accept "positive," "neutral," "negative" for charge. Accept "1," "1," "very small/negligible" for mass.

2. Uranium-235 nuclide notation: ²³⁵₉₂U

(a) State the number of protons in this nucleus. [1 mark]

• Answer: 92

(b) Calculate the number of neutrons in this nucleus. [1 mark]

• Answer: 235 − 92 = 143 neutrons
• Marking note: Award mark for correct subtraction and answer.

(c) State the number of electrons in a neutral atom of uranium-235. [1 mark]

• Answer: 92 (equal to the number of protons)

3. Complete the table for alpha, beta, and gamma radiation.

[4 marks — 1 mark per row; deduct 0.5 per error within a row]

Marking note: Accept equivalent descriptions for nature. Ionising and penetrating must match the standard hierarchy.

4. State one similarity and one difference between the plum pudding model and the nuclear model of the atom.

[2 marks]

Similarity (any one):

• Both models contain positive and negative charges.
• Both models describe the atom as being made of subatomic particles.

Difference (any one):

• Plum pudding model: positive charge is spread throughout the atom; Nuclear model: positive charge is concentrated in a small nucleus.
• Plum pudding model: electrons are embedded in the positive sphere; Nuclear model: electrons orbit the nucleus.

Marking note: 1 mark for a valid similarity, 1 mark for a valid difference.

5. Explain why an atom is electrically neutral overall.

[1 mark]

• Answer: An atom has an equal number of positively charged protons and negatively charged electrons, so the charges cancel out.
• Marking note: Accept "same number of protons and electrons" or equivalent.

Section B: Radioactive Decay and Half-Life (10 marks)

6. Half-life = 8 days; initial activity = 640 counts per second.

(a) Define the term "half-life." [2 marks]

• 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.
• Marking note: 1 mark for "time taken," 1 mark for "half the nuclei/activity to decay."

(b) Calculate the activity after 24 days. [2 marks]

• Number of half-lives = 24 ÷ 8 = 3
• Activity = 640 × (½)³ = 640 × ⅛ = 80 counts per second
• Answer: 80 counts per second
• Marking note: 1 mark for correct number of half-lives, 1 mark for correct final activity.

(c) State how many half-lives have passed after 32 days. [1 mark]

• Answer: 32 ÷ 8 = 4 half-lives

7. Background count rate = 30 counts per minute.

(a) Explain why it is necessary to measure the background count rate. [1 mark]

• Answer: To subtract the background radiation from the measured count rate to obtain the count rate due to the source alone / to account for natural environmental radiation.
• Marking note: Accept any answer that references subtracting background or obtaining true source count.

(b) Calculate the corrected count rate. [1 mark]

• Corrected count rate = 270 − 30 = 240 counts per minute
• Answer: 240 counts per minute

8. Decay curve analysis.

(a) Determine the half-life from the graph. [1 mark]

• Activity halves from 800 to 400 in 6 hours, and from 400 to 200 in another 6 hours.
• Answer: 6 hours

(b) Estimate the activity after 18 hours. [1 mark]

• 18 hours = 3 half-lives
• Activity = 800 × (½)³ = 800 × ⅛ = 100 counts per minute
• Answer: 100 counts per minute

(c) Explain why the activity never reaches exactly zero. [2 marks]

• Answer: Radioactive decay is a random process. The number of undecayed nuclei halves each half-life, approaching zero but never reaching it mathematically. In practice, the activity eventually becomes indistinguishable from background radiation.
• Marking note: 1 mark for "random process / exponential decay," 1 mark for "approaches but never reaches zero / becomes indistinguishable from background."

9. Beta particle emission.

(a) State what happens to the mass number and atomic number. [2 marks]

• Mass number: remains unchanged / stays the same
• Atomic number: increases by 1
• Marking note: 1 mark each.

(b) Write the nuclear equation for beta decay of carbon-14 into nitrogen. [1 mark]

• Answer: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e (or ¹⁴₆C → ¹⁴₇N + β⁻)
• Marking note: Accept any correct notation showing mass number 14, atomic number increasing from 6 to 7, and beta particle.

10. State what is meant by the term "background radiation" and give one natural source of it.

[2 marks]

Meaning:

• Background radiation is the low-level radiation that is always present in the environment from natural and artificial sources.

Natural source (any one):

• Cosmic rays from space
• Radioactive rocks in the ground (e.g., granite)
• Radon gas
• Food and drink (e.g., bananas contain potassium-40)

Marking note: 1 mark for correct meaning, 1 mark for a valid natural source.

Section C: Nuclear Reactions and Applications (10 marks)

11. Distinguish between nuclear fission and nuclear fusion.

[4 marks — 2 marks per process]

Nuclear fission:

• The splitting of a heavy nucleus (e.g., uranium-235) into two lighter nuclei
• Triggered by neutron bombardment
• Releases a large amount of energy and additional neutrons
• Can lead to a chain reaction

Nuclear fusion:

• The joining/combining of two light nuclei (e.g., hydrogen isotopes) to form a heavier nucleus
• Requires very high temperatures and pressures to overcome electrostatic repulsion
• Releases even more energy than fission per unit mass
• Occurs naturally in stars (including the Sun)

Marking note: Award 1 mark for "splitting heavy nucleus" and 1 mark for "releases energy/neutrons" for fission. Award 1 mark for "joining light nuclei" and 1 mark for "requires high temperature / occurs in stars" for fusion.

12. Advantage and disadvantage of nuclear power.

[2 marks]

Advantage (any one):

• Does not produce greenhouse gases / carbon dioxide during operation
• Produces a very large amount of energy from a small mass of fuel
• Reliable baseload power (not dependent on weather)

Disadvantage (any one):

• Produces radioactive waste that must be stored safely for thousands of years
• Risk of catastrophic accidents (e.g., meltdown)
• High initial construction and decommissioning costs
• Limited fuel supply (non-renewable uranium)

Marking note: 1 mark for a valid advantage, 1 mark for a valid disadvantage.

13. Control rods in a nuclear reactor.

(a) Explain the purpose of absorbing neutrons. [2 marks]

• Answer: Control rods absorb excess neutrons to control the rate of the fission chain reaction. By adjusting the depth of the control rods, the number of neutrons available to cause further fission is regulated, keeping the reaction stable and preventing it from becoming uncontrolled.
• Marking note: 1 mark for "control rate of reaction," 1 mark for "absorb neutrons to prevent uncontrolled chain reaction."

(b) Name a material commonly used for control rods. [1 mark]

• Answer: Boron / cadmium / hafnium
• Marking note: Accept any one correct material.

14. Medical applications of radiation.

(a) Which radiation is most suitable for treating cancerous tumours inside the body? Explain. [2 marks]

• Answer: Gamma radiation is most suitable because it has high penetrating power and can reach tumours deep inside the body. Alpha and beta radiation would be absorbed by the skin and superficial tissues before reaching the tumour.
• Marking note: 1 mark for "gamma," 1 mark for "high penetrating power / can reach deep tumours."

(b) Explain why a short half-life is important for a radioactive tracer. [2 marks]

• Answer: A short half-life means the radioactive material decays quickly and does not remain in the patient's body for a long time, minimising the radiation dose received. The activity is high enough for imaging during the procedure but falls to safe levels soon after.
• Marking note: 1 mark for "decays quickly," 1 mark for "minimises radiation dose to patient."

15. Smoke detector with alpha source.

(a) Explain how the smoke detector works. [2 marks]

• Answer: The alpha particles ionise the air between two electrodes, creating a small, constant current. When smoke enters the detector, the smoke particles absorb or block the alpha particles, reducing the ionisation and thus the current. This drop in current triggers the alarm.
• Marking note: 1 mark for "ionises air / creates current," 1 mark for "smoke reduces current / triggers alarm."

(b) State why an alpha source is used rather than a beta or gamma source. [1 mark]

• Answer: Alpha particles have high ionising ability, so they produce a strong, easily measurable current. They also have low penetrating power, making the detector safe as the radiation cannot escape the casing. Beta and gamma would be too penetrating and less ionising.
• Marking note: Accept "high ionising ability" or "low penetrating power / easily stopped."

Section D: Electromagnetic Spectrum and Quantum Physics (10 marks)

16. List the following types of electromagnetic radiation in order of increasing wavelength: gamma rays, radio waves, visible light, X-rays.

[2 marks]

Shortest wavelength: Gamma rays X-rays Visible light Longest wavelength: Radio waves

Marking note: 2 marks for completely correct order. 1 mark if one pair is swapped. 0 marks if more than one error.

17. State two properties that are common to all types of electromagnetic waves.

[2 marks]

Any two from:

• They all travel at the speed of light in a vacuum (3.0 × 10⁸ m/s).
• They are all transverse waves.
• They can all travel through a vacuum.
• They all transfer energy.
• They can all be reflected, refracted, and diffracted.

Marking note: 1 mark for each correct property.

18. A photon of light has a frequency of 6.0 × 10¹⁴ Hz. The Planck constant is 6.63 × 10⁻³⁴ J s.

(a) State the equation linking the energy of a photon (E), Planck constant (h), and frequency (f). [1 mark]

• Answer: E = hf (or E = h × f)

(b) Calculate the energy of this photon. [2 marks]

• E = hf = (6.63 × 10⁻³⁴ J s) × (6.0 × 10¹⁴ Hz)
• E = 3.978 × 10⁻¹⁹ J (accept 3.98 × 10⁻¹⁹ J or 4.0 × 10⁻¹⁹ J)
• Answer: 3.98 × 10⁻¹⁹ J
• Marking note: 1 mark for correct substitution, 1 mark for correct answer with unit.

19. Explain what is meant by the photoelectric effect.

[2 marks]

• Answer: The photoelectric effect is the emission of electrons from the surface of a metal when electromagnetic radiation (light) of sufficiently high frequency shines on it.
• Marking note: 1 mark for "emission of electrons," 1 mark for "from a metal surface / when light shines on it."

20. State one practical application of the photoelectric effect.

[1 mark]

Any one from:

• Solar cells / photovoltaic cells
• Photodiodes / light sensors
• Image sensors in digital cameras
• Night vision devices
• Photomultiplier tubes

Marking note: Accept any valid application.

END OF ANSWER KEY