AQA A-Level Physics Paper 2, June 2023: Question 9

✅ Correct Answer

C — Charles’s law

💡 Key Knowledge: The Gas Laws

• Charles’s Law: For a fixed mass of gas at constant pressure, volume is directly proportional to absolute temperature ( V ∝ T ).
• Boyle’s Law: Pressure is inversely proportional to volume ( p ∝ 1/V ) at constant temperature (isothermal). Since temperature does not change, it cannot be used to find absolute zero.
• Brownian Motion: Provides evidence for the particulate nature of matter and random molecular motion, not quantitative values for absolute zero.
• Rutherford Scattering: Alpha particle deflection used to determine the structure and size of the atomic nucleus.

📐 Graphical Extrapolation to Absolute Zero

In the required practical for Charles’s law:

1. A sealed column of dry air is trapped in a capillary tube by a bead of concentrated sulfuric acid (maintaining constant pressure p ).
2. The tube is heated in a water bath across a range of temperatures in Celsius ( θ ).
3. The length of the air column (proportional to volume V ) is plotted on the y-axis against temperature θ / °C on the x-axis.
4. The resulting straight line is extrapolated backwards to V = 0 (the x-intercept).
5. The temperature where gas volume would hypothetically drop to zero corresponds to absolute zero, approximately -273 °C (or 0 K ).

🧠 Exam Technique: Elimination Strategy

For multiple-choice questions on practical physics:

• Look at the physical quantities involved in the question. Absolute zero is a temperature.
• Eliminate options from entirely different topics: Rutherford scattering is Nuclear Physics; Brownian motion is qualitative observation.
• Distinguish between the thermal laws: Boyle's law is defined at constant temperature, so no temperature variation is measured.
• This leaves Charles’s law (or the Pressure Law, if listed) as the only valid choice.