AQA A-Level Physics Paper 2, June 2025: Question 8

1 mark · Medium difficulty · Multiple Choice

Calculate the new volume of gas in a syringe when a weight is suspended from the plunger, using Boyle's law.

Practise this question

Question

Multiple-choice question 8 showing a diagram of a Boyle's law apparatus. A sealed syringe is clamped vertically to a stand with the sealed nozzle facing upwards. The plunger extends downwards from the barrel, and a 2.5 N weight is suspended from the bottom of the plunger via a loop of string. The syringe has a cross-sectional area of 1.0 × 10⁻⁴ m² and initially contains 12 cm³ of gas at atmospheric pressure of 1.0 × 10⁵ Pa. Four multiple-choice options are given for the new volume: A (3 cm³), B (9 cm³), C (16 cm³), and D (48 cm³).

Mark scheme

Show the mark scheme Mark scheme for question 8 displaying the answer key: Question number 8, correct option C (16 cm³), testing assessment objective AO2.

How to answer it

Boyle's Law & Pressure Equilibrium in a Syringe

📋 What This Question Tests

This question evaluates your ability to combine mechanical equilibrium with thermal physics:

  • Free-Body Force Balance: Identifying the direction of atmospheric pressure, internal gas pressure, and suspended weights acting on a movable plunger.
  • Pressure-Force Relationship: Applying p = F / A to calculate pressure contributions.
  • Boyle’s Law: Using p₁V₁ = p₂V₂ for an ideal gas at constant temperature.
Question 08 • Multiple Choice

Plunger Equilibrium & Gas Expansion

AQA A-Level Physics • Thermal Physics / Required Practical 9 Context

✅ Correct Answer

C: 16 cm³

Because the syringe is clamped vertically with the sealed end at the top, hanging a weight pulls the plunger downward, expanding the gas and reducing its internal pressure below atmospheric pressure.

Mark Award: 1 Mark (AO2 — Application of knowledge to practical context).

💡 Key Knowledge

  • Boyle’s Law: For a fixed mass of gas at constant temperature, p × V = constant .
  • Equilibrium of the Plunger:
    Downwards forces = Upwards forces
    F_gas + W = F_atm
    p_gas × A + W = p_atm × A
  • Rearranged Gas Pressure:
    p_gas = p_atm − (W / A)

📐 Step-by-Step Calculation

Step 1: Calculate the pressure exerted by the suspended weight

Δp = W / A = 2.5 N / (1.0 × 10⁻⁴ m²) = 25 000 Pa = 0.25 × 10⁵ Pa

Step 2: Determine the new gas pressure (p₂)

The downward weight pulls the plunger outwards, opposing atmospheric pressure pushing in from below:
p₂ = p_atm − Δp = (1.0 × 10⁵ Pa) − (0.25 × 10⁵ Pa) = 0.75 × 10⁵ Pa

Step 3: Apply Boyle's Law to calculate the new volume (V₂)

Since temperature is constant: p₁V₁ = p₂V₂
(1.0 × 10⁵ Pa) × (12 cm³) = (0.75 × 10⁵ Pa) × V₂
V₂ = (1.0 × 10⁵ × 12) / (0.75 × 10⁵) = 12 / 0.75 = 16 cm³

❌ Common Errors & Traps

  • Adding instead of subtracting: Thinking the weight compresses the gas leads to p₂ = 1.25 × 10⁵ Pa , giving V₂ = 9.6 ≈ 9 cm³ (Option B). Always check the diagram orientation!
  • Neglecting atmospheric pressure: Using only the weight's pressure ( 0.25 × 10⁵ Pa ) gives 48 cm³ (Option D).
  • Unit conversion errors: Wasting time converting volume to m³ and back. Because pressure units cancel out directly, volume can stay in cm³.

🧠 Exam Technique & Intuition

  • Physical Sanity Check: The weight hangs from the plunger pulling downwards away from the sealed top. This must expand the trapped gas. Therefore, V₂ > 12 cm³ . Options A (3 cm³) and B (9 cm³) can be instantly eliminated!
  • Free-Body Sketch: Draw arrows on the plunger: weight acts down, internal gas pressure pushes down, external atmosphere pushes up.

Topics

Physics · Required Practicals · 3.6 Further mechanics and thermal physics (A-level only) · A-Level practicals (7–12)

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.