AQA AS Level Physics Paper 2, June 2024: Question 8

✅ Correct Answer: Row A

The correct option is A: The positron has the greatest charge-to-mass ratio, and the alpha particle has the smallest charge-to-mass ratio.

💡 Key Knowledge

• Charge-to-mass ratio formula: Specific Charge = Charge (Q) / Mass (m), measured in C kg⁻¹.
• Positron: Tiny mass (approx 9.11 × 10⁻³¹ kg) carrying a full elementary charge (+e). High charge packed into a very small mass yields a huge ratio.
• Proton: Similar charge (+e) to a positron, but roughly 1836 times more massive, resulting in a much smaller ratio.
• Alpha particle: Comprises 2 protons and 2 neutrons (mass ≈ 4u = 4 × 1.66 × 10⁻²⁷ kg) with a charge of +2e. Because its mass scales up much faster than its charge relative to the proton/positron, its ratio is the smallest.

🧠 Exam Technique

You do not need to calculate exact decimal values in an exam setting. Think logically about proportions:

• To maximise the ratio, you want the smallest possible denominator (mass) for a given numerator (charge). Hence, the positron wins.
• To minimise the ratio, you want the largest possible denominator for the charge. The heavy alpha particle wins here.

❌ Common Errors

• Confusing the proton and positron masses (assuming they are identical because both are subatomic particles).
• Failing to account for the alpha particle containing four nucleons (2 protons + 2 neutrons), leading to an incorrect mass estimate of 2u instead of 4u.