AQA AS Level Physics Paper 2, November 2021: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify the correct products of the decay of a free neutron from a multiple-choice list.

Practise this question

Question

Multiple-choice question 14 asking for the products when a free neutron decays, worth 1 mark. The options are A: p + e^- + v_e, B: p + e^+ + anti-v_e, C: p + e^- + anti-v_e, and D: p + e^+ + v_e, each with a selection box next to it.
Question text

14 What are the products when a free neutron decays?

[1 mark]

p + e− + v

A e

B p + e+ + v

e

C p + e− + v

e

p + e+ + v

D e

Mark scheme

Show the mark scheme The mark scheme table shows question number 14 with the correct answer C corresponding to the product combination p + e^- + anti-v_e.

14 C p + e− + v

e

How to answer it

Products of Free Neutron Decay

What this question tests

This question assesses your knowledge of fundamental particle physics, specifically weak nuclear interactions, radioactive decay of subatomic particles, and the conservation laws (charge, lepton number, and baryon number) governing beta-minus decay.

Question 1.4 [1 mark]

Free Neutron Decay Equation

✅ Correct Answer

Option C: p + e⁻ + v̄ₑ (proton + electron + electron antineutrino)

Marks available: 1 out of 1 for selecting C.

💡 Key Knowledge

  • A free neutron is unstable and decays via the weak interaction (beta-minus decay).
  • Quark transformation: A down quark changes into an up quark ( d → u + e⁻ + v̄ₑ ).
  • Baryon number and charge must be conserved on both sides of the equation.

🧠 Exam Technique

  • Check conservation of charge: Neutron (0) → Proton (+1) + Electron (-1) + Antineutrino (0). Total charge = 0 = 0.
  • Check lepton number: Neutron (0) → Proton (0) + Electron (+1) + Electron antineutrino (-1). Total lepton number = 0 = 0.
  • Quickly eliminate options containing a positron ( e⁺ ) or a neutrino ( vₑ ) instead of an antineutrino.

❌ Common Errors

  • Confusing the electron neutrino ( vₑ ) with the electron antineutrino ( v̄ₑ ), which violates lepton number conservation.
  • Mistaking beta-minus decay products for beta-plus decay products ( p → n + e⁺ + vₑ ), which occurs inside certain unstable nuclei but not for a free neutron.

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.