AQA AS Level Physics Paper 2, June 2022: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify the Feynman diagram representing the process of electron capture among four options.

Practise this question

Question

A multiple choice question asking which of four Feynman diagrams (A, B, C, D) represents the process of electron capture. Diagram A shows an electron-neutrino interacting with an electron via a W+ boson to produce a neutron and proton. Diagrams B, C, and D show alternative particle interactions with different exchange bosons or particle arrows.
Question text

08 Which diagram represents the process of electron capture?

[1 mark]

A B

C D

A

B

C

D

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 08 is A, backed by Assessment Objective 1 (AO1), showing the matching Feynman diagram for electron capture.

08 A (AO1)

How to answer it

Identifying Feynman Diagrams: Electron Capture

What this question tests

This question assesses your knowledge of particle interactions (specifically weak interactions), your ability to interpret Feynman diagrams correctly, and your understanding of conservation laws (charge, lepton number, and baryon number) applied to particle equations like p + e⁻ → n + vₑ.

Question 08

Exam Breakdown: Part 08

Identify the Feynman diagram representing electron capture

✅ Correct Answer: Diagram A

Diagram A correctly illustrates an electron (e⁻) interacting with a proton (p). The electron enters from the bottom left, emits an electron neutrino (vₑ) upward, and turns into a W⁺ exchange boson (or equivalently, absorbs/interacts via weak force). The proton absorbs the exchange boson and turns into a neutron (n).

💡 Key Knowledge

  • Definition: Electron capture happens when an inner-shell atomic electron is captured by a nucleus (proton turns into a neutron, releasing an electron neutrino).
  • Equation: p + e⁻ → n + vₑ
  • Arrow Rules: Fermions (quarks and leptons) must have arrows pointing forward in time (upwards). Antiparticles have arrows pointing backwards (downwards relative to flow of time, though standard AS diagrams show incoming/outgoing arrows explicitly).

🧠 Exam Technique

Break down the diagram into three checks:

  1. Incomers: What goes in? For electron capture, a proton ( p ) and an electron ( e⁻ ) must be coming together.
  2. Outcomers: What comes out? A neutron ( n ) and an electron neutrino ( vₑ ).
  3. Vertex Check: Ensure particle types and arrow directions match conservation rules at every junction.

❌ Common Errors

  • Confusing an electron neutrino ( vₑ ) with an antineutrino ( v̄ₑ ). Diagram C features an antineutrino, which violates lepton conservation for this reaction.
  • Mixing up beta-minus decay diagrams with electron capture diagrams due to superficial similarities in the layout of straight lines.
Mark Scheme Note: 1 mark is awarded exclusively for selecting A. No working is required for multiple-choice questions, but visual recognition of vertex arrows is vital.

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.