AQA AS Level Physics Paper 2, June 2023: Question 14
1 mark · Easy difficulty · Multiple Choice
Identify which physical quantity is not conserved when an electron and a positron annihilate each other.
Practise this questionQuestion
Question text
14 An electron and a positron annihilate each other.
Which quantity is not conserved in the annihilation?
[1 mark]
A electric charge
B kinetic energy
C lepton number
D momentum
Mark scheme
Show the mark scheme
14 B kinetic energy
How to answer it
Electron-Positron Annihilation Conservation Laws
What this question tests
This question assesses your understanding of conservation laws in particle interactions, specifically distinguishing between quantities that are strictly conserved (like charge, lepton number, and momentum) and total energy versus kinetic energy during particle-antiparticle annihilation.
Annihilation and Conserved Quantities
✅ Correct Answer
B: kinetic energy
Kinetic energy is not conserved during annihilation. Total energy (including rest mass energy converted into photons) is conserved, but kinetic energy changes into electromagnetic energy (photons).
💡 Key Knowledge
- Electric Charge: Conserved in all particle interactions. An electron (e⁻) and positron (e⁺) have charges of -1e and +1e, summing to 0 before and after.
- Lepton Number: Conserved. Electron lepton number is +1 for the electron and -1 for the positron, summing to 0.
- Momentum: Always conserved in interactions according to fundamental physical laws.
🧠 Exam Technique
Read the stem very carefully—pay close attention to negative qualifiers like "not". Quickly evaluate each option against fundamental conservation laws taught in particle physics modules.
❌ Common Errors
Students frequently confuse total energy with kinetic energy. While total energy (and rest mass energy) must be conserved, kinetic energy is converted into photon energy during annihilation, meaning kinetic energy itself is not conserved.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.