A Level Physics 11.2 Fundamental Particles MCQs – 30 Practice Questions with Answers (Cambridge 9702)

These 30 original practice MCQs cover Cambridge AS & A Level Physics 9702 Section 11.2, Fundamental Particles: quarks and hadrons, leptons and antiparticles, and the conservation laws that decide whether a particle reaction can happen. Work through each set, then open the Show Answer toggle to check your answer with a full explanation.

Formula recap: Quark charges: up, charm and top = +2/3 e; down, strange and bottom = −1/3 e. Baryons are three quarks (baryon number B = +1), antibaryons are three antiquarks (B = −1) and mesons are a quark-antiquark pair (B = 0). See the full A Level Physics Formula Sheet for every equation you need.

Quarks & Hadrons

Q1. What is the electric charge of an up quark, in units of the elementary charge e?

  • (A) +2/3 e
  • (B) +1/3 e
  • (C) −1/3 e
  • (D) +1 e
Show Answer

Answer: A – The up quark carries charge +2/3 e. The up-type quarks (up, charm, top) all have +2/3 e, while the down-type quarks (down, strange, bottom) have −1/3 e.

Q2. A proton consists of two up quarks and one down quark (uud). What is the total charge of the proton?

  • (A) 0
  • (B) +1/3 e
  • (C) +1 e
  • (D) +5/3 e
Show Answer

Answer: C – Charge = (+2/3) + (+2/3) + (−1/3) = +3/3 = +1 e, exactly the known charge of the proton.

Q3. A neutron consists of one up quark and two down quarks (udd). What is the total charge of the neutron?

  • (A) −1 e
  • (B) 0
  • (C) −1/3 e
  • (D) +1/3 e
Show Answer

Answer: B – Charge = (+2/3) + (−1/3) + (−1/3) = 0. The neutron is electrically neutral, as its name suggests.

Q4. Which statement correctly describes a baryon?

  • (A) A particle made of one quark and one antiquark
  • (B) A fundamental particle that is not made of quarks
  • (C) A particle made of two quarks
  • (D) A particle made of three quarks
Show Answer

Answer: D – Baryons are hadrons made of three quarks. The proton (uud) and neutron (udd) are the most familiar baryons. A quark-antiquark pair is a meson, and a particle not made of quarks is a lepton.

Q5. Which combination makes a meson?

  • (A) One quark and one antiquark
  • (B) Three quarks
  • (C) Three antiquarks
  • (D) Two quarks and one antiquark
Show Answer

Answer: A – Mesons are hadrons built from one quark and one antiquark. This gives them baryon number B = (+1/3) + (−1/3) = 0.

Q6. A positive pion (π+) consists of an up quark and an anti-down quark. Given that the anti-down quark has charge +1/3 e, what is the charge of the π+?

  • (A) +1/3 e
  • (B) +1 e
  • (C) 0
  • (D) +2/3 e
Show Answer

Answer: B – Antiquark charges are the negatives of the quark charges, so anti-down = −(−1/3 e) = +1/3 e. Total charge = (+2/3) + (+1/3) = +1 e.

Q7. What is the charge of a baryon made from three down quarks (ddd)?

  • (A) 0
  • (B) −1/3 e
  • (C) −1 e
  • (D) +1 e
Show Answer

Answer: C – Charge = 3 x (−1/3 e) = −1 e. This combination is the Delta-minus particle.

Q8. A baryon consists of one up quark and two strange quarks (uss). The strange quark has charge −1/3 e. What is the charge of this baryon?

  • (A) +1 e
  • (B) −2/3 e
  • (C) +2/3 e
  • (D) 0
Show Answer

Answer: D – Charge = (+2/3) + (−1/3) + (−1/3) = 0. This neutral baryon is the Xi-zero.

Q9. What is the baryon number of a single quark, and of a single antiquark?

  • (A) Quark +1, antiquark −1
  • (B) Quark +1/3, antiquark −1/3
  • (C) Quark 0, antiquark 0
  • (D) Quark +1/3, antiquark +1/3
Show Answer

Answer: B – Each quark has baryon number +1/3 and each antiquark −1/3. Three quarks therefore give a baryon B = +1, while a quark-antiquark meson gives B = 0.

Q10. The Delta-plus-plus particle is a baryon made of three up quarks (uuu). What is its charge?

  • (A) +2 e
  • (B) +1 e
  • (C) +2/3 e
  • (D) 0
Show Answer

Answer: A – Charge = 3 x (+2/3 e) = +2 e, one of the largest charges carried by any single hadron.

Leptons & Antiparticles

Q11. Which of the following particles is a lepton?

  • (A) Proton
  • (B) Neutron
  • (C) Electron
  • (D) Pion
Show Answer

Answer: C – The electron is a lepton: a fundamental particle that is not made of quarks. The proton, neutron and pion are all hadrons built from quarks.

Q12. What is the antiparticle of the electron?

  • (A) Positron
  • (B) Proton
  • (C) Photon
  • (D) Neutrino
Show Answer

Answer: A – The positron has the same mass as the electron but the opposite charge, +1 e. It was the first antiparticle to be discovered.

Q13. Which statement about a particle and its antiparticle is correct?

  • (A) They have the same charge and the same mass
  • (B) They have opposite charges and different masses
  • (C) The antiparticle always has zero charge
  • (D) They have the same mass but opposite charges
Show Answer

Answer: D – An antiparticle has exactly the same mass as its particle but opposite charge (and opposite baryon and lepton numbers). The neutron and antineutron are both neutral, but differ in baryon number.

Q14. An electron and a positron, both almost at rest, meet and annihilate. What is produced?

  • (A) One gamma photon
  • (B) A proton and a neutron
  • (C) A single electron with double the energy
  • (D) Two gamma photons
Show Answer

Answer: D – In annihilation the particle and antiparticle disappear and their mass-energy is converted into two γ photons travelling in opposite directions.

Q15. Why does the annihilation of a stationary electron-positron pair produce two photons rather than one?

  • (A) Momentum must be conserved, and a single photon would carry momentum while the initial pair had (almost) none
  • (B) One photon is needed for each particle
  • (C) Photons are always created in pairs
  • (D) Charge would not be conserved with one photon
Show Answer

Answer: A – The pair has total momentum of (almost) zero. A single photon must carry momentum, so momentum could not be conserved. Two photons moving in opposite directions have total momentum zero while still carrying away the energy.

Q16. In pair production, a single gamma photon passing close to an atomic nucleus can produce…

  • (A) a proton and an antiproton
  • (B) two electrons
  • (C) an electron and a positron
  • (D) a neutron and a neutrino
Show Answer

Answer: C – Pair production converts the energy of one photon into a particle-antiparticle pair, most commonly an electron and a positron. The nucleus is needed so that momentum is conserved, and the photon must have at least the combined rest-mass energy of the pair.

Q17. What is the charge of an anti-up quark?

  • (A) +2/3 e
  • (B) −2/3 e
  • (C) −1/3 e
  • (D) 0
Show Answer

Answer: B – Antiquark charges are the negatives of the corresponding quark charges, so the anti-up quark has charge −2/3 e.

Q18. Which of the following is NOT a lepton?

  • (A) Muon
  • (B) Tau particle
  • (C) Electron neutrino
  • (D) Proton
Show Answer

Answer: D – The muon, the tau and the neutrinos are all leptons, in three families alongside the electron. The proton is a baryon, a hadron made of quarks.

Q19. What are the charge and baryon number of an electron neutrino?

  • (A) Charge 0, baryon number 0
  • (B) Charge 0, baryon number +1
  • (C) Charge −1 e, baryon number 0
  • (D) Charge +1 e, baryon number 0
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Answer: A – Neutrinos are uncharged leptons, so both their charge and their baryon number are zero. They do carry lepton number +1.

Q20. In the decay n → p + e− + electron antineutrino, which check confirms that charge is conserved?

  • (A) After the decay the total charge is +1 e
  • (B) Before: 0. After: (+1) + (−1) + 0 = 0
  • (C) The antineutrino carries the missing charge
  • (D) Charge does not need to be conserved in decays
Show Answer

Answer: B – The neutron has charge 0. After the decay the charges add to (+1 e) + (−1 e) + 0 = 0, so the total charge is unchanged, as it must be in every reaction.


Particle Reactions & Conservation Laws

Q21. Which equation correctly represents β− (beta-minus) decay of a neutron?

  • (A) n → p + e+ + electron neutrino
  • (B) n → p + e− + electron antineutrino
  • (C) n → p + e− + electron neutrino
  • (D) p → n + e− + electron antineutrino
Show Answer

Answer: B – In β− decay a neutron changes into a proton, emitting an electron and an electron antineutrino. Option C is wrong because the emitted lepton partner is the antineutrino, which keeps lepton number conserved: 0 = 0 + 1 + (−1).

Q22. At the quark level, what change takes place inside the neutron during β− decay?

  • (A) An up quark changes into a down quark
  • (B) A down quark changes into a strange quark
  • (C) An up quark changes into a strange quark
  • (D) A down quark changes into an up quark
Show Answer

Answer: D – The neutron (udd) becomes a proton (uud), so one down quark changes into an up quark, emitting the electron and antineutrino. The charge works out: −1/3 e (down) becomes +2/3 e (up), a rise of +1 e balanced by the −1 e electron.

Q23. In β+ (beta-plus) decay a proton changes into a neutron. What is the quark-level change?

  • (A) An up quark changes into a down quark
  • (B) A down quark changes into an up quark
  • (C) A strange quark changes into an up quark
  • (D) A down quark changes into a strange quark
Show Answer

Answer: A – The proton (uud) becomes a neutron (udd), so one up quark changes into a down quark, emitting a positron and an electron neutrino.

Q24. Which of the following reactions is NOT allowed because it violates conservation of lepton number?

  • (A) n → p + e− + electron antineutrino
  • (B) p → n + e+ + electron neutrino
  • (C) n → p + e− + electron neutrino
  • (D) e− + e+ → 2γ
Show Answer

Answer: C – Lepton number before = 0. In option C the electron has lepton number +1 and the neutrino also has +1, giving a total of +2 after the decay, so lepton number is not conserved. In β− decay the partner must be an antineutrino (lepton number −1), as in option A.

Q25. During β− decay, what happens to the total baryon number?

  • (A) It increases from 0 to +1
  • (B) It decreases from +1 to 0
  • (C) It changes from +1 to −1
  • (D) It stays at +1, because a neutron (B = +1) is replaced by a proton (B = +1) and leptons have B = 0
Show Answer

Answer: D – Baryon number is conserved in all reactions. Neutron and proton are both baryons with B = +1, and the electron and antineutrino are leptons with B = 0.

Q26. A hadron has the quark content udd. What is this particle?

  • (A) Proton
  • (B) Antineutron
  • (C) Neutron
  • (D) Positive pion
Show Answer

Answer: C – Three quarks make it a baryon, and its charge is (+2/3) + (−1/3) + (−1/3) = 0, so it is the neutron.

Q27. What is the quark content of an antineutron?

  • (A) udd
  • (B) uud
  • (C) One anti-up quark and two anti-up quarks
  • (D) One anti-up quark and two anti-down quarks
Show Answer

Answer: D – The antiparticle of a hadron is built from the corresponding antiquarks. The neutron is udd, so the antineutron is anti-up, anti-down, anti-down, with charge (−2/3) + (+1/3) + (+1/3) = 0 and baryon number −1.

Q28. The positive kaon (K+) is a meson made of an up quark and an anti-strange quark. Given that the anti-strange quark has charge +1/3 e, what is the charge of the K+, and what identifies it as a strange particle?

  • (A) Charge 0; it contains an up quark
  • (B) Charge +1 e; it contains a strange antiquark
  • (C) Charge +1 e; it contains three quarks
  • (D) Charge +2/3 e; it contains a down quark
Show Answer

Answer: B – Charge = (+2/3) + (+1/3) = +1 e. Particles containing a strange quark or antiquark are called strange particles; the strange quark itself carries strangeness −1 and the anti-strange +1.

Q29. Which of the following reactions is NOT allowed because it violates conservation of charge?

  • (A) n → p + e+ + electron antineutrino
  • (B) n → p + e− + electron antineutrino
  • (C) p → n + e+ + electron neutrino
  • (D) e+ + e− → 2γ
Show Answer

Answer: A – In option A the charge before is 0 but the charge after is (+1) + (+1) + 0 = +2 e, so charge is not conserved and the reaction cannot occur. All the other reactions conserve charge.

Q30. A baryon has the quark content uus. What are its charge and baryon number?

  • (A) Charge 0, baryon number +1
  • (B) Charge +1 e, baryon number 0
  • (C) Charge +1 e, baryon number +1
  • (D) Charge +2 e, baryon number +1
Show Answer

Answer: C – Charge = (+2/3) + (+2/3) + (−1/3) = +1 e. It has three quarks, so it is a baryon with B = 3 x (+1/3) = +1. This is the Sigma-plus particle.

Keep practising: work through more section-by-section question sets on the Physics MCQs hub, and keep every key equation in one place with the A Level Physics Formula Sheet.

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