CCEA A Level Physics
CCEA A Level Physics 5.8 Fundamental Particles Exam Questions
Free CCEA A Level Physics revision for 5.8 Fundamental Particles: what you need to know, common mistakes and exam-style questions with answers. Covers fundamental forces, exchange particles, gauge bosons, leptons, hadrons, baryons, mesons, quarks, conservation laws and beta decay in the quark model.
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About CCEA A Level Physics 5.8 Fundamental Particles
Fundamental particles ends Unit A2 2: Fields, Capacitors and Particle Physics and the CCEA A level content. You need to name the four fundamental forces and their exchange particles, and classify particles as gauge bosons, leptons or hadrons, with hadrons split into baryons and mesons. Quark structure explains the charges of protons, neutrons and mesons, and you must apply conservation of charge, baryon number and lepton number to decide whether reactions can occur. Beta decay is then described in terms of quarks and the W boson. Examiners commonly ask for quark compositions, conservation checks on given reactions and a quark-level account of beta decay.
What you need to know
- Fundamental forces Strong (holds quarks and nucleons together), electromagnetic, weak (responsible for beta decay) and gravitational. Exchange particles: gluon, photon, W+, W- and Z0 bosons, and the hypothesised graviton.
- Particle classes Gauge bosons carry forces. Leptons (electron, muon, tau and their neutrinos) are fundamental and do not feel the strong force. Hadrons are made of quarks and feel the strong force.
- Baryons and mesons Baryons contain three quarks, e.g. proton uud and neutron udd. Mesons contain a quark and an antiquark, e.g. a pi-plus meson is u and anti-d.
- Quark properties The up quark has charge +2/3 e and the down quark -1/3 e; each quark has baryon number +1/3 and each antiquark -1/3 with opposite charge.
- Conservation laws In all interactions charge, baryon number and lepton number are conserved. Leptons have lepton number +1, antileptons -1, and all other particles 0.
- Beta decay in the quark model In beta-minus decay a down quark changes into an up quark via the weak interaction, emitting a W- boson which decays into an electron and an electron antineutrino: n becomes p + e- + antineutrino.
Common mistakes in 5.8 Fundamental Particles
- Calling a proton or neutron a fundamental particle; they are made of quarks.
- Forgetting the antineutrino in beta-minus decay, which means lepton number is not conserved.
- Saying leptons feel the strong force; only hadrons do.
- Giving a meson three quarks; mesons are quark-antiquark pairs.
- Mixing up exchange particles, such as saying the photon carries the weak force.
Try three 5.8 Fundamental Particles exam-style questions
1. Show that the quark composition uud gives the proton a charge of +1e. (2 marks)
Show the mark scheme answer
Charge = (+2/3) + (+2/3) + (-1/3); = +1 (in units of e).
2. Use conservation laws to show whether p + p becomes p + p + n is possible. (3 marks)
Show the mark scheme answer
Charge: +2 before, +2 after, conserved; baryon number: 2 before, 3 after, not conserved; so the reaction cannot occur.
3. Describe beta-minus decay in terms of quarks and exchange particles. (3 marks)
Show the mark scheme answer
A down quark in a neutron changes to an up quark, so the neutron becomes a proton; via the weak interaction with a W- boson as exchange particle; the W- decays into an electron and an electron antineutrino.
5.8 Fundamental Particles FAQs
What does CCEA A Level Physics 5.8 Fundamental Particles cover?
The four fundamental forces and exchange particles, classification of particles into gauge bosons, leptons and hadrons, quark structure, conservation of charge, baryon number and lepton number, and beta decay in the quark model.
Which unit is 5.8 examined in?
It is in Unit A2 2: Fields, Capacitors and Particle Physics.
How do I check whether a particle reaction can happen?
Add up charge, baryon number and lepton number on each side. If any of them differs, the reaction cannot occur. Show each total clearly for full marks.
