Fundamental particles
You already know the atom holds protons, neutrons and electrons — but the twentieth century revealed a deeper layer. The proton and the neutron are NOT fundamental: each is built from three even smaller particles called quarks. In this Advanced Subsidiary lesson we meet the up, down and strange quarks and their strange fractional charges (+2/3, −1/3, −1/3), build the proton as u u d and the neutron as u d d and prove the charges add up, and sort every particle into hadrons (baryons and mesons, made of quarks) versus leptons (the electron and neutrino, which are fundamental). We meet antiparticles, learn how the weak interaction changes one quark into another, and re-read beta-minus and beta-plus decay as a single quark flipping type — d → u or u → d — while an electron or positron and a (anti)neutrino carry away the balance. Throughout, we lean on one unbreakable rule: charge is conserved in every particle reaction.
By the end you should be able to (NSSCAS Physics (AS) 4.2):
- Appreciate that protons and neutrons are not fundamental particles, because each is made of quarks
- Describe a simple quark model of hadrons in terms of up, down and strange quarks and their antiquarks, recalling the fractional charges +2/3 (up) and −1/3 (down and strange)
- Describe the proton (u u d) and the neutron (u d d) in terms of the simple quark model and show that the quark charges add up correctly
- Classify particles as hadrons (baryons and mesons) or leptons (electron and neutrino), and distinguish particles from antiparticles
- Appreciate that there is a weak interaction between quarks that gives rise to beta decay
- Describe beta-minus and beta-plus decay in terms of a simple quark model, as a down quark changing to an up quark (or up to down) with emission of the appropriate lepton and (anti)neutrino
- Apply the conservation of charge to particle reactions, using it to check whether a proposed reaction is allowed
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.