Particle Physics · revision map · A-Level 9702, Topic 11

TOPIC 11 · PARTICLE PHYSICS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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PARTICLE PHYSICS11
The Nuclear Atom

Firing alpha particles at thin gold foil revealed a tiny, dense, positive nucleus at the centre of mostly empty space.

  • Most pass straight through → the atom is largely empty.
  • A few deflect through large angles → the nucleus is positively charged.
  • Very few bounce back → the nucleus is tiny and dense, holding nearly all the mass.
gold foil nucleus most pass through deflected bounced back
nucleus ≈ 10−15 m · atom ≈ 10−10 m
Nuclide Notation, Isotopes & Decay

Label the nucleus, then balance the books. Proton number Z sets the element; nucleon number A = protons + neutrons.

A Z X A = nucleons Z = protons X = symbol
  • Neutrons = A − Z.
  • Isotopes: same Z, different N.
  • Mass unit: u · 1 u = 1.66 × 10−27 kg.

Every decay conserves nucleon number and charge; A and Z balance on both sides.

A = Z + N
Fundamental Particles & Forces

Matter is built from two families of fundamental particles, held together by the four forces.

Quarks
up (+⅔e), down (−⅓e)
Leptons
electron, neutrino
  • Quarks feel the strong force; this binds nucleons.
  • The weak force changes quark flavour in beta decay.
  • Electromagnetic force acts on charge; gravity on mass.
  • Leptons are not made of quarks; they are fundamental.
Hadrons: Baryons & Mesons

Quarks combine to make hadrons, which feel the strong force.

  • Baryons = three quarks (qqq).
  • Mesons = a quark and an antiquark (q q̄).
proton = uud
neutron = udd
u u d u: +⅔e · +⅔e d: −⅓e proton (uud) total charge +1e
Antiparticles & Beta Decay

Every particle has an antiparticle: same mass, opposite charge. The positron is the anti-electron.

  • Beta-minus: a neutron becomes a proton as a down quark turns into an up quark, emitting an electron and an antineutrino.
  • Beta-plus: a proton becomes a neutron as an up quark turns into a down quark, emitting a positron and a neutrino.
  • Both conserve charge and baryon number.
β−: n → p + e− + ν̄e · d → u
β+: p → n + e+ + νe · u → d d u down up e− ν̄e
BUILDS ON · Forces · The atom (IGCSE) → PARTICLE PHYSICS → LEADS TO · Quantum physics · Nuclear physics
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