AQA Physics A-Level: Section 2 - Particles and Radiation

3.2.1 Particles

3.2.1.1 Constituents of the Atom

• Atoms consist of protons, neutrons, and electrons.
• Protons and neutrons form the nucleus (nucleons); electrons orbit the nucleus.
• Properties of particles described in SI and relative units:
  • Proton: Charge = +1.6 x 10^-19 C, Relative Mass = 1
  • Neutron: Charge = 0, Relative Mass = 1
  • Electron: Charge = -1.6 x 10^-19 C, Relative Mass = 0.0005
• Specific charge: Charge-mass ratio, important for protons.
• Proton number (Z): Number of protons.
• Nucleon number (A): Number of protons and neutrons.
• Isotopes: Atoms with the same protons but different neutrons, e.g., carbon-14 for carbon dating.

3.2.1.2 Stable and Unstable Nuclei

• Strong Nuclear Force (SNF): Stabilizes nucleus by counteracting proton repulsion.
  • Range: Attractive up to 3 fm, repulsive below 0.5 fm.
• Unstable Nuclei: Decay to become stable; type of decay depends on nucleon composition:
  • Alpha Decay: Large nuclei, reduces proton number by 2 and nucleon number by 4.
  • Beta-minus Decay: Neutron-rich nuclei, increases proton number by 1.
  • Discovery of neutrinos to conserve energy during beta decay.

3.2.1.3 Particles, Antiparticles, and Photons

• Antiparticles: Have same mass but opposite properties to particles (e.g., positron for electron).
• Photons: Massless packets of electromagnetic radiation energy, energy proportional to frequency.
• Annihilation: Particle-antiparticle collision converting mass to energy (e.g., PET scanner uses).
• Pair Production: Photon converts to matter and antimatter.

3.2.1.4 Particle Interactions

• Four fundamental forces: gravity, electromagnetic, weak nuclear, strong nuclear.
• Exchange Particles: Carry energy/momentum between particles.
  • Strong Force: Gluons, acts on hadrons.
  • Weak Force: W bosons, acts on all particles.
  • Electromagnetic Force: Virtual photons, acts on charged particles.
• Weak force responsible for beta decay, electron capture.

3.2.1.5 Classification of Particles

• Particles: Hadrons (experience strong force) and Leptons (do not experience strong force).
• Hadrons: Composed of quarks, classified into baryons, antibaryons, and mesons.
  • Baryons: 3 quarks (e.g., protons, neutrons).
  • Mesons: Quark-antiquark pairs.
• Baryon Number: Conserved in interactions, 1 for baryons, -1 for antibaryons.
• Lepton Number: Conserved, with types for electron and muon.
• Strange Particles: Created by strong interaction, decay via weak interaction (e.g., kaons).

3.2.1.6 Quarks and Antiquarks

• Fundamental particles forming hadrons; types: up, down, strange.
• Quark Properties:
  • Up (u): +2/3 charge, baryon number +1/3.
  • Down (d): -1/3 charge, baryon number +1/3.
  • Strange (s): -1/3 charge, baryon number +1/3, strangeness -1.
• Meson Combinations: Various quark-antiquark pairs, e.g., kaons.

3.2.1.7 Applications of Conservation Laws

• Conservation laws: energy, momentum, charge, baryon number, lepton numbers.
• Strangeness conserved in strong interactions, not necessarily in weak.
• Example: Beta-minus decay, conservation of properties.

3.2.2 Electromagnetic Radiation and Quantum Phenomena

3.2.2.1 The Photoelectric Effect

• Photoelectrons emitted when light above a threshold frequency shines on metal.
• Wave theory vs. photon model of light:
  • Photon Model: EM waves travel as photons, energy proportional to frequency.
  • Threshold frequency needed for photon energy to eject electrons.
• Work Function: Minimum energy to emit electrons, denoted by ( \phi ).
• Stopping Potential: Potential needed to stop photoelectrons.

3.2.2.2 Collisions of Electrons with Atoms

• Electrons exist in discrete energy levels; can be excited or ionized.
• Excitation: Electrons absorb energy, move to higher energy levels.
• Ionization: Electrons gain sufficient energy to be ejected.
• Fluorescent tubes: Use excitation and de-excitation to emit light.

3.2.2.3 Energy Levels and Photon Emission

• Line Spectrum: Discrete energy levels evidenced by specific photon energies.
• Line Absorption Spectrum: Shows absorbed photon energies corresponding to energy level differences.

3.2.2.4 Wave-Particle Duality

• Light and electrons exhibit both wave and particle properties.
• De Broglie Hypothesis: Particles have wave-like properties, wavelength related to momentum.
• Wave-particle duality accepted through evidence like electron diffraction.

These notes summarize the key concepts from the AQA Physics A-Level curriculum, specifically focusing on particles and radiation, conservation laws, and quantum phenomena.