AQA A Level Physics 3.2 Particles and Radiation Exam Questions

About AQA A Level Physics 3.2 Particles and Radiation

Particles and Radiation is usually the first new content on the AQA A Level Physics course and is examined on Paper 1. Section 3.2.1 covers the constituents of the atom, stable and unstable nuclei, antiparticles, the four fundamental interactions, the classification of hadrons and leptons, quarks and the conservation laws used to check whether a reaction can happen. Section 3.2.2 covers the photoelectric effect, collisions of electrons with atoms, energy levels and line spectra, and wave-particle duality. Examiners like Feynman diagrams, checking conservation of baryon number, lepton number and strangeness, and photoelectric calculations in both joules and electronvolts. The ideas return later in nuclear physics and the Turning Points option.

What you need to know

• 3.2.1 Constituents of the atom Know the charge, mass and specific charge (charge divided by mass, in C kg-1) of protons, neutrons, electrons and nuclei, and use nucleon number A and proton number Z in nuclide notation. Isotopes have the same Z but different numbers of neutrons.
• 3.2.1 Particles and antiparticles Every particle has an antiparticle with the same rest energy but opposite charge, baryon number and lepton number. In pair production a photon of energy at least 2 times the rest energy creates a particle-antiparticle pair; in annihilation the pair becomes two photons.
• 3.2.1 Interactions The four fundamental interactions are gravity, electromagnetic, weak and strong. Exchange particles: virtual photon (electromagnetic), W+ and W- bosons (weak), pions (strong between nucleons). Draw Feynman diagrams for beta decay, electron capture and electron-proton collisions.
• 3.2.1 Classification and quarks Hadrons (baryons qqq and mesons q anti-q) feel the strong force; leptons (electron, muon, neutrinos) do not. The proton is uud, the neutron udd, pions and kaons are mesons; kaons contain a strange quark and decay into pions. Strangeness is conserved in strong interactions but can change by 1 in weak interactions.
• 3.2.1 Conservation laws Charge, baryon number, electron lepton number and muon lepton number are always conserved, along with energy and momentum. Use these to decide if a reaction is possible and to identify missing particles.
• 3.2.2 Photoelectric effect Use hf = work function + Ek(max). Below the threshold frequency no electrons are emitted whatever the intensity. Stopping potential Vs satisfies eVs = Ek(max). The photon model explains why emission is instantaneous and why intensity changes only the number of electrons.
• 3.2.2 Energy levels and spectra Electrons in atoms occupy discrete energy levels. Excitation and ionisation happen by electron collision or photon absorption; de-excitation emits photons with hf = E1 - E2, giving line spectra. Know how a fluorescent tube works and convert between eV and J (1 eV = 1.60 x 10^-19 J).
• 3.2.2 Wave-particle duality Electron diffraction is evidence that particles behave as waves. The de Broglie wavelength is h/mv, so faster electrons have shorter wavelengths and diffract less.

Common mistakes in 3.2 Particles and Radiation

• Saying that higher intensity light gives electrons more kinetic energy. Intensity only increases the number of photons per second, so more electrons are emitted; Ek(max) depends on frequency.
• Forgetting to convert between electronvolts and joules, for example subtracting a work function in eV from a photon energy in J. Put both in the same unit first.
• Treating mesons as baryons or giving kaons a baryon number of 1. Mesons have baryon number 0 because they are a quark and an antiquark.
• Writing that strangeness is conserved in all interactions. It is conserved in strong interactions only, so kaon decay must be a weak interaction.
• Drawing the exchange particle in beta-minus decay as a W+ boson. A neutron changes to a proton by emitting a W-, which decays into an electron and an electron antineutrino.

Try three 3.2 Particles and Radiation exam-style questions