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LearnAQA7408First teaching 2015Paper 1 · Paper 1Particles and radiation

Paper 1 · Module

Particles and radiation

6 subtopics≈ 225 min in total

Start the module: Atoms, nuclei and radiation Open graphing calculator

Atoms, nuclei and radiation
The α-scattering experiment showed that an atom's mass and positive charge are concentrated in a tiny nucleus. A nuclide is written $^A_Z\text{X}$ ( $A$ nucleons, $Z$ protons). Unstable nuclei emit α (a helium nucleus), β⁻ (an electron plus an antineutrino), β⁺ (a positron plus a neutrino) or γ (a photon). Nucleon number and charge are conserved in every decay.
≈ 35 min
Photons and the photoelectric effect
Light comes in quanta called photons, each of energy $E=hf=\dfrac{hc}\lambda$ ( $h=6.63\times10^{-34}$ J s) and momentum $p=\dfrac Ec$ . When photons hit a metal, each can release one electron if its energy exceeds the work function $\Phi$ : $hf=\Phi+\tfrac12mv_{\max}^2$ . Below the threshold frequency $f_0=\dfrac\Phi h$ , no electrons are emitted, however bright the light. Brighter light means more photons, so more electrons, but not faster ones.
≈ 40 min
Fundamental particles
Protons and neutrons are made of quarks. There are six flavours: up ( $+\tfrac23e$ ), down ( $-\tfrac13e$ ), strange ( $-\tfrac13e$ ), charm ( $+\tfrac23e$ ), top ( $+\tfrac23e$ ) and bottom ( $-\tfrac13e$ ), each with an antiquark of opposite charge. Hadrons are baryons (three quarks) or mesons (quark + antiquark). Leptons, such as electrons and neutrinos, are fundamental. In β⁻ decay a down quark becomes an up quark; in β⁺, up becomes down.
≈ 35 min
Wave–particle duality
Light behaves as a wave (diffraction, interference) and as particles (the photoelectric effect). Matter does too: de Broglie proposed that any moving particle has a wavelength $\lambda=\dfrac hp=\dfrac h{mv}$ . Electron diffraction through thin graphite proves it: electrons form rings like waves passing through a grating. Faster electrons have shorter wavelengths, so the rings get smaller.
≈ 35 min
Energy levels and line spectra
Electrons in an isolated atom can only have certain discrete energies, called energy levels (negative values, with zero meaning the electron is free). When an electron drops from level $E_1$ to a lower level $E_2$ , it emits a photon with $hf=E_1-E_2$ . Because only certain differences exist, only certain wavelengths appear: a line spectrum. An absorption spectrum shows dark lines where the atoms absorb exactly those photon energies from a continuous spectrum.
≈ 35 min
Particle interactions
Every particle has an antiparticle with the same mass and opposite charge. They can annihilate into photons, and a photon with enough energy can create a particle–antiparticle pair: $E_{\min}=2mc^2$ . Forces act by exchange particles (photon, W bosons, gluons). A reaction can happen only if charge, baryon number and lepton number are all conserved; strangeness is conserved in strong interactions.
≈ 45 min