How much of AQA A-Level Physics is module 3.2 Particles and radiation?

Particles and radiation is one of the largest content modules and is examined mainly on Paper 1. It splits into two halves: particle physics (the atom, forces, hadrons, leptons and quarks) and quantum phenomena (the photoelectric effect, energy levels and wave-particle duality). It rewards careful learning of definitions and conservation laws, with a smaller set of calculations around photon energy, specific charge and de Broglie wavelength.

What conservation laws do I need for particle interactions?

Charge, baryon number and lepton number are always conserved. Lepton number is conserved separately for each family (electron and muon). Strangeness is conserved in the strong and electromagnetic interactions but can change by zero or plus-or-minus one in the weak interaction. Checking each of these quickly tells you whether a proposed interaction is allowed.

Why is the photoelectric effect such an important topic?

It is the clearest evidence for the particle (photon) nature of light, because the wave model cannot explain the threshold frequency or the instant emission of electrons. The photoelectric equation, the work function and the link between photon energy and frequency appear repeatedly, and the topic links forward to energy levels and wave-particle duality.

What calculations recur across module 3.2?

Specific charge from charge and mass, photon energy with E equals hf or hc over lambda, the energy in annihilation and pair production from rest energies in MeV, the energy of an emitted photon from a difference of energy levels, and the de Broglie wavelength from momentum. Converting between electronvolts and joules and between MeV and joules is essential, so practise it until it is automatic.

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AQA A-Level Physics 3.2 Particles and radiation: a complete overview of the atom, particles, quarks and quantum phenomena

A deep-dive AQA A-Level Physics guide to module 3.2 Particles and radiation. Covers the constituents of the atom, the strong force and radioactive decay, antiparticles and photons, exchange particles and Feynman diagrams, the classification of hadrons and leptons, quarks, the photoelectric effect, energy levels and wave-particle duality, with the conservation laws and calculations AQA repeats.

Generated by Claude Opus 4.822 min read3.2Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What module 3.2 actually demands
The atom and radioactive decay
Antimatter, photons and forces
Hadrons, leptons and quarks
Quantum phenomena
How module 3.2 is examined
Check your knowledge

What module 3.2 actually demands

Particles and radiation is where AQA A-Level Physics meets modern physics. The module runs from the structure of the atom, through the fundamental forces and the particles that carry them, to the quark model of matter, and then into the quantum phenomena that revealed the dual nature of light and matter. The examiners test two linked skills: precise recall of definitions and conservation laws, and a small but important set of calculations.

This guide walks through the nine topics of the module in specification order, then sets out the exam patterns AQA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

The atom and radioactive decay

The module opens with the constituents of the atom: protons, neutrons and electrons, their relative charges and masses, proton and nucleon number, isotopes, nuclide notation and specific charge. It then introduces the strong nuclear force, attractive from about $3 \text{ fm}$ to about $0.5 \text{ fm}$ and repulsive below that, which holds the nucleus together against electrostatic repulsion.

Unstable nuclei decay by emitting alpha, beta-minus or gamma radiation. The continuous energy spectrum of beta particles was the evidence that predicted the neutrino, needed to conserve energy and momentum.

Antimatter, photons and forces

Every particle has an antiparticle of equal mass and rest energy but opposite charge. Light comes in photons of energy $E = hf$ . These ideas drive annihilation (a particle and antiparticle produce two photons) and pair production (a photon makes a particle-antiparticle pair), both calculated from rest energies in MeV.

The four fundamental forces act through exchange particles: the virtual photon (electromagnetic), the W bosons (weak) and the pion (strong, between nucleons). Feynman diagrams represent these interactions, with charge, baryon number and lepton number conserved at every vertex.

Hadrons, leptons and quarks

Particles are classified as hadrons (which feel the strong force, split into baryons and mesons) or leptons (which do not). The conservation laws for charge, baryon number, lepton number and strangeness decide which interactions are allowed.

Underlying the hadrons are quarks: up ( $+\frac{2}{3}$ ), down ( $-\frac{1}{3}$ ) and strange ( $-\frac{1}{3}$ , strangeness $-1$ ). The proton is uud and the neutron is udd, and beta-minus decay is a down quark turning into an up quark.

Quantum phenomena

The photoelectric effect shows the particle nature of light: there is a threshold frequency below which no electrons are emitted, explained by $hf = \phi + E_{k(max)}$ . Energy levels in atoms are discrete, so de-excitation emits photons of fixed energy ( $hf = E_1 - E_2$ ), giving line spectra and powering the fluorescent tube.

Finally, wave-particle duality unites the two pictures: light is both wave and particle, and de Broglie showed matter has a wavelength $\lambda = \frac{h}{p}$ , confirmed by electron diffraction.

How module 3.2 is examined

A typical AQA profile for particles and radiation:

Recall and definitions. Relative charges and masses, types of radiation, the properties of quarks and leptons, and the meaning of coherence and work function.
Conservation-law questions. Deciding whether an interaction is allowed by checking charge, baryon number, lepton number and strangeness.
Calculations. Specific charge, photon energy, annihilation and pair-production energies, energy-level transitions and de Broglie wavelength.
Explanation questions. Why the photoelectric effect needs the photon model, how the neutrino was predicted, and how electron diffraction shows the wave nature of matter.

Check your knowledge

A mix of recall and calculation questions covering module 3.2. Attempt them, then check against the solutions.

State the relative charge and relative mass of a proton, a neutron and an electron. (3 marks)
State the range over which the strong nuclear force is attractive. (1 mark)
Write the equation for beta-minus decay of a neutron, including the antineutrino. (2 marks)
Calculate the minimum energy of each photon produced when an electron and a positron annihilate (rest energy $0.51 \text{ MeV}$ each). (2 marks)
Name the exchange particle for the weak interaction and for the electromagnetic interaction. (2 marks)
State the quark composition of the proton and the neutron. (2 marks)
Light of frequency $7.0 \times 10^{14} \text{ Hz}$ strikes a metal of work function $3.1 \times 10^{-19} \text{ J}$ . Find the maximum kinetic energy of an emitted electron. (3 marks)
State the de Broglie equation and explain what electron diffraction demonstrates. (2 marks)

Solutions

Eight questions spanning the constituents of the atom, radioactive decay, antimatter, exchange particles, quarks and quantum phenomena. Work through each in turn.

Step 1: Q1 - Relative charge and mass of proton, neutron and electron

These are fundamental data that must be recalled precisely. The proton and neutron each have a relative mass of 1 (to the nearest whole number), while the electron's mass is negligible by comparison at roughly 1/1840 of a proton mass.

Proton: charge $+1$ , relative mass $1$ .
Neutron: charge $0$ , relative mass $1$ .
Electron: charge $-1$ , relative mass approximately $\dfrac{1}{1840}$ .

Step 2: Q2 - Range over which the strong nuclear force is attractive

The strong nuclear force has a very short range and changes character depending on the separation of nucleons. It is attractive over most of its range but becomes repulsive at very small separations, preventing nuclear collapse.

The strong nuclear force is attractive from about $3$ fm down to about $0.5$ fm; it is repulsive below $0.5$ fm.

Step 3: Q3 - Beta-minus decay equation for a neutron

In beta-minus decay a down quark converts to an up quark, so a neutron becomes a proton. The equation must conserve charge, baryon number and lepton number, which requires an electron (the beta particle) and an electron antineutrino.

{}^{1}_{0}\text{n} \rightarrow {}^{1}_{1}\text{p} + {}^{0}_{-1}\text{e} + \overline{\nu}_e

Step 4: Q4 - Minimum photon energy in electron-positron annihilation

When an electron and a positron annihilate, conservation of energy requires the total rest energy of the pair to appear as photon energy. The minimum occurs when the particles are at rest and exactly two photons are produced, sharing the energy equally so that momentum is also conserved.

E_{\text{total}} = 2 \times 0.51 = 1.02 \text{ MeV}

Each photon carries a minimum of $0.51$ MeV.

Final answer: minimum photon energy $= 0.51$ MeV.

Step 5: Q5 - Exchange particles for fundamental forces

Each fundamental force is mediated by a specific exchange particle. Knowing which particle carries which force is a recall requirement.

Weak interaction: the W bosons ( $\text{W}^+$ and $\text{W}^-$ ).
Electromagnetic interaction: the virtual photon.

Step 6: Q6 - Quark composition of the proton and neutron

The proton and neutron are both baryons composed of three quarks (up, u, and down, d). The charges of the three quarks must sum to the total charge of the baryon.

Proton: uud (charge $= +\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$ ).
Neutron: udd (charge $= +\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$ ).

Step 7: Q7 - Maximum kinetic energy from the photoelectric effect

The photoelectric equation states that the photon energy $hf$ is shared between the work done in escaping the metal (the work function $\phi$ ) and the maximum kinetic energy of the emitted electron. First calculate $hf$ , then subtract $\phi$ .

hf = 6.63 \times 10^{-34} \times 7.0 \times 10^{14} = 4.64 \times 10^{-19} \text{ J}

E_{k(\text{max})} = hf - \phi = 4.64 \times 10^{-19} - 3.1 \times 10^{-19} = 1.5 \times 10^{-19} \text{ J}

Final answer: $E_{k(\text{max})} = 1.5 \times 10^{-19}$ J.

Step 8: Q8 - De Broglie equation and what electron diffraction shows

The de Broglie equation assigns a wavelength to any particle by relating it to the particle's momentum. Electron diffraction provides experimental evidence for this wave nature: if electrons pass through a crystal and form a diffraction pattern, they must be behaving as waves.

\lambda = \frac{h}{p}

Electron diffraction (electrons forming a diffraction pattern when passing through a crystal lattice) demonstrates that matter has a wave nature, because diffraction is a wave property.

Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)