Particles, Antiparticles & Photons

Antimatter, the photon model, annihilation, pair production, and the four fundamental interactions.

Spec Points Covered
  • State the properties of a photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. and calculate photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. energyThe capacity to do work. Measured in joules (J). using $E = hf$ and $E = hc/\lambda$.
  • State the antiparticle of the electron, proton, neutron and neutrino.
  • Explain what is meant by rest mass-energyThe capacity to do work. Measured in joules (J)..
  • Describe annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energyThe capacity to do work. Measured in joules (J)., typically producing two gamma photons. and calculate the minimum energy/frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of the photons produced.
  • Describe pair productionThe creation of a particle-antiparticle pair from a high-energy photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. in the presence of a nucleus. The photon energy must be at least 2mc². and calculate the minimum photon energy required.
  • Name the four fundamental interactions and their exchange particles.
  • Describe the electromagnetic, strong and weak interactions in terms of exchange particles.
  • Draw and interpret Feynman diagrams for electromagnetic repulsion, beta decay, and electron capture.
Notes
01 Every particle has an antimatter counterpart with opposite charge 3.2.1.3 → 02 A photon is a quantum of electromagnetic energy Photon 3.2.1.3 → 03 Annihilation converts mass into photon energy Annihilation 3.2.1.3 → 04 Pair production creates a particle-antiparticle pair from a photon Pair production 3.2.1.3 → 05 Four fundamental interactions govern all particle physics 3.2.1.4 → 06 Exchange particles carry the fundamental forces 3.2.1.4 → 07 The weak interaction causes beta decay 3.2.1.4 → 08 Feynman diagrams represent particle interactions visually 3.2.1.4 →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Photon energy (frequency)
$$E = hf$$
  • Where:
    • $E$ = photon energy (J)
    • $h$ = Planck's constant (6.63 × 10⁻³⁴ J s)
    • $f$ = frequency (Hz)
Fundamental equation linking energy to frequency.
Minimum photon energy in annihilation
$$\begin{aligned} E_{\min} &= hf_{\min} \\ &= E_0 \end{aligned}$$
  • Where:
    • $E_min$ = minimum energy of one produced photon (J)
    • $E_0$ = rest mass-energy of one particle (J)
Two photons are produced. Each photon carries at least the rest mass-energy of one particle.
Photon energy (wavelength)
$$E = \frac{hc}{\lambda}$$
  • Where:
    • $E$ = photon energy (J)
    • $h$ = Planck's constant (J s)
    • $c$ = speed of light (m s⁻¹)
    • $λ$ = wavelength (m)
Derived from E = hf and c = fλ. Use when given wavelength instead of frequency.
Minimum photon energy for pair production
$$\begin{aligned} E_{\min} &= hf_{\min} \\ &= 2E_0 \end{aligned}$$
  • Where:
    • $E_min$ = minimum energy of incident photon (J)
    • $E_0$ = rest mass-energy of one particle (J)
Factor of 2 because both the particle and the antiparticle must be created.
Q Retrieval Practice All 12 Questions →
Q1. State three properties that a particle and its antiparticle share, and one property that differs.
  • Same: mass, rest mass-energy, magnitude of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
  • Different: sign of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). (opposite).
Q2. Name the antiparticle of the electron.
The positron (e⁺).
Q3. Define a photon.
A massless discrete packet (quantum) of electromagnetic energy.
Q4. Write two equations for photon energy.
E = hf and E = hc/λ, where h = Planck's constant, f = frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz)., c = speed of light, λ = wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)..
Q5. Define annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energy, typically producing two gamma photons. and state the products.
When a particle meets its corresponding antiparticle, both are destroyed and their mass converts into two gamma-ray photons emitted in opposite directions.