How do the four fundamental forces act between particles through the exchange of virtual particles?
The four fundamental interactions, the concept of exchange particles (gauge bosons), the W bosons, the photon and the pion, and the use of Feynman diagrams to represent interactions such as beta decay and electron-proton collisions.
A focused answer to AQA A-Level Physics 3.2.1.4, covering the four fundamental forces, exchange (gauge) bosons, the W bosons and the photon, and how to draw and read Feynman diagrams for beta decay and electron-proton interactions.
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What this dot point is asking
AQA specification point 3.2.1.4 wants you to know the four fundamental interactions, explain forces as the exchange of virtual particles, name the exchange particle for each force, and draw Feynman diagrams for beta-minus and beta-plus decay and for electron-proton collisions.
The four fundamental forces
The W bosons are unusual exchange particles because they have mass, which limits the range of the weak force to a very short distance; the photon is massless, giving the electromagnetic force its infinite range.
Exchange particles
A force between two particles is modelled as the exchange of a virtual particle. One particle emits the exchange particle and the other absorbs it, transferring momentum and energy, which produces the force between them. These exchange particles are also called gauge bosons, and they are described as virtual because they exist only fleetingly during the interaction and are not directly observed.
Feynman diagrams
To draw one correctly, label every line with the particle and check that each quantum number balances on the two sides of each vertex.
In beta-plus decay a proton becomes a neutron, emitting a that decays into a positron and an electron neutrino. In electron capture and electron-proton collisions a W boson is again exchanged.
Try this
Q1. Name the exchange particle for the electromagnetic force and for the weak interaction. [2 marks]
- Cue. Virtual photon for electromagnetic; W bosons for the weak interaction.
Q2. State the conservation rules that must hold at every vertex of a Feynman diagram. [2 marks]
- Cue. Charge, baryon number and lepton number are conserved.
Q3. State why the weak force has a very short range. [1 mark]
- Cue. Its exchange particles (the W bosons) have a large mass.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe how the Feynman diagram for beta-minus decay represents the interaction, naming the exchange particle and the products, and state the conservation laws that apply at each vertex.Show worked answer β
At the first vertex a neutron changes into a proton, emitting a boson. Charge is conserved: the neutron () becomes a proton () plus the ().
The boson is the exchange particle (an internal line). At the second vertex it decays into an electron and an electron antineutrino, conserving lepton number ( and sum to zero).
Charge, baryon number and lepton number are conserved at every vertex.
Markers reward the neutron-to-proton change emitting a , the decay into electron plus antineutrino, and naming the conserved quantities.
AQA 20213 marksState the four fundamental interactions and name the exchange particle associated with the electromagnetic force, the weak interaction and the strong force between nucleons.Show worked answer β
The four fundamental interactions are gravity, the electromagnetic force, the weak nuclear force and the strong nuclear force.
The electromagnetic force is carried by the virtual photon; the weak interaction by the W bosons ( and ); and the strong force between nucleons is modelled at A-Level by the pion.
Markers reward naming all four interactions and correctly pairing the photon, W bosons and pion with their forces.
Related dot points
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Sources & how we know this
- AQA A-level Physics (7408) specification β AQA (2017)