AQA A Level Physics

Revision Notes

2.3.3 The Weak Interaction

The Weak Interaction

  • The weak interaction is responsible for the radioactive decay of atoms
  • The exchange particle that carries this force is the W, W+ or Z0 boson
    • The type of exchange particle depends on the type of interaction

β decay

  • β and β+ decay are examples of the weak interaction in action
  • In β decay, a neutron turns into a proton emitting an electron and an anti-electron neutrino

2.3.3 Beta Minus Equation

  • The W boson is the exchange particle in this interaction

Beta Minus Decay

Feynman diagram showing beta minus decay. The W boson is the exchange particle

  • In β+ decay, a proton turns into a neutron emitting a positron and an electron neutrino

2.3.3 Beta Plus Equation

  • The W+ boson is the exchange particle in this interaction

Beta Plus Decay

Feynman diagram showing beta plus decay. The W+ boson is the exchange particle

Electron Capture & Electron–Proton Collisions

  • Electrons and protons are attracted to each other via the electromagnetic interaction
    • However, when they interact with each other, it is the weak interaction that facilitates the collision
  • Both electron capture and electron-proton collisions have the same decay equation

2.3.3 Electron Capture Equation

  • Electron capture is when an atomic electron is absorbed by a proton in the nucleus resulting in the release of a neutron and an electron neutrino
    • This decay is mediated by the W+ boson
  • Electron-proton collisions are similar; when an electron collides with a proton, a neutron and an electron neutrino are emitted
    • This decay is mediated by the W boson

Electron Capture and Collision, downloadable AS & A Level Physics revision notes

Feynman diagrams for electron capture and an electron-proton collision. These are equal except for the sign of the W boson

Exam Tip

Notice that the sign of the W boson matches that of the beta decay. The W boson is exchanged in beta minus decay and W+ boson is exchanged in beta plus decay. The sign should reflect the total charge of the products (right-hand side) in the equation.

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