Particle Interactions

There are four fundamental forces;

  • gravity
  • electromagnetic force
  • strong nuclear force
  • weak nuclear force

To explain the forces between particles we use the concept of exchange particles or bosons.

Gravity – gravitons

All particles with mass attract each other with the force of gravity, the mechanism by which particles attract each other is through the exchange of particles called gravitons (as yet undetected).

Electromagnetic force – virtual photons

The virtual photon is the exchange particle (or boson) which carries the electromagnetic force between charged particles. Particles with electric charges can either attract or repel each other by exchanging particles called virtual photons.

Strong nuclear force – gluons

Gluons are the exchange particles involved in the strong nuclear force interaction.

Weak nuclear force – W+, W- bosons

W and Z  bosons ( in A-level we just need the W+ and W- bosons) are the exchange particles  involved in the weak nuclear force interaction.

The weak force acts within the  nucleus, quarks and leptons excerpt forces on each other by exchanging bosons. The weak nuclear force is very weak and acts over a very small distance.

Feynman diagrams

Feynman diagrams are visual representations of particle interactions which also show the exchange particles involved.

Repulsion between electrons (e-)


Above two electrons exchange a photon (g) as they repel each other.

A proton (p) and an electron (e-) combine to form a neutron (n) and a neutrino (n).


The exchange particle in the above interaction is a W+ boson.


More Feynman Diagrams for A-level

In diagrams 4, 5 and 6, the exchange particle in each case could be the oppositely charged W particle travelling in the opposite direction.

Links to other pages in this topic;

Constituents of the Atom

Stable and Unstable Nuclei

Particles, Antiparticles & Photons

Particle Interactions

Classification of Particles

Quarks & Anti-quarks