AQA A Level Physics

Revision Notes

2.3.1 The Four Fundamental Interactions

The Four Fundamental Interactions

  • There are four fundamental interactions, or fundamental forces, that exist. These are:
    • Gravity
    • Electromagnetism
    • Strong Nuclear (or Strong Interaction)
    • Weak Nuclear
  • Gravity is the weakest of these forces, whilst the strong interaction is the strongest (hence the name)
  • They also have different ranges:
    • The electromagnetic and gravitational interactions have an infinite range
    • The weak force has a range of ~ 10–8 m
    • The strong force has a range of ~ 10–15 m

The Four Fundamental Interaction, downloadable AS & A Level Physics revision notes

The fundamental interactions in order of strength

  • The gravitational interaction only affects particles with mass
  • The electromagnetic interaction only affects particles with charge
  • The weak interaction affects all particles
  • The strong interaction only affects hadrons

Exchange Particles

  • When two particles interact, there cannot be instantaneous action at a distance
    • This means one particle needs to “know” that the other is there
  • This is the idea behind exchange (or virtual) particles
  • When two particles exert a force on each other, a virtual particle is created
  • Virtual particles only exist for a short amount of time and carry the fundamental force between each particle
  • A force can be attractive or repulsive. An analogy of exchange particles would be:
    • Two people are on skateboards and a ball is passed between them. Due to this, they start to move away from each other. The ball represents an exchange particle creating repulsion
    • However, if one person throws a boomerang to the other, they will start to move closer together. The boomerang represents an exchange particle creating attraction

Exchange Particles Analogy

Both skateboarders can create repulsion or attraction by throwing a ball or boomerang between them. The ball and boomerang represent exchange particles.

  • Each fundamental interaction is transmitted by its own exchange particle
    • These are also called gauge bosons

Table of Exchange Particles


  • Since gravity is so weak, it only has a noticeable effect on large masses, therefore, gravity does not play a part in particle interactions
  • The theorised exchange particle for the gravitational force is the graviton, however, this has not yet been discovered

Exam Tip

You will be expected to remember which exchange particle mediates which fundamental interaction for your exam. This is particularly important when drawing Feynman diagrams, as the correct exchange particle will be expected to be on them.

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