The Four Fundamental Interactions (AQA A Level Physics): Revision Note

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 up to 10^–18 m

  • The strong force has a range of ~ 10^–15 m

• 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

• Each fundamental interaction is transmitted by its own exchange particle

  • These are also called gauge bosons

• 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