There are four fundamental forces;
- 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 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
Classification of Particles
Quarks & Anti-quarks
You may have learnt that there are many different types of forces such as friction and tension. However only four fundamental forces are at a basic level responsible for all interactions in the universe.
Strong Nuclear Force
This type of force is only experienced by baryons (hadrons with 3 quarks). The exchange particles for this force are gluons. This force is responsible for keeping the protons and neutrons in a atomic nucleus together. The strong nuclear force only acts over a short range of 3fm. Between the distance of 3fm and 0.5fm separation the strong nuclear force is attractive. Below 0.5fm separation the strong nuclear force is repulsive.
Weak Nuclear Force
This type of force is experienced by both baryons and leptons. The exchange particle for this type of force is the W or the K boson. This force is responsible for decay interactions (such as beta decay) and interactions where strangeness is not conserved or where quarks change flavour (or type).
This type of force is experienced by particles which are charged (such as electrons and protons). The exchange particle for this type of force is the virtual photon. This force is responsible for the repulsion of two electrons and attraction of a electron to a proton.