Interactions between particles


  • Classify proton, neutron and electron into lepton and hadron.
  • State the fundamental forces of nature and their relative strengths.
  • Understand how a force can be caused by the exchange of a particle.
  • Accept that electromagnetic interactions are the result of the exchange of virtual photons.
  • Construct simple Feynman diagrams to represent electromagnetic interactions.
  • Understand that the strong force must involve a massive particle.



  • Feynman diagrams are a useful tool in particle physics. If the diagram is possible then so is the interaction.
  • Classification of particles into different groups helps predict interactions.


  • Is the driving force behind particle physics to develop new technology or are these just spin offs?

International Mindedness

  • Big particle accelerators like the one at CERN are international projects.


So far in the course we have introduced the proton, neutron, photon, neutrino and electron so best to start by classifying them and the interactions they take part in.

  • Electromagnetic for charged particles
  • Strong for nucleons
  • Weak for neutrinos

Probably worth memorising relative strength and range of the different forces.

Although throwing a sack of potatoes from one canoe to another isn't a correct picture of an exchange force it gets the idea across and the introduction of a boomerang adds a bit of fun. HL student will know about Heisenbergs uncertainty principle so can understand why virtual particles have to be short lived.

Students only need to deal with simple, one vertex Feynman diagrams (hopefully). The two electron - one photon example shows nicely how interactions can be predicted by rotating the arms.

I have been trying to find an applet for building Feynman diagrams but can't find one, here is an animation (time on the y axis). In the end I made my own simple version in GeoGebra.


Particle interactions test

Text Book

pages 313 - 316


Exercises 36 - 37

Vocabulary list

Exchange force


Feynman diagrams in GeoGebra

Move the electron and positron around to make different interactions. I couldn't make a wavy line so you have to make do with a dashed one for a photon.

And with 4 particles

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