AS AQA Phys Unit 1- Particle Interaction

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Electromagnetic force

Forces can be measured using newtonmeters and force sensors. Forces are caused by particle exchange and exchange particles are called gauge bosons. 

When a single force acts on an object, it changes the momentum of an object:

Momentum of an object = mass * velocity

When two objects interact, they exert equal and opposite forces on each other. Momentum is transferred between the objects by these forces if no other forces act on them. E.g. if two protons approach each other they repel. The American physicist, Richard Feynmann stated the electromagnetic force (repulsion) between two charged objects is due to the exchange of virtual photons -these are gauge bosons of the electromagnetic force. These photons cannot be seen directly but if they were intercepted the force would stop acting (this can be seen in a Feynmann diagram).

Feynmann diagram: Gauge bosons are represented by 'wriggly' lines, other particles by straight lines.

Particle exchange

There are two forms of particle exchange. These can be explained by imagining two people on skateboards with a ball or boomerang acting as the exchange particle (gauge boson):

1) Repulsion - Each time a ball is thrown or caught the people are pushed apart. This happens because the ball carries momentum.

2) Attraction - Each time the boomerang is thrown or caught the people are pushed together.

Gauge bosons only exist for a very short time as they are virtual particles.


All forces in nature are caused by four fundamental forces. Each one has a specialised gauge boson, the names of which must be learnt:

Interaction type: strong Gauge boson: gluon Particles affected: hadrons only

electromagnetic, photon, charged particles only

weak, W^+ boson W^- boson Z^0 boson, all types

gravity, graviton?, all types

Gravity is usually ignored in particle physics as it only applies on a large scale (i.e. planets, stars etc.) - there is no evidence for a graviton.

Weak nuclear force

This is the force which…


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