Particle Interactions

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The Four Fundamental Forces

Nature is governed by four fundamental forces:

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  • Gravity is the weakest of the four forces.
    • It is so weak that its effects are only noticeable for huge masses like stars and planets.
  • Gravity is a purely attractive force.
  • Gravity is experienced by all matter.
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Strong nuclear force

  • The strong nuclear force is the strongest of the four fundamental forces.
  • But, it can only be felt over a very short range (a few fm).
  • The strong nuclear force is only experienced by hadrons (e.g. protons and neutrons).
  • The strong nuclear force is attractive for separations above about 0.5 fm, but strongly repulsive for separations less than about 0.5 fm.
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Weak nuclear force

  • The weak nuclear force affects all types of particles.
  • The weak nuclear force is responsible for beta-plus and beta-minus decay, as well as electron capture interactions.
  • The weak nuclear force is a very weak, very short range force.
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Electromagnetic force

  • The electromagnetic force is very strong and has a very large range.
  • The electromagnetic force is responsible for interaction between charged objects like electrons and protons.
  • Most everyday forces we experience are because of the electromagnetic force.
    • When you touch an object, the force you feel is because of the repulsion between the outer electrons on the object and your hand.

Exchange Particles

Two objects cannot interact instantaneously over a finite distance. For the objects to feel a force, an exchange particle must travel from one to the other.

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Types of exchange particle

  • Each fundamental force has its own exchange particle.
  • The gluon/pion is the exchange particle for the strong nuclear force.
  • The exchange particle of the electromagnetic force is called a 'virtual photon' (virtual because they only exist for a very short time).
  • The weak nuclear force has three exchange particles: the W+, W- and Z0 bosons.
  • Exchange particles are sometimes called gauge bosons.
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  • Think of two particles as skaters on an ice rink (with no friction).
  • If one skater throws a ball straight at the other, both skaters will be pushed further apart as momentum is exchanged.
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  • Think again about our two skaters.
  • Imagine this time that the boomerang is thrown away from himself.
  • The boomerang circles round to the other skater and pushes him towards the first skater, bringing them closer.

Feynman Diagrams

We can represent particle interactions using Feynman diagrams.

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General rules

  • Particles start at the bottom and move upwards.
  • Particles have straight lines. Exchange particles have wiggly lines.
  • Hadrons must stay on the left, leptons on the right.
  • Particles cannot cross paths, they can only interact via an exchange particle.
  • The charge entering a node must equal the charge leaving the node.
  • A W+ boson travelling from left to right is the same as a W- boson travelling from right to left.
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Electromagnetic repulsion

  • Two electrons repel each other because of the electromagnetic force.
  • The exchange particle is a virtual photon.
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Beta-minus decay

  • n → p + e- + νe
  • A neutron decays into a proton and W- boson which then decays into an electron and an electron antineutrino.
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Beta-plus decay

  • p → n + e+ + νe
  • A proton decays into a neutron and W+ boson, which then decays into a positron and an electron neutrino.
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Electron capture

  • p + e- → n + νe
  • A proton interacts with an electron via a W+ boson, producing a neutron and an electron neutrino.
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Electron-proton collision

  • p + e- → n + νe
  • The equation is identical to electron capture but the diagram is different.
    • The diagram shows an electron colliding with a proton via the W- boson (travelling the other way to a W+ boson), producing a neutron and an electron neutrino.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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