CIE A Level Physics

Revision Notes

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First teaching 2020

Last exams 2024

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11.2.1 Fundamental Particles

Test Yourself

Fundamental Particles: Quarks

  • Quarks are fundamental particles that make up other subatomic particles such as protons and neutrons
  • Protons and neutrons are in a category of particles called hadrons
    • Hadrons are defined as any particle made up of quarks

  • Fundamental means that quarks are not made up of any other particles. Another example is electrons
  • Quarks have never been observed on their own, they’re either in pairs or groups of three
  • There are six flavours (types) of quarks that exist:

Quarks, downloadable AS & A Level Physics revision notes

The six flavours of quarks

Properties of Quarks

  • The charge of a hadron is determined by the sum of the charges of its quarks
  • Each flavour of quark has a certain relative charge:

 Charge of quarks, downloadable AS & A Level Physics revision notes

Each flavour of quark has a charge of either +⅔e or -⅓e

  • For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

+⅔e + ⅔e - ⅓e = +1e

  • The equivalent antiparticle of the quark is the anti-quark
  • These are identical to quarks except with opposite relative charges

Charge of anti-quarks, downloadable AS & A Level Physics revision notes

Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks

Worked example

Particles are made up of a combination of three quarks or two quarks. Which quark combination would not give a particle a charge of -1 or 0?

A. up, strange, strange

B. charm, charm, down

C. top, anti-up

D. anti-up, anti-up, anti-strange

Exam Tip

You will be expected to remember the charge of each quark. However, instead of memorising the charges of anti-quarks too, just remember they are identical but with opposite signs.

Fundamental Particles: Leptons

  • Leptons are a group of fundamental (elementary) particles
  • This means they are not made up of any other particles (no quarks)
  • There are six leptons altogether:

 

Leptons, downloadable AS & A Level Physics revision notes

The six leptons are all fundamental particles

  • The muon and tau particle are very similar to the electron but with slightly larger mass
  • Electrons, muon and tau particles all have a charge of -1e and a mass of 0.0005u
  • There are three flavours (types) of neutrinos (electron, muon, tau)
  • Neutrinos are the most abundant leptons in the universe
    • They have no charge and negligible mass (almost 0)

  • Leptons interact with the weak interaction, electromagnetic and gravitational forces
  • However, they do not interact with the strong force
  • Although quarks are fundamental particles too, they are not classed as leptons
  • Leptons do not interact with the strong force, whilst quarks do

Worked example

Circle all the anti-leptons in the following decay equation.WE - Leptons question image, downloadable AS & A Level Physics revision notes

Worked example - leptons, downloadable AS & A Level Physics revision notes

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