2.1. Particles

• What are the main constituents of an atom?

  ● Proton

  ● Neutron

  ● Electron
• What is meant by specific charge?

  The charge to mass ratio: Specific charge = charge / mass Units C/kg.
• What is the specific charge of a proton?

  Protons have charge +1.6 x 10^-19 and mass

  1.67 x 10^-27 kg

  Specific charge = 1.6 x 10^-19/1.67 x 10^-27

  = 9.58 x 10^7 C/kg
• What is the letter associated with a proton number?

  Z
• What is a nucleon?

  A constituent of the nucleus: a proton or a neutron.
• What letter represents nucleon number?

  A.
• What is an isotope?

  A version of an element with the same number of protons but a different number of neutrons.
• State a use of radioactive isotopes.


  Carbon dating - the proportion of carbon-14 in a material can be used to estimate its age.
• What is the strong nuclear force?

  The fundamental force that keeps the nucleus stable by counteracting the electrostatic force of repulsion between protons.
• Describe the range of the strong force?

  ● Repulsive up to 0.5 fm

  ● Attractive from 0.5-3 fm

  ● Negligible past 3 fm
• What makes a nucleus unstable?

  Nuclei which have too many of either protons or neutrons or both.
• How do nuclei with too many nucleons decay?

  Alpha decay (emission of a helium nucleus formed of 2 protons and 2 neutrons).
• How do nuclei with too many neutrons decay?

  Beta minus decay in which a neutron decays to a proton by the weak interaction (quark character has changed from udd to uud).
• How was the existence of the neutrino

  hypothesised?

  The energy of particles after beta decay was lower than before, a particle with 0 charge (to conserve charge) and negligible mass must carry away this excess energy, this particle is the neutrino.
• What is meant by beta minus decay?

  When a neutron turns into a proton, the atom releases an electron and an anti-electron neutrino.
• What is an alpha particle?

  A particle contains two protons and two neutrons, the same as a helium nucleus
• What is an antiparticle?

  For each particle there is an antiparticle with the same rest energy and mass but all other properties are the opposite of its respective particle.
• True or false: ‘Every particle has a antiparticle’

  True.
• What is the name of the antiparticle of an electron?

  Positron
• What is the antiparticle of π0 (pion with 0 charge)?


  π0, its antiparticle is itself.
• What occurs when a particle and antiparticle meet?

  Annihilation:

  The mass of the particle and antiparticle is converted back to energy in the form of 2 gamma ray photons which go in opposite directions to conserve momentum.
• What is pair production?

  A gamma ray photon is converted into a particle-antiparticle pair.
• Name the 4 fundamental forces?

  ● Gravity

  ● Electromagnetic

  ● Weak nuclear

  ● Strong nuclear
• The virtual photon is the exchange particle of which force?

  The electromagnetic force.
• What type of particles are affected by the strong nuclear force?

  Hadrons.
• What is the exchange particle of the

  weak nuclear force?

  The W boson (W+ or W-).
• What does the electromagnetic force act on?

  It acts on charged objects, for example when a positively charged ball repels another positively charged ball.
• When does weak nuclear interaction occur?

  When quark character changes (a quark changes into another quark), it affects all types of particles.
• Which properties must be conserved in particle interactions?

  ● Energy

  ● Charge

  ● Baryon number

  ● Lepton number

  ● Momentum

  ● Strangeness (only for strong interactions)
• What is a hadron?

  Both baryons and mesons are hadrons, hadrons are made of 2 or more quarks held together by the strong nuclear force.
• What are the classes of hadrons?

  ● Baryons (three quarks)

  ● Mesons (1 quark, 1 antiquark)
• The pion and kaon are both examples of which class of particle?

  Mesons.
• The pion can be an exchange particle for which force?

  The strong nuclear force.
• What particle does a kaon decay into?

  A kaon decays into a pion.
• Give some examples of baryons?


  ● Proton - uud

  ● Neutron - ddu
• What is significant about a proton?

  ● It is the only stable baryon

  ● All baryons will eventually decay into protons
• What are some example of leptons?

  ● Electron

  ● Muon

  ● Neutrino

  ● (the antiparticles of the above)
• What does a muon decay into?

  An electron and two types of neutrino.
• What is the strangeness value of a strange quark?

  -1
• True or false: ‘Strangeness is always conserved in a weak interaction’

  False.Strangeness is only conserved in the strong interaction, in weak interactions it can change by 0, -1 and +1.