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Simple model of the atom
Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.
Charge and mass of proton, neutron and electron in SI units
Proton: charge +1.60x10^-19 C, mass 1.67x10^-27 kg Neutron: charge 0 C, mass 1.67x10^-27 kg Electron: charge -1.60x10^19 C , mass 9.11x10^-31 kg
Charge and mass of proton, neutron and electron in relative units
Proton: charge +1, mass 1 Neutron: charge 0, mass 1 Electron: charge -1, mass 1/1840
What is specific charge?
Charge divided by mass, unit C kg-1
What is proton number (atomic number)?
Number of protons in the nucleus, symbol Z
What is nucleon number (mass number)?
Number of protons and neutrons in the nucleus, symbol A
Example of nuclide notation
What are isotopes?
Atoms with the same number of protons and different number of neutrons
What is the role of the strong nuclear force?
Overcomes electrostatic repulsion between protons and keeps the nucleus stable
How does the strong nuclear force vary with separation?
Closer than 0.5fm - repulsive Between 0.5-3.0fm - attractive Further than 3.0fm - no effect / zero
What is alpha decay?
Unstable nucleus emits alpha particle (helium nucleus) Equation: X(A,Z) -> Y(A-4,Z-2) + α(4,2)
What is beta (minus) decay?
A neutron in the nucleus changes into a proton and emits fast-moving electron and electron antineutrino Equation: X(A,Z) -> Y(A,Z+1) + e-(0,-1) + _νe(0,0)
Why was the existence of the neutrino hypothesised?
To account for conservation of energy in beta decay. Observation showed energy of particles after beta decay was less than it was before. Some of the energy must had been carried away by undetected particles (neutrino).
What is antiparticle?
For every type of particle, there is a corresponding antiparticle
Comparison of particle and antiparticle masses, charge and rest energy
Particle and its corresponding particle have equal masses and rest energy, but opposite charge.
Antiparticles of the electron, proton, neutron and neutrino
Positron, antiproton, antineutron, antineutrino
Photon model of electromagnetic radiation
Electromagnetic waves are emitted as discrete wavepackets and each wavepacket is referred to as a photon. E = hf, E = hc/λ E = photon energy, J h = planck constant, 6.63x10^-34 J s f = frequency, Hz c = speed of light, 3.00x10^8 m s-1 λ = wavelength, m
What is annihilation?
A particle and a corresponding antiparticle meet and their mass is converted into radiation energy as two photons. Two photons are produced in this process to conserve momentum.
What is pair production?
A photon interacts with a nucleus or an electron and creates a particle-antiparticle pair, its radiation energy is converted into mass.
Energies involved in annihilation and pair production
Rest energy and kinetic energy of the particle-antiparticle pair is equal to the energy of the photon / two photons
What are the four fundamental interactions?
Gravity, electromagnetic, weak nuclear, strong nuclear/interaction
What is the concept of exchange particles?
Exchange particles are transferred between particles when a force acts between them. Exchange particles transfer energy and momentum.
What is the electromagnetic force?
The force that acts between charged particles
What is the exchange particle of the electromagnetic force?
Virtual photons - they have zero mass, infinite range and no charge.
What is the weak interaction?
The force that is responsible for β- decay, β+ decay, electron capture and electron-proton collisions.
What is exchange particle of the weak interaction?
W bosons - they have a non-zero rest mass, a short range of no more than 0.001fm, and are positively charged (W+ boson) or negatively charged (W- boson).
Feynman diagram: β- decay
Feynman diagram: β+ decay
Feynman diagram: electron capture
Feynman diagram: electron-proton collisions
What are hadrons?
Particles that are subject to the strong interaction
The two classes of hadrons
Baryons / antibaryons and mesons
What are (anti)baryons?
Particles that consist of three (anti)quarks
Examples of baryons and antibaryons
Baryons: proton, neutron Antibaryons: antiproton, antineutron
What are mesons?
Particles that consist of one quark and one antiquark
What is baryon number?
A quantum number that must be conserved in all interactions. Baryon: 1 Antibaryon: -1 Non-baryon: 0
What is the only stable baryon?
The proton, into which other baryons eventually decay
What is the pion?
The exchange particle of the strong nuclear force
What is the kaon?
A strange particle that can decay into pions
What are leptons?
Leptons are fundamental particles and are not subject to the strong interaction
Example of leptons and anti-leptons
Leptons: Electron (e-), muon (μ-), electron neutrino (νe), muon neutrino (νμ) Their antiparticles: Positron (e+), anti-muon (μ+), electron antineutrino (_νe), muon antineutrino (_νμ)
What is lepton number?
A quantum number that must be conserved in all interactions; lepton number for electron leptons and muon leptons must be conserved in all interactions. Lepton: 1 Anti-lepton: -1 Non-lepton: 0
What is the muon?
A particle that decays into an electron
What are strange particles?
Strange particles contain strange quark. They are produced through the strong interaction and decay through the weak interaction.
What is strangeness?
A quantum number to reflect the fact that strange particles are always created in pairs. It is conserved in strong interaction but can change by 0, +1 or -1 in weak interaction. Strange quark: -1 Anti-strange quark: 1
Properties of quarks and antiquarks
Quark combinations of hadrons
Proton: u u d Neutron: u d d Antiproton: _u _u _d Antineutron: _u _d _d π+: u _d π-: _u d π0: u _u, d _d, s _s K+: u _s K-: _u s K0: d _s _K0: _d s
Decay of the neutron
n -> p + e- + _ve
Change of quark character in β- decay
d -> u + e- + _ve
Change of quark character in β+ decay
u -> d + e+ + ve
What are conserved in interactions?
Energy, charge, momentum, baryon number, lepton number are conserved in all interactions. Strangeness is not conserved in weak interaction.
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