Physics Section 1 - Particles and Radiation

Physics Section 1 - Particles and Radiation

1. 1 ~ Matter and Radiation

   • 1.1 Inside the atom
     • positively charged nucleus composed of nucleons (p / n)
       • because +ve charge, held in atom by electrostatic force of attraction
       • Rutherford - most mass in nucleus
     • electrons (negatively charged) surround the nucleus
     • Comparisons
       • e has much smaller mass than p or n
       • p and n have almost equal mass
       • e has equal and opposite charge to p (n uncharged)
     • Isotopes
       • proton no. is called the atomic number (z) of the element
         • (bottom no.)
       • atom with same no. of p, different no.of n
       • total no. of p + n is nucleon no. (A) or sometime mass no.
         • almost numericallyequal to the mass of atom in relative units
         • (top no.)
       • label isotopes according to atomic no. Z, mass no. A, and chemical symbol of element
       • no. of neutrons = A-Z
       • each type of nucleus is called a nuclide
     • Specific charge
       • charge/mass
       • C kg^-1
   • 1.2 Stable and unstable nuclei
     • Strong nuclear force
       • overcomes electrostatic force of repulsion of p and n in nucleus
       • range no more than 3-4 femtometres (fm) (x10^-15)
         • same as diameter of nucleus
       • same effect between 2 p as 2 n or p + n
     • Radioactive decay
       • Alpha radiation
         • alpha particles - 2 p + 2 n
           • A=4
           • Z=2
         • alpha particle emitted from unstable nucleus
         • A decreases by 4, Z decreases by 2
           • ∴ different element
       • Beta radiation
         • beta -
           • beta - particle emitted
             • A=0
             • Z=-1
           • antineutrino emitted
           • Z increases by 1
             • ∴ different element
         • beta +
           • beta + particle emitted
             • A=0
             • Z=1
             • neutrino emitted
             • Z decreases by 1
               • ∴ different element
       • Gamma radiation
         • electromagnetic radiation emitted by unstable nucleus
         • can pass through thick metal plates
         • no mass or charge
         • emitted by nucleus with too much energy following alpha or beta emission
   • 1.3 Photons
     • Electromagnetic waves
       • all objects emit infrared radiation
       • In vacuum, all travel at 3.0 x 10^8 m/s
       • λ=c/f
       • transverse (right angles)
       • vibrate in phase
       • wavelength often measured in nm
     • Photons
       • em waves emitted by a charged particle when it loses energy
         • can happen when fast-moving electron stops, slows, or changes direction
         • can happen when e in a shell of an atom moves to a different shell of lower energy
       • em waves emitted as short bursts of waves
         • referred to as a photon
           • photon theory established by Einstein in 1905 to explain photoelectric effect
             • this is the emission of e from a metal surface when light is directed at surface
             • photon energy = hf
     • Laser power
       • laser beam consists of photons of same frequency
       • power of beam = nhf
         • n is no. of photons in the beam passing a fixed point each second
   • 1.4 Particles and antiparticles
     • Antimatter
       • antimatter and matter particles destroy each other when they meet and radiation is released
         • use this effect in a positron emitting tomography hospital scanner
           • two gamma photons emitted as result used to build op image
           • positron emitted when proton changes into a neutron in an unstable nucleus with too many protons
         • annihilation
       • predicted in 1928 by English Paul Dirac before first antiparticle discovered
         • total mass converted into photons
         • same rest mass
         • exactly opposite charge
         • also predicted pair production (opposite of annihilation)
           • photon must have sufficient energy
       • Einstein E=mc^2
         • when stationary, rest mass corresponds to rest energy
     • Particles, antiparticles and E=mc^2
       • 1Mev=1.60x10^-13J
         • 1ev is the energy transferred when an electron is moved through a p.d. of 1 volt
       • Annihilation produces two photons
         • Energy of each photon = hf (min) = E0
       • Pair production particle and antiparticle produced
         • min energy of photon needed = hf (min) = 2E0
   • 1.5 Particle interactions
     • Electromagnetic force
       • American Richard Feynman
         • em force between two charge particles is due to the exchange of virtual photons
         • Feynman diagrams
           • virtual photon represented by wave
     • Weak nuclear force
       • causes n to change to p in beta- decay and vice versa
       • W bosons
         • non-zero rest mass
         • v. short fange 0.001fm
         • charged ±
     • Strong nuclear force
       • hold n and p in the nucleus together
     • Beta decay
       • W- boson decays into beta - particle and an antineutrino
       • W+ boson decays into beta + particle and a neutrino
       • Charge always conserved
     • Electron capture
       • sometimes proton in proton-rich nucleus turns into neutron as result of interacting through weak interaction with inner-shell electron from outside nucleus
         • W+ boson
         • can also happen at high speed and with W- boson is electron has sufficient energy
       • p + e > n + v
     • Force carriers
       • W boson carries weak nuclear force
       • photon carries electrostatic force
       • pion carries strong nuclear force
       • graviton (not yet observed)

2. 2 ~ Quarks and Leptons

   • 2.1 The particle zoo
     • The Particle Zoo
     • Space invaders
       • cosmic rays are high-energy particles that travel through space from the stars, inc. the Sun
         • create new short-lived particles and antiparticles as well as photons
         • Investigation showed that most are fast-moving protons or small nuclei
         • collide with gas atoms in atmosphere, creating showers of particles and antiparticles that can be detected at ground level
       • cloud chambers and other detectors, new types of short-lived particles and antiparticles discovered
         • muon (heavy electron)
           • x200 rest mass of electron
         • pion (π meson)
           • rest mass greater than muon but less than proton
         • kaon (K meson)
           • rest mass greater than pion but less than proton
     • A strange puzzle
       • both pions and kaons produced in twos through the strong interaction
         • decay of kaons took longer than expected and included pions as product
       • decay modes
         • kaon can decay into πons/ a muon and antineutrino/ antimuon and neutrino
         • charged πon can decay into muon and antiuneutrino/ antimuon and neutrino
         • π0 decays into high-energy photons
         • muon decays into electron and antineutrino
         • antimuon decays into positron and neutrino
         • decay always observes conservation of energy, momentum and charge
   • 2.3 Leptons at work
     • Lepton collisions
       • leptons and antileptons can interact to produce hadrons
     • Neutrino types
       • travel almost as fast as light
       • produced in much smaller numbers when particles in accelerators collide
       • pairs produced in beta decays different to ones produced from muon decays
         • use different symbols and neutrino symbol for electron and muon neutrinos
     • Lepton rules
       • can change into other leptons through weak interaction and can be annihilated or produced in particle-antiparticle interactions
       • Still appear to be fundamental
       • in an interaction between a lepton and a hadron, a neutrino or antineutrino can change into or from a corresponding charged lepton
         • electron neutrino + n -> p +e-
       • in muon decay, muon changes into a muon neutrino and electron is created to conserve charge and corresponding antineutrino created to conserve lepton no.
       • lepton no. conserved in any change
     • A weak puzzle
       • muon can't change into muon antineutrino and electron can only be created with electron antineutrino
       • apply lepton conservation seperateley to electrons and muons
   • 2.2 Particle Sorting
     • Classifying particles and antiparticles
       • The Particle Zoo
         • The Particle Zoo
       • bar for antiparticle unless charged
       • hadrons
         • can interact through all 4 fundamental interations
           • interact though strong interaction and through em interaction if charged
         • apart from proton (stable), hadrons tend to decay through weak interaction
       • leptons
         • interact through weak interaction, gravitational interaction, and through em interaction if charged
     • Energy matters
       • using conservation of energy, rest energy of products = total energy before - kinetic energy of products
     • Baryons and mesons
       • when kaons created, short-lived particles with greater rest masses than protons may also be produced
         • created through strong interaction so hadrons
           • however, decay into protons and well as into πons
       • baryons are protons and all other hadrons that decay into protons (direct/indirect)
       • mesons are hadrons that do not include protons in their decay products (eg koans and πons)
       • hadrons as made from quarks/antiquarks
   • 2.4 Quarks and antiquarks
     • Strangeness
       • kaons are strange particles
       • although strange particles all decay through weak interaction:
         • those that decay into πons only were referred to as koans
         • others found to have different rest masses, all greater than proton's rest mass and decay either in sequence or directly into protons and πons
         • strange particles created in twos
       • conserved in strong interaction, can change by ±1 or not change in weak interactions
     • Quark model
       • properties of hadrons explained assuming they are composed of smaller particles known as quarks and antiquarks
       • See table on third page
         • The Particle Zoo
     • Quark combinations
       • mesons
         • 1 quark, 1 antiquark
       • baryons and antibaryons
         • 3 quarks for baryons, 3 antiquarks for antibaryons
     • Quarks and beta decay
       • in beta - decay, n in neutron-rich nucleus changes into a p, releasing an e and an electron antineutrino
         • d quark changes to u quark
       • in beta + decay, p in proton-rich nucleus changes into n, releasing a positron and an electron neutrino
         • u quark changes to d quark
   • 2.5 Conservation rules
     • Particles and properties
       • conservation of energy and conservation of charge
       • conservation of lepton no.s
       • conservation of strangeness in strong interaction
     • Are baryons and mesons conserved?
       • +1 baryon no. to baryons and -1 for antibaryons
       • baryon no. = 0 for meson or lepton
       • total baryon no. always conserved

3. 3 ~ Quantum Phenomena

   • 3.1 The photoelectric effect
     • Discovery of the photoelectric effect
       • Heinrich Hertz
         • sparks produced in spark gap detector when radio waves transmitted stronger when UV radiation directed at spark gap
         • further investigations on effect of em radiation on metals showed electrons emitted from metal surface when em radiation above certain f was directed at metal
           • known as photoelectric effect
     • Puzzling problems
       • these problems meant light couldn't be described as waves
         • photoelectric emission only occurs when em radiation is above threshold f. as λ=c/f, wavelength had maximum value
         • no. of e emitted /second proportional to intensity of incident radiation but f must still be greater than threshold
         • emission occurs without delay as soon as radiation is directed at the surface, provided f exceeds threshold
     • Einstein's explanation of the photoelectric effect
       • light composed of wave packets or photons
       • E=hf=hc/λ
       • electron can leave metal surface if energy gained from a single photon exceeds to work function of the metal
         • hf=E(kmax)+φ
         • f(min)=φ/h
     • Stopping potential
       • min. potential to stop photoelectric emission is called the stopping potential Vs
         • max. KE of emitted electron reduced to 0 as each emitted e must do extra work equal to e x Vs to leave the metal surface. Hence max. KE = e x Vs
   • 3.2 More about photoelectricity
     • Into the quantum world
       • Max Plank suggested energy of each vibrating atom is quantised (certain level of energy allowed)
         • h - Plank's constant
     • More about conduction electrons
       • conduction electrons in metal move at random
       • φ of metal is min. energy needed by a conduction electron to escape from the metal surface when the metal is at zero potential
     • The vacuum photocell
       • a vacuum photocell is a glass tube that contains a metal plate, referred to as the photocathode, and a smaller metal electrode referred to as the anode
       • when light of frequency greater than threshold for metal directed at photocathode, electrons emitted from cathode and attracted to anode. microammeter in circuit can be used to measure photoelectric current - proportional to no. of electrons/second that transfer from cathode to anode
       • no. of photoelectrons/second transferring from cathode to anode =I/e. e is charge of electron
       • photoelectric current proportional to intensity of light incident to cathode. intensity is measure of energy/second carried by incident light which is proportional to no. of photons/second incident on cathode
       • E(kmax)=hf-φ
       • max. KE of photoelectrons emitted for given f of light can be measured using a photocell
   • 3.3 Collisions of electrons with atoms
     • Ionisation
       • an ion is a charged atom
         • no. of e ≠ no. of p
         • formed from uncharged atoms by adding/removing e from atom
       • processes of ionisation
         • alpha, beta, gamma radiation create ions when they pass through substances and collide with the atoms of the substance
         • e passing through a flourescent tube create ions when they collide with the atoms of the gas or vapour in the tube
     • The electron volt
       • unit of energy equal to work done when an e is moved through a pd of 1 V
         • wd=qV
         • when moved through 1 V wd=1.6 x 10^-19J
     • Excitation by collision
       • gas atoms can absorb energy from colliding electrons without being ionised
       • excitation happens at certain energies which are characteristic of the atoms of the gas
       • electrons move from inner shells to outer shells
   • 3.4 Energy levels in atoms
     • Electrons in atoms
       • trapped by electrostatic force of attraction to nucleus
       • lowest energy state is called the ground state
       • when electron moves to higher energy shell, it is in an excited state
       • energy level diagrams
     • De-excitation
       • electron configuration in an excited atom is unstable because an electron that moves to an outer shell leaves a vacancy in the shell it moves from
         • vacancy filled by electron from outer shell transferring to it
           • when this happens, electron emits a photon
             • ∴ atom move to a lower energy level (de-excitation)
       • use energy levels to determine energy of photon
         • hf=E1-E2
     • Excitation using photons
       • electron can absorb a photon and move to an outer shell where a vacancy exists - but only if energy of photon is exactly equal to the gain in the electron's energy
     • Fluorescence
       • atom in excited state can de-excite directly or indirectly to the ground state, regardless of how excitation took place
       • an atom can absorb photons of certain energies and then emit photons of the same or lesser energies
       • this overall process explains why certain susbstances fluoresce (glow with visible light when they absorb UV radiation)
         • atoms in substance absorb UV photons and become excited
           • when they de-excite, visible photons are emitted
             • when source of UV radiation is removed, substance stops glowing
       • fluorescent tube is a glass tube with a fluorescent coating on the inner surface
         • contains mercury vapour at low pressure
         • when on, emits visible light because:
           • ionisation and excitation of mercury atoms occur as they collide with electrons in the tube
           • mercury atoms emit UV photons, as well as visible photons and photons of much less energy, when they de-excite
           • UV photons absorbed by atoms of fluorescent coating, causing excitation of atoms
           • coating atoms de-excite in steps and emit visible photon
   • 3.5 Energy levels and spectra
     • A colourful spectrum
       • white light has continuous spectrum
       • line spectrum
         • specific wavelengths for different elements
         • photons producing lines all have the same energy
         • photon emitted when atom de-excites
         • hf=E1-E2
   • 3.6 Wave-particle duality
     • The dual nature of light
       • light part of em spectrum
       • wave-like nature observed when diffraction of light takes place
       • particle-like nature is observed in the photoelectric effect
     • Matter waves
       • if light has dual wave-particle nature, perhaps particles of matter also have dual wave-particle nature
       • electrons in beam can be deflected by a magnetic field
         • evidence that electrons have particle-like nature
         • idea that matter particles have a wave-like nature was first considered by Louis de Broglie (1923)
           • de Broglie hypothesis
       • de Broglie hypothesis
         • matter particles have a dual wave-particle nature
         • wave-like nature characterised by de Broglie wavelength λ=h/p=h/mv
     • Evidence for de Broglie's hypothesis
       • 3 years after hypothesis, it was demonstrated that a beam of electrons can be diffracted
         • narrow beam of electrons directed at thin metal foil. the rows of atoms cause electrons in beam to be diffracted
           • diffracted in certain directions only and form pattern of rings

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