knowledge-3 POP

• Hadrons:

  • affected by the strong nuclear force
  • not fundamental
• leptons:

  • fundamental
  • not affected by the strong nuclear force
  • can change into other leptons via the weak interaction
• Baryons (type of hadron):

  • made up of three quarks
  • eventually decay into protons
• mesons (type of hadron):

  • made up of one quark and anti-quark pair
  • instable and do not include protons in their decay products
• Nucleons (type of baryon):

  • protons and neutrons
  • antiparticles are antiprotons and antineutrons
• other baryons:

  • sigma plus, minus and zero
  • contain strange quark
• pions (type of meson):

  • the exchange particle for strong nuclear force
  • contain quarks and anti-quarks only (no strange quarks)
• kaons (type of mesons):

  • have a short lifetime
  • decay into pions
  • contain quarks and antiquarks (including strange particle)
• electrons:

  • antiparticle is the positron
• muons:

  • heavy and decay intro electrons
  • -1 charge
• neutrinos:

  • antiparticle is the anti-neutrino
• strangeness is a property only hadrons have, leptons have a strangeness of 0.
• strangeness is conserved in strong interaction but can change by 0 , +1 , -1 in weak interactions.
• strange particles:

  • are always produced through strong interaction and are always produced in pairs to conserve strangeness
  • always decay via weak interaction.
• in beta decay a neutron changes into a proton , releasing an electron and an electron antineutrino. This is because a down quark becomes a up quark via weak interaction.
• Beta plus decay has the reverse change in quarks compared to beta minus.
• the photoelectric effect is the emission of electrons from the surface of a metal when electromagnetic radiation above a certain frequency is incident on it.
• E=hf= hc/wavelength

  • e= energy
  • h= plancks constant
  • f= frequency of em radiation
  • c = speed of em radiation.
• electrons emitted due to photoelectric effect are called photoelectrons.
• the minimum energy an electron needs to break bonds and escape the metal surface is called the work function.
• the minimum frequency for photoelectric emission of electrons to take place is called the threshold frequency.
• photoelectric effect : below threshold frequency, no electrons are emitted from the metal.

  • when light is incident on the metal surface lots of photons hit the surface but only one electron can absorb one photon
  • if the energy gained by the electron is greater than the metals work function then the electron is emitted from the surface.
• photelectric effect: photoelectrons are emitted with a range of kinetic energies

  • the intensity of radiation is the number of photons per second per unit area
  • increasing intensity results in more photoelectrons because there are more individual photons to be absorbed by electrons but each photon has the same energy and can only interact with one electron.
  • if frequency is increased beyond threshold excess energy is gained by the photoelectron and becomes kinetic energy.
  • electrons lose different amounts of energy dependent on the energy level they are emitted from.
• stopping potential is the minimum potential needed to stop photoelectric emission.
• stopping potential:

  A photocell uses photoelectric effect to produce the current in a circuit. light of an appropriate frequency is shone upon the cathode , which emits electrons that travel across the vacuum to the anode. if the anode is made negative by applying an external p.d the photelectric effect can be slowed down and stopped.
• stopping potential:

  graph of stopping potential against threshold frequency has properties:

  • gradient = h/e
  • y-intercept = -work function/h
• energy levels and electrons:

  • electrons in their lowest energy inside an atom is called ground state
  • electrons move up energy levels if they absorb enough energy from collisions with photons. this is known as excitation
  • electrons move down energy levels when they emit photons. this is know as de-excitation
  • energy absorbed or = hf = e1 -e2
• line spectra: provides evidence for transitions between discrete energy levels in atoms

  • each coloured line in line emission corresponds to a wavelength
  • the photons that produce each line all have the same energy unique to the line
  • each photon is emitted when an atom is de-excited
  • an atom only emits particular wavelengths of light because the electrons in it can only emit photons with energies equal to the difference in two energy levels.
• Fluorescence: when a fluorescent substance absorbs UV radiation, their electrons absorb uv photons and move to higher energy levels

  • they emit photons of visible light as they drop back to lower energy levels
  • a fluorescent tube is a glass tube filled with mercury vapour with a fluorescent coating on its inner surface.
  • when in use a high voltage is applied to accelerate free electrons in the mercury vapour.
  • mercury atoms are ionized and excited through collision emitting uv photons.
• ionization: ionization is where an electron gains enough energy to leave the atom and become a free electron

  • ionization energy is the energy required to emit an electron from the ground state of an atom.
• Wave-particle duality:

  light has a dual nature- it can behave as either a wave or particle depending on the circumstances

  • diffraction models light as a wave
  • photoelectric effect models light as a particle
  • de Broglie hypothesized that particles of matter should have wave properties
  • his equation = h/p = h/mv
• diffraction of electrons:

  the observation that electrons can be diffracted to produce interference patterns provides evidence of their wave properties.
• diffraction of electrons:

  • electron diffraction can be observed through firing a beam of electrons at a thin sheet of graphite in a vacuum
  • the pattern formed on the screen is a result of electrons being diffracted as they pass through rows of atoms
  • they then interfere with each other producing maxima and minima
  • wavelength = h/mv