Particles: quantum

• 1 eV is equal to 1.6e-19 J
• photons are packets of EM waves with a discrete (fixed) amount of energy
• the definition of a work function is the minimum energy required to liberate an electron from the surface of a metal . every metal has its own work function value.
• 1. the definition of the threshold frequency is the minimum photon frequency that will cause photoelectrons to be released.
  2. threshold frequency (Hz) = work function(J)/ plancks constant(Js)
• photoelectrons have a range of kinetic energies due to some electrons are not already at the surface therefore some energy is used to bring them to the surface first
• stopping potential is the (reversed) potential difference needed across the plates to stop the fastest moving photoelectron from reaching the other plate after being released.
• • emitted electrons have a range of kinetic energies up to a maximum value because the energy of a photon is fixed/ energy given to an electron is fixed
  • electron has to over come work function
  • maximum kinetic energy is the energy of photon minus the work function.
  • deeper electrons require energy to get to the surface.
• the photoelectric effect is a phenomenon that shows that light behaves as a particle
• describe the photoelectric effect

  when light above a certain frequency is shone on a metal, electrons are released - these electrons are called "photoelectrons"
• the energy of a photon is determined by its frequency
• if a photon has a frequency higher than the threshold frequency the electron will be liberated and the remaining energy is the kinetic energy of the electron
• what is an electron volt

  the kinetic energy of an electron that has been accelerated from rest through a potential difference of 1V
• $hf=$$ϕ +$$Ek$

  photoelectric effect
• The threshold frequency couldn’t be explained by the wave theory, as it suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave
• the photon model of light suggests:

  • em waves travel in discrete packets called photons, which have an energy which is directly proportional to frequency
  • each electron can absorb a single photon, therefore a photoelectron can only be emitted if the frequency is above the threshold frequency
  • if the intensity of the light is increased, if the frequency is above the threshold, more photoelectrons are emitted per second.
• measuring stopping potential allows you to find the maximum kinetic energy of the released photoelectrons, as $Ek(max) =$$e Vs$
• Electrons in atoms can only exist in discrete energy levels
• Excitation: The process of an electron taking in exactly the right quantity of energy to move to a higher energy level
• Ionisation: The process of an atom losing an orbital electron and becoming charged
• electrons can gain enough energy to be removed from the atom entirely, this is called ionisation
• Ionisation occurs if the energy of the free electron is greater than the ionisation energy.
• If an electron becomes excited, it will quickly return to its original energy level (the ground state), and therefore release the energy it gained in the form of a photon
• An example of a practical use of excitation is in a fluorescent tube in order to produce light. Fluorescent tubes are filled with mercury vapour, across which a high voltage is applied.

  
• fluorescent tubes:

  • the voltage accelerates free electrons through the tube, which collide with the mercury atoms causing them to become ionised, releasing more free electrons.
  • The free electrons collide with the mercury atoms, causing them to become excited. When they de-excite they release photons.
  • The (phosphorous) fluorescent coating on the inside of the tube, absorbs these UV photons.
  • therefore electrons in the atoms of the coating become excited and de-excite releasing photons of visible light.
• for a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value, called the threshold frquency
• • the photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value
  • the maximum kinetic energy value increases with the frequency of the radiation
• the intensity of radiation is the amount of energy per second hitting an area of the metal.

  the maximum kinetic energy of the photoelectrons is unaffected by varying the intensity of the radiation
• wave theory cant explain the fact that the kinetic energy depends only on the frequency in the photoelectric effect.
• the photon model of light could be used to explain the photoelectric effect
• einstein saw these photons of light as having a one-to-one, particle like interaction with an electron in a metal surface.

  each photon would transfer all its energy to one specific electron
• Ground State

  The most stable energy level that an electron can exist in
• electrons in an atom can move up and down between energy levels by absorbing or emitting a photon
• the energy carried by a photon emitted after a transition is equal to the difference in energies between the two levels of the transition.
• electrons also (excitation) move up energy levels if they absorb a photon with the exact energy difference between the two levels.
• By passing the light from a fluorescent tube through a diffraction grating or prism, you get a line spectrum.
• each line in the spectrum will represent a different wavelength of light emitted by the tube
• line spectra provide evidence that the electrons in atoms exist in discrete energy levels
• the spectrum of white light is continuous
• continuous spectra


  all the wavelengths are allowed because the electrons are not confined to energy levels in the object producing the continuous spectrum

  the electrons are not bound to atoms and are free.
• line absorbtion spectra

  • occurs when light with a continuous spectrum of energy(white light) passes through a cool gas.