4.5 - Quantum physics

• Photon

  A quantum pack of electromagnetic energy
• Photo-electron

  An electronic that has escaped form the atoms shells after gaining energy from light
• Threshold frequency

  The minimum frequency of a photon that can release an atomic electron
• Ground state

  The minimum energy an electron in orbit can have
• Excitation

  An electron gaining energy, enabling it to jump up energy levels
• Wave particle duality

  All moving particles have wavelike properties and particle like properties
• Photo electric effect

  The emission of electrons from the surface of a metal when electromagnetic waves of higher frequency of the threshold frequency are incident on the metal
• State one peice of evidence of wave like properties of electrons

  Electron diffraction
• What material is used for electron diffraction

  Polycrystalline graphite
• 1 electron volt is equal to how many joules 


  1.6x10^-19
• 1 electron volt is equal to how many joules 


  1.6x10^-19
• De broglie equation

  h/p
• De-excitation

  When an electron moves back down to its original energy level level, by giving out the exact amount of energy
• De-excitation

  When an electron moves back down to its original energy level level, by giving out the exact amount of energy
• Define an electronvolt

  The kinetic energy gained by a electron when accelerated through a pd of 1V
• Describe the photoelectric effect (4 marks)

  wave model can't explain instantaneous emission Only photons with energy above the work function will cause photoelectron emission Frequency is proportional to energy of a photon hf-work function=KEmax Individual photons absorbed by Individual electrons
• Differences between photons and electrons

  Electrons have mass photons have zero mass Electrons have charge photons are unchanged Photons travel at the speed of light
• Does red or blue light give out more photons per second

  Red as photons have less energy
• Einstein equation for photoelectric effect

  hf= work function + max kinetic energy
• Example of a wave acting like a particle

  Photoelectric effect
• How do we give orbiting electrons energy

  Heat it Absorb incident photons Collides with incident particles
• Gold leaf electroscope

  Charge a gold leaf electroscope with a plastic rod Charge is accumulated on the surface of the zinc plate and on the gold leaf Due to the electrostatic repulsion the gold leaf rises
• Interaction between photons and electrons

  There is a 1 to 1 interaction between photons and electrons
• Interaction between photons and electrons

  There is a 1 to 1 interaction between photons and electrons
• What happens if energy of photon is greater than the work function

  Electron will be emitted
• What happens if we use visible light during the gold leaf electroscope

  No electrons are emitted as the photon energy is below the work function
• What happens if we use visible light during the gold leaf electroscope

  No electrons are emitted as the photon energy is below the work function
• Why is UV light used for the gold leaf electroscope

  The photon energy is greater than the work function As UV has a higher frequency
• What happens if we increase the intensity of the UV light (assuming light is above threshold frequency)

  Higher intensity means More photons are striking the surface so more electrons are emitted
• What happens if we increase the intensity of the UV light (assuming light is above threshold frequency)

  Higher intensity means More photons are striking the surface so more electrons are emitted
• Work function

  The minimum energy required to release an electron from the surface of a metal
• Line spectrum

  Light emitted from atoms produces a line spectrum A series of lines against a dark background
• Absorption line spectrum

  A series of dark lines against a bright background
• Does the maximum kinetic energy of a released electron depend on the intensity of light hitting the surface?

  No. Energy transferred is due to a one-to-one interaction, and so depends on frequency, not intensity