CIE A Level Physics复习笔记22.1.5 The Work Function

Photoelectric Emission

  • The work function Φ, or threshold energy, of a material is defined as:

The minimum energy required to release a photoelectron from the surface of a material

  • Consider the electrons in a metal as trapped inside an ‘energy well’ where the energy between the surface and the top of the well is equal to the work function Φ
  • A single electron absorbs one photon
  • Therefore, an electron can only escape the surface of the metal if it absorbs a photon which has an energy equal to Φ or higher
  • Different metals have different threshold frequencies, and hence different work functions
  • Using the well analogy:
    • A more tightly bound electron requires more energy to reach the top of the well
    • A less tightly bound electron requires less energy to reach the top of the well

Photoelectric Emission, downloadable AS & A Level Physics revision notes

In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy

  • Alkali metals, such as sodium and potassium, have threshold frequencies in the visible light region
    • This is because the attractive forces between the surface electrons and positive metal ions are relatively weak
  • Transition metals, such as manganese and iron, have threshold frequencies in the ultraviolet region
    • This is because the attractive forces between the surface electrons and positive metal ions are much stronger

Laws of Photoelectric Emission

  • Observation:
    • Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
  • Explanation:
    • Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
    • Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
    • Therefore, the gold leaf loses negative charge more rapidly
  • Observation:
    • Using a higher frequency light source does not change how quickly the gold leaf falls
  • Explanation:
    • The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
    • In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
    • So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency
  • Observation:
    • Using a filament light source causes no change in the gold leaf’s position
  • Explanation:
    • If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
    • A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released
  • Observation:
    • Using a positively charged plate causes no change in the gold leaf’s position
  • Explanation:
    • If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
    • Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
    • Any electrons emitted will be attracted back by positive charges on the surface of the metal
  • Observation:
    • Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal
  • Explanation:
    • A single photon interacts with a single electron
    • If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously

Intensity & Photoelectric Current

  • The maximum kinetic energy of the photoelectrons is independent of the intensity of the incident radiation
  • This is because each electron can only absorb one photon
  • Kinetic energy is only dependent on the frequency of the incident radiation
  • Intensity is a measure of the number of photons incident on the surface of the metal
  • So, increasing the number of electrons striking the metal will not increase the kinetic energy of the electrons, it will increase the number of photoelectrons emitted

Photoelectric Current

  • The photoelectric current is the number of photoelectrons emitted per second
  • Photoelectric current is proportional to the intensity of the radiation incident on the surface of the metal
  • This is because intensity is proportional to the number of photons striking the metal per second
  • Since each photoelectron absorbs a single photon, the photoelectric current must be proportional to the intensity of the incident radiation

KE & Photocurrent Graphs, downloadable AS & A Level Physics revision notes

Kinetic energy of photoelectrons is independent of intensity, whereas the photoelectric current is proportional to intensity and independent of frequency

 

 

 

 

转载自savemyexams

更多Alevel课程
翰林国际教育资讯二维码