AQA A Level Physics

Revision Notes

2.4.2 Threshold Frequency & Work Function

Threshold Frequency

  • The photoelectric effect is the phenomena in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation
  • Electrons removed from a metal in this manner are known as photoelectrons
  • The photoelectric effect provides important evidence that light is quantised or carried in discrete packets
    • This is shown by the fact each electron can absorb only a single photon
    • This means only the frequencies of light above a threshold frequency will emit a photoelectron

The Photoelectric Effect, downloadable AS & A Level Physics revision notes

The photoelectric effect: photons of sufficient energy are able to liberate electrons from the surface of a metal

Threshold Frequency & Wavelength

  • The threshold frequency is defined as:

The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

  • The threshold wavelength, related to threshold frequency by the wave equation, is defined as:

The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal

Exam Tip

A useful analogy for threshold frequency is a fairground coconut shy:

  • One person is throwing table tennis balls at the coconuts, and another person has a pistol
  • No matter how many of the table tennis balls are thrown at the coconut it will still stay firmly in place – this represents the low frequency quanta
  • However, a single shot from the pistol will knock off the coconut immediately – this represents the high frequency quanta

Coconut Shy Photoelectric Effect, downloadable AS & A Level Physics revision notes

The Work Function

  • 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 metal

  • 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 from the surface of the metal if it absorbs a photon which has an energy equal to Φ or higher

Energy Well (1), downloadable AS & A Level Physics revision notes Energy Well (2), downloadable AS & A Level Physics revision notes Energy Well (3), downloadable AS & A Level Physics revision notes

  • 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
  • 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 zinc 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

Stopping Potential

  • Stopping potential, Vs, is defined as:

The potential difference required to stop photoelectron emission from occurring

  • The photons arriving at the metal plate cause photoelectrons to be emitted
    • This is called the emitter plate
  • The electrons that cross the gap are collected at the other metal plate
    • This is called the collector plate

Stopping Potential, downloadable AS & A Level Physics revision notes

This set up can be used to determine the maximum kinetic energy of the emitted photoelectrons

  • The flow of electrons across the gap results in an e.m.f. between the plates that causes a current to flow around the rest of the circuit
    • Effectively, it becomes a photoelectric cell producing a photoelectric current
  • If the e.m.f. of the variable power supply is initially zero, the circuit operates only on the photoelectric current
  • As the supply is turned up, the emitter plate becomes more positive (because it is connected to the positive terminal of the supply)
  • As a result, electrons leaving the emitter plate are attracted back towards it
    • This is because the p.d. across the tube opposes the motion of the electrons between the plates
  • If any electrons escape with enough kinetic energy, they can overcome this attraction and cross to the collector plate
    • And if they don’t have enough energy, they can’t cross the gap
  • By increasing the e.m.f. of the supply, eventually a p.d. will be reached at which no electrons are able to cross the gap – this is the stopping potential, Vs
  • At this point, the energy needed to cross the gap is equal to the maximum kinetic energy KEmax of the electrons

KEmax = eVS

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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