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Photoelectric Effect | Physics Form 4 Coursework e-Content CDs

Background

In the earlier topic on the Cathode Ray Tube, we learnt that electrons can be emitted from a metal surface by the effect of heat energy, a process known as thermionic emission. In this topic, we shall learn about emission of electrons from a metal surface by use of light energy, a process known as photoelectric effect.

Objectives

By the end of this topic, you should be able to:

  • Perform and describe simple experiments to illustrate the photoelectric effect
  • Explain the factors affecting photoelectric effect
  • Apply the equation E = hf to calculate the energy of photons
  • Define threshold frequency, work function and the electric volt
  • Explain photoelectric emission using Einstein's equation E = hf = hf0 + ½ mv2
  • Explain the applications of photoelectric effect
  • Solve numerical problems involving photoelectric emissions







    • Introduction

      When light with a frequency that has sufficient energy is radiated on a metal surface, electrons are emitted. This phenomenon has wide applications in life such as construction of solar panels for green energy, automatic counters, street lights' controls and alarms.

      Factors affecting photoelectric emission

      The following factors affect the emission of photo electrons from a metal surface:


      • The intensity of the incident radiation
      • The type of the metal surface
      • The frequency of the incident radiation

      Play the animation below and observe how these factors affect emission of photo electrons.

      Observations
      a)
      When the intensity of the light is increased, more electrons are ejected from the metal surface.
      Explanation
      Increasing the intensity of light implies that more energy reaches the metal surface. This energy is used in ejecting more electrons which are attracted to the anode.

      b) By using different metal surfaces, it is observed that different intensities of the emitted electrons are achieved. The least emission of electrons is achieved with nickel while lead has the highest emission with the same light being shown on the metal surfaces.
      Explanation
      The energy required to remove electrons from a metal surface differs from one metal to another.

      c) With different frequencies of the incident light, the number of electrons emitted is seen to vary. Higher frequencies emit electrons with higher energies.

      Threshold frequency

      This is the minimum frequency of light below which no electrons are ejected from a metal surface no matter how intense the light is. This is represented by fo. Play the animation below to observe the ability of photons to eject photoelectrons depending on the amounts of energy they possess.

      Photons

      Electromagnetic waves such as light travel as packets of energy called photons. Packets for a particular wave carry an equal amount of energy. Click on the play button of the animation below to observe photons packed with different amounts of energy in motion.

      Work function

      This is defined as the least energy required to eject an electron from a metal surface. It can be calculated from the equation: E = hfo and is denoted by Wo.The work function varies from one metal to another.

      The electron Volt (eV)

      This is a unit of energy equivalent to the energy gained by an electron accelerated through a potential difference of one Volt. Its value is 1.6 x 10-19 Joules. It is more convenient to use this unit when dealing with small amounts of energy. 1 eV = 1.6 x 10-19 Joules

      Einstein's equation

      This equation gives the kinetic energy of the electrons emitted during photoelectric effect. The energy of the incident photon is transferred to the work done in removing the electron from the metal surface and the rest is transferred to the kinetic energy of the electron. This implies that:

      Observation

      When the photon strikes the metal surface, an electron is ejected from the surface.

      Explanation

      The energy of the photon is above the work function of the metal. It therefore ejects the electron from the metal surface and the rest of the energy is converted into the kinetic energy of the electron.

      a) Photo emissive devices

      These are devices which have a photo emissive cell. In a photo emissive cell, the incident light on the cathode ejects electrons which move to the anode constituting an electric current.

       


      Examples of such devices include

      • Signals from a photocell receiving light from the sound track recorded of a movie film which is amplified and used to drive loudspeakers.
      • Photocells can be used in counting items on a conveyor belt as they pass. When the item passes between the light source and the cell, they block the light reducing the cell output. This increases after the item has passed. An external source is used to count the light pulses electronically. This can also be modified to work as an alarm.

      b) Photo conductive devices

      These are devices with a light dependant resistor (LDR) which changes its resistance depending on the intensity of light falling on it. The change in resistance alters the current on the device which is operated depending on the light intensity. Examples of such devices include light operated security lamps and gates. Click on the play botton to make your observations

      c) Photo voltaic cells

      These are cells that generate an e.m.f when light falls on them. They consist of two different materials placed together to form a junction. When light falls on the junction, electrons are freed from one end of the material to the other creating a potential difference. Solar cells are constructed from layers of semi conductor materials that generate sufficient current to charge batteries.

      Examples of other devices operating with photo voltaic cells include traffic lights, watches, calculators, cameras, light sensors among others.

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