Edexcel International A Level Physics

Revision Notes

2.51 Resistance & Illumination

Test Yourself

Modelling the Variation of Resistance with Illumination

  • Light can cause a change in conductivity of some semi-conductors
  • When light is absorbed by the material it causes more electrons to be available for conduction


  • An increase in the number of conduction electrons reduces the resistance

Resistance & Illumination for LDRs

  • A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
  • Its resistance automatically changes depending on the light energy falling onto it (illumination)
  • As the light intensity increases, the resistance of an LDR decreases

LDR diagram, downloadable AS & A Level Physics revision notes

Resistance of an LDR depends on the light intensity falling on it

  • This is shown by the following graph:

LDR graph, downloadable AS & A Level Physics revision notes

Graph of light intensity and resistance for an LDR

  • LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
    • In the dark, its resistance is very large (millions of ohms)
    • In bright light, its resistance is small (tens of ohms)

LDR Street Lights, downloadable IGCSE & GCSE Physics revision notes

LDRs are used for automatic street lights

Worked example

The graphs show various possible relationships between current and voltage through a component.

Which graph best represents the relationship between the current and voltage of an LDR?

Step 1: Consider the relationship between light intensity and resistance

    • As light intensity increases, resistance decreases in an LDR
    • If the resistance decreases then the potential difference will increase

Step 2: Consider a relevant equation

    • Ohm’s law states that V = IR
    • The resistance is equal to V/I or 1/R = I/V = gradient of the graph
      • Since R decreases, the value of 1/R increases, so the gradient must increase

Step 3: State the conclusion

    • Therefore, I increases with changing V with an increasing gradient
    • This is seen in graph A

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