OCR AS Physics

Revision Notes

4.2.4 The Light-Dependent Resistor (LDR)

The Light-Dependent Resistor (LDR)

  • 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
  • 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 diagram, downloadable AS & A Level Physics revision notes

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

Worked Example

Which graph best represents the way in which the current I through an LDR depends upon the potential difference V across it?

    • As the potential difference across the LDR increases, the light intensity increases causing its resistance to decrease
    • 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
    • Therefore, I increases with the p.d with an increasing gradient

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