Resistance in a light-dependent resistor
- A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
- Its resistance automatically changes depending on light (illumination)
- As the light intensity increases, the resistance of an LDR decreases
- This is shown by the following graph:
LDR graph
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)
Resistance of an LDR
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 current 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
The resistance of the thermistor is equal to the resistance of R at room temperature.When the temperature of the thermistor decreases, which statement is correct?