Resistance & Illumination (Edexcel International A Level Physics)
Revision Note
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
Resistance of an LDR depends on the light intensity falling on it
This is shown by the following 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)
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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