Resistance in Sensory Resistors (Cambridge (CIE) A Level Physics): Revision Note
Exam code: 9702
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
Resistance in a thermistor
A thermistor is a non-ohmic conductor and sensory resistor
Its resistance changes depending on its temperature
As the temperature increases, the resistance of a thermistor decreases
This is shown by the following graph:
Graph of temperature and resistance for a thermistor
Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
As the thermistor gets hotter, its resistance decreases
As the thermistor gets cooler, its resistance increases
Resistance through a thermistor
The resistance through a thermistor is dependent on the temperature of it
Worked Example
A thermistor is connected in series with a resistor R and a battery.
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?
A. The p.d across the thermistor increases
B. The current in R increases
C. The current through the thermistor decreases
D. The p.d across R increases
Answer: A
The resistance of the thermistor increases as the temperature decreases
Since the thermistor and resistor R are connected in series, the current I in both of them is the same
Ohm’s law states that V = IR
Since the resistance of the thermistor increases, and I is the same, the potential difference V across it increases
Therefore, statement A is correct
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