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 represents the variation of current with potential difference for a light-dependent resistor (LDR) in constant light intensity?
Answer: B
An LDR is a type of resistor whose resistance decreases as light intensity increases
For a given light intensity, the LDR obeys Ohm's law, i.e. it has constant resistance
If resistance is constant, then current and potential difference will be directly proportional, since V = IR
The graph is therefore a straight line rather than a curve
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 NTC 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 resistance of the thermistor decreases
D. The p.d across R increases
Answer: A
The resistance of the thermistor increases as the temperature decreases
Increasing the thermistor’s resistance increases the total resistance of the series circuit
This reduces the current supplied by the battery
The p.d. across a component is given by Ohm’s law V = IR
Although the total current decreases, the fraction of the p.d. across the thermistor increases because its resistance becomes larger relative to R
Therefore, the p.d. across the thermistor increases
Hence, A is correct
B is incorrect because the current decreases as the total resistance of the circuit is increased
C is incorrect because for a negative temperature coefficient thermistor, the resistance increases as the temperature decreases
D is incorrect because the p.d. decreases due to resistor R now having a smaller share of the total resistance
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