Thermistors (OCR AS Physics): Revision Note
Exam code: H156
Thermistors
A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature
Most thermistors are negative temperature coefficient (ntc) components
This means that if the temperature increases, the resistance of the thermistor decreases (and vice versa)
The resistance through a thermistor is dependent on the temperature of it
The temperature-resistance graph for a thermistor is shown below
Graph of temperature against 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
A digital thermometer uses a thermistor
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
Examiner Tips and Tricks
Make sure you remember the shape of the temperature-resistance graph for a thermistor, as it is a common exam question to draw and interpret this.The graph should not touch the x-axis, as this implies 0 resistance which is only possible in superconductors.
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