Resistance in a Thermistor (AQA A Level Physics)
Revision Note
Temperature & Resistance
All solids are made up of vibrating atoms
The higher the temperature, the faster these atoms vibrate
Electric current is the flow of free electrons in a material
The electrons collide with the vibrating atoms which impede their flow, hence the current decreases
Metal atoms and free electrons at low and high temperatures
So, if the current decreases, then the resistance will increase (from V = IR)
Therefore, for a metallic conductor which obeys Ohm's law:
An increase in temperature causes an increase in resistance
A decrease in temperature causes a decrease in resistance
This is not the case for components such as a thermistor
For a thermistor, an increase in temperature causes a decrease in resistance
Applications of a Thermistor
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 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
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 remains constant
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
B and C are both incorrect – total resistance has increased so current has decreased
D is incorrect because the p.d. across R has decreased – the thermistor's resistance has increased so it receives a greater proportion of the cell's emf (and therefore R receives a smaller proportion)
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