A LevelPhysicsAQARevision NotesElectricityResistance & ResistivityResistance in a Thermistor

Resistance in a Thermistor (AQA A Level Physics): Revision Note

Exam code: 7408

Written by: Dan Mitchell-Garnett

Reviewed by: Tim

Updated on 25 September 2026

Guided study available on this topic

Understand this topic with deeper explanations and understanding checks.

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 vibrating slightly at a low temperature of 20 degrees celsius and strongly at a high temperature of 70 degrees celsius, with free electrons moving between them. Metal atoms vibrate more at 70 degrees, causing more collisions with green electrons flowing right, making flow harder and resistance higher. — **At higher temperatures, metal ions vibrate more and get in the way of the free electrons**

So, if the current decreases, then the resistance will increase (from $V = I R$ )
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 resistance against temperature for a thermistor, with resistance falling as temperature rises. Thermistor circuit symbol below. — **The resistance of a thermistor falls as its temperature rises**

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 thermistor symbol where a low temperature corresponds to high resistance, and a high temperature corresponds to low resistance. — **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.

A circuit diagram containing a battery connected in series with a thermistor, shown as the upper rectangular component with a diagonal line, and a resistor labelled “R” below.

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

[1]

Answer:

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 $V = I R$
Since the resistance of the thermistor increases, and $I$ is the same, the potential difference $V$ across it increases
Therefore, statement A is correct [1 mark]
- 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 e.m.f. (and therefore $R$ receives a smaller proportion)

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