# 5.2.2 Resistance in a Thermistor

### 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

### Application 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
• 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 decreases
D.     The p.d across R increases

• 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 ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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