Resistance in a Thermistor (AQA A Level Physics)

• 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

• 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

     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)