- 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
- 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
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