3.5.1.4
NTC thermistor resistance decreases with temperature because of more charge carriers
Resistivity & Superconductivity — AQA A-Level Physics
Key Definition
NTC thermistor — A semiconductor component whose resistance decreases as temperature increases. NTC stands for negative temperature coefficient.
- A thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). is made from a semiconductorA material with conductivity between a conductor and insulator; resistance decreases as temperature increases (e.g. silicon, germanium). material.
- At low temperatures, most electrons are bound to atoms. Few are free to conduct.
- At higher temperatures, thermal energyThe capacity to do work. Measured in joules (J). frees more electrons from their atoms.
- The number of charge carriersParticles that carry electric charge through a material, such as electrons in metals or ions in electrolytes. increases significantly.
- Although lattice vibrations also increase (as in metals), the effect of more chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers dominates.
- Net result: resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). decreases as temperature increases.
- The resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω).-temperature graph is a steep decay curve, not a straight line.
- Applications: temperature sensors in ovens, fire alarms, digital thermometers, car engine management systems.
- In a potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). Informal term for potential difference. across one of the resistors., a thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). allows V_out to vary automatically with temperature.
Common Mistake
MEDIUM
Students often: Explaining thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). behaviour using the metal mechanism: 'atoms vibrate less so resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). decreases'.
Instead: The thermistor mechanism is about charge carrierA particle that carries electric charge through a material. In metals, these are free (delocalised) electrons. number increasing, not about vibrations decreasing. More charge carriers means more current per voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb., so resistance falls.
Instead: The thermistor mechanism is about charge carrierA particle that carries electric charge through a material. In metals, these are free (delocalised) electrons. number increasing, not about vibrations decreasing. More charge carriers means more current per voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb., so resistance falls.
Examiner Tips and Tricks
- For full marks: metals and thermistors have different mechanisms.
- Metal: more vibrations causes more collisions causes higher R.
- Thermistor: more thermal energyThe capacity to do work. Measured in joules (J). releases more charge carriers causes lower R.
- Never mix them.