OCR.4.1.2
thermistor (NTC)
Energy, Power & Resistance - OCR A-Level Physics
- Metals have a positive temperature coefficientA material whose resistance increases as temperature increases (e.g. metals).: 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). increases with temperature.
- Higher temperature means ions vibrate more. Free electronsElectrons not bound to any particular atom, free to move through a conductor. Also called delocalised or conduction electrons. collide more often.
- More collisions slow chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers. Less currentThe rate of flow of charge. Measured in amperes (A). 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.. Higher R.
Key Definition
thermistor (NTC)
A semiconductor component whose resistance decreases as temperature increases. NTC = negative temperature coefficient.
A semiconductor component whose resistance decreases as temperature increases. NTC = negative temperature coefficient.
- A thermistorA component whose resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). decreases significantly as temperature increases (negative temperature coefficient). is made from a semiconductor, not a metal.
- Higher temperature releases more charge carriers (free electronsElectrons not bound to any particular atom, free to move through a conductor. Also called delocalised or conduction electrons.) in the semiconductor.
- More charge carriers means higher 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 resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). falls.
- Key distinction: in metals, ion vibrations dominate. In semiconductorsMaterials with resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). between conductors and insulators. Their number densityMass per unit volume of a material. Measured in kg m⁻³. of charge carriers increases with temperature., carrier number dominates.
Key Definition
light-dependent resistor (LDR)
A resistor whose resistance decreases as light intensity increases.
A resistor whose resistance decreases as light intensity increases.
- Both LDRsLight-dependent resistors — components whose resistance decreases as light intensity increases. and thermistorsComponents whose resistance changes significantly with temperature. NTC thermistors decrease in resistance as temperature rises. are used in sensing circuits.
- LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases.: more photons release more charge carriers. Same mechanism as a thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). but triggered by light, not heat.
R-T graph showing metal resistance rising linearly and NTC thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). resistance falling as a curve.
Graph: Resistance vs Temperature for a metal (straight line, positive gradient) and an NTC thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). (curved, decreasing).
Common Mistake
MEDIUM
Saying the thermistor resistance decreases because 'electrons move faster'. The correct reason is that the number densityMass per unit volume of a material. Measured in kg m⁻³.The number of charge carriers per unit volume in a material. Measured in m⁻³. of charge carriers increases.
Related:Sensing Circuits
Current Electricity