Thermistors and LDRs
You should be able to recognise the circuit symbols for a thermistor and an LDR (light-dependent resistor), and know how the The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms. of these components can be changed.
The thermistor
Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases.
At low temperatures, the resistance of a thermistor is high, and little current can flow through them.
At high temperatures, the resistance of a thermistor is low, and more current can flow through them.
The LDR
LDRs (light-dependent resistors) are used to detect light levels, eg in automatic security lights. Their resistance decreases as the light intensity increases.
In the dark and at low light levels, the resistance of an LDR is high, and little current can flow through it.
In bright light, the resistance of an LDR is low, and more current can flow through it.
LDRs are also useful for controlling how long the shutter should remain open on a digital camera. Changes in the resistance are measured and, if the light level is low, the shutter stays open for longer than if the light level is high.