Explain that the bimetallic strip used in the thermostat is based on the different rates of expansion of different metals on heating.
Bimetal strips:
A bimetal strip consists of two thin strips of different metals such as brass and iron joined together. On heating the strip, brass expands more than iron. This unequal expansion causes bending of the strip.
(a) A bimetal strip of brass and iron
(b) Bending of the brass-iron bimetal strip on heating due to the difference in their thermal expansion.
Uses of bimetal strips:
- Bimetal thermometers are used to measure temperatures, especially in furnaces and ovens.
- Bimetal strips are also used in thermostats.
- Bimetal thermostat switch is used to control the temperature of the heater coil in an electric iron.
DO YOU KNOW
Anomalous expansion of water:
Water on cooling below 4°C begins to expand until it reaches 0°C. On further cooling, its volume increases suddenly as it changes into ice at 0°C. When ice is cooled below 0°C, it contracts i.e. its volume decreases like solids. This unusual expansion of water is called the anomalous expansion of water.