The change in temperature of a body causes expansion or contraction of that body. Most of the substances expand on heating and contract on cooling. This is called thermal expansion. It has been observed that bottles which are tightly sealed with a metallic lid are easily opened when the bottle is kept upside down in a hot water for some time in such a way that just the lid is immersed in water. In this way metallic lid expands and opens easily. Also the level of mercury rises in thermometer when it is put in warm water and when the thermometer is taken out the mercury level drops.
In case of gases, when balloon is partially inflated in a cool room it expands to full size when put in warm water. This all is due to thermal expansion.
So, increase in dimension of a body due to increase in temperature is thermal expansion. The expansion in length is called linear expansion. The expansion in area is called area expansion. The expansion in volume is called volume expansion. Coefficient of thermal expansion measures the fractional change in size due to small change in temperature.
Metals expand more and have relatively high values of coefficient of linear expansion.
The fractional change in volume of a substance due to change in temperature is called coefficient of volume expansion.
In terms of thermal expansion water shows a typical behaviour. It contracts on heating between 0 degree C and 4 degree C. The volume of given amount of water decreases as it is cooled from room temperature, until its temperature reaches 4 degree C. Below 4 degree C, the volume increases and density decreases.
This means that water has a maximum density at 4 degree C. Due to this property water bodies like lakes and ponds freezes at the top first.
At ordinary temperature gases expand more than solid and liquid. The coefficient of gas expansion is dependent on temperature. For an ideal gas, the coefficient of volume expansion at constant pressure can be found from the ideal gas equation:
PV = μRT
At constant pressure
PΔV = μR ΔT
ΔV/V = ΔT/T
i.e. αv = 1/T for ideal gas
At 0 °C, αv = 3.7 × 10-3 K–1, which is much larger than that for solids and liquids.
There is relation between the coefficient of volume expansion and coefficient of linear expansion. If thermal expansion of a rod is prevented by fixing its ends rigidly the rod will undergo compressive strain due to external forces provided by the rigid support at the ends. The corresponding stress set up in the rod is called thermal stress.
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