Most of the substances solids, liquid and gases expand on heating and contract on cooling. Their thermal expansions and contractions are usually small and are not noticeable. However, these expansions and contractions are important in our daily life.
The kinetic energy of the molecules of an object depends on its temperature. The molecules of a solid vibrate with larger amplitude at high temperature than at low temperature. Thus, on heating, the amplitude of vibration of the atoms or molecules of an object increases. They push one another farther away as the amplitude of vibrations increases. Thermal expansion results an increase in length, breadth and thickness of a substance.
Linear Thermal Expansion In Solids:
It has been observed that solids expand on heating and their expansion is nearly uniform over a wide range of temperature. Consider a metal rod of length L° at certain temperature T°. Let its length on heating to a temperature T becomes L. Thus
Increases in length of the rod = ΔL = L – L0
Increase in temperature = ΔT = T – T°
It is found that change in length ΔL of the solid is directly proportional to its original length length L°, and the change in temperature Δ T. That is
ΔL ∝ L0ΔT
ΔL =αL0ΔT …….(1)
L – L0=αL0ΔT
or L =L0(1+αΔT) …..(2)
Where α is called the coefficient of linear thermal expansion of the substance.From equation (1) we get
α = ΔL/L°ΔT
Thus, we can define the coefficient of linear expansion α of a substance as the fraction increase in the length per kelvin rise in temperature.
Below given table of linear thermal expansion of some materials:
Volume Thermal Expansion:
The volume of a solid also changes with the change in temperature and is called volume thermal expansion or cubical thermal expansion. Consider a solid of initial volume V°. On heating the solid to a temperature T, let its volume becomes V, then
Change in the volume of a solid ΔV = V – V°
and change in temperature ΔT = T – T°
Like linear expansion, the change in volume ΔV is found to be proportional to its original volume V° and change in temperature ΔT. Thus
ΔV ∝ V° ΔT
ΔV=βV° ΔT ……(3)
V – V°=βV° ΔT
V = V°(1 + βΔT)
Where β is the temperature coefficient of volume expansion.From equation (3) we get
β = ΔV/V° Δ
Thus, we can define the temperature coefficient of volume expansion β as the fractional change in its volume per kelvin change in temperature. The coefficients of linear expansion and volume expansion are related by the equation:
β = 3 α
Values of β for different substances are given in table:
Consequences of Thermal Expansion:
Why gaps are left in railway tracks? The expansion of solids may damage the bridges, railway tracks and roads as they are constantly subjected to temperature changes. So provision is made during contraction for expansion and contraction with temperature. Foe example, railway tracks buckled on a hot summer day due to expansion if gaps are not left between sections.
Bridges made of steel girders also expand during the day and contract during night. They will bend if their ends are fixed. To allow thermal girder rests on rollers in the gap left for expansion.
Overhead transmission lines are also given a certain amount of sag so that they can contract in winter without snapping.
Application of Thermal Expansion:
Thermal expansion is used in our daily life. In thermometers, thermal expansion is used in temperature measurements. To open the cap of a bottle that is tight enough,immerse in it hot water for a minute or so. Metal cap expands and becomes loose. It would now be easy to turn it to open.
To join steel plates tightly together, red hot rivets are forced through holes in the plates. The end of hot rivets is then hammered. On cooling, the rivets contract and bring the plates tightly gripped.
Iron rims are fixed on wooden wheels of carts. Iron rims are heated. Thermal expansion allows them to slip over the wooden wheel. Water is poured on it to cool. The rim contracts and becomes tight over the wheel.
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.
bimetal strips are used for various purposes. Bimetal thermometers are used to the measure temperature especially in furnaces and ovens. Bimetal strips are used in thermostats. Bimetal thermostat is used to control the temperature of heater coil in an electric iron.
Thermal Expansion of Liquids:
The molecules of liquids are free to move in all directions within the liquid.On heating a liquid, the average amplitude of vibration of its molecules increases. The molecules push each other and need more space to occupy. The accounts for the expansion of the liquid when heated. The thermal expansion in liquids is greater than solids due to the weak forces between their molecules. Therefore, the coefficient of volume expansion of liquids is greater than solids.
Liquids have no definite shape of their own. A liquid always attains shape of their container in which it is poured.Therefore,when a liquid is heated, both liquid and the container undergo a change in their volume. Thus, there are two types of thermal volume expansion for liquid.
- Apparent volume expa0°Cnsion
- Real volume expansion
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.