gas continued to be a gas. The absolute minimum temperature for an ideal gas, therefore, inferred by extrapolating the straight line to the axis, as in Fig. . .
This temperature is found to be – . ° C and is designated as absolute zero . Absolute zero is the foundation of the Kelvin temperature scale or absolute scale temperature water the level of mercury falls again. Similarly, in case of gases, a balloon partially inflated in a cool room may expand to full size when placed in warm water.
On the other hand, a fully inflated balloon when immersed in cold water would start shrinking due to contraction of the air inside. It is our common experience that most substances expand on heating and contract on cooling. A change in the temperature of a body causes change in its dimensions. The increase in the dimensions of a body due to the increase in its temperature is called 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 (Fig. .
). Fig. . Thermal Expansion.
If the substance is in the form of a long rod, then for small change in temperature, ∆ T , the fractional change in length, ∆ l/l , is directly proportional to ∆ T . ( . ) where α is known as the coefficient of linear expansion (or linear expansivity) and is characteristic of the material of the rod. In Table .
, typical average values of the coefficient of linear expansion for some material in the temperature range ° C to °C are given. From this Table, compare the value of α l for glass and copper. We find that copper expands about five times more than glass for the same rise in temperature. Normally, metals expand more and have relatively high values of α l .
Table . Values of coefficient of linear expansion for some material Material ααααα l ( – K – ) Aluminium . Brass . Iron .
Copper . Silver .