Cubic expansivity of a gas at constant pressure
It may be remembered that when the linear expansivity of a solid was defined on page 162 it was not specified that the original length of the rod of material should be measured at 0 “C. Owing to the smallness of the linear expansion of most substances,
it makes very little difference to the result obtained for the expansivity whether we start with the rod at 0 “C or at ordinary temperature. In the case of a gas, however, the situation is different. The expansion of gases is very much larger than that of solids, and consequently the value of the cubic expansivity obtained will depend on the starting temperature. Hence, in order to make a satisfactory comparison between different gases, the cubic expansivity is always calculated in terms of an original volume at 0 “C, The cubic expansivity of a gas is thus defined as follows:
The cubic expansivity of a gas at constant pressure is the fraction of its volume at o °C by which the volume of a fixed mass of gas expands per kelvin rise in temperature.