[Encyclios]

Intensive and extensive properties of a thermodynamic system

Each characteristic of a thermodynamic system is called property; the properties of a system are divided into:

• intensive properties: these are those that do not depend on the size of the system, for example: temperature, pressure, density (if a system is large, or small, it does not influence the temperature, pressure or density of the system);
• extensive properties: these are those that depend on the size, or extent, of the system, for example: mass, volume, total energy (if a system is large, or small, it influences the mass, the volume and the total energy of the same, as the mass will be larger, or smaller, the volume will be larger, or smaller, the total energy contained will be larger or smaller).

A state variable is extensive if its value depends on the mass of the system; these variables are also called additive because their total value can be obtained as the sum (or integral) of the values of the various parts of the system.

A variable is intensive if its value is a local function of a particular point of the system. For example, pressure is an intensive variable: it can be different in every point of the system, its total value in the system is not obtained by summing the value of the pressures in the various parts. On the contrary, volume is an extensive variable: it is not locally definable, and if we divide the system into parts the total volume is the sum of the partial volumes.

For each extensional variable it is possible to define a corresponding specific (or more properly mass) variable, i.e. the ratio of the variable in question to the mass of the system. Generally, the extensional variables are indicated with a capital letter, the corresponding specific variables with the corresponding lower case letter. As a first example, the volume of the system is indicated with V, and is measured in m³; the corresponding specific variable (specific volume) is indicated with v = V/M, is measured in m³/kg, and represents the volume of the mass unit.