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The State of A System

In thermodynamics, the state of a system is completely defined by specifying the various macroscopic properties of the system such as temperature, pressure, volume and composition. If any of these properties is changed he system is said to be in a different state. These macroscopic properties are, therefore, called state variables.

If these properties are fixed, all other physical properties of the system are automatically fixed. Further, if the system is homogeneous and consists of a single substance, the composition is fixed (because it is 100%), and hence the state of the system depends only upon pressure, volume and temperature. In actual practices, it is not necessary to specify even these three thermodynamic properties because experiments have shown that hence three properties of a simple homogeneous system of definite mass are related to one another. The relationship between them is called the equation of state. For example, the equation of state for an ideal gas I PV = RT where P is the pressure, V is the volume of mole, T is the absolute temperature and R is a constant. Thus out of P, V and T, if any two are fixed, the third is automatically fixed. Hence we conclude that the state of a simple homogeneous system ay be completely defined by specifying only two of the properties out of P,V and T. The two variables which are generally varied are pressure and temperature and these are called ‘independent variables’. The third viz. volume whose value depends upon the values of pressure and temperature is called ‘dependent variable’.

In statistical mechanics, the state of a system is completely described by specifying the state of the individual atoms or molecules. In classical statistics, the state of an individual particle is described by specifying its position coordinates (s ,y, z) and the components of its momentum (px, py, px). In quantum statistics, the state of the individual particles is described by specifying the particular quantum state (energy level) to which the particle or a group of particles belong.

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