This article gives you an insight into important concepts related to the thermodynamic properties of a system.
Introduction
Properties are also termed as state variables which are used to define or specify the state of a system. They are also known as point functions and are independent of the past history of the system or path of the process. Properties are exact differential. Properties of a thermodynamic system can be classified into intensive and extensive properties. Intensive properties are also called as intrinsic properties and are independent of mass. All specific properties are intensive properties. Some of the other examples of intensive properties are pressure, temperature, density, velocity, viscosity, thermal conductivity, molecular weight etc. Extensive properties are also known as extrinsic properties and they are dependent on mass. Some of the examples of extensive properties are volume, internal energy, entropy, enthalpy, Gibbs free energy, electric charge, magnetization etc. While deciding the type of property, we should not change the system under consideration.
Important Concepts
- Important conditions to be fulfilled by the thermodynamic properties to specify the state of a system are, properties must be uniform within the system or throughout the system and it should be invariant with time at least for a temporary period. The uniformity of properties throughout the system indicates that there should not be any internal or external disbalance in the system. The system should not interact with its surroundings so that it is invariant with time. Such a condition or equilibrium can only be achieved by creating an isolating boundary between system and surroundings or by making system properties same as that of surroundings, in other words, by maintaining a dead state.
- The above concept gives us an insight that when systems are not in a dead state, the state of the system can be defined only for an isolated system. For all systems, interacting with surroundings, they are not in equilibrium. Unfortunately for all our applications, the interaction between system and surroundings is our goal and from this only we are benefitted. So in order to tackle this, for all our thermodynamic studies, we consider intermediate states of the system while it is interacting with the surroundings to be in equilibrium in a limiting case which is known as quasistatic or reversible. So this way only we can specify the state of the system.
- Even if system contracts to a point, intensive properties have a finite value. For example, we can define temperature at a point or pressure at a point.
- However, when system contacts to a point, the value of an extensive property is zero. This is because the mass of the system tends to zero at a point. For example, we cannot define internal energy of a system at a point.
