CONCEPT OF THERMODYNAMIC SYSTEM AND ENVIRONMENT

Thermodynamic system is a part of the universe under thermodynamic study which can be one, two or three dimensional and may consist of one or more objects called components of the system. A system may be made up of radiation or radiation may be one of the components of the system. Remaining part of the universe surrounding the system and interacting with it is known as its environment. The boundary separating the system and its environment is called wall of the system. The type of interactions between the system and its
environment depends on the nature of its wall.

The macroscopic description of any system in physics is done in terms of its measurable properties. For example, in kinematics of rotational motion, position and velocity are macroscopic properties which are called mechanical coordinates. Potential and kinetic energy of a rigid body are determined using these mechanical coordinates with respect to some system of coordinate axes. Similarly, in thermodynamics, macroscopic quantities are thermodynamic coordinates which help in determining internal state of the system. Thermodynamic state of a system is determined from values of its mechanical and thermal properties like pressure, volume, Temperature, internal energy, etc.

Interaction between a system and its environment is called a thermodynamic process. If no interaction occurs, the system is called an isolated system. Thermal and mechanical properties of an isolated system remain constant and the system is said to be in a definite thermodynamic equilibrium state. During interaction with the environment, thermal and mechanical properties of the system change continuously at the end of which the system attains an equilibrium state. The amount of heat energy exchanged during the interaction, known as heat, is denoted by Q and the mechanical energy exchanged, called work, is denoted by W.

The thermodynamic equilibrium state of a gas is decided by its pressure, volume, temperature and quantity, not all of which are independent. These are called thermodynamic or state variables. The mathematical equation of state gives the relations between these variables, e.g., PV = μRT is the equation of state of an ideal gas. Thermodynamic state variables are of two types:
( i ) Extensive variables which depend upon the dimensions of the system, e.g., mass, volume, internal energy,  ( ii ) Intensive variables which are independent of the dimensions of the system, e.g., temperature.