Concepts of classical thermodynamics

Thermodynamic system

A thermodynamic system is defined as the quantity of matter or a region in space upon which attention is concentrated in the analysis of a problem. Here quantity of matter may be gas, solid or liquid, magnetic field, electric field or even photons. Minimum quantity of matter required to analyze a thermodynamic system is the order of Avogadro’s number.

Surroundings/Environment

Everything external to the system is called the surroundings or environment. The system is separated from the surroundings by the system boundary. The boundary may be fixed or flexible.

Fig 1.1 Thermodynamic System

System and surroundings together constitute the universe. Size of the universe depends on the size of the system and surroundings. A pin head can constitute a system and the bulb containing the pinhead may form the universe.

Fig 1.2 Thermodynamic System for a Pin Head

If we consider the earth (lithosphere) to be a system, the atmosphere may be termed as the surroundings. Hence earth and its atmosphere can be said to be the universe. Similarly, if we consider the solar system as our system, the milkyway may become the surroundings and universe may be infinite.