A thermodynamic system (Fig. 1.2(a)) is defined as a fixed quantity of matter or a region in space upon which attention is focussed for the study of thermodynamic properties of the matter or substance. Everything outside the system, which interacts with the system is called surroundings or environment. The interface between the system and surrounding is the boundary. The boundary may be real or imaginary and may be fixed or moving. Mass and energy transfer can take place through boundary.
There are three types of system.
• Closed System • Open System • Isolated System
1.3.1 Closed System
A system in which the mass is constant and only energy can flow through the boundary in or out, is called a closed system.
Fig. 1.2 (b) is the closed system. Here there is no mass transfer.
1.3.2 Open System
A system in which the mass crosses the boundary in addition to heat and work transfer is called an ope11 system. Fig. 1.2 (c) is an open system.
1.3.3 Isolated System
A system where there is no interaction between the system and surroundings is called isolated system. There is no mass transfer and no energy transfer. Therefore it is the fixed mass and fixed energy system. Fig. 1.2 (d) is an isolated system.