Page 16 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

1.4 Definitions of system, boundary, surrounding, open system and close

system

A thermodynamic system, or simply a system, is defined as a quantity of matter or a
region in space chosen for study. The fluid contained by the cylinder head, cylinder walls

and the piston may be said to be the system.

The mass or region outside the system is called the surroundings. The surroundings may
be affected by changes within the system.

The boundary is the surface of separation between the system and its surroundings. It

may be the cylinder and the piston or an imaginary surface drawn as in Fig. 1.4-1, so as to
enable an analysis of the problem under consideration to be made.

Boundary
Surrounding

System

Figure 1.4-1 System, surroundings and boundary

A system can either to be close or open, depending on whether a fixed mass or a fixed
volume in space is chosen for study. A close system (also known as a control mass)

consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass

can enter or leave a close system, as shown in Fig. 1.4-2. But energy, in the form of heat

or work can cross the boundary, and the volume of a close system does not have to be

fixed.

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