1. Basic Concepts and Definitions

1.2 System and surroundings

In thermodynamics, a refers to a selected quantity of matter in the case of closed systems or a selected region in space in the case of open systems, see Figure 1.2.1. The rest of the universe outside the system is called , and the surface that separates the system and its surroundings is called A boundary may be fixed or movable, real or imaginary, rigid or flexible.

A system and its surroundings are separated by a boundary.
Figure 1.2.1 System and surroundings

A system interacts with its surroundings through two mechanisms:

  1. Mass transfer
  2. Energy transfer (i.e., in the form of heat and work)

A system of a fixed mass is a , which can only interact with its surroundings through energy transfer. Mass cannot cross the boundary of a closed system. For example, a sealed bottle of soft drink, Figure 1.2.2, can be modelled as a closed system because there is a fixed amount of liquid in the bottle. When you take the bottle out of your cooler, the liquid will warm up slowly due to the temperature difference between the bottle and the ambient air (surroundings). In other words, the system (the liquid in the bottle) interacts with its surroundings (the ambient air) through energy transfer (in the form of heat transfer). Figure 1.2.3 illustrates a piston-cylinder device, which can also be modelled as a closed system. The amount of the fluid in the cylinder (the system) remains constant as the piston moves. Only the transfer of energy, in the form of heat and work, may happen across the system boundary consisting of the cylinder walls and the lower surface of the piston.

An unopened bottle of drink to illustrate the concept of closed system. The drink is the closed system. The bottle is the boundary, separating the drink from its surroundings.
Figure 1.2.2  A sealed bottle of soft drink as an example of closed systems
Piston cylinder device as an example of closed system
Figure 1.2.3 Piston cylinder device as an example of closed systems

An , also called , is a selected region in space. An open system always exchanges mass with its surroundings. It may exchange energy with its surroundings in the form of heat and work, but energy transfer is not a necessary condition for a system to be an open system. In other words, an open system doesn’t have to exchange heat or work with its surroundings at all. Figure 1.2.4 illustrates an open system, which typically encloses a device that involves mass flow through its inlet and outlet. Figure 1.2.5 illustrates the outdoor condensing unit of an air conditioner. It may be treated as an open system because the coolant can enter and leave the condensing unit (the system) via its connecting coolant lines.


Control volume allowing mass and energy to flow across the system boundary
Figure 1.2.4  Open system (also called control volume)
Figure 1.8 Outdoor condensing unit of an air conditioner as an example of open system (control volume)
Figure 1.2.5 Outdoor condensing unit of an air conditioner as an example of open systems

If a system doesn’t allow the exchange of mass and energy with its surroundings, it is called an . An isolated system is an idealized, hypothetical system. In reality, no device is absolutely isolated.


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