In general, Thermodynamics involves the study of various systems. A system is a collection of objects, and there are a lot of hurdles in the way it is defined. However, in thermodynamics, it is a much more convenient to define it. A thermodynamic system is defined as a volume in space or a well defined set of materials (matter). The imaginary outer edge of the system is called its boundary.
A system in thermodynamics is nothing more than the collection of matter that is being studied.
In science and engineering, a system is the part of the universe that is being studied, while the environment is the remainder of the universe that lies outside the boundaries of the system. It is also known as the surroundings, and in thermodynamics, as the reservoir. Depending on the type of system, it may interact with the environment by exchanging mass, energy (including heat and work), linear momentum, angular momentum, electric charge, or other conserved properties.
The Concept of a “System”
A thermodynamic system is a quantity of matter of fixed identity, around which we can draw a boundary, the boundaries may be fixed or moveable. Work or heat can be transferred across the system boundary.
The rest of the universe which might be in a position to exchange energy and matter with the system is called the surroundings.
When working with devices such as engines it is often useful to define the system to be an identifiable volume with flow in and out. This is termed a control volume.
Three types of thermodynamic systems are distinguished depending on the kinds of interaction and energy exchange taking place between the system and its surrounding environment:
Isolated systems: are completely isolated in every way from their environment. They do not exchange heat, work or matter with their environment. An example of an isolated system would be an insulated rigid container, such as an insulated gas cylinder.
Closed systems: are able to exchange energy (heat and work) but not matter with their environment. A greenhouse is an example of a closed system exchanging heat but not work with its environment. Whether a system exchanges heat, work or both is usually thought of as a property of its boundary.
Open systems: exchanging energy (heat and work) and matter with their environment. A boundary allowing matter exchange is called permeable. The ocean would be an example of an open system.