Thermodynamic Systems

In general, a system is a collection of objects, and there is a lot of subtlety in the way it is defined, as in set theory. However, in thermodynamics, it is a much more straightforward concept. 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.

As can be seen from the definition, the boundary can be fixed or moving. A system in which matter crosses the boundary is called an opensystem. Similarly, a system in which no matter enters or leaves (i.e. crosses the boundary) is called a closed system.

The above image shows a piston cylinder arrangement, where a gas is compressed by the piston. The dotted lines represent the system boundary. As can be seen, due to an opening in the cylinder, gas can escape outside as the piston moves inwards, and gas enters the system when the piston moves outwards. Thus, it is an open system.

Now consider a similar system, but one in which gas cannot escape. In practice, there might be some space between the piston and the cylinder, but we can ignore it for modeling purposes. Thus the model of this configuration is a closed system.

The region outside the system is called the surroundings. The system and the surroundings together are called the Universe. A system which does not exchange matter or energy with the surroundings is called an isolated system.

Another term sometimes used instead of system is control volume. In the case of a closed system, in which the mass of matter inside the system remains constant, the control volume is referred to as control mass. A control volume is said to be enclosed by a control surface.