• A thermodynamic system is defined as a finite quantity of matter or a prescribed region of space upon which attention is focused for study. 

Figure: Thermodynamic System

• Thermodynamic systems are distinguished into three classes, namely:

  1. Closed system

  2. Open system

  3. Isolated system

Closed System

Figure: Closed System

• Is a system of fixed mass. There will be no mass transfer across the system boundary. But energy transfer will be there into or out of the system.

• For example, a certain quantity of fluid in a closed beaker constitutes a closed system, as in figure. If the fluid is heated its temperature increases and the fluid will raise inside the beaker but here fluid will not flow out of the beaker as it is closed and the boundary of the system changes. Energy in the form heat crosses the boundary of the system during the process, but the matter i.e fluid that comprises the system is fixed.

Open System

• Is one in which matter crosses the boundary of the system. But energy transfer will be there into or out of the system.

Figure: Open System

• For example, a certain quantity of fluid in an open beaker constitutes an open system, as in figure. If the fluid is heated its temperature increases and the fluid will raise and get displaced out of the beaker and the boundary of the system changes. Energy in the form of heat and mass in the form of fluid crosses the boundary of the system during the process. Most engineering devices are generally open systems.

Isolated System

Figure: Isolated System

• Is one in which there will be no interaction between the system and the surroundings. It is of fixed mass and energy. There will be no mass or energy transfer across the system boundary.

Picture: Thermodynamic System Examples