There are different types of systems, and depending how you define the system, your analysis could be affected. The first type of system that I am going to discuss is the closed system. For a system to be considered a close system there has to be a fixed amount of control mass within the system where the mass cannot cross the boundary layer. Energy, however, can cross the boundary layer of a closed system, and since energy can cross the systems boundary either the pressure in the system or the volume will increase or decrease depending if the energy is flowing in or out of the system. For the pressure to increase the volume of the system is fixed which means the boundary is fixed, while if the volume increases the system boundary system will have to have a moving boundary. An example of a closed system can be seen below.

There is a special type of closed system called and isolated system. Like a closed system, and isolated system has a fixed amount of control mass which cannot cross the boundary layer. However, unlike closed systems isolated systems prevent energy from crossing the boundary layer. Now in reality there is no such system that is perfectly isolated. However, well insulated systems can come very close to becoming isolated.

The next and final type of system is called an open system, or sometimes referred to as a control volume. An open system has both real and imaginary surfaces for its boundary layer. This means mass as well as energy can cross the imaginary boundary layer of the system. An example of an open system would be a nozzle; refer to the image below.

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