Definition of systems:
A Thermodynamic system can easily be defined as a quantity of matter or a region in the universe selected for study. The mass or region outside the system is defined as surrounding.
The region or the space that keeps the system a part of its surrounding is called boundary of the system. Boundary is placed evenly on the system and surrounding to ensure contact between two. Mathematically boundary has 0 thicknesses. So it is clear that boundary neither occupies volume in space nor it has some mass.
Thermodynamic System or simply system could be closed or open. It depends on whether constant mass or constant volume in space has been chosen for study.
Open and Closed Systems
Types of Systems
A closed system is one that has a constant amount of mass and no mass crosses the boundary of the system .i.e. mass neither enters or leaves the system. This system is also called control mass.
In closed system energy can cross the boundary of the system in the form of heat and work. So it means that volume of the system doesn’t remain fixed. In the special cases when the energy and mass are not allowed to cross the boundary then the system is called an isolated system.
Mass cannot crosses the boundary of the system but energy can
Consider an example of a closed piston cylinder in which gas is enclosed. Our system or focus of attention is gas inside in the piston cylinder. When the piston is heated, the inner surface of the piston cylinder that serves as the boundary moves up as pressure builds up inside the boundary. Since mass is not allowed to enter or leave the system. Only energy in the form of moving boundary interacts. Everything outside the gas including the piston is surrounding.
An open system is one in which there is proper selection of region in the space. this system is often called control volume. in this system both mass and energy are allowed to cross the boundary. This system is usually enclosed by a device that allows mass flows such as nozzle, compressor or turbines.
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