An open system or a control volume is a properly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Both mass and energy can cross the boundary of a control volume. A closed system or a control mass or just system consists of a fixed amount of mass, and no mass can cross its boundary. No mass can enter nor leave the system, but energy in the form of heat or work can cross the boundary , and the volume of a closed system does not have to be fixed. Equilibrium implies a state of balance. In this state there are no unbalanced potentials or driving forces within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings. Thermal equilibrium - the temperature is same throughout the entire system. Mechanical equilibrium- related to pressure. A system is a mechanical equilibrium if there is no change in pressure at any point of the system with time. The pressure may vary within the system with elevation due to gravitational effects. For example, the higher
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