1: Systems and boundaries (open, closed, isolated)

Section 1: Fundamental Concepts

1: Systems and Boundaries (Open, Closed, Isolated)

The analysis of any thermodynamic problem begins by defining a system. A system is a specified region of space, a finite quantity of matter, or a chosen control volume upon which attention is focused. Everything external to the system is termed the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary. The boundary may be fixed or movable, and it may be permeable or impermeable to mass and energy.

Systems are classified based on the interactions (mass and energy) that occur across their boundaries:

1. Closed System (Control Mass)
A closed system is defined by a fixed amount of matter (control mass). No mass can cross its boundary. However, energy in the form of heat or work can cross the boundary. For example, the gas trapped inside a piston-cylinder assembly is a classic closed system. As the piston moves, work is done across the boundary, and heat may be transferred through the cylinder walls, but the mass of gas inside remains constant.

2. Open System (Control Volume)
An open system, or control volume, is a region of space through which mass can flow. It has mass crossings at its boundaries, called inlets and outlets. Energy transfers also occur across its boundary. Most engineering devices (turbines, compressors, heat exchangers) are analyzed as open systems because they operate with a continuous flow of mass.

3. Isolated System
An isolated system is a special, ideal case of a closed system. It is one for which no interactions of any kind occur with the surroundings. No mass, heat, or work crosses its boundary. The energy and mass within an isolated system remain constant. A perfectly insulated thermos flask is a good approximation, though truly isolated systems do not exist; they are useful conceptual tools for applying conservation principles.

The choice of system boundary is crucial and is made by the analyst to simplify the problem. Correctly identifying the type of system is the essential first step in applying the laws of thermodynamics.