Thermodynamics is a subsection of physics that deals with energy and its relationship with properties of matter. It is concerned with the different forms of energy and their transformation between one another. It provides the general laws that are the basis for energy conversion, transfer, and storage.
Systems, System Boundaries, Surroundings:
A thermodynamic system, or briefly a system, is a quantity of matter or a region in space chosen for a thermodynamic investigation. Some examples of systems are an amount of gas, a liquid and its vapor, a mixture of several liquids, a crystal or a power plant. The system is separated from the surroundings, the so-called environment, by a boundary (real or imaginary). The boundary is allowed to move during the process under investigation, e.g., during the expansion of a gas, and matter and energy may cross the boundary. Energy can cross a boundary with matter and in the form of heat transfer or work. The system with its boundary serves as a region with a barrier in which computations of energy conversion processes take place. Using an energy balance relationship (the first law of thermodynamics) applied to a system; energies that cross the system boundary (in or out), the changes in stored energy, and the properties of the system are linked. A system is called closed when mass is not allowed to cross the boundary, and open when mass crosses the system boundary. While the mass of a closed system always remains constant, the mass inside an open system may also remain constant when the total mass flow in and the total mass flow out are equal.
Description of States, Properties, and Thermodynamic Processes:
A system is characterized by physical properties, which can be given at any instant, for example, pressure, temperature, density, electrical conductivity, and refraction index. The state of a system is determined by the values of these properties. The transition of a system from one equilibrium state to another is called a change of state. The distinction between a closed and an open system corresponds to the distinction between a Lagrangian and an Eulerian reference system in fluid mechanics. In the Lagrangian reference system, which corresponds to the closed system, the fluid motion is examined by dividing the flow into small elements of constant mass and deriving the corresponding equations of motion.