The first law of thermodynamics states that energy can neither be created nor destroyed, only altered in form. For any system, energy transfer is associated with mass crossing the control boundary, external work, or heat transfer across the boundary. These produce a change of stored energy within the control volume. The mass flow of a fluid is associated with the kinetic, potential, internal, and “flow” energies that affect the overall energy balance of the system. The exchanges of external work and heat complete the energy balance. That is why the first law of thermodynamics is referred to as the conservation of energy principle, meaning that energy can neither be created nor destroyed, but rather transformed into various forms as the fluid within the control volume changes. A system is a region in space (control volume) through which a working fluid may or may not pass. The various energies associated with the fluid are then observed as they cross the boundaries of the system and the balance is made. As discussed in Chapter 1, a system may be one of three types:

1. Isolated system;

2. Closed system;

3. Open system.