ABSTRACT
In Chapter 2 it was shown that energy transfers from one system to another as either work or heat. Energy transfer from one system to another is the only way that a system gains or loses energy. The rst law of thermodynamics states that energy is not generated or destroyed; it only changes from one form to another or transfers from one system to another. This important physical principle can be expressed by a simple mathematical equation. Consider a system with total initial energy Ei as shown in Figure 3.1. The total energy of this system after some time, during which total energy Ein enters the system and total energy Eout leaves the system, will be Ef. The first law of thermodynamics requires that the difference between the total energy entering this system and the total energy leaving it be equal to the difference between its final and initial energies [1-3]:
E E E Ef iin out− = − . (3.1)
The difference between final and initial energies of a system is called change of energy of that system or energy accumulation in that system; ∆E E Ef isystem = − . Therefore, the first law of thermodynamics can be expressed by the following equation:
E E Ein out system− = ∆ . (3.2)
This is a simple yet powerful equation that is the basis of all the energy and heat transfer analyses. It is also known as energy balance equation.
