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The Law of Conservation of Energy

This law provides the basis for establishing relationships in all branches of engineering. It is also known as the First Law of Thermodynamics and postulates that energy can neither be created nor destroyed. Hence, for any system enclosed by boundaries as shown schematically in Fig. 1-2, the following relationship results from this law

Figure 1-2. Energy balance in bounded system.

In Eq. 1-2 the gain in the internal energy may be positive or negative; part or all of the internal energy may be reversible, or part or all of it may be irreversible.

Reversible and irreversible processes

A reversible process is defined as one for which the system and surroundings can be completely restored to the respective initial states after a process has occurred.1 Spontaneous changes that occur in nature are considered irreversible because they proceed in only one direction. To effect a reverse process identical in all respects with the spontaneous change, it is necessary to introduce an external source of some form. Water flows spontaneously through a pipe from a tank at one level to one at a lower level. However, to move the water through the pipe back into the tank at the higher level a pump is required and an external source of energy would have to be used to drive the pump to reverse the process. The following are examples of irreversible processes

  • Flow of ground water into wells.
  • Leakage in dams.
  • Flow of water in pipes.
  • Energy dissipation as heat in electrical circuits.
  • Combustion processes.
  • Diffusion processes.
  • Firing of a projectile.
  • Production of sound in a loud-speaker.
  • Braking of automobiles.
  • Corrosion.

1 George A. Hawkins, Thermodynamics for All Engineering Students. Conference on Thermodynamics at Pennsylvania State University, June 27-29, 1955.

Last Update: 2011-02-16