Second law of thermodynamics

In physics, the second law of thermodynamics is a statement about the quality and direction of energy flow, and it is closely related to the concept of entropy. It establishes the fact that certain transfers of energy, such as electrical heating of a liquid, can only proceed in one direction. Electricity can heat a cool liquid, but a heated liquid, by itself, cannot create electricity. In the first case, the energy is of high quality but in the second case it is of low quality.

The Kelvin-Planck statement of the second law of thermodynamics says, “It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.” One of the consequences of this is that no heat engine can have an efficiency of 100%. That includes idealized heat engines with no friction and no other dissipative effects.

The second law of thermodynamics is important to engineers because it provides a way to determine the quality, as well as the amount of degradation of energy during a process. It is also used to determine the theoretical upper limits for the performance of many commonly used engineering systems like refrigerators and internal combustion engines. Any device that violates the second law of thermodynamics is called a perpetual motion machine of the second kind. One example of this would be a device that can do work such as pumping water, simply by taking energy from the air.