The KelvinPlanck 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.
The second law can be stated in terms of entropy as follows: the entropy of an isolated system increases in time.
Explanations for the second law The second law of thermodynamics is essentially a macroscopic law. It has remained somehow mysterious up to these days to derive it from the microscopic laws, and none of the explanations is really fully satisfactory. The problem is that microscopic laws are all reversible in time, and the second law is not.
Boltzmann has first addressed this question.He has given explanation by means of Statistical mechanics, frist for diluted gases in his Htheorem[?]. He does not derive the second law of thermodynamics from mechanics alone, but also from the probability arguments. His idea is to use the coarse graining[?], grouping of microstates into macrostates, and then to make a statement about what is most probable to happen for a macrostate  for some microstates the entropy will decrease, but this happens with low probability.
The Ergodic hypothesis was important for Boltzman approach. It says that, over long periods of time, the time spent in some region of the phase space of microstates with the same energy is proportional to the volume of this region, i.e. that all accessible microstates are equaly probable over long period of time. Equivalently, it says that time average and average over the statistical ensemble are the same.
In Quantum mechanics, the ergodicity approach can also be used. However, there is alternative explanation, which involves Quantum collapse  it is a straightforward result that quantum measurement increases entropy of the ensamble. Thus, second law of thermodynamics is intimately related to quantum measurement theory and quantum collapse  and none of them is completely understood.
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