The second law of thermodynamics is a profound principle of nature which affects the way energy can be used. There are several approaches to stating this principle qualitatively. Here are some approaches to giving the basic sense of the principle.

1. Heat will not flow spontaneously from a cold object to a hot object.

 

. Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy

 

. You cannot create a heat engine which extracts heat and converts it all to useful work

 

4. There is a thermal bottleneck which contrains devices which convert stored energy to heat and then use the heat to accomplish work. For a given mechanical efficiency of the devices, a machine which includes the conversion to heat as one of the steps will be inherently less efficient than one which is purely mechanical.