The Second Law of Thermodynamics (part 3)

A Third Statement of the Second Law

            A third way of stating the second law is to say that it is impossible to create a refrigerator that uses no power. This is equivalent to saying that, by itself, heat always flows from a warmer place to a cooler place. Recall that the purpose of a refrigerator (or air conditioner) is to take heat out of a cool place and move it to a warm place. According to this statement of the second law, this won’t happen spontaneously. Therefore, a common way for a refrigerator to function is to establish a region that is even colder than the space to be cooled, and a separate region that is even hotter than the spot where the heat is to go. However, in doing this, we have added something to our original setup, i.e. the refrigerator along with its associated input of power. 
Now, if it were possible to build a refrigerator that did its job without any input of power, we could take such a refrigerator, enclose it in a box, and use it to make heat flow spontaneously from a cooler place to a warmer place. Such a device has never been demonstrated and would violate the second law. Thus, saying that heat always flows down a temperature gradient is equivalent to saying that it is impossible to build a refrigerator that requires no power. 
            In a similar way, this statement of the second law implies, and is implied by the first statement that we used, i.e. that it is impossible to convert all heat into work. Recall that a heat engine is a device that operates between a hot reservoir and a cold reservoir and produces work. But if it were possible to build a refrigerator that did not require any work to operate, we could imagine a composite device consisting of a normal heat engine combined with a refrigerator that uses no work. 
The net result of the composite device would be a heat engine that changed all heat into work which would violate the second law. 
Alternately, we could imagine a composite device where a normal refrigerator was connected to a device that converts all heat into work. The combined device would violate our third statement of the second law since it would move heat from a cool place to a warm place without any input of work.








Summary
          The Second Law of Thermodynamics is based on the experience of many years of observations and is as solidly grounded as the First Law of Thermodynamics. It can be expresed in many different ways but all the expressions imply one another. The second law establishes values for different forms of energy, allowable directions for processes, and theoretical limits on all heat engine efficiencies.

Second Law, part 1

Second Law, part 2