Temperature > 06 - Second Law of Thermodynamics

The second law of thermodynamics involves the operation of heat engines, that is, situations in which heat is transformed into other forms of energy.

The study of heat engines left physicists quite intrigued by situations that did not occur as expected.

One of these situations that did not occur was the spontaneous transfer of heat from a cold body to a hot body. What always happens is the opposite: the spontaneous transfer of heat from a hot body to a cold body. Someone could mention the refrigerator as the transfer of heat from a cold region to a hot region, but it is not spontaneous. For it to occur, an engine must be used to perform the work.

The other situation that does not occur is the complete transformation of heat into work. Heat engines work using two sources of different temperatures, so that part of the heat removed from the hot source is sent to the cold source. It is not possible to transform all the heat removed from the hot source into work.

These two situations, also called prohibitions, gave rise to the second law of thermodynamics:

Heat flows spontaneously from a hot body to a cold body; the reverse only occurs when work is performed, and no heat engine that operates in cycles can remove heat from a source and transform it entirely into work.

By comparing the first and second laws of thermodynamics, we can say that while the first law of thermodynamics establishes the conservation of energy in any transformation, the second law establishes conditions for thermodynamic transformations to occur.