Among the two laws of thermodynamics, the second is the one that has greater application in machine building and use in industry, because it deals directly with the performance of thermal machines.
Two apparently different statements illustrate the 2nd Law of Thermodynamics, the Clausius statements and Kelvin-Planck:
- Clausius statement:
Heat cannot flow spontaneously from one lower temperature body to another higher temperature body.
As a consequence, the natural direction of heat flow is from the highest to the lowest temperature, and for the flow to be reversed it is necessary for an external agent to work on this system.
- Kelvin-Planck Statement:
It is impossible to build a machine that, operating in a thermodynamic cycle, converts all the amount of heat received into work.
This statement implies that it is not possible for a thermal device to have a 100% efficiency, that is, however small, there is always an amount of heat that does not become effective work.
Thermal machines were the first mechanical devices to be widely used in industry around the 18th century. In the earliest form, heating was used to turn water into steam, capable of moving a piston, which in turn moved an axis that made mechanical energy usable for the industries of the time.
We call a thermal machine the device that, using two thermal sources, causes the thermal energy to become mechanical energy (work).
The heat source provides an amount of heat that in the device turns into work plus an amount of heat that can't be used as work .
Thus it is valid that:
The absolute value of the amount of heat is used because, in a machine that aims for cooling, for example, these values will be negative.
In this case, the heat flow happens from the lowest temperature to the highest. But according to the 2nd Law of Thermodynamics, this flow does not happen spontaneously, so it is necessary to have an external work, like this: