Second Law of Thermodynamics
Revised 24 May 2019
For a heat engine to be 100% thermally efficient, the engine is required to reject no heat to a reservoir. This requirement has proved to be a physical impossibility. The energy of heat comprises the internal energy of random molecular motion as opposed to the ordered kinetic motion of mechanical work. It is not feasible to completely convert the random molecular motion of heat to ordered molecular motion of mechanical work because the motions of each individual molecule would need to be fully controlled. Furthermore, part of the portion of heat that can be converted into mechanical work dissipates to the heat form of energy due to the unavoidable friction present in all machines. The degree of convertibility of energy - stored work - into applied work is often called availability. The Second Law of Thermodynamics can be stated as:
All physical processes proceed in such a way that the availability of the energy involved decreases.
The Second Law of Thermodynamics is not deduced from the First Law of Thermodynamics, but stands as a separate law of nature. The First Law of Thermodynamics denies the possibility of creating or destroying energy, but does not preclude the possibility of running a power station that extracts heat from the atmosphere. The Second Law of Thermodynamics denies any possibility of perpetual motion of the second kind whereby a machine utilizes the internal energy of one, and only one, heat reservoir.
The Second Law of Thermodynamics may appear equally simple to understand as the First Law of Thermodynamics, but the Second Law of Thermodynamics is very deceptive (Andrews, 1980) and there are many far reaching implications which connect back to this law. An outcome of the Second Law of Thermodynamics is that all forms of energy ultimately degrade into dispersed heat energy. There is no process whose sole result is the complete transformation of energy into another form of energy which is of higher grade than heat. This is why it is impossible to drive a steamship across the ocean by extracting heat from the ocean or why the perpetual motion machine does not exist. In other words, there are some fixed limits to technological innovation, placed there by fundamental laws of nature.