Irreversible process

A physical process is irreversible when it is not reversible. The opposite is a reversible process. Although all microscopic elementary reactions are reversible, all macroscopic processes, if you look hard enough, irreversible. The irreversibility arises from the improbability of the initial state and establishes the unique direction of time, see arrow of time.

As an example of an irreversible process serves a glass, which falls from the table onto the floor and shatter. According to Rudolf Clausius, this process is irreversible since it can not spontaneously occur in the reverse direction. In fact, we have never observed such as the splinters of glass together spontaneously sat down again, and the newly formed glass jumped on a table.

However, this definition is still incomplete, which first Max Planck recognized. To illustrate the Planckian Irreversibilitätsbegriff, we imagine that the shattered glass is melted, a new glass is made, which is then placed on the table. It has now been restored, but in a different way obviously the output state ( on the table glass). During the melting and forming of the glass, however, additional irreversible processes have taken place; the attempt to make an irreversible process has reversed, so in turn left a deep trace of irreversibility in the area.

A good definition of irreversibility is thus the following: There is no way to make an irreversible process in any way undo, and simultaneously reset all the tools for about used again in its initial state.

This Planck's formulation of irreversibility is much stronger than that of Clausius, as all freedoms and any means may be used during the reversal of a process. If one assumes that there is definitely irreversible processes in nature, such as the conversion of mechanical work into heat ( eg friction ), it follows that thermodynamic states is a natural order in terms of their temporal ( irreversible possess ) sequence. This order can be for equilibrium states by a measure, the thermodynamic entropy expressed. After the Second Law of Thermodynamics, all processes are irreversible, in which entropy is produced.

Conversely, the laws of thermodynamics can be derived starting from the Irreversibilitätsbegriff - thermodynamics is a theory of irreversibility. This access is called axiomatic thermodynamics. Max Planck and Max Born promoted the further development of this approach by Constantin Carathéodory.