Reversible process

A reversible process is a thermodynamic change in the state of bodies , which could take place in reverse at any time without the bodies or their surroundings experiencing permanent changes . In ideal reversible processes, no entropy is generated, the entropy production is consequently zero: ${\displaystyle \Delta S=0.}$

In contrast, call real irreversible processes of energy dissipation (eg. Friction ) an entropy production inside the system out, which is always positive here . ${\displaystyle \Delta S>0}$

Whether a process is reversible or irreversible is defined by the entropy flow generated in the system and not by the change in entropy of the overall system, which depends on entropy flows across the system boundary in the form of heat or material flows (cf. second law of thermodynamics ).

In classical mechanics , all processes are reversible. In thermodynamics, on the other hand, changes of state are irreversible or irreversible if they move towards a state of equilibrium in which there are no longer any temperature or pressure differences and from which they no longer move due to a lack of potential differences; this is mostly the case in reality.

The second law of thermodynamics says that the maximum possible work of the system can only be done by a reversible process through heat supplied or removed .

For reversible processes, the change in entropy S applies : ${\displaystyle dS_{\mathrm {rev} }}$ 

${\displaystyle dS_{\mathrm {rev} }={\frac {\delta Q_{\mathrm {rev} }}{T}}}$

It is

• ${\displaystyle \delta Q_{\mathrm {rev} }}$the amount of heat converted
• T is the absolute temperature at which the process takes place.

From this it can be concluded for reversible cycle processes (e.g. for the ideal Carnot process ) that there is no change in entropy:

${\displaystyle \Rightarrow \Delta S=\oint {\frac {\delta Q_{\mathrm {rev} }}{T}}=0}$

In contrast, the following applies to the change in entropy of the system of irreversible processes:

${\displaystyle dS_{\mathrm {irrev} }>dS_{\mathrm {rev} }}$

Examples of irreversible changes of state are

• the heat conduction at finite temperature differences
• the mixing of gases or liquids (compensation of partial pressure differences )
• Throttling (converting pressure into motion)
• Friction (conversion of movement into heat).

literature

• Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4 .
• Wolfgang Nolting: Basic Course Theoretical Physics 4th Special Theory of Relativity and Thermodynamics 6th Edition, Springer-Verlag, Berlin 2005, ISBN 3-540-24119-1

Individual evidence

1. ↑ See Weigand, Bernhard: Thermodynamik kompakt, Heidelberg 2013, p. 28 ff.