# How can I achieve isothermal compression

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### Irreversible and reversible volume work

The gas can be returned to its original state by applying the appropriate masses to the stamp. This compression runs along the paths marked with K in the following diagrams for the cases A, B and D (E = expansion) treated above.

After running through a cycle A B A, the path of expansion and compression encloses the area hatched in the diagram (except for case D). It is like that of the work that is released by lowering only the uppermost, externally attached mass to the level of the original stamp position. This amount of energy is lost to the environment as potential energy, but it remains as thermal energy (friction of the punch on the piston wall). Since thermal energy (= disordered energy) cannot easily be converted back into mechanical work (= usable energy), the environment has suffered a loss of work due to such a cycle.

A change of state A B is called irreversible if the reverse process B A takes place in such a way that mechanical work (ordered particle energies) of the environment is converted into heat (disordered particle energies) of the environment or another, corresponding change has occurred.

An exception is the case D, in which the pressure equilibrium persists during the expansion. Here is the top mass, so that there is no loss of mechanical energy. After each cycle, both the system and the environment return to their original state.

A change of state A B is called reversible if the reverse process B A is possible in such a way that not only the system but also the environment returns to the original state. Mechanical energy from the environment is not converted into thermal energy from the environment.

It should be noted that every change in state that can be carried out in the laboratory is irreversible, since it must take place in a finite time. As the above examples show, however, in practice we can approximate the reversible ideal value relatively well. In particular, the statement is permitted and also useful (even if the strict meaning of the word is incorrect) that a process is 95% reversible if the practical value reaches the 0.95 multiple of the ideal value.