|Any process||Earth-bound process|
|English chemical thermodynamicist Edward Guggenheim's 1933 definition of what constitutes the differences between a natural process and an unnatural process, based on the Clausius inequality (that which governs the known universe), for all systems (left) and for standard earth-bound systems ( (right), for the latter of which: differential Gibbs free energy decrease (dG < 0) quantifies what is "natural" whereas differential Gibbs free energy increase (dG > 0) quantifies what is "unnatural". |
“We call an action natural when it does not differ from the habitual mode of action.”
Rossini notation (1950) Modern notation Description δu < 0 ΔG < 0 If the value of δu is negative, useful energy is obtainable from the system and we know the change is a naturally occurring one in the direction toward the state of equilibrium. δu > 0 ΔG > 0 If the value of δu is positive, useful energy is required to be supplied to the given system to bring about the desired change and we know that the change is an unnatural one in the direction away from equilibrium.