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According to a Wikipedia article on Thermodynamic potential:

When the entropy $S $and "external parameters" (e.g. volume) of a closed system are held constant, the internal energy $U$ decreases and reaches a minimum value at equilibrium. This follows from the first and second laws of thermodynamics and is called the principle of minimum energy. The following three statements are directly derivable from this principle.

When the temperature $T$ and external parameters of a closed system are held constant, the Helmholtz free energy $F$ decreases and reaches a minimum value at equilibrium.

When the pressure $p$ and external parameters of a closed system are held constant, the enthalpy $H$ decreases and reaches a minimum value at equilibrium.

When the temperature $T$, pressure $p$ and external parameters of a closed system are held constant, the Gibbs free energy $G$ decreases and reaches a minimum value at equilibrium.

In this context, what are the 'external parameters'? Is it just the volume? Does it mean $dV=0$?

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  • $\begingroup$ external parameters are temperature and pressure. If you put a cut of coffee outside its temperature will change. This is because the temperature of the cup is defined by the temperature of the environment, hence an external parameter. $\endgroup$ – lakehal May 16 at 19:31

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