Second law of thermodynamics

The second law of thermodynamics for a reversible change (that is to say that the system evolves through a succession of equilibrium states) can be written as

$dQ=TdS$

Thus for a closed system, having a fixed number of atoms/molecules, one has

$dU=TdS-PdV$

where $U$ is the internal energy. For an open system:

$dU=TdS-PdV+\mu dN$

where $\mu$ is the chemical potential.

For $U(S,V)$ one has the following total differential

$dU={(\frac{\partial U}{\partial S})}_{V}dS+{(\frac{\partial U}{\partial V})}_{S}dV$

See also

• Entropy
• Fluctuation theorem
• H-theorem

References

1. Nicolas Léonard Sadi Carnot "Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance" (1824), printed in Annales scientifiques de l'École Normale Supérieure Sér. 2, 1 pp. 393-457 (1872)