In a recent article1 a thermodynamic derivation of Young’s and Laplace’s equations for the mechanical equilibrium of fluids was given. The derivation used a simplified global approach for incompressible fluids and was compared to the derivation originally given by Gibbs. At the end of Sec. II it was asserted that the relation,

δPdv=ΩPδNdΩ,
(1)

[Eq. (23) of Ref. 1] used in Gibbs’ derivation2 is incorrect and examples to support this assertion were given in Appendix B.

In the following I suggest that in the context of Gibbs’ derivation, Eq. (1) is correct and hence the derivation that follows leading to Young’s and Laplace’s equations (and in Ref. 3) is also correct.

Gibbs did not believe Eq. (1) to be correct for arbitrary real (or virtual) variations of the system. This point is also...

AAPT members receive access to the American Journal of Physics and The Physics Teacher as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.