Editing Second law of thermodynamics (section)

=== Planck's principle ===
In 1926, Max Planck wrote an important paper on the basics of thermodynamics.<ref name="Planck 1926"/><ref>Uffink, J. (2003), pp. 129–132.</ref> He indicated the principle
:: The internal energy of a closed system is increased by an adiabatic process, throughout the duration of which, the volume of the system remains constant.<ref name="Munster 45"/>{{sfnp |Lieb|Yngvason|1999|p=49}}

This formulation does not mention heat and does not mention temperature, nor even entropy, and does not necessarily implicitly rely on those concepts, but it implies the content of the second law. A closely related statement is that "Frictional pressure never does positive work."<ref>[[Clifford Truesdell|Truesdell, C.]], Muncaster, R.G. (1980). ''Fundamentals of Maxwell's Kinetic Theory of a Simple Monatomic Gas, Treated as a Branch of Rational Mechanics'', Academic Press, New York, {{ISBN|0-12-701350-4}}, p. 15.</ref> Planck wrote: "The production of heat by friction is irreversible."<ref>[[Max Planck|Planck, M.]] (1897/1903), p. 81.</ref><ref>[[Max Planck|Planck, M.]] (1926), p. 457, Wikipedia editor's translation.</ref>

Not mentioning entropy, this principle of Planck is stated in physical terms. It is very closely related to the Kelvin statement given just above.<ref>Lieb, E.H., Yngvason, J. (2003), p. 149.</ref> It is relevant that for a system at constant volume and [[Mole (unit)|mole numbers]], the entropy is a monotonic function of the internal energy. Nevertheless, this principle of Planck is not actually Planck's preferred statement of the second law, which is quoted above, in a previous sub-section of the present section of this present article, and relies on the concept of entropy.

A statement that in a sense is complementary to Planck's principle is made by Claus Borgnakke and Richard E. Sonntag. They do not offer it as a full statement of the second law:
:: ... there is only one way in which the entropy of a [closed] system can be decreased, and that is to transfer heat from the system.<ref>Borgnakke, C., Sonntag., R.E. (2009), p. 304.</ref>

Differing from Planck's just foregoing principle, this one is explicitly in terms of entropy change. Removal of matter from a system can also decrease its entropy.