Editing Max Planck (section)

=== Entropy ===
[[Thermodynamics]], also known as the "mechanical theory of heat" at the end of the 19th century, had emerged at the beginning of this century from an attempt to understand the functioning of steam engines and to improve their efficiency. In the 1840s, several researchers independently discovered and formulated the law of conservation of energy, which is now also known as the [[first law of thermodynamics]]. In 1850, [[Rudolf Clausius]] formulated the so-called [[second law of thermodynamics]], which states that a voluntary (or spontaneous) transfer of energy is only possible from a warmer to a colder body, but not vice versa. In England at this time [[William Thomson, 1st Baron Kelvin|William Thomson]] came to the same conclusion.

Clausius generalized his formulation further and further and came up with a new formulation in 1865. To this end, he introduced the concept of [[entropy]], which he defined as a measure of the reversible supply of heat in relation to the absolute temperature.

The new formulation of the second law, which is still valid today, was: "Entropy can be created, but never destroyed". Clausius, whose work Planck read as a young student during his stay in Berlin, successfully applied this new law of nature to mechanical, thermoelectric and chemical processes.

In his dissertation in 1879, Planck summarized Clausius' writings, pointing out contradictions and inaccuracies in their formulation and then clarifying them. In addition, he generalized the validity of the second law to all processes in nature; Clausius had limited its application to reversible processes and thermal processes. Furthermore, Planck dealt intensively with the new concept of entropy and emphasized that entropy is not only a property of a physical system, but at the same time a measure of the irreversibility of a process: If entropy is generated in a process, it is irreversible, since entropy cannot be destroyed according to the second law. In reversible processes, the entropy remains constant. He presented this fact in detail in 1887 in a series of treatises entitled "On the Principle of the Increase of Entropy".<ref>{{cite journal |last1=Vlasak |first1=Weldon |title=Planck's theory and thermodynamics |journal=Chemical Innovation |date=February 2001 |volume=31 |issue=2 |pages=56–59 |url=https://pubsapp.acs.org/subscribe/archive/ci/31/i02/html/02learning.html |access-date=7 August 2024}}</ref>

In his study of the concept of entropy, Planck did not follow the molecular, probabilistic interpretation that prevailed at the time, as these do not provide absolute proof of universality. Instead, he pursued a phenomenological approach and was also skeptical of atomism. Even though he later abandoned this attitude in the course of his work on the law of radiation, his early work impressively shows the possibilities of thermodynamics in solving concrete physicochemical problems.<ref name="Hoffmann29f">Hoffmann: ''Max Planck.'' Munich 2008, p.&nbsp;29.</ref><ref name="Hartmann156f">Hartmann: ''Max Planck als Mensch und Denker.'' 3. revised edition, Basel 1953, p.&nbsp;156.</ref>

Planck's understanding of entropy included the realization that the maximum of entropy corresponds to the equilibrium state. The accompanying conclusion that knowledge of the Entropy allows all laws of thermodynamic equilibrium states to be derived corresponds to the modern understanding of such states. Planck therefore chose equilibrium processes as his research focus and, based on his habilitation thesis, researched the coexistence of aggregate states and the equilibrium of gas reactions, for example. This work on the frontier of chemical thermodynamics also received great attention due to the rapidly expanding chemical work at that time.

Independently of Planck, [[Josiah Willard Gibbs]] had also discovered almost all the knowledge Planck gained about the properties of physicochemical equilibria and published them from 1876 onwards. Planck was unaware of these essays, and they did not appear in German until 1892. However, both scientists approached the topic in different ways, while Planck dealt with irreversible processes, Gibbs looked at equilibria. This approach was finally able to prevail because of its simplicity, but Planck's approach is attributed the greater universality.<ref>Hoffmann: ''Max Planck.'' Munich 2008, p.&nbsp;31&nbsp;f.</ref>