How was the second law of thermodynamics discovered?

The second law of thermodynamics began as an empirical observation regarding heat and energy conversion, notably through Sadi Carnot's 1824 work on heat engines. It was later given rigorous mathematical form and the concept of entropy by Rudolf Clausius in the 1850s. Statistical mechanics later provided a microscopic explanation using probability distributions.

What we know

• The first formulation of the second law was Carnot's theorem, presented by Sadi Carnot in 1824 based on caloric theory, showing an upper limit on the efficiency of heat engines. [1, 2]
• The first rigorous definition of the second law based on entropy was established by Rudolf Clausius in the 1850s. [1, 2]
• Statistical mechanics later provided a microscopic explanation of the second law using probability distributions of states for large assemblies of atoms or molecules. [2, 5]

Where to go next

• whoWho was Sadi Carnot?
• how it workedHow does statistical mechanics explain entropy?
• the bigger pictureWhat are the other laws of thermodynamics?
• what caused itHow did the steam engine influence thermodynamics?
• a case in pointWhat is Carnot's theorem?
• an unexpected connectionHow is thermodynamics connected to information theory?The history of thermodynamics is intertwined not only with classical mechanics and chemistry, but also with distant fields like information theory.

Sources

• [1]second law of thermodynamics (Wikidata Q177045) · CC0 1.0
• [2]Second law of thermodynamics · CC BY-SA 4.0
• [3]Laws of thermodynamics · CC BY-SA 4.0
• [4]Third law of thermodynamics · CC BY-SA 4.0
• [5]History of thermodynamics · CC BY-SA 4.0