Who introduced the concept of entropy in 1865?

Rudolf Clausius introduced the concept of entropy in 1865. He gave the quantity its name and presented its first mathematical formulation while developing the second law of thermodynamics.

Clausius defined the entropy change for a reversible process through the relation dS = δQᵣₑᵥ/T, where δQᵣₑᵥ is reversible heat transfer and T is absolute temperature. Entropy is a state function, so its change depends only on the initial and final states, even though heat and work themselves depend on the path taken.

The word entropy comes from the Greek root associated with transformation. Clausius used it to describe how energy becomes less available for producing useful work during real processes. His 1865 work also helped unify the first and second laws, including the idea that the entropy of an isolated system does not decrease.

Sadi Carnot’s earlier work on heat engines laid important foundations, while James Joule studied the relationship between heat and mechanical work. Lord Kelvin also shaped thermodynamics, but he did not introduce entropy. Later, Ludwig Boltzmann and J. Willard Gibbs supplied statistical interpretations connecting entropy with microscopic arrangements and probability.