annotated color version of the original 1824 carnot heat engine
showing the hot body (boiler), working body (system, steam), and cold body (water), the letters labeled according to the stopping points in carnot cycle
thermodynamics is a branch of physics that deals with heat and temperature, and their relation to energy, work, radiation, and properties of matter. the behavior of these quantities is governed by the four laws of thermodynamics which convey a quantitative description using measurable macroscopic physical quantities, but may be explained in terms of microscopic constituents by statistical mechanics. thermodynamics applies to a wide variety of topics in science and engineering, especially physical chemistry, chemical engineering and mechanical engineering, but also in fields as complex as meteorology.
historically, thermodynamics developed out of a desire to increase the efficiency of early steam engines, particularly through the work of french physicist nicolas léonard sadi carnot (1824) who believed that engine efficiency was the key that could help france win the napoleonic wars. scots-irish physicist lord kelvin was the first to formulate a concise definition of thermodynamics in 1854 which stated, "thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency."
the initial application of thermodynamics to mechanical heat engines was quickly extended to the study of chemical compounds and chemical reactions. chemical thermodynamics studies the nature of the role of entropy in the process of chemical reactions and has provided the bulk of expansion and knowledge of the field. other formulations of thermodynamics emerged. statistical thermodynamics, or statistical mechanics, concerns itself with statistical predictions of the collective motion of particles from their microscopic behavior. in 1909, constantin carathéodory presented a purely mathematical approach in an axiomatic formulation, a description often referred to as geometrical thermodynamics.