For nearly two centuries, from the late seventeenth century to the mid-nineteenth, physicists believed heat to be a subtle, indestructible fluid, the caloric: it passed from one body to another like water from one vessel to another, but could be neither created nor annihilated. The model worked well for describing mixtures, thermal equilibria, even Carnot’s steam engines.
The crack opened with the work of Rumford, Mayer and finally Joule: friction seemed to produce heat, and no caloric could have been drawn from any reservoir. It was then understood that heat is not a substance: it is a way of transferring energy, exactly like work.
The difference between heat and work
Both heat and work are flows of energy, not stored energy. The difference lies only in the “how”: work happens in an ordered way (a force applied to a piston), heat in a disordered way (microscopic collisions between molecules). What is stored in the body is the internal energy ; heat and work are the two ways of changing it.
From this reinterpretation the first law is born: . It is one of the most elegant cases in the history of science where a paradigm shift (from caloric to energy) left every calculation untouched, simply reinterpreting them (Simonyi 2012, ch. 4; Battimelli and Stilli 1999).
See also Riferimenti bibliografici.
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Topics: Thermodynamics Concepts: First law of thermodynamics · Internal energy
Related exercises: Nitrogen heated in a rigid cylinder · Heating under the piston · Heat as a substance?