When all is said and done, the First Law of Thermodynamics establishes energy as a conserved property of nature (see Chapter 3).

Historically, however, the First Law emerged from a narrower perspective involving Rudolf Clausius’s review of Sadi Carnot’s and Émile Clapeyron’s theoretical examination of the idealized steam engine and the piston-in-cylinder process within. As part of this review, Clausius performed the one of the first and most famous energy balances in history and concluded that the internal energy (U)—a newly created concept by Clausius—of the working substance within a piston-in-cylinder assembly can change only through the addition of heat (Q) to the system or through work (W) done by the system or both. If no Q and no W, energy (U) does not change.

The following slide is one I created to illustrate this principle in a recent presentation. In addition to showing Clausius’s famous 1st-Law equation, I include his equally famous defining equation of entropy. It was Gibbs who formalized Clausius’s work and so concluded the creation of classical thermodynamics.

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