It is known that a work W > 0 produced by an engine over a cycle equals to a net heat Q Σ Σ = | Q H | − − | Q C | absorbed over a cycle. Thus, with the sign convention for a heat Q transferred in a thermodynamic process ( Q > 0 for an absorption and Q < 0 for a dissipation) we get: W − − Q Σ Σ = W − − | Q H | + | Q C | = W − − Q H − − Q C = 0 Since this equality holds over an entire Carnot cycle, it gave Clausius the hint that at each stage of the cycle the difference between a work and a net heat would be conserved, rather than a net heat itself. Which means there exists a state function U with a change of d U = δ δ Q − − d W . It is called an internal energy and forms a central concept for the first law of thermodynamics.