 In the traditional approach classical thermodynamics has two laws, the First and Second  Laws.[2]  The First Law expresses the conservation of energy and is founded upon the impossibility of creating a machine that can create energy. The law uses the concept of the internal energy of a system, \(U\), which is a function of the system’s macroscopic variables, e.g., temperature, volume. For thermally isolated (adiabatic) systems—think of systems such as coffee in a thermos—the law states that this function, \(U\), is such that the work \(W\) delivered to a system’s surroundings is compensated by a loss of internal energy, i.e., \(dW = -dU\). When James Joule and others showed that mechanical work and heat were interconvertible, consistency with the principle of energy conservation demanded that heat, \(Q\), considered as a different form of energy, be taken into account. For non-isolated systems we extend the law as \(dQ = dU + dW\), where \(dQ\) is the differential of the amount of heat added to the system (in a reversible manner).