The empirical conclusion that heat and internal energy belong to the general category of energies, help extending the law of conservation of mechanical energy, which states that potential and kinetic energies are fully inter-convertible. As already discussed in chapter 1, a thermodynamic system may possess any other forms of energy such as surface energy, electrical energy, and magnetic energy, etc. Thus one may arrive at an extended postulate that all forms are energies are inter-convertible. This constitutes the basis of the First Law of Thermodynamics, which may be stated as follows:
Energy can neither be destroyed nor created, when it disappears in one form it must re-appear at the same time in other forms.
It must be said that there is no formal proof of the first law (or indeed of other laws of thermodynamics) is possible, but that no evidence have been found to date that violates the principle enunciated by it.
For any thermodynamic process, in general one needs to account for changes occurring both within a system as well as its surroundings. Since the two together forms the “universe” in thermodynamic terms, the application of the first law to a process leads to the following mathematical form:
∆(Total energy of the universe) = 0
∆(T otalen erg y o f thesyste m) +∆(T otalen erg y o f the surrounding s) = 0
Where Δ ≡ finite change occurring during the thermodynamic process