Where Q is the applied heat from surrounding to the system and W is part of the energy that system expend to perform work on the surrounding. While Q is reminiscent of the external energy, we show that W in the form of expansion work in absence of additional works (W non-pv ) is equal to the potential energy change in classical mechanics theory. Below we elaborate on this similarity with an illustrative explanation of each work with the aid of a schematic representation for these two theories in Figure . Imagine we apply external energy to a piston that has separated two boxes initially at equilibrium with each other (E k,i = 0). According to Newtonian mechanics, the piston moves in the direction of the external energy in the absence of other forces 1. After the piston reaches a new equilibrium state (E k,f = 0), the stored energy in the upper box will be: