The"genuine' physics energy concept is an abstract, mathematical one. There is a system(for example, system I in figure 14.1) In this system, quantities of state defined, one of them being energy (EI). No matter what happens this system, there is no change in the quantity of state energy. If the results of measurements are put into the appropriate energy formula, the same number is produced. If this number is not the outcome of a calculation, one has to search for transformations in the system that have not been regarded so far or else ther has been an interaction with another system(system II). If the latter is the case in one system the energy number increases whereas in the other system it decreases accordingly.