The energy changes of the various parts of the supersystem
during a thermodynamic event, measured in a reference frame
fixed in the laboratory, are AE”, and AE, (i = 1, 2, ) The firstlaw expression of the local formulation (eq 1] is replaced by a statement of the conservation of energy within the supersystem:
Ess = Esys + Ei = 0 (3)
A rearranged version of eq 3 allows the system’s energy change
to be detennined from measurements in its surroundings:
Esys = -Ei (4)