Energy is a Property of the System

Refer to Figure 6.2 again and consider Eq. (6.5)

	(6.8)

and depend on path function following by the system. The quantity is the same for both the process and connecting the states 2 and 1. The quantity does not depend on path followed by the system, depends on the initial and final states. Hence is an exact differential.

Differential of property of the system

This property is the internal energy of system, U

(6.9)

Energy of an Isolated System

An isolated system is one in which there is no interaction of the system with the surroundings for an isolated system

(6.10)