The following explanation can be given to justify internal energy as a thermodynamic property:

Consider a system, which changes from its state 1 to state 2 by following the path A, and returns from state 2 to state1 by following the path B (Fig. 1.10). So the system undergoes a cycle.

Fig. 1.10 Thermodynamic processes

For path A:

QA = ΔEA + WA

1.6

For path B:

QB = ΔEB + WB

1.7

The processes A and B, together constitute a cycle for which