ΣWcycle = ΣQcycle

1.8

or,

WA + WB = QA + QB

1.9

or,

QA - WA = -(QB - WB)

1.10

From Eqs. (1.6) and (1.7), it yields

ΔEA = -ΔEB

1.11

similarly, had the system returned from state 2 to state 1 by following the path C instead of path B,

ΔEA = -ΔEC

1.12

From Eqs. (1.11) and (1.12),

ΔEB = ΔEC

1.13

Thus, it is seen that the change in energy between two states of a system is the same, whatever path the system follow in undergoing the change of state. Therefore, energy has a definite value for energy state of the system. Hence it is a point function and is a property of the system. The energy is an extensive property. The specific energy (J/kg) is an intensive property. The cyclic integral of any thermodynamic property is zero (∫dE = 0).