Second law of thermodynamics while applied to a process give rise to a property, called entropy. Absolute entropy (S) or entropy is a measure of energy dispersion in a system.

Following are basic features of entropy:

• Entropy transfer is associated with heat transfer (direction of flow of entropy is same as that of heat flow)
• Entropy is proportional to mass flow in an open system
• Entropy does not transfer with work

Besides flow, entropy may be generated in a system (Table 1.7)

Table 1.7 Entropy linked with energy

Heat → Work	Entropy transfers
Work → Work	No entropy transfer
Work → Heat	No entropy transfers Entropy generates

When work is dissipated, energy dispersion decreases. As a result entropy increases. Consideration of entropy affords a more convenient and straightforward means of establishing the reversibility and irreversibility of processes.

Entropy in an isolated system

•  No interaction of the system with surrounding.

Fig. 1.14 An isolated system