Now let us redefine a system which includes our earlier system and its surroundings. Then

for an isolated system

(23.14)

Through generalization we can write for an isolated system

(23.15)

Examine the statement critically. For a reversible process it is equal to zero. For an irreversible process it is greater than zero.

Similarly it is possible to write . The equality sign holds good if the process undergone is reversible; the inequality sign holds good if the process undergone is irreversible.

Clausius summarized these as:

• Die Energie der Welt ist konstant.
• Die Entropie der Welt strebt einem Maximum .

This means:

• The energy of the universe in constant (first law).
• The entropy of the universe tends towards a maximum (second law)