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3.6

Entropy is not conserved

Entropy
  • Unlike energy functions, entropy is not conserved in natural process or in isolated systems.
  • For example, consider heat transfer between two objects with different temperature TI and TII (TI > TI)in an isolated system. The two objects are connected by a thermal conductor such as a copper wire. The heat will flow from the high temperature side (I) to the low temperature side (II). Assuming no heat loss, the heat from I will go into II, thus \(Q\)\(I\)=−\(Q\)\(II\)

  • The internal energy change of the whole system (I+II) is thus zero.
    ∆\(U\)\(I+II\)=\(∆U\)\(I\)+\(∆U\)\(II\)=\(Q\)\(I\)+\(Q\)\(II\)=0
    The energy is conserved.
  • Let’s consider the entropy change of the whole system (I+II).

Insert the relation \(Q\)\(I\)=−\(Q\)\(II\)

The entropy of this isolated system is not conserved. This heat flow is spontaneous and irreversible and the irreversibility cause entropy increase.

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Thermodynamics in Energy Engineering

Hanyang University