Entropy
A quantity that can be created but never destroyed.
Lesson at a glanceShow the key points
- ΔS = ΔQ/T (26.5); reversible: S_f − S_i = ∫dQ/T (26.6).
- Reversible adiabatic process: S constant.
- Entropy measures disorder.
- Entropy is not conserved: it can be created but not destroyed. Second law: the entropy of an isolated system never decreases.
Entropy is the bookkeeping quantity that tells which way processes run. Energy is always conserved; entropy can only be created.
Defining entropy
Like pressure or internal energy, entropy S has a definite value in each equilibrium state. Give a system at absolute temperature T a small amount of heat ΔQ:
In a reversible adiabatic process no heat flows, so the entropy stays the same.
100 g of ice melts at 0°C (L = 3.36 × 10⁵ J/kg). By how much does its entropy change?
Entropy and disorder
Entropy measures disorder. If every molecule of a gas moved with the same velocity in the same direction, its entropy would be lower than in the real gas, where they move randomly every which way.
The second law in terms of entropy
Entropy is not conserved. It can be created, and once created it can never be destroyed.
1000 J of heat flows from a large body at 400 K to another at 300 K. Find the total entropy change.
- 1ΔS = ΔQ/T; for a reversible process S_f − S_i = ∫dQ/T. A reversible adiabatic process keeps S constant.
- 2Entropy measures disorder.
- 3The entropy of an isolated system never decreases: it can be created, never destroyed.
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