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Entropy change
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Entropy of the universe never decreases.
ΔS ≥ 0For heat flow Q from hot T_h to cold T_c: ΔS = Q/T_c − Q/T_h > 0. Total entropy increases in any real process.
1000 J from 400 K to 300 K: +0.83 J/K.
Entropy of a part can decrease if the surroundings gain more.
In more detail
The entropy change of a system during a reversible process is the heat added divided by the absolute temperature. Heating a substance increases its entropy.
Real-world uses
- Calculating entropy changes in melting ice or boiling water.
- Analysing efficiencies of engines and refrigerators.
- Scientists use the increase of entropy to explain why heat spreads out and why a cup of coffee cools rather than heats up.
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Frequently asked questions
Why divide by temperature?
Because the same heat causes a smaller entropy change at higher temperature.
What is the entropy change in melting?
Latent heat divided by the melting temperature.
Can entropy decrease locally?
Yes. A fridge or a living cell can reduce entropy locally, but this is paid for by a larger entropy increase elsewhere.
Related topics in Heat & Thermodynamics
Otto cycle · Diesel cycle · Entropy · Statistical entropy · Free energy · Third law