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Entropy and the second law

The second law says the entropy of an isolated system can never decrease – it can only increase or stay constant in an ideal, reversible process. This explains why heat always flows from hot to cold on its own, and never the other way around.

ΔSisolated≥0\Delta S_{isolated} \ge 0the second law
ΔS=Q/T\Delta S = Q/Tentropy change at constant temperature (kelvin)

Symbols

SSentropyJ/K
QQheat transferredJ
TTtemperatureK

Example

500 J of heat is transferred from a reservoir at 400 K. The entropy change there is ΔS=−500/400=−1.25\Delta S = -500/400 = -1.25 J/K, and a colder reservoir receiving the heat gets a larger positive increase.

Remember the temperature in kelvin in ΔS=Q/T\Delta S = Q/T, and that heat leaving a system has a negative sign.
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← Enthalpy · Carnot efficiency →

Part of Thermodynamics: Heat and entropy.