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Entropy
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A measure of disorder or the number of microstates.
dS = δQ/TEntropy S measures disorder or the number of ways to arrange microscopic states; for reversible heat transfer dS = δQ/T. It allows us to state the second law quantitatively.
Melting 1 kg of ice at 273 K: ΔS = 334000/273 = 1224 J/K.
Entropy isn't "chaos", it's a measure of accessible states.
In more detail
Entropy is a measure of disorder or the number of microscopic arrangements of a system. Its change for reversible heat transfer is dS = dQ/T, and entropy tends to increase in natural processes.
Real-world uses
- Entropy explains why heat flows from hot to cold.
- Chemists use entropy changes to predict whether reactions proceed spontaneously.
- Chemists use entropy to explain why ice melts and why mixed gases do not separate by themselves.
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Frequently asked questions
Is entropy disorder?
It is often described as disorder, but more precisely it counts microscopic arrangements.
Can entropy decrease?
Locally yes, such as in a fridge, but the total entropy of an isolated system does not decrease.
Is entropy the same as disorder?
It is often described that way, but more precisely it counts the number of microscopic arrangements consistent with the same macroscopic state.
Related topics in Heat & Thermodynamics
Refrigerators and heat pumps · Otto cycle · Diesel cycle · Entropy change · Statistical entropy · Free energy