What is the second law of thermodynamics?
Answer
The second law of thermodynamics states that the entropy of an isolated system always tends to increase and heat cannot flow on its own from a colder to a hotter body.
The working
Core idea
It sets the direction of natural processes: they always proceed towards increasing disorder, or entropy, in an isolated system.
Clausius statement
Heat cannot spontaneously flow from a colder body to a hotter body without external work being done on the system.
Kelvin-Planck statement
It is impossible to build an engine that converts all the heat absorbed from a source completely into work with no other effect.
Consequence
No heat engine can be 100% efficient; the maximum efficiency is set by the Carnot limit, η = 1 − T2/T1.
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