Lesson 10 of 13 · 6 min
Second law of thermodynamics
NCERT §11.9
The workshop fridge hums, keeping cold drinks cold on a hot afternoon, while its back panel is warm. Riya wonders why the drinks never cool themselves by dumping heat into the warm room on their own.
The lesson in notes
In short
The first law is energy conservation, but many processes that conserve energy are never seen. A book lying on a table never jumps up by itself, even though the table could, in principle, cool slightly and hand that energy to the book as mechanical energy.
The principle that rules out such processes, while allowing everything else the first law allows, is the second law of thermodynamics.
It sets a basic limit: the efficiency of a heat engine can never be 1, and the coefficient of performance of a refrigerator can never be infinite.
Kelvin–Planck statement: no process can have, as its only outcome, taking heat from a reservoir and turning all of it into work. A perfect heat engine is impossible.
Clausius statement: no process can have, as its only outcome, moving heat from a colder body to a hotter one. A perfect refrigerator or heat pump is impossible.
The two statements can be shown to be completely equivalent: breaking either one would let you break the other.