Physics · Ch 11 — Thermodynamics
Second Law of Thermodynamics
Second Law of Thermodynamics
The first law of thermodynamics is simply a statement of energy conservation, and by itself it places no restriction at all on the direction in which a process can occur -- as far as the first law is concerned, heat flowing spontaneously from a cold body to a hot one is exactly as permissible as the reverse (familiar) direction, since either way, energy is conserved. Yet this reverse flow is never observed to happen spontaneously in nature. This missing restriction on the direction of natural processes is supplied by the second law of thermodynamics, which can be expressed in either of two forms, each phrased around a different practical device, and each provably equivalent to the other.
The Kelvin-Planck statement (phrased in terms of heat engines):
No process is possible whose sole result is the absorption of heat from a single reservoir, and the complete conversion of that heat into an equal amount of work.
In plain terms: no heat engine, however cleverly designed, can be perfectly (100%) efficient. A heat engine that took in heat from just one reservoir (say, the ocean, or the atmosphere) and turned every joule of it into useful work, with no other change anywhere, would not violate the first law at all -- yet the Kelvin-Planck statement says such an engine is nonetheless impossible. Every real heat engine must reject some heat to a second, colder reservoir; it is this unavoidable rejection of waste heat that keeps every real engine's efficiency strictly below 100%.
The Clausius statement (phrased in terms of refrigerators):
No process is possible whose sole result is the transfer of heat from a colder body to a hotter body.
In plain terms: heat cannot move "uphill," from cold to hot, entirely on its own -- doing so always requires that external work be supplied to force it, as happens inside a refrigerator or an air conditioner (Section 11.13). If heat could move spontaneously from cold to hot with no other effect, refrigeration and air conditioning would need no electricity at all. …
| Aspect | Kelvin-Planck statement | Clausius statement |
|---|---|---|
| Framed in terms of | Heat engines | Refrigerators / heat pumps |
| What it forbids | Converting heat from one reservoir completely into work, with no other effect | Heat flowing from a colder to a hotter body, with no other effect |
| Practical consequence | Every heat engine must reject some heat to a cold sink; always | Every refrigerator or heat pump needs external work input to move heat 'uphill' |