Heat Engines and the Second Law
An engine takes heat from a hot reservoir, does work and dumps the rest cold, and no arrangement of parts can skip the dumping. The second law states that limit in terms of what cannot be built.
What a learner can do afterwards
- Computes the efficiency of an engine from the heat taken in and rejected
- States the engine and refrigerator forms of the second law and shows they say the same thing
- Explains why a device converting heat entirely into work is impossible
1 · Read
An engine takes heat Qh from a hot store, turns part into work W, and dumps the rest Qc to a cold store. Efficiency is W over Qh, which equals one minus Qc over Qh. The dumping is not bad design: car engines all need cooling because no arrangement of parts can skip it.
An engine with efficiency 0.60 does 300 J per cycle. The heat drawn is 300 over 0.60, which is 500 J, and the heat dumped is 500 minus 300, which is 200 J. Even this idealised engine throws away 200 J every cycle.
The second law states the limit two ways. Engine form: no cycle can turn heat from one store fully into work. Refrigerator form: heat cannot crawl from cold to hot without work put in. The two say the same thing: a cheat device for one form could drive a cheat device for the other.
Measure temperatures in kelvin before any ratio. Efficiency always lands below one, and any claim above one breaks the law. A refrigerator is not a reversed cheat: its performance still costs work.
Work is heat in minus heat out, efficiency stays below one, and both forms of the second law ban the same cheats.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.