ratio of what we get out to what we have to put in—that is, e = QW/QH. Since W =
Qnet = QH − |QC|, we have
Notice that unless QC = 0, the engine’s efficiency is always less than 1. Here is
another form of the second law of thermodynamics.
For any cyclic heat engine, some exhaust heat is always produced. Because QC ≠ 0,
no cyclic heat engine can operate at 100% efficiency; it is impossible to completely
convert heat into useful work.
- A heat engine draws 800 J of heat from its high-temperature
source and discards 600 J of exhaust heat into its cold-temperature
reservoir each cycle. How much work does this engine perform per
cycle, and what is its thermal efficiency?
Here’s How to Crack It
The work output per cycle is equal to the difference between the heat energy drawn
in and the heat energy discarded.
W = QH − |QC| = 800 J − 600 J = 200 J.The efficiency of this engine is