21.0 - Introduction
Physics often seems to be about possibilities, but it is also about limits. You can
think of the first law of thermodynamics as stating a limit: An engine cannot do more
work during an engine cycle than the heat added to it. If it did, it would defy the
principle of conservation of energy. If such an engine existed, it would be a source
of “free energy.” Alas, no such engine exists.
If an engine cannot perform an amount of work greater than the energy added to it,
can an engine just “break even”? This is not an idle question: The efficiency of an
engine, how much work it does per amount of energy added, is a crucial measure of
an engine’s utility.
Automobiles, for instance, can take advantage of about one-fourth of the energy
produced by the combustion of gasoline to do useful work, making them about 25%
efficient.
The second law of thermodynamics establishes the theoretical limit of efficiency: It
states that no heat engine can be 100% efficient; that is, no engine can convert all
of the heat supplied to it into an equal amount of work. In theory, engines could be
built with efficiencies of 99.9%, but never 100%. This limit cannot be reached with any conceivable improvements in engineering; it is a
theoretical limit that in principle can never be realized. The second law of thermodynamics and the topic of efficiency are two areas of focus for
this chapter.
The concept of entropy provides another way to study thermal processes. In general terms, entropy is a measure of how ordered a system is,
and is another useful tool for understanding the efficiency and limits of engines.
The simulation on the right lets you explore the relationship between heat, work and efficiency in an engine cycle. During the engine cycle, the
engine will do work and return to its initial condition at the end of the cycle. The heat engine will perform one cycle when you press GO. You
can set the amount of heat transferred from the hot reservoir to the engine during the cycle, and the amount of heat the engine expels to the
cold reservoir. At the end of the cycle, the work done by the engine and the engine's efficiency are calculated and displayed.
You can add from 50 to 500 joules of heat. These are small amounts of heat, appropriate for a toy or model engine. More than 500 joules will
exceed the safety limits of the engine.
We want you to observe two principles at work in this process. First, apply what you learned about the first law of thermodynamics to this
engine. What relationship do you expect between the net heat transferred to the engine and the net work done by the engine? (The engine
does positive work on the piston as the system expands and raises the piston, and a smaller amount of negative work as the piston falls.) Make
a hypothesis and then test it with the simulation.
Second, you will encounter the second law of thermodynamics: No engine is 100% efficient. The energy that flows out of the engine in the form
of heat to the cold reservoir cannot be used to do useful work. The greater the heat that flows out to the cold reservoir, the less efficient the
engine. In the interest of realism, the simulation requires a realistic amount of heat to flow to the cold reservoir.
Even the most efficient practical engines, like those in electric generation plants, run at less than 60% efficiency. See how efficient you can
make the simulation engine; it can be much more efficient than the average engine. But, try as you might, you will find that reaching 100%
efficiency is a goal you cannot achieve.