3: Heat engines and the second law

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We all know of engines. We could talk about them in general terms - the thing that runs our car, and such as that. If we’re really literate, we can talk about exactly how a piston takes gasoline and a spark and generates a great deal of power that keeps us moving forward.

But in most of the contexts we think about engines in our modern life, we don’t really want to think of engines. We want engines to work, and that’s all; they serve the role of black boxes under the hoods of our cars, functioning in ways that we don’t care about to get us from place X to place Y as fast as possible.

Maybe I shouldn’t speak for you. Maybe you really care about the piston and the cam, all the time. But I know I don’t.

So starting out a new section of this course with a definition of an engine is awkward - at least for me. But it’s necessary because of the process energies we’ve been deliberately developing over the last section - heat and work. An engine is a system that transforms heat energy into work performed by the system.

We care about engines because they’re practical applications of many of the ideas we’ve been talking about in the course thus far. They’re the kinds of systems that bring many of our core ideas to bear.

If we’re not careful, though, we won’t bring those ideas to bear usefully. As we get used to the kinds of processes we need to study engines, we can impose conditions on the behavior of them that are unrealistic - and not merely unrealistic in the real world, but unrealistic even in the context of the kind of theoretical study we’re trying to build.

So our primary goal, even theoretically, is to think about the engine as practically as possible. We want to be aware of the very real problems that come with engine operation, and we want to solve those problems - or, at least, consider those problems - as completely as we can. If we do this seriously enough, we will come to some conclusions about how well we can transform heat energy into practical work - and those conclusions will complete our picture of thermodynamics.