Cochran and Heron1 have tested student understanding of heat engines and refrigerators in terms of the pipeline diagram of heat flows and work. This diagram is familiar from textbooks and draws on analogies with fluid flow between reservoirs. The diagram nicely illustrates conservation of energy but is silent on the other constraint involved in a Carnot cycle: the constancy of the entropy. Not surprisingly, about two-thirds of the students failed to give correct responses and explanations. I suggest that this outcome results in large part from the pipeline diagram’s exclusive emphasis on energy conservation.
A generation ago a wedge diagram was proposed2 that simultaneously represents conservation of energy and constancy of entropy in a Carnot cycle. The wedge diagram also inspired extensions to other cycles, irreversibility, and the second law of thermodynamics,3 but it has not yet been adopted in a physics text. As implied by the...
