Pedagogic niceties in the treatment of unconventional thermodynamic cycles, especially those involving (negatively sloping) diagonal linear transitions in a P/V state diagram and/or those implying supposedly superefficient heat-engine operation, are discussed as a means of stimulating student interest and comprehension, as well as promoting fresh insights, correcting erroneous notions, and provoking further enquiry. In particular, a novel (ostensibly all-adiabatic) engine using two ideal gases of mutually differing atomicities as working substance is analyzed qualitatively and quantitatively. Emphasis is placed on the crucial role of the second law of thermodynamics in a determination of heat-engine operation.

Although the ideal Carnot engine rejoices in the largest efficiency of any heat engine operating between given temperature extremes, alternative closed-loop four-step cycles in which a variable-temperature, straight-line transition replaces the Carnot high-temperature isothermal expansion appear ostensibly to offer a higher thermodynamic efficiency. This arises from the fact that higher maximal temperatures are attained; the efficiency of the equivalent Carnot engine operating between such temperature extremes reinstates Carnot supremacy by slightly transcending the unconventional engine's efficiency. Our investigations stress the importance of the second, as well as the first, law of thermodynamics, and consider operating characteristics quantitatively for specimen engines of various design.

for their support throughout the process. I. Introduction Around the world, forces are combining to reshape power systems at a rate faster than would have been expected 20 years ago. Power systems appear poised for a revolution. Yet the pathway to transformation is highly sensitive to each local situation, and the technical, economic, and political factors at play-whether global or local-warrant careful examination.

In a typical engineering or physics curriculum, the momentum equation is used for the determination of jet engine thrust. Even a simple thrust analysis requires a heavy emphasis on mathematics that can cause students and engineers to lose a physical perspective on thrust. This article provides for this physical understanding using only static pressures that act on engine surfaces. Such an alternative, but equivalent, method can offer insights into some special examples of jet engine thrust that contradict commonly held beliefs. One such example is provided by the engine bellmouth that is used for testing jet engines on the ground. The static pressure distribution clearly shows that the engine bellmouth actually experiences forward thrust. Another example is provided by the conic exhaust nozzle that is used at the end of some jet engines. The static pressure distribution shows that the conic nozzle does not experience any forward thrust (although the nozzle increases the overall thrust of an engine through higher pressures upstream of the nozzle). Following these examples, a basis for conceptualizing propulsive efficiency is discussed. This illustrates that it is more efficient to have a smaller acceleration of a large amount of air than a larger acceleration of a smaller amount of air. The momentum equation and static pressure distributionStudents frequently describe thrust as the rate of change in momentum of air. This explanation is, of course, correct. Yet students, and even engineers, seem unable or unwilling to provide more depth or go any further. It is as if thrust is a 'magical' or 'mathematical' result that is due to the change in momentum of air, and that is it.However, thrust can surely be recognized beyond such a mathematical scope by returning to the principles of physical interactions and Newton's third law.

In this note, we show that casual application of the principle of least action leads to the erroneous conclusion that things ought to fall upwards. We will analyze this paradox and show which lesson can be learned from it.