Teaching By Analogy: The Use Of Effort And Flow Variables

Students seem to realize that flow can only occur between points where the effort variable is higher at one and lower at another. They have had sufficient experience with electricity, heat, and fluids to know that electric current must flow from higher voltage to lower, that heat must flow from higher temperature to lower, and that water must flow downhill. If it is explained to them that higher to lower potentials needed for flow to occur actually expresses the second law of thermodynamics in another, more general, way, then a very abstruse concept can be made more real.