ABSTRACT
This chapter explores the principles of pathways and cycles in thermodynamics. By measuring work and heat in slow processes, we can compute state functions, such as energy and entropy. State functions sum to zero around thermodynamic cycles. But path functions, like heat and work, do not sum to zero around cycles. The chapter broaden our toolkit to include heat, work, processes, pathways, and cycles. This is crucial for understanding cyclic energy conversions—in engines, motors, refrigerators, pumps, rechargeable batteries, hurricanes, ATP-driven biochemical reactions, oxygen transport around our body, and geophysical cycles of carbon and water. Thermodynamic logic often seems complex. This apparent complexity arises because the fundamental quantities that predict equilibria are not directly measurable. Work describes energy transport through mechanical means. At the core of thermodynamics are thermodynamic cycles. Cycles are described by state diagrams, which show how certain state variables change throughout the cycle.
