Historically, classical thermodynamics was developed during attempts to improve the efficiency of steam engines. Those investigations led to the well-known zeroth, first, and second laws of thermodynamics. Although these laws are presented independently, linkages between the zeroth and second laws have been pointed out in the physics teaching…

What if energy flowed from cold to hot regions? How would thermodynamics be different? We examine such an imagined cold-to-hot world, abbreviated here as a C2H world, and show that the unfamiliar direction of energy flow implies that two thermally interacting objects at different temperatures will not equilibrate. We find that there is no…

The first law of thermodynamics does not forbid heat engines with 100% thermodynamic efficiency; it is the second law of thermodynamics which states that heat engines cannot obtain 100% efficiency. Can the first law ever show this second law result? Yes. We show that the first law alone can show that some heat engines cannot achieve efficiencies…