Speaker
Description
Suppose you have a beaker of water at 100°C and a second beaker containing an equal amount of water at 0°C. If you bring them into thermal contact and wait, both will reach 50°C. That heat exchange is irreversible. There is another way to bring these samples of water into thermal contact so that they exchange temperatures. The 100°C mass ends up at 0°C, and the 0°C sample at 100°C. Clearly this heat exchange is reversible. How do you do it? No heat exchanges with any other object (such as a heat engine) occur. It's not magic; it's just physics. (Although some people might say those are synonyms.) Think carefully about what a reversible heat exchange between two objects requires. Are you sure it's necessary that the two objects be infinitesimally different in temperature?
