In nature almost all thermodynamic transformations are irreversible. Some everyday examples: coffee cools down in the air, gas from a spray can expands freely when the valve is opened, a red-hot block of iron plunged into water brings everything to thermal equilibrium. These are processes that occur spontaneously in one direction only: we have never seen cold coffee heat itself up by cooling the room.
Key idea — What makes a transformation irreversible
During an irreversible transformation the system (gas, liquid or solid) passes through states that are not equilibrium states. It makes no sense to speak of a “curve” in the - diagram, because and are not even well defined while the transformation is under way: during a free expansion, for instance, the particles at the front invade the vacuum before those behind them, and pressure does not have a single value throughout the container.
Why this isn’t discouraging. The central point of the chapter is that conservation of energy always works, even when the transformation is abrupt, explosive or ill-defined. There is no need to follow the path: the initial state A and the final state B are enough, both being well-defined equilibrium states. Between A and B we can apply exactly the method already seen in mechanics: energy exchange diagrams and energy balances.
This is an important conceptual distinction. A reversible isotherm and a free expansion can take the gas between the same initial and final states (for example same , doubled volume), but they change the universe in completely different ways: in the first the gas exchanges heat and work with the surroundings, in the second it does not. The first law alone does not distinguish the two: to understand which can be reversed and which cannot will require a new quantity, entropy, introduced in the next chapter.
Connections
Topics: Termodinamica Concepts: Trasformazioni termodinamiche · Primo principio della termodinamica Objects: Gas ideale
Related exercises: Same temperature, different heat · True or false on transformations · Free expansion of a gas