Thermodynamic Processes

Any operation that changes a system from one state to another is called a thermodynamic process -- heating, cooling, expansion, compression, fusion (melting) and vaporisation are everyday examples. These processes are further classified along several independent axes.

Reversible vs Irreversible. A reversible process is one where the system (and its surroundings) can be brought back exactly to their initial state from the final state, leaving absolutely no net change anywhere in the universe. Two conditions must both hold: the process must occur infinitesimally slowly, and the system must remain in equilibrium with its surroundings at every single instant along the way. An irreversible process, by contrast, cannot be fully undone -- the system and surroundings are not in mutual equilibrium during the change. In practice, every process that happens in nature spontaneously is irreversible; a perfectly reversible process is an idealised limiting case, useful for deriving exact formulas (as Section ~7.2.6 does for expansion/compression work) even though it is never fully achieved in reality.

Classification by which variable is held fixed:

• Adiabatic process ($q=0$): no heat crosses the boundary at all -- the system is thermally insulated from its surroundings. If the system does work in an adiabatic process, its own temperature must fall (since it can't replace the lost energy with incoming heat); if work is done ON it, its temperature rises.
• Isothermal process ($dT=0$): the system's temperature stays constant throughout, even though heat IS exchanged with the surroundings to compensate for any work done -- typically achieved experimentally by running the process inside a thermostat.
• Isobaric process ($dP=0$): the system's pressure stays constant from initial to final state. …