Isothermal, adiabatic, isobaric, isochoric: the thermodynamic transformations we study are idealised. In reality there is no truly isothermal transformation (it would require contact with an infinite heat reservoir held at exactly the same temperature), nor a truly adiabatic one (it would require a perfect insulator), nor a truly quasi-static one (it would require being infinitely slow).
And yet these “limiting cases” are useful: they work like ideal straight lines in geometry, which, though they do not exist in nature, allow us to build theorems with which to reason about real cases.
"Almost as if..."
The gas in our room never follows an isobar exactly, but it behaves “almost as if it did” when the transformation is slow compared with the timescales of internal equilibrium. The key is always the comparison between the time of the transformation and the time the system takes to re-equilibrate: if the former is much longer, the process is quasi-static and the ideal transformations become excellent approximations.
It is the same logic that distinguishes, for example, the rapid compression of a bicycle pump (nearly adiabatic: there is no time to exchange heat) from the slow one (nearly isothermal: the gas has time to give up heat to its surroundings). Idealisations are not useless fictions: they are the extremes within which all real cases live.
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Topics: Thermodynamics Concepts: Thermodynamic transformations
Related exercises: Same temperature, different heat · Estimate: work of a hot air bubble · Ranking: heat absorbed in four transformations