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In thermodynamics, one primarily studies systems that are in so-called "thermal equilibrium", a state in which certain prescribed "thermodynamic" quantities such as temperature, heat, chemical potential, etc. are considered static in time. In fact, most modern textbooks admit that the study of systems not in thermal equilibrium is extremely complicated, and is the primary focus of fields such as nonlinear dynamics.

With this in mind, why isn't thermodynamics called thermostatics? If this were done, then we would have a proper word for the study of systems that are out of thermal equilibrium, thermodynamics.

I know that, historically, thermodynamics was the study of heat and engines and such, so I can see where it got its name from. However, I don't see why it hasn't changed its name since then. Thermodynamics as is currently taught has been around for over 100 years!

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    $\begingroup$ "Within thermodynamics, thermostatics is the physical theory that deals with equilibrium states, and with transformations where time is not an explicit variable; it ignores the flows, i.e. the time derivatives of quantities such as the energy or the number of particles" ueltschi.org/teaching/chapthermostatics.pdf There is also non-equilibrium thermodynamics. $\endgroup$
    – Conifold
    Commented Oct 5, 2016 at 0:01
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    $\begingroup$ Also, you could consider dynamic equillibria. Even though the net change in heat may be zero, there is still heat flow, it's just that flow out and flow in are equal. Like in Chemistry, an equillibrium reaction is still a reaction, even though there is no net conversion from products to reacts. $\endgroup$
    – Brayton
    Commented Oct 5, 2016 at 5:16
  • $\begingroup$ Probably because there is not a need for a sharp distinction between systems without heat flow and those with. The same study covers them both. $\endgroup$
    – Mary
    Commented Jun 6, 2021 at 17:04
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    $\begingroup$ It is hard to watch a steam engine work and not think it is dynamic. $\endgroup$
    – Jon Custer
    Commented Jun 7, 2021 at 13:07

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Thermodynamics really studies CHANGES of states, not just the states themselves. These changes are slow. The technical term is "adiabatic". It started with Carnot cycle, describing the action of a heat engine. A heat engine cycle passes through several different states.

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Dynamical systems are described by a set of differential/difference equations(ordinary, partial, deterministic, stochastic, etc). Although most of these equations must be solved numerically, the solutions describe the evolution of state variables in time and/or space.

In the Thermodynamics textbooks I have examined that kind of structure does not exist unless or until one looks at Chemical Kinetics and dynamics.

So Thermodynamics appears to be a fuzzily defined discipline with thousands of empirically acquired steady-state data.

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