Slow Processes

Whenever a system is caused to undergo a procedure, at least one of its measurable properties, let say x, is subjected to a change x in some time duration, tp. This duration can be termed as the process time. When tp is zero, Δx is impressed as a step function and the system continues equilibrium only after the relaxation time T has gone.

When tp is raised and the similar Δx is impressed more steadily, smaller deviations from equilibrium are examined during the procedure. Eventually, when tp is made long sufficient for a specified x, the state of the system at each instant of the procedure is so closely the equilibrium state equivalent to the instantaneous value of x which deviations from any of the equations of state cannot be detected by measurement. Under these situations, the procedure is termed as slow. Therefore all the properties of a system are stated and all equations of state are satisfied at every instant throughout a slow process.

We are now in a place to answer the question increased earlier in this sub-section. Thermodynamics can be employed to explain a process only when it is slow in the logic discussed above. Processes that are not slow are termed as fast. An application of thermodynamics to fast procedures will lead to error as, by definition, there are measurable deviations from the equilibrium relations on which the thermodynamics depend.

We will sum up the above conclusions as shown below:

The process is slow when tp significantly exceeds y, whenever both refer to the similar x, that is,

tp >> T in a slow process.

It should be emphasized that fast and slow processes are stated in relation to the corresponding relaxation times of the systems under learning. For one system a procedure with tp of 1 ms might be slow. For another, a fast procedure could have tp equivalent to many hours. In most of the texts on thermodynamics, slow processes are alternatively termed as quasi-static or fully-resisted. The quasi-static procedure is also at times termed as a quasi-equilibrium process, and symbolizes a process carried out in such a way that at every moment the system departs only infinitesimally from an equilibrium state. When there are finite departures from equilibrium, the procedure is non-quasi-static. It will be observe later that a quasi-static procedure in an internally reversible process. The fully-resisted process is also explained as one is which the speed of motion of the system boundary is much less than the velocity of sound in the fluid, allowing the fluid pressure at any moment to be very near uniform during the system.