Quasi Static Process

3 Key excerpts on "Quasi Static Process"

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  eBook - ePub

  Physics of Cryogenics

  An Ultralow Temperature Phenomenon

  • Bahman Zohuri(Author)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  The above finite unbalanced force may cause the system to pass through nonequilibrium states. If it is desired, during a process, to describe every state of a system by means of system-wide thermodynamic coordinates, then the process must not be performed using a finite unbalanced force or torque. Under these circumstances, the external forces acting on a system are varied only slightly so that the unbalanced force is infinitesimal, and the process proceeds infinitesimally slowly. A process performed in this mode is said to be quasi-static.

  If all of the states through which the system passes can be described by means of thermodynamic coordinates referring to the system as a whole, and an equation of state for all these states is valid, the process is called quasi-static. A quasi-static process is an idealization that is applicable to any thermodynamic system, including electric and magnetic ones. The conditions for such a process can never be achieved in the real world, but can often be approached with almost any degree of accuracy.

  Classical thermodynamics does not quantify how infinitesimally slowly a process must take place to be considered quasi-static. Molecular gas kinetics requires only that the process proceed slowly compared to the speed of the molecules in the gas. This allows system properties to be equilibrated across the system faster than the system configuration changes. Examples of processes that seem rapid but can be treated as quasi-static are the expansion of combustion products in a gasoline engine, or the expansion of the exhaust gases of a chemical rocket.

  The reason for the introduction of a quasi-static process is to allow calculations without addressing the complications of friction within the system. This approach is no different from that of Newtonian's mechanics with its massless springs and ideal pulleys, or that of circuit theory with wires with no resistance, or batteries with constant voltage. Later reversible processes will be considered that are synonymous quasi-static processes because dissipative processes are ignored.

  4.4. Quasi-Equilibrium Work due to Moving Boundary

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  eBook - PDF

  Energy Conversion Statics

  • H. K. Messerle, Henry G. Booker, Nicholas Declaris(Authors)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  This temperature gradient represents definitely a nonequilibrium state and the energy flux tries to equalize temperature across the body in order to achieve an equi-librium state. Processes associated with such a situation are known as irreversible processes since there are considerable energy losses involved and we are dealing with irreversible thermodynamics. Irreversible thermodynamics is not dealt with in this text and we are restricting our study to quasi-static processes. Electrical and mechanical machinery as well as thermal engines are generally designed to operate under static or, relatively speaking, slowly varying conditions. The processes involved then approach quasi-static conditions. Hence quasi-static or steady state processes have received considerable attention in the past. As will be shown in later chapters, even dynamic processes in electromechanics fall within the definition of quasi-static processes if they are lossless. Consequently, these processes concern the major bulk of practical energy conversion applications in fields ranging from electrical and mechanical to caloric and thermo-dynamic machinery. In Chapters II and III we have shown that a general theory can be developed describing equilibrium states and incremental changes of states. This theory was based on the laws of energy conservation and dissipation. The field was formalized with the aid of four postulates. Hence a unified basis for the treatment of quasi-static energy conversion processes is available. The general theory developed above is now used to establish practical relations for quasi-static process. This will lead to a number of theorems, most of which have been used in the past, although different fields have used different approaches. The general theory here allows a unified presentation and an extension of the usual fragmental treatments.

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  Statistical and Thermal Physics

  With Computer Applications, Second Edition

  • Harvey Gould, Jan Tobochnik(Authors)
  • 2021(Publication Date)
  • Princeton University Press

    (Publisher)

  The name “thermodynamics” is a misnomer, because 66 • THERMODYNAMIC CONCEPTS AND PROCESSES thermodynamics treats only equilibrium states and not dynamics. Nevertheless, thermodynamics discusses processes that must occur over some interval of time. It is also confusing that we can consider processes that did not actually happen. In this case, the calculation of S in Problem 2.39 is identical to what we would do for an isothermal nonadiabatic quasistatic process for which the gas does work, even though in the actual process, no work was done by the gas and no heating or cooling occurred. However, the initial and final states and the change in the entropy are the same for the actual process and the calculated process. Quasistatic adiabatic processes. We have discussed that the entropy does not change in a quasistatic adiabatic processes, but we repeat this statement here to emphasize its importance. If a process is adiabatic, then Q = 0, and if the process is also qua-sistatic, then S = Q / T = 0, and there is no change in the entropy. Consider again the free expansion of an ideal gas. Initially the gas is confined to one chamber and then allowed to expand freely into the second chamber to fill the entire container. This process is certainly not quasistatic, and thus S > 0, even though Q = 0. Suppose that we imagine this process to be performed very slowly by dividing the second chamber into many small chambers separated by partitions and puncturing each partition in turn, allowing the expanded gas to come into equilibrium. In the limit of an infinite number of partitions, it might seem that such a process would be quasistatic. However, this conclusion must be incorrect, because it would imply that dS = dQ / T = 0. It is important to understand that an infinitesimal process is not necessarily quasistatic. The puncture of a partition would cause the wall to move suddenly, thus creating turbulence and causing changes that are not near equilibrium.

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