at conditions of interest to us, a given molecule can undergo roughly 10 molecular collisions per second, so that, if ten collisions are needed to come to equilibrium, the equilibration time is on the order of 10-9 seconds. This is generally much shorter than the time scales associated with the bulk properties of the flow(say the time needed for a fluid particle to move some significant fraction of the lighten of the device of interest). Over a large range of parameters, therefore, it is a very good approximation to view the thermodynamic processes as consisting of such a succession of equilibrium states 8)Reversible process For a simple compressible substance Work If we look at a simple system, for example a cylinder of gas and a piston, we see that there can be two pressures, Ps, the system pressure and Px, the external pressure The work done by the system on the environment is Work=Pdv This can only be related to the system properties if Px=Ps. For this to occur, there cannot be any friction, and the process must also be slow enough so that pressure differences due to accelerations are not significant P with friction ∫PxdV≠0 but jPs dV=0 Work during an irreversible Under these conditions, we say that the process is reversible. The conditions for reversibility are a) If the process is reversed, the system and the surroundings will be returned to the b) To reverse the process we need to apply only an infinitesimal dP. a reversible process can be altered in direction by infinitesimal changes in the external conditions (see Van Ness, Chapter 2) 9)Work For simple compressible substances in reversible processes, the work done by the system on the environment is PdV. This can be represented as the area under a curve in a Pressure volume diagram 0-40-4 at conditions of interest to us, a given molecule can undergo roughly 1010 molecular collisions per second, so that, if ten collisions are needed to come to equilibrium, the equilibration time is on the order of 10-9 seconds. This is generally much shorter than the time scales associated with the bulk properties of the flow (say the time needed for a fluid particle to move some significant fraction of the lighten of the device of interest). Over a large range of parameters, therefore, it is a very good approximation to view the thermodynamic processes as consisting of such a succession of equilibrium states. 8) Reversible process For a simple compressible substance, Work = ∫PdV. If we look at a simple system, for example a cylinder of gas and a piston, we see that there can be two pressures, Ps, the system pressure and Px, the external pressure. ￾￾ ￾￾Ps Px The work done by the system on the environment is Work = ∫PxdV. This can only be related to the system properties if Px ≈ Ps. For this to occur, there cannot be any friction, and the process must also be slow enough so that pressure differences due to accelerations are not significant. Work during an irreversible process ≠ ∫Ps dV ∫PxdV ≠ 0 but ∫Ps dV = 0 Ps (V) Px with friction P Vs ➀ ➀ ➀ ➁ ➁ Under these conditions, we say that the process is reversible. The conditions for reversibility are that: a) If the process is reversed, the system and the surroundings will be returned to the original states. b) To reverse the process we need to apply only an infinitesimal dP. A reversible process can be altered in direction by infinitesimal changes in the external conditions (see Van Ness, Chapter 2). 9) Work: For simple compressible substances in reversible processes , the work done by the system on the environment is ∫PdV. This can be represented as the area under a curve in a Pressure￾volume diagram: