Thermodynamics Lecture (2) PDF

10/23/2024 Thermodynamics Lecture (2) 1 1 Chapter (1): State and Equilibrium ❑Consider a system not undergoing any change. At this point, all the properties...

10/23/2024 Thermodynamics Lecture (2) 1 1 Chapter (1): State and Equilibrium ❑Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, ❑Which gives us a set of properties that completely describes the condition, or the state of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. 1  10/23/2024 ❑Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. ❑A system in equilibrium experiences no changes when it is isolated from its surroundings. ❑There are many types of equilibrium: ❑A system is not in Thermodynamic Equilibrium unless the conditions of all the types of equilibrium are satisfied. ❑For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Fig. 1–11. ❑Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. ❑a system is in Chemical Equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. 2  10/23/2024 ❖Processes and Cycles Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process (Fig. 1–13). To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. When a process proceeds in such a manner that the system remains infinitely close to an equilibrium state at all times, it is called a quasistatic, or quasi- equilibrium, process. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts. This is illustrated in Fig. 1–30. When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, 3  10/23/2024 Thus, creating a high-pressure region there. Because of this pressure difference, the system can no longer be said to be in equilibrium, and this makes the entire process nonquasi-equilibrium. However, if the piston is moved slowly, the molecules will have sufficient time to redistribute and there will not be a molecule pile up in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process. Quasi-static process: 1. In thermodynamics, a quasi-static process is referred to as a slow process. 2. It is a process that happens at an infinitely slow rate. 3. A quasi-static process has all of its states in equilibrium. 4. A quasi-static process is one in which the system is in thermodynamic equilibrium with its surroundings at all times. 4  10/23/2024 Figure 1–31 shows the P-V diagram of a compression process of a gas. Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasiequilibrium processes only. Quasi-static and non-quasi-static processes between states A and B of a gas. In a quasi- static process, the path of the process between A and B can be drawn in a state diagram since all the states that the system goes through are known. In a non-quasi-static process, the states between A and B are not known, and hence no path can be drawn. It may follow the dashed line as shown in the figure or take a very different path. 5  10/23/2024 The prefix iso- is often used to designate a process for which a particular property remains constant. ▪ Isothermal: constant temperature, ▪ Process: a series of changes of state. ▪ Isobaric: constant pressure, and ▪ Cycle: series of processes, which returns to the original state. The cycle is a ▪ Isochoric: constant volume. thermodynamic “round trip”. ❑A system is said to have undergone a cycle if it returns to its initial state at the end of the process. That is, for a cycle the initial and final states are identical. Chapter (2) Energy, Energy Transfer. Forms of Energy Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear. Their sum represent the total energy E of a system. The total energy of a system on a unit mass basis is denoted by e and is expressed as 6  10/23/2024 The energy that a system possesses as a result of its motion relative to some reference frame is called kinetic energy (KE). or, on a unit mass basis. The energy that a system possesses as a result of its elevation in a gravitational field is called potential energy or, on a unit mass basis, The total energy of a system consists of the kinetic, potential, and internal energies and is expressed as: or, on a unit mass basis, Most closed systems remain stationary during a process and thus experience no change in their kinetic and potential energies. Closed systems whose velocity and elevation of the center of gravity remain constant during a process are frequently referred to as stationary systems. 7  10/23/2024 m m The change in the total energy ΔE of a stationary system is identical to V the change in its internal energy ΔU. Control volumes typically involve fluid flow for long periods of time, and it is convenient to express the energy flow associated with a fluid stream  Here in the rate form. The mass flow rate m , which is the amount of mass flowing through a cross section per unit time. It is related to the volume flow rate 𝑽ሶ Here  is the fluid density, Ac is Mass flow rate: the cross-sectional area of flow, Volume A.x Avt and Vavg is the average flow V (volume flow r ate = )= = = AV tim e t t velocity normal to Ac m m The dot over a symbol is used to indicate time rate. V Then the energy flow rate associated with a fluid flowing at a rate of m Energy flow rate:  Here which is analogous to E = me. 8 10/23/2024 9