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Note for Thermodynamics - TD By Mahabub Alom

  • Thermodynamics - TD
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CHAPTER 1: THERMODYNAMIC SYSTEMS: BASIC CONCEPTS 1.1 Introduction The word “Thermodynamics” originates from its Greek roots (therme, heat; dynamis, force). As a ld subject it is concerned with quantification of inter-relation between energy and the change of state of any real world system. The extent of such change of state due to transfer of energy to or from the system is captured through the basic equations of thermodynamics which are derived or starting from a set of fundamental observations known as “Laws of Thermodynamics”. The laws are essentially ‘postulates’ that govern the nature of interaction of real systems and energy. They are products of human experiential observations to which no exceptions have been found so far, W and so are considered to be “laws”. The scope of application of the laws of thermodynamics ranges from the microscopic to the macroscopic order, and indeed to cosmological processes. Thus, all processes taking place in the universe, whether in non-living or living systems, are subject to the laws of thermodynamics. Historically speaking, thermodynamics, is an extension of Newtonian mechanics which TU considered mechanical forces (or energy) as the agent of change of state of a body (anything possessing mass), the state being defined by its position and momentum with respect to a frame of reference. With the discovery steam power which propelled the so-called ‘Industrial Revolution’ of the 18th century, it became evident that not only the direct application of mechanical energy can change the state of a system, but that fluids themselves can act as JN reservoir of energy, which can be harnessed to effect changes in the real world to human advantage. It was this observation that laid the foundations of thermodynamics, which now constitutes a generalized way of understanding and quantifying all changes that occur during ll processes taking place in the universe as a result of application of energy in any form. 1.2 Thermodynamic System: Select Definitions A It may be evident from the foregoing introduction, that for the purpose of any thermodynamic analysis it is necessary to define a ‘system’. A system, in general, is any part of the universe which may be defined by a boundary which distinguishes it from the rest of the universe. Such a thermodynamic system is usually referred to as control volume as it would possess a volume and

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would also contain a definite quantity of matter. The system boundary may be real or imaginary, and may change in shape as well as in size over time, i.e., increase or decrease. A system can either be closed or open. A closed system does not allow any transfer of mass (material) across its boundary, while an open system is one which does. In either case ld energy transfer can occur across the system boundary in any of its various forms; for example, heat, work, electrical / magnetic energy, etc. However, for most real world systems of interest to chemical engineers the primary forms of energy that may transfer across boundaries are heat and or work. In contrast to closed or open systems, a system which is enclosed by a boundary that allows neither mass nor energy transfer is an isolated system. All matter external to the system constitutes the surroundings. The combination of the W system and surroundings is called the universe. For all practical purposes, in any thermodynamic analysis of a system it is necessary to include only the immediate surroundings in which the effects are felt. A very common and simple example of a thermodynamic system is a gas contained in a JN TU piston-and-cylinder arrangement derived from the idea of steam engines, which may typically ll Fig. 1.1 Example of simple thermodynamic system exchange heat or work with its surroundings. The dotted rectangle represents the ‘control A volume’, which essentially encloses the mass of gas in the system, and walls (including that of the piston) form the boundary of the system. If the internal gas pressure and the external pressure (acting on the moveable piston) is the same, no net force operates on the system. If, however, there is a force imbalance, the piston would move until the internal and external pressures

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equalize. In the process, some net work would be either delivered to or by the system, depending on whether the initial pressure of the gas is lower or higher than the externally applied pressure. In addition, if there is a temperature differential between the system and the surroundings the ld former may gain or lose energy through heat transfer across its boundary. This brings us to a pertinent question: how does one characterize the changes that occur in the system during any thermodynamic process? Intuitively speaking, this may be most readily or done if one could measure the change in terms of some properties of the system. A thermodynamic system is, thus, characterized by its properties, which essentially are descriptors of the state of the system. Change of state of a system is synonymous with change in the W magnitude of its characteristic properties. The aim of the laws of thermodynamics is to establish a quantitative relationship between the energy applied during a process and the resulting change in the properties, and hence in the state of the system. Thermodynamic properties are typically classified as extensive and intensive. A property TU which depends on the size (i.e., mass) of a system is an extensive property. The total volume of a system is an example of an extensive property. On the other hand, the properties which are independent of the size of a system are called intensive properties. Examples of intensive properties are pressure and temperature. The ratio of an extensive property to the mass or the property per unit mass (or mole) is called specific property. The ratio of an extensive property to JN the number of moles of the substance in the system, or the property per mole of the substance, is called the molar property. Specific volume (volume per mass or mole) V = V t / M Molar Volume (volume per mole) V = V t / N ll where, A V t = total system volume (m3 ) M = total system mass (kg) N = total moles in system (kg moles) ..(1.1)

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1.3 Types of Energies associated with Thermodynamic Processes: We know from the fundamentals of Mechanics, that the energy possessed by a body by virtue of ld its position or configuration is termed potential energy (PE). The potential energy of a body of mass m which is at an elevation z from the earth’s surface (or any particular datum) is given by: ..(1.2) Where, g is the acceleration due to gravity (= 9.81 m/s2). or PE = mgz The energy possessed by a body by virtue of its motion is called the kinetic energy (KE). For a KE = W body of mass m moving with a velocity u, the kinetic energy of the body is given by: 1 mu 2 2 ..(1.3) TU It follows that, like any mechanical body, a thermodynamic system containing a fluid, in principle may possess both PE and KE. It may be noted that both PE and KE are expressed in terms of macroscopic, directly measurable quantities; they, therefore, constitute macroscopic, mechanical forms of energy that a thermodynamic system may possess. As one may recall from the basic tenets of mechanics, PE and KE are inter-convertible in form. JN It may also be noted that PE and KE are forms of energy possessed by a body as a whole by virtue of its macroscopic mass. However, matter is composed of atoms /molecules which have the capacity to translate, rotate and vibrate. Accordingly, one ascribes three forms intra-molecular energies: translational, rotational and vibrational. Further, energy is also associated with the motion of the electrons, spin of the electrons, intra-atomic (nucleus-electron, nucleus-nucleus) ll interactions, etc. Lastly, molecules are also subject to inter-molecular interactions which are electromagnetic in nature, especially at short intermolecular separation distances. All these forms A of energy are microscopic in form and they cannot be readily estimated in terms of macroscopically measurable properties of matter. It needs to be emphasized that the microscopic form of energy is distinct from PE and KE of a body or a system, and are generally independent of the position or velocity of the body. Thus the energy possessed by matter due to the

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