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Note for Thermodynamics - TD By Mahabub Alom

  • Thermodynamics - TD
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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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microscopic modes of motion is referred to as the internal energy of the matter. The microscopic variety of energy forms the principal consideration in case of transformations that occur in a thermodynamic system. Indeed, as mentioned earlier, it is the realization that matter or fluids the basis of the 18th century Industrial Revolution. ld possessed useful form of microscopic energy (independent of macroscopic KE or PE) that formed As we will see later, the majority of practical thermodynamic systems of interest are the or ones that do not undergo change of state that entails significant change in its macroscopic potential and kinetic energies. Thus, it may be intuitively obvious that in a very general sense, when a thermodynamic system undergoes change of state, the attendant change in the internal W energy is responsible for the energy leaving or entering the system. Such exchange of energy between a thermodynamic system and its surroundings may occur across the system boundary as either heat or work or both. Thermodynamic Work: TU Work can be of various forms: electrical, magnetic, gravitational, mechanical, etc. In general work refers to a form of energy transfer which results due to changes in the external macroscopic physical constraints on a thermodynamic system. For example, electrical work results when a charge moves against an externally applied electrical field. As we will see later, it is mechanical work that is most commonly encountered form in real thermodynamic JN systems, for example a typical chemical plant. In its simplest form, such work results from the energy applied to expand the volume of a system against an external pressure, or by driving a piston-head out of a cylinder against an external force. In both the last examples, work transfer takes place due to the application of a differential (or finite) force applied on the system boundary; the boundary either contracts or expands due to the application of such a force. In ll effect this results in the applied force acting over a distance, which results in mechanical energy A transfer. Consider the system in fig.1.2, where a force F acts on the piston and is given by pressure x piston area. Work W is performed whenever this force translates through a distance.

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