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Thermodynamics and Fluid Mechanics

by Suhas MondalSuhas Mondal
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Suhas Mondal
Suhas Mondal
Basic Thermodynamics – Module 1 Lecture 1: Introduction Introduction The most of general sense of thermodynamics is the study of energy and its relationship to the properties of matter. All activities in nature involve some interaction between energy and matter. Thermodynamics is a science that governs the following:     Energy and its transformation Feasibility of a process involving transformation of energy Feasibility of a process involving transfer of energy Equilibrium processes More specifically, thermodynamics deals with energy conversion, energy exchange and the direction of exchange. Areas of Application of Thermodynamics: All natural processes are governed by the principles of thermodynamics. However, the following engineering devices are typically designed based on the principles of thermodynamics. Automotive engines, Turbines, Compressors, Pumps, Fossil and Nuclear Power Plants, Propulsion systems for the Aircrafts, Separation and Liquefication Plant, Refrigeration, Airconditioning and Heating Devices. The principles of thermodynamics are summarized in the form of a set of axioms. These axioms are known as four thermodynamic laws: The zeroth law, the first law, the second law and the third law. 1
• The Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures. • The First Law deals with the conservation of energy and introduces the concept of internal energy. • The Second Law of thermodynamics provides with the guidelines on the conversion of internal energy of matter into work. It also introduces the concept of entropy. • The Third Law of thermodynamics defines the absolute zero of entropy. The entropy of a pure crystalline substance at absolute zero temperature is zero. Different Approaches in the Study of Thermodynamics Thermodynamics can be studied through two different approaches: (a) Macroscopic Approach and (b) Microscopic Approach 2
Macroscopic Approach Consider a certain amount of gas in a cylindrical container. The volume (V) can be measured by measuring the diameter and the height of the cylinder. The pressure (P) of the gas can be measured by a pressure gauge. The temperature (T) of the gas can be measured using a thermometer. The state of the gas can be specified by the measured P, V and T . The values of these variables are space averaged characteristics of the properties of the gas under consideration. In classical thermodynamics, we often use this macroscopic approach. The macroscopic approach has the following features. • • • The structure of the matter is not considered. A few variables are used to describe the state of the matter under consideration. The values of these variables are measurable following the available techniques of experimental physics. Microscopic Approach On the other hand, the gas can be considered as assemblage of a large number of particles each of which moves randomly with independent velocity. The state of each particle can be specified in terms of position coordinates ( xi , yi , zi ) and the momentum components ( pxi , pyi , pzi ). If we consider a gas occupying a volume of 1 cm3 at ambient temperature and pressure, the number of particles present in it is of the order of 1020 . The same number of position coordinates and momentum components are needed to specify the state of the gas. The microscopic approach can be summarized as: • • A knowledge of the molecular structure of matter under consideration is essential. A large number of variables are needed for a complete specification of the state of the matter. SI Units SI is the abbreviation of Système International d' Unités. The SI units for mass, length, time and force are kilogram, meter, second and newton respectively. The unit of length is meter, m, defined as 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the orange-red line of the spectrum of Krypton-86. The unit of time is second, s. The second is defined as the duration of 9 192 631 770 cycles of the radiation associated with a specified transition of the Cesium 133 atom. The unit of mass is kilogram, kg. It is equal to the mass of a particular cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures. The amount of substance can also be expressed in terms of the mole (mol). One kilomole of a substance is the amount of that substance in kilograms numerically equal to its molecular weight. The number of 3
kilomoles of a substance, n , is obtained by dividing the mass (m) in kilograms by the moleculare weight (M), in kg/ kmol. The unit for temperature is Kelvin, K . One K is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Quite often the Celsius, oC , is used to express the temperature of a substance. The SI unit of force, called the newton, N is a secondary unit. The, N , is the force required to accelerate a mass of 1 kilogram at the rate of 1 meter per (second)2 . 1 N = (1kg) (1m/s2 )= 1kg m/s2 The smaller or bigger quantities are expressed using the following prefixes Factor 1012 109 106 103 102 Prefix tera giga mega kilo hecto Symbol T G M k h Factor 10-2 10-3 10-6 10-9 10-12 Prefix centi milli micro nano pico Symbol c m μ n p Pressure Pressure is the normal force exerted by a system against unit area of the boundary surface. where δA approaches zero. The unit for pressure in SI is pacsal, Pa 4

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