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By Prof. Dr. Said Mohamed Attia First Law of Thermodynamics

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By Prof. Dr. Said Mohamed Attia First Law of Thermodynamics Part 3 First Law of Thermodynamics By Prof. Dr. Said Mohamed Attia Heat & Thermodynamics Prof. Dr. Said M. Attia Outlines 1. Thermodynamic terminology. 1. System 2. Surroundings 3. Boundary 4. Thermodynamic Processes. 5. Thermodynamic equilibrium 2. Work in Thermodynamics 3. Internal Energy 4. First law of thermodynamics 1. Molar Specific heat of gas at constant volume and constant pressure. 2. Isotheral process 3. Isobaric process 4. Isochoric process 5. Adiabatic process. Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies □ System • An amount of matter in a container of a certain volume SURROUNDINGS □ Surroundings • Mass or region outside the system SYSTEM □ Boundary • The real or imaginary surface that separates the system from the surroundings BOUNDARY Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Isolated System: Neither the energy nor the mass can be exchanged with the surroundings Closed System: Mass remains constant but energy can exchanged with the surroundings Types of Systems Types Open System: Both energy and matter can be exchanged with the surroundings Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Macroscopic System: The word macroscopic means ‘on a large scale”. A system is said to be macroscopic when it consists of a large number of molecules, atoms or ions Macroscopic Properties: The properties associated with a macroscopic system are called macroscopic properties. These properties are pressure, volume, temperature, density, viscosity, surface tension, refractive index, electric conductivity etc. State of the System: When macroscopic properties of a system About the Systems About have definite values, the system is said to be in a definite state. The state of a system (or thermodynamic state) at a specific time is defined using thermodynamic variables such as pressure, volume, temperature, and internal energy, these quantities called state variables. Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Properties of a system Any characteristic of a system, e.g., temperature, pressure, volume, etc. خواص ممتدة تعتمد على Extensive Property خواص ممركزة تعتمد على Intensive Property كمية المادة وحجمها نوع المادة وﻻ تعتمد على كمية المادة وﻻ حجمها It is a bulk property, meaning that it is a It depends on the amount of the material in local physical property of a system that does the system. not depend on the system size or the amount of material in the system. E.g., Pressure, Temperature, E.g., Volume, Mass, No. of moles, Density, Viscosity, Conductivity, Energy, Heat capacity, Entropy Surface tension, Boling point, refractive index, density, etc. Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Thermodynamics Equilibrium A system in which the macroscopic properties do not undergo any change with time is said to be in thermodynamic equilibrium. Three types of equilibrium; In Thermal equilibrium • There is no flow of heat from one position of the system to another. This is possible if the temperature remains the same throughout in all parts of the system. In Mechanical equilibrium • There is no mechanical work is done by one part of the system on another part of the system. This is possible if the pressure remains the same throughout in all parts of the system. In Chemical equilibrium • the composition of the various phases in the system remains the same throughout. Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Thermodynamics Process A thermodynamic process may be defined as the operation by which a system changes from one state (푃1, 푉1, 푇1) to another state (푃2, 푉2, 푇2). In a process, at least one property of the system changes and give a path along which the variables of the system changes. Isothermal Process • Process take place at constant temperature. (∆푇 = 0) Isobaric Process • Process take place at constant pressure. (∆푃 = 0) Isochoric Process • Process take place at constant volume. (∆푉 = 0) Adiabatic Process • Process take place without heat exchange between system and surroundings. (∆푄 = 0) Heat & Thermodynamics Prof. Dr. Said M. Attia Some thermodynamic Terminologies Thermodynamics Process Cyclic Process • It is the process in which the system undergoes a number of changes (process), then finally returns to its initial state again. (∆푈 = 0) Reversible Process • It is the process that can be reversed by changes in the system without loss or waste of energy. Irreversible Process • It is the process that can not be reversed by changes in the system without loss or waste of energy. Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics □ Work (W) is the scalar product of a force times the displacement 풅풙 □ 푊표푟푘 = 퐹표푟푐푒 . 푑푠푝푙푎푐푒푚푒푛푡 □ For small changes; 푨 푭 □ 푑푊 = 퐹. 푑푥 □ If the force F is applied on a piston of Area A, then the piston move a distance 푑푥, as shown in the figure. □ The work done to change the volume by Δ푉 퐹 □ 푑푊 = . 퐴d푥 = 푃. 푑푉 퐴 □ A positive sign for W defines work done on a system (compression case) □ A negative sign for W defines work done by a system against force (expansion case) Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics □ We can talk about the work done on or by a system when some process has occurred in which energy has been transferred to or from the system. □ The total work done on the gas as its volume change from 푉푖 to 푉푓 is given by 푉 (*) 푑푊 = −푃푑푉 ∴ 푊 = − ׬ 푓 푃푑푉 □ 푉푖 풅푽 □ Equation (*) equal to the area under the curve on the PV diagram. □ The work done on the gas from initial state to final state depends on the path between these state. Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics □ The value of the work done depends on the path of the thermodynamic operation. □ In the figure, the work done through the path a from point 1 to point 2 is given by □ 푊 = 2푃표 2푉표 − 푉표 = 2푃표푉표 □ the work done through the path b from point 1 to point 2 is given by □ 푊 = 푃표 2푉표 − 푉표 = 푃표푉표 □ The work done is not a property of the system. Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics Special cases for the work=PV 1- The work at constant volume • In this case, 푉 = 푐표푛푠푡푎푛푡, 푎푛푑 푑푉 = 0, and 푃 푓 푊 = − ׬ 푃푑푉 = 0 • 푖 푃푓 푓 • In this case, there is no work done, 푃푖 푉 푉푖 = 푉푓 2- The work at zero pressure (free expansion) • In this case, 푃 = 0, and 푃 푓 푊 = − න 푃푑푉 = 0 푖 In this case, the system expands freely, 푓 • and there is no work done, 푃 = 0 푉 푉푖 푉푓 Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics Special cases for the work=PV 3- The system expands against a constant Pressure • In this case, 푃 = 푐표푛푠푡푎푛푡, 푎푛푑 Δ푉 = 푉푓 − 푉푖, and 푓 (푊 = − ׬푖 푃푑푉 = −푃(푉푓 − 푉푖 • • The work done is negative, when the system expands, 푃 • The work done is positive, when the system contract. 푓 푃푖 = 푃푓 푉 푉푖 푉푓 Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics Special cases for the work=PV 4- The work done in a cyclic process ➢In a cyclic process, the system starts and returns to the same thermodynamic state. ➢The net work involved is the enclosed area on the 푃푉 diagram. ➢ The work done is negative, If the cycle goes clockwise, the system does work on the expense of the internal energy. ➢The work done is positive, If the cycle goes counterclockwise, work is done on the system every cycle, and the internal energy increases. ➢ The work done through the 푃 푃 path 퐴퐵 and 퐶퐷 is 퐵 퐶 퐶 퐵 푃푓 푃푓 ➢ 푊퐴퐵 = 푊퐶퐷 = 0, because Δ푉 = 0. 푃 ➢ The net work done, 푖 퐴 퐷 푃푖 퐷 퐴 푉 푉 푉 푉 ➢ 푊푛푒푡 = 푊퐵퐶 − 푊퐷퐴 푖 푓 푉푓 푉푖 work done is negative work done is positive Heat & Thermodynamics Prof. Dr. Said M. Attia Work in Thermodynamics Special cases for the work=PV 5- The system expands against a variable Pressure • If 푇 푠 푐표푛푠푡푛푎푡, in this case, 푃 푓 • 푊 = − 푃푑푉 푓 ׬푖 푃푓 • For ideal gas 푃푉 = 푛푅푇 = 푐표푛푠푡푎푡 푛푅푇 푃푖 • Then 푃 = , then 푉 푉푓 푛푅푇 푉푓 1 푉푓 푊 = − ׬ 푑푉 = −푛푅푇 ׬ 푑푉 = −푛푅푇 ln 푉 푉 푉 • 푉푖 푉 푉푖 푉 푉푖 푓 푉푖 푉 푃 • 푊 = −푛푅푇 ln 푓 = −푛푅푇 ln 푖 푉푖 푃푓 • The work done is negative, when the system expands, • The work done is positive, when the system contract. Heat & Thermodynamics Prof. Dr. Said M. Attia Internal energy (U) □ Internal energy (U) is all energies associated with the kinetic and potential energies of the molecules, electronic energies, and energy of nuclei. □ Internal Energy (U) = Kinetic Energy + Potential 푲. 푬. 풂풕 푻 Energy ퟏ □ Kinetic energy includes □ the K.E. of random translation, □ the K.E. of random rotation, and 푲. 푬. 풂풕 푻ퟐ □ the K.E. of random vibration motion of molecules, □ The potential energy which include all binding energy between molecules, electrons, etc. Heat & Thermodynamics Prof. Dr. Said M. Attia Enthalpy (H) □ Enthalpy, H, is a property of a thermodynamic system, that equal the system's internal energy plus the product of its pressure and volume, i.e., □ 퐻 = 푈 + 푃푉 (Enthalpy) □ 푑퐻 = 푑푈 + 푃푑푉 + 푉푑푃 □ That is the change in enthalpy equals the change in the internal energy and change in the work done □ At constant pressure, 푑푃 = 0, then, □ Δ퐻 = Δ푈 + 푃Δ푉 □ For liquid and solids, (Δ푉 ≈ 0), then, □ Δ퐻 = Δ푈 □ It is an extensive quantity. The unit of measurement for enthalpy in the International System of Units (SI) is the joule. Other historical conventional units still in use include the British thermal unit (BTU) and the calorie.
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