chapter 1 concepts of thermodynamics and properties of gases
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Chapter 1 Concepts of Thermodynamics and Properties of Gases. Introduction. Thermodynamics is a branch of science and engineering that deals with interaction of energy mainly in the forms of heat and work. There are different forms of energy : all the energy cannot be used as a work. - PowerPoint PPT PresentationTRANSCRIPT
Basic Mechanical Engineering, First Editionby Dr Pravin Kumar
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Chapter 1
Concepts of Thermodynamics and Properties of Gases
Basic Mechanical Engineering, First Editionby Dr Pravin Kumar
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IntroductionThermodynamics is a branch of science and engineering that deals with interaction of energy mainly in the forms of heat and work.
There are different forms of energy:• all the energy cannot be used as a work. • the convertibility of energy into work depends on its
availability.
Thermodynamics is studied in two forms:• Classical• Statistical
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•Classical thermodynamics is concerned with the macrostructure of matter and addresses the major characteristics of large aggregations of molecules and not the behaviour of individual molecules.
•Statistical thermodynamics is concerned with the microstructure of the matter and addresses behaviour of individual molecules of the matter.
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Important TerminologiesThermodynamics: It is the field of thermal engineering that studies the properties of systems that have a temperature and involve the laws that
o govern the conversion of energy from one form to another.
o the direction in which heat will flow. o the availability of energy to do work.
System: System is the fixed quantity of matter and/or the region that can be separated from everything else by a well-defined boundary/surface.
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Important Terminologies (Cont…)State: At any instant of time, the condition of a system is
called state. The state at a given instant of time is defined by the properties of the system such as pressure, volume, temperature, etc.
Property: It is any quantity whose numerical value depends
on the state but not on the history of the system. There are two types of properties: extensive and intensive. o Extensive properties depend on the size or extent of
the system. Volume, mass, energy and entropy are the examples of extensive properties.
o Intensive properties are independent of the size or extent of the system. Pressure and temperature are the examples of intensive properties.
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• Change in Stateo Thermodynamic system
undergoes changes due to flow of mass and energy. The mode in which the changes in the state of a system takes place is known as process such as isobaric (constant pressure) process, isochoric (constant volume) process, isothermal (constant temperature) process, adiabatic (constant entropy) process, etc.
o The path is loci of series of state changes from initial state to final state during a process.
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Process: Two states are identical if, and only if, the properties of the two states are same. When any property of a system changes in value there is a change in state and the system is said to undergo a process. Cycle: When a system from a given initial state goes into a sequence of processes and finally returns to its initial state, it is said to have undergone a cycle.Phase: Phase refers to a quantity of matter that is homogeneous throughout in its chemical composition and physical structure.
o A system can contain one or more phases. o A pure substance is one that is uniform and invariable
in chemical composition. o A pure substance can exist in more than one phase,
but its chemical composition must be the same in each phase.
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Equilibrium: In thermodynamics, the concept of equilibrium includes not only a balance of forces but also a balance of other influencing factors, such as thermal equilibrium, pressure equilibrium, phase equilibrium, etc. Zeroth law of thermodynamics is law of thermal equilibrium, which states that if a system A is in thermal equilibrium with systems B and C, then systems B and C will be in thermal equilibrium as well.Quasi-static Process: When a process proceeds in such a way that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static process.
o A quasi-static process can be understood as sufficiently slow process that allows the system to adjust internally so that properties in one part of the system do not change faster than those at other parts.
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Temperature: Temperature is a property of a substance by which it can be differentiated from other substance in terms of degree of hot or cold.
°C = °K − 273.15°R = 1.8°K °F = °R − 459.67 °F = 1.8°C + 32
• Internal Energy: The Internal Energy (U) of a system is the total energy content of the system. o It is the sum of the kinetic, potential, chemical,
electrical, and all other forms of energy possessed by the atoms and molecules of the system.
o The Internal Energy (U) is path independent and depends only on temperature for an ideal gas.
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Work: Work in thermodynamics may be defined as any quantity of energy that flows across the boundary between the system and surroundings which can be used to change the height of a mass in the surroundings.Heat: Heat is defined as the quantity of energy that flows across the boundary between the system and surroundings because of a temperature difference between system and surroundings. The characteristics of heat are as follows:
o Heat is transitory and appears during a change in state of the system and surroundings. It is not a point function.
o The net effect of heat is to change the internal energy of the system and surroundings in accordance to first law.
o If heat is transferred to the system, it is positive and if it is transferred from the system it is negative.
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Enthalpy: Enthalpy, h, of a substance is defined as h = u + PV. It is intensive properties of a substance and measured in terms of kJ/kg.Specific Heat at Constant Volume (Cv): The rate of change of internal energy with respect to absolute temperature at constant volume is known as specific heat at constant volume (Cv).
Specific Heat at Constant Pressure (CP): The rate of change of enthalpy with respect to absolute temperature when pressure is constant is known as specific heat at constant pressure (Cp).
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First Law of Thermodynamics
First Law of Thermodynamics: The first law of thermodynamics is equivalent to law of conservation of energy. It deals with the transformation of heat energy into work and vice versa. o When a small amount of work (dw) is supplied to a
closed system undergoing a cycle, the work supplied will be equal to the heat transfer or heat produced (dQ) in the system.
o If Q amount of heat is given to a system undergoing a change of state and W is work done by the system and transferred during the process, the net energy (Q – W) will be stored in the system named as internal energy or simply energy of the system (∆U).
Q – W = ∆U
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Sign Convention: The convention is adopted that Q indicates the heat added to the system and W the work done by it. Thus,
o dQ > 0, heat added to system or system absorbs heat.
o dQ < 0, heat removed from system or system rejects heat.
o dW > 0, work is done by system.o dW < 0, work is done on the system.o ∆U > 0, internal energy of system increases.o ∆U < 0, internal energy of system decreases.
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Non-flow Processes
• Constant Volume Process
• Constant Pressure Process
• Constant Temperature Process
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• Adiabatic Process
• Polytropic Process
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Application of First Law of Thermodynamics in Steady Flow Process and Variable Flow Process
Steady Flow Process: In a steady flow process, thermodynamic properties at any section remain constant with respect to time; it can vary only with respect to space.
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Variable Flow Process: In some flow process, mass flow rate is not steady but varies with respect to time. In such a case, the difference in energy flow is stored in system as ∆Ev.
• Limitations of First Law of ThermodynamicsFirst law of thermodynamics does not tell about the following:o How much of the given quantity of heat is changed into
work?o In which direction is changing take place (heat to work
or work to heat)?o Under which condition the changing will take place?
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The Second Law of ThermodynamicsKelvin–Planck Statement: It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir.Clausius Statement: It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower temperature body to higher temperature body.
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The Carnot Cycle1–2 Reversible Isothermal Expansion2–3 Reversible Adiabatic Expansion3–4 Reversible Isothermal Compression4–1 Reversible Adiabatic Compression
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The Clausius Inequality
Entropy: Defining entropy in an exact word or line is impossible. It can be viewed as a measure of molecular disorder or molecular randomness. As a system becomes more disordered, the positions of the molecules become less predictable and the entropy increases. Thus, the entropy of a substance is lowest in the solid phase and highest in the gas phase.
• SGEN > 0 for an irreversible (real) process• SGEN = 0 for a reversible (ideal) process• SGEN < 0 for an impossible process
or
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Third Law of Thermodynamics
Third law of thermodynamics is law of entropy. It is a statement about the ability to create an absolute temperature scale, for which absolute zero is the point at which the internal energy of a solid is zero. Third law of thermodynamics states that it is impossible to reduce any system to absolute zero in a finite series of operations.
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Gas LawsBoyle’s law is stated as ‘volume and pressure of a sample of gas are inversely proportional to each other at constant temperature’.
• Charle’s law can be stated as ‘volume of a sample of gas is directly proportional to the absolute temperature when pressure remains constant’.
• Gay–Lussac’s law states that the pressure of a sample of gas is directly proportional to the absolute temperature when volume remains constant’.
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• Combined Gas Law
•Gas Constant
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