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W Q
I II
I II
I
P1
II
P2
22
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Jilin University Thermal energy engineering department
§ 12 The Concept of State
The thermodynamic state of a system is defined by specifying values of a set of measurable properties sufficient to determine all other properties For fluid systems; typical properties are pressure; volume and temperature
interaction p → 0;and 2 molecules
are point masses; i e ; zero volume
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Jilin University Thermal energy engineering department
§ 14 Gas Equation of State
§ 13 Equilibrium State
A system in thermodynamic equilibrium satisfies:
• mechanical equilibrium no unbalanced forces • thermal eferences • chemical equilibrium
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Jilin University Thermal energy engineering department
§ 11 Thermodynamic System
Mass cannot cross the boundaries of a closed system; but energy can
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Jilin University Thermal energy engineering department
§ 12 The Concept of State
A process between states 1 and 2 and the process path
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Jilin University Thermal energy engineering department
§ 11 Thermodynamic System
A control volume may involve fixed; moving; real; and imaginary boundaries
➢ In reality; there are no truly reversible processes; however; for analysis purposes; one uses reversible to make the analysis simpler; and to determine maximum theoretical efficiencies
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Jilin University Thermal energy engineering department
§ 12 The Concept of State
Absolute; gage; and vacuum pressures
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Jilin University Thermal energy engineering department
p
Evaluating the work
p1
W pdVpAdx
Integral for a quasistatic process
p2 V1
V2 V = Ax
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Jilin University Thermal energy engineering department
§ 17 Heat
§ 11 Thermodynamic System
System; surroundings; and boundary
• Closed system
Control mass
• Control volume
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Jilin University Thermal energy engineering department
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Jilin University Thermal energy engineering department
§ 13 Equilibrium State
The PV diagram of a compression process
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Jilin University Thermal energy engineering department
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Jilin University Thermal energy engineering department
§ 15 Quasistatic Process and reversible
process
➢ A reversible process for a system is defined as a
§ 14 Gas Equation of State
F(p,v,T)0
Gas equation of state
The ideal gas equation of state
For 1kg
pvRgT
For 1mol
pVm RT
This implies: 1 very little molecular
process that; once having taken place; can be reversed; and in so doing leaves no change in either the system or surroundings ➢ In other words the system and surroundings are returned to their original condition before the process took place
§ 15 Quasistatic Process and reversible
process
Only infinitesimal unbalanced forces exist; so that the process can be viewed as taking place in a series of quasiequilibrium states
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Jilin University Thermal energy engineering department
§ 16 The work of reversible process
Interactions
Surroundings
Mass Flow Heat
SysMteam
f(Pk, k =1...N)=0
Jilin University Thermal energy engineering department
Engineering Thermodynamics
Chapter One Basic Concepts
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Jilin University Thermal energy engineering department
The universal gas constant
R RgM 8.314 kJ/(kmol K )
M = Molecular weight, kg/kmol
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Jilin University Thermal energy engineering department
§ 15 Quasistatic Process and reversible process
Evaluating the δQ TdS
heat
Integral for a quasistatic process
A resistor heating water
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Jilin University Thermal energy engineering department
§ 18 Cyclic Processes
§ 12 The Concept of State
Temperature scales
Kelvin Scale
Triple point of water at P = 1 atm
T = 273 15 K
Steam point of water at P = 1 atm
T = 373 15 K
Celsius Scale
Direction of Motion Piston Wall
Note: Pgas > Pambient
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Jilin University Thermal energy engineering department
§ 16 The work of reversible process
Work
Mass Flow
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Jilin University Thermal energy engineering department
§ 16 The work of reversible process
Deformation of the system boundary
x
System: A gas at p,V, and T.
A gas does a differential amount of work W as it forces the piston to move by a differential amount ds
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Jilin University Thermal energy engineering department
The term quasi can be taken to mean as if
For this to be true the process must be slow in relation to the time needed for the system to come to equilibrium internally
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Jilin University Thermal energy engineering department
§ 13 Equilibrium State
State Postulate The thermodynamics state of a simple compressible system is completely specified by two independent ; intensive properties
• Triple point of water at P = 1 atm T = 0 oC
• Steam point of water at P = 1 atm T = 100 oC
• 1 oC = 1 K
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Jilin University Thermal energy engineering department
§ 12 The Concept of State
Comparison of temperature scales •Celsius scale •Kelvin scale •Fahrenheit scale •Rankine scale
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Jilin University Thermal energy engineering department