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3CHAPTERCHAPTER
The First Law ofThermodynamics:
Closed Systems
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Chapter SummaryChapter Summary
The first law of thermodynamics is essentially
an expression of the conservation of energy
principle. Energy can cross the boundaries of
a closed system in the form of heat or work.
If the energy transfer across the boundaries of
a closed system is due
to a temperature difference, it is heat;
otherwise, it is work.
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Heat is transferred in three ways: conduction,convection, and radiation.
Conduction is the transfer of energy from the more
energetic particles of a substance to the adjacent less
energetic ones as a result of interactions between theparticles.
Convection is the mode of energy transfer between a
solid surface and the adjacent liquid or gas that is in
motion, and it involves the combined effects of
conduction and fluid motion. Radiation is the energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the
changes in the electronic configurations of the atoms
or molecules.
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The three modes of heat transfer are expressed
as:
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Various forms of work are expressed as follows:
Electricalwork: (kJ)
Boundarywork: (kJ)
Gravitationalwork(=DPE): (kJ)
Accelerationwork(=DKE): (kJ)
Shaftwork: (kJ)
Spring work: (kJ)
Chapter SummaryChapter Summary3-25
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For the ploytropicprocess (Pvn = constant) of real
gases, the boundary work can be expressed as:
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The energy balance foranysystemundergoing
anyprocesscan be expressed as:
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The energy balances foranysystemundergoing
anyprocesscan be expressed in the rateformas:
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For a constant-pressureprocess, .
Thus
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The amount of energy needed to raise the
temperature of a unit of mass of a substance by
one degree is called the specificheatatconstant
volume Cv for a constant-volume process and thespecificheatatconstantpressure Cp for a
constant pressure process. They are defined as:
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For ideal gases u, h, Cv, and Cp are functions of
temperature alone. The #uand #h of ideal gases
can be expressed as:
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For ideal gases Cv, and Cp are related by:
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The specificheatratiok is defined as:
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Forincompressiblesubstances(liquids and
solids), both the constant-pressure and constant-
volume specific heats are identical and denoted by
C:
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The uand h of incompressible substances are
given by
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The refrigeration and freezing of foods is a major
application area of thermodynamics.
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Heat TransferHeat Transfer3-1
(Fig. 3-3)
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Adiabatic ProcessAdiabatic Process
(Fig. 3-4)
3-2
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Convection: Heat TransferConvection: Heat Transfer
(Fig. 3-8)
3-3
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Convection: CoolingConvection: Cooling
(Fig. 3-9)
3-4
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RadiationRadiation
(Fig. 3-10)
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Heat and WorkHeat and Work
(Fig. 3-15)
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Path FunctionsPath Functions
(Fig. 3-16)
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Boundary WorkBoundary Work
(Fig. 3-27)
3-8
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Net Work per CycleNet Work per Cycle
(Fig. 3-29)
3-9
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Schematic/Diagram for Ex. 3-8Schematic/Diagram for Ex. 3-8
(Fig. 3-31)
3-10
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Schematic/Diagram for Ex. 3-9Schematic/Diagram for Ex. 3-9
(Fig. 3-32)
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Schematic/Diagram for thePolytropic ProcessSchematic/Diagram for thePolytropic Process
(Fig. 3-33)
3-12
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Schematic/Diagram for Ex. 3-13Schematic/Diagram for Ex. 3-13
(Fig. 3-43)
3-13
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Stretching a Liquid FilmStretching a Liquid Film
(Fig. 3-45)
3-14
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System Energy ChangeSystem Energy Change
(Fig. 3-52)
3-15
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Energy Change for a CycleEnergy Change for a Cycle
(Fig. 3-54)
3-16
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Closed-Systems, First-LawClosed-Systems, First-Law
(Fig. 3-55)
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Formal Definitions ofCv and CpFormal Definitions ofCv and Cp
(Fig. 3-72)
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Specific Heats for Some GasesSpecific Heats for Some Gases
(Fig. 3-76)
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Three Ways to Calculate uThree Ways to Calculate u3-20
(Fig. 3-80)
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Typical Freezing Curve (food)Typical Freezing Curve (food)
(Fig. 3-91)
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