thermo lecture ch1
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Thermodynamics I
Dimensions, Units, System
and Equlibrium
Lecture 1
9/21/2009
Objectives
Identify the unique vocabulary associated withthermodynamics through the precise definition
of basic concepts to form a sound foundationfor the development of the principles ofthermodynamics.
Review the metric SI and the English unitsystems.
Get familiar with the terminology ofthermodynamics
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THERMODYNAMICS AND
ENERGY Thermodynamics: The science of
energy. Energy: The ability to cause changes.
The name thermodynamicsstemsfrom the Greek words therme(heat)and dynamis(power).
Conservation of energy principle:During an interaction, energy canchange from one form to another butthe total amount of energy remains
constant.
Energy cannot be created ordestroyed; it can only changeforms (the first law).
Applications
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Laws of Thermodynamics The first law of
thermodynamics: Anexpression of the conservationof energy principle. The first law asserts that energyis
a thermodynamic property.
The second law ofthermodynamics: It assertsthat energy has qualityas wellas quantity, and actualprocesses occur in the directionof decreasing quality of energy.
Macroscopic and microscopic point of
view
Classical thermodynamics: A macroscopicapproach to the study of thermodynamics thatdoes not require a knowledge of the behavior of
individual particles. It provides a direct and easy way to the solution of
engineering problems and it is used in this text.
Statistical thermodynamics: A microscopicapproach, based on the average behavior oflarge groups of individual particles.
It is used in this text only in the supporting role.
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Dimensions and Units Dimensions: Any physical quantity.
The magnitudes assigned to thedimensions are called units.
primary or fundamentaldimensions : mass m, length L, time t, and
temperature T
secondary dimensions, orderived dimensions. velocity V, energy E, and volume V
Systems of Unit
Metric SI system: A simple and logicalsystem based on a decimal relationship
between the various units. English system: It has no apparent
systematic numerical base, and variousunits in this system are related to eachother rather arbitrarily.
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Dimensions Primary (basic) dimensions
Length, L
Time, T(t)
Mass, M
Temeprature, (T)
Secondary dimensions
Developed from primary dimensions
Area = L2
Velocity=LT-1
Density=ML-3
Ways to specify
primary dimensions
There are three basic
systems ofdimensions
MLT (MLT)
FLT (FLT
FMLT
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Dimensional homogeneity
All theoretically derived equations are
dimensionally homogeneous, that is, the
dimensions of the left side of the equation
must be the same as those on the right hand
side, and all additive separte terms must have
the same dimensions
e.g. V=V0+at
Common systems of units
lb/ft2 (psf) or
lb/in2(psi)
Pa (N/m2)bar (106 dyne/cm2)P
R(oF)R(oF)KK
lblbN (newton)dyneF
sluglbmkggM
ssssT
ftftmcmL
British
Gravitational
System (BG)
English Engineering
System (EE)
International
System (SI)
CGS
U.S. systemMetric system
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Unit conversion SI system should
be used in mostcases, but othersystems aresometimes
preferred in a fewfields.
Use SI system in
your work for theclass unless aparticular unitsystem is specified.
Conversion table and constants
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Some SI and English Units
The SI unit prefixes are used in allbranches of engineering.
The definition of the force units.
Work = Force Distance1 J = 1 Nm
1 cal = 4.1868 J1 Btu = 1.0551 kJ
The relative magnitudes of the forceunits newton (N), kilogram-force
(kgf), and pound-force (lbf).
The weight of a unit
mass at sea level.
A body weighing60 kgf on earthwill weigh only 10kgf on the moon.
W weightm massg gravitationalacceleration
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Unity Conversion Ratios
All nonprimary units (secondary units) can beformed by combinations of primary units.Force units, for example, can be expressed as
They can also be expressed more conveniently
as unity conversion ratios as
Unity conversion ratios are identically equal to 1 and
are unitless, and thus such ratios (or their inverses)can be inserted conveniently into any calculation toproperly convert units.
Dimensional homogeneity
All equations must be dimensionally homogeneous.
To be dimensionally
homogeneous, all theterms in an equationmust have the same unit.
System
Closed system
Control volume
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System System: A quantity of matter
or a region in space chosenfor study.
Surroundings: The mass orregion outside the system
Boundary: The real orimaginary surface thatseparates the system from its
surroundings. The boundary of a system
can be fixedor movable.
Systems may be consideredto be closedor open.
Closed System
Also called Isolated SystemorControl Mass
A fixed amount of mass, and
no mass can cross itsboundary.
Always contains the samematter/mass.
No mass can cross theboundary, but energyexchange is allowed.
Composition of the massmay change, e.g. combustion.
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Control Volume Also called Open system
A properly selected region in space.
It usually encloses a device that involves massflow such as a compressor, turbine, or nozzle.
Both mass and energy can cross the boundary ofa control volume.
Control surface is the boundaries of a controlvolume. It can be real or imaginary.
An open system (acontrol volume) with oneinlet and one exit.
Property, State, Process and
Equilibrium
Property A macroscopic characteristics of a system
Assigned at a given time (state) without the knowledge of
the previous behavior (process) of the system. State
The condition of a system as described by its properties.
2 independent properties define a state.
Steady state means properties of the system do not changewith time.
Process A transformation from one state to another.
Equilibrium A state of balance.
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Properties of a System Property: Any characteristic of a system.
A quantity if, and only if, its change in value between two statesis independent of the process.
Some familiar properties are pressure P, temperature T,volume V, and mass m.
Properties are considered to be either intensiveorextensive.
Intensive properties: Those that are independent ofthe mass of a system, such as temperature, pressure,
and density. Extensive properties: Those whose values depend on
the sizeor extentof the system.
Specific properties: Extensive properties per unitmass.
State Postulate
The number of properties required to fixthe state of a system is given by thestate postulate:
The state of a simple compressiblesystem is completely specified by
TWOindependent, intensiveproperties.
Simple compressible system: If asystem involves no electrical, magnetic,gravitational, motion, and surfacetension effects.
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ProcessProcess: Any change that a system
undergoes from one equilibrium stateto another.
Path: The series of states through which asystem passes during a process.
To describe a process completely, oneshould specify the initial and finalstates, as well as the path it follows,and the interactions with the
surroundings.Quasistatic or quasi-equilibrium process:
When a process proceeds in such amanner that the system remainsinfinitesimally close to an equilibrium
Types of Process Process diagrams plotted by employing
thermodynamic properties as coordinatesare very useful in visualizing the processes.
Some common properties that are used ascoordinates are temperature T, pressure
P, and volume V(or specific volume v). The prefix iso- is often used to designate a
process for which a particular propertyremains constant.
Isothermal process: A process duringwhich the temperature Tremains constant.
Isobaric process: A process during whichthe pressure Premains constant.
Isochoric (or isometric) process: Aprocess during which the specific volume vremains constant.
Cycle: A process during which the initialand final states are identical.
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Diagrams of Processes
P
v
Steady Flow Process Steadyimplies no change with time.
The opposite of steady is unsteady, ortransient.
A large number of engineering devicesoperate for long periods of time under thesame conditions, and they are classified assteady-flow devices
During a steady-flow process, fluid propertieswithin the control volume may change withposition but not with time.
Under steady-flow conditions, the mass andenergy contents of a control volume remainconstant.
Steady-flow process: A process duringwhich a fluid flows through a control volumesteadily.
Steady-flow device: turbines, pumps, boilers,condensers, and heat exchangers or powerplants or refrigeration systems.
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Equilibrium Thermodynamics deals with equilibriumstates.
Equilibrium: A state of balance.
In an equilibrium state there are no unbalancedpotentials (or driving forces) within the system.
Thermal equilibrium: If the temperature is the samethroughout the entire system.
Mechanical equilibrium: If there is no change inpressure at any point of the system with time.
Phase equilibrium: If a system involves two phasesand when the mass of each phase reaches anequilibrium level and stays there.
Chemical equilibrium: If the chemical composition of asystem does not change with time, that is, no chemicalreactions occur.
Equilibrium
A system at two different states.A closed system reaching thermalequilibrium.
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Important Properties
Density
Temperature
Pressure
Density
Density
Specific volume
Specific weight
Specific gravity (SG)
The ratio of the density of a substance to the density of some standardsubstance at a specified temperature (usually water at 4C).
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Zeroth law of thermodynamics
The zeroth law of thermodynamics: If twobodies are in thermal equilibrium with a thirdbody, they are also in thermal equilibrium witheach other.
By replacing the third body with a thermometer,the zeroth law can be restated as two bodies
are in thermal equilibrium if both have the sametemperature reading even if they are not incontact.
Temperature Scale
All temperature scales are based on some easilyreproducible states such as the freezing and boilingpoints of water: the ice pointand the steam point.
Ice point: A mixture of ice and water that is inequilibrium with air saturated with vapor at 1 atmpressure (0C or 32F).
Steam point: A mixture of liquid water and watervapor (with no air) in equilibrium at 1 atm pressure(100C or 212F).
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Temperature Scales Celsius scale: in SI unit system
Fahrenheit scale: in English unit system
Thermodynamic temperature scale: Atemperature scale that is independent of theproperties of any substance.
Kelvin scale (SI) Rankine scale (E)
A temperature scale nearly identical to the Kelvin
scale is the ideal-gas temperature scale. Thetemperatures on this scale are measured using aconstant-volume gas thermometer.
Absolute Zero: 0 K P versus Tplots of the experimental data
obtained from a constant-volume gasthermometer using four different gases atdifferent (but low) pressures.
A constant-volume gas thermometerwould read 273.15C at absolute zeropressure.
The reference temperature in the originalKelvin scale was the ice point, 273.15 K,which is the temperature at which waterfreezes (or ice melts).
The reference point was changed to amuch more precisely reproducible point,thetriple point of water (the state atwhich all three phases of water coexist inequilibrium), which is assigned the value273.16 K.
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Conversions Between Temperature
Scales
Pressure
A normal force exerted by a fluid per unit area
Unit:
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The standard atmosphere At sea level the U.S. Standard Atmosphere
Conditions are listed as follows:
The standard atmosphere
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Pressure Measurements: Mercury
Manometer
Evangelista Torricelli
(1608-1647), 1644
Patm = h+Pvaporh
Pvapor=0.000023 lb/in2
(abs) at 68oF, can be
neglected
PRESSURE
The normal stress (or pressure) on thefeet of a chubby person is much greater
than on the feet of a slim person.
Somebasicpressuregages.
Pressure: A normal force exertedby a fluid per unit area
68 kg 136 kg
Afeet=300cm2
0.23 kgf/cm2 0.46 kgf/cm2
P=68/300=0.23 kgf/cm2
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The standard atmosphere
The temperature decreases linearly with
height (up to 11 km above sea level, called
troposhere) is according to
T=(288-0.006507 z) K
=(519-0.00357z) oR
From height of 11 km to 20.1 km, theatmosphere is isothermal at a temperature of -
56.5 oC which is called stratosphere.
Measurement of pressure Absolute pressures are always
positive
Gage pressures can bepositive (negative) if the
pressure is above (below) theatmospheric pressure
A negative gage pressure isreferred to as a suction orvacuum pressure. In the text,pressures will be assumed tobe gage pressures unlessspecifically designatedabsolute 10 psi or 100 kPa : gage
pressures
10 psia or 100 kPa (abs):absolute pressure
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Absolute, Gage and Vacuum Pressures
Absolute pressure: The actual pressure at agiven position. It is measured relative toabsolute vacuum (i.e., absolute zero pressure).
Gage pressure: The difference between theabsolute pressure and the local atmosphericpressure. Most pressure-measuring devices arecalibrated to read zero in the atmosphere, and
so they indicate gage pressure. Vacuum pressures: Pressures below
atmospheric pressure.
Absolute, Gage and Vacuum Pressures
Throughout
this text, thepressure Pwill denoteabsolutepressureunlessspecifiedotherwise.
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ManometryU-tube manometer The pressure PA is large =>
a heavy gage fluid (mercury)can be used
The pressure PA is small =>a lighter gage fluid (water)can be used
The difference in pressurebetween two containers or
two points in a given systemP1+1h1-2h2-3h3=P5PA-PB=2h2+3h3-1h1
ManometryManometry InclinedInclined--tubetube
manometermanometer
(P1-P2)A=Alg =>P=lsin
PA+1h1-2l2sin-3h3=PB=>PA-PB=2l2sinq+3h3-1h1
For gases of A and B, then
As 0
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Other Pressure Measurement Devices Bourdon tube: Consists of a hollow metal tube bent like a hook whose
end is closed and connected to a dial indicator needle.
Pressure transducers: Use various techniques to convert the pressureeffect to an electrical effect such as a change in voltage, resistance, orcapacitance.
Pressure transducers are smaller and faster, and they can be more sensitive,reliable, and precise than their mechanical counterparts.
Strain-gage pressure transducers: Work by having a diaphragm deflectbetween two chambers open to the pressure inputs.
Piezoelectric transducers: Also called solid-state pressure transducers,work on the principle that an electric potential is generated in a crystalline
substance when it is subjected to mechanical pressure.
HW Ch #1
1-2, 1-15, 1-25, 1-32, 1-34, 1-37, 1-41