Chapter1-Basic Concepts of Thermodynamics -W1
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Basic Concepts ofThermodynamics
Chapter 1
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What is Thermodynamics?
Basic science that deals with energy. We all
know that energy is neither created nor
destroyed but it is transformed.
Among other things well look at Power Production
Refrigeration
Relationships between the properties of matter
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The Science of Energy
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Two ways to look at systems
MacroscopicClassical Thermodynamics
Atomic or MolecularStatistical Thermodynamics
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Dimensions and Units
Dimensions are names that characterize
physical quantities.
Units are those arbitrary magnitudes and
names assigned to dimensions, which are
adopted as standards for measurement
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Weight vs. Mass
Mass is an amount of matterWeight is a measure of how much force
is applied to the matterYour mass is the same on the earth and
on the moonYour weight is different!We often are not careful to make a
distinction between mass and weight
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F=ma
m is mass
a is acceleration
On the surface of the earth we usuallycall the acceleration g g=9.8 m/sec2
g= 32.174 ft/sec2
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Weight on the surface of the
earth
F = m g
In metric the units becomeKg m/sec2 which is the same as
N
(Nx0.2248) lbf
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Systems
Quantity of matter or a region of spaceClosed SystemOpen System
Surroundings everything thats not thesystem
BoundaryStationaryMoving
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Closed System
System
E
E
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Open System
(Control Volume)
WaterHeat
er
Matter
Matter
Energy
Energy
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Analyzing Systems
The approach is different for closed and
open systems
Energy is a lot harder to handle thanmatter, because it exists in many forms
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Energy = E
Macroscopic formsRespect to some outside reference frame
MicroscopicRelated to the molecular structure
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Macroscopic Energy
Kinetic Energy (KE)KE = mV2/2
Potential Energy (PE)PE = mgz
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Other Kinds of Macroscopic
Energy
Magnetic
Electrical
Surface TensionThese are specialized, and we dont
usually need to include them
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Microscopic Energy
Kinetic energy of individual molecules
Potential energy of individual molecules
Binding forcesChemical Energy
Nuclear Energy
Etc
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Both macroscopicand microscopicforms of energy arestatic they can be
stored in a system
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PEKEUE ++=
2
2
2mVKE =
mgzPE =
mgzmVUE ++= 2
2
2
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A unit mass basis is often
more convenient
e = E/m
u = U/m
ke = KE/m = V2
/2pe = PE/m = gz
gzVupekeue ++=++=2
2
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Dynamic Energy
When energy moves from place to
place we treat it differently
The only forms of energy that can crossa system boundary without matter
transfer are:
Heat (Q)Work (W)
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Heat
A system can not contain heat
Heat only exists as energy crossing a system
boundary
What we think of as a systems heat content
is Thermal Energy
Heat is energy transferred through a
temperature differenceAll other forms of energy transfer are work!!
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Properties of a System
IntensiveDoes not depend on the systems size
Temperature
Pressure
ExtensiveDepends on the systems size
VolumeMass Total Energy
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We often define properties in
terms of other properties
Density = m/V
Specific Volume v = V/m = 1/
We can define most extensiveproperties per unit mass
Called specific properties u = U/m is the specific internal energy
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State
At a given state, all the properties of a
system have a fixed value
If you change a property, you havechanged the state
Thermodynamics deals with
equilibrium states
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Process Diagram
P
T
State1
State2
ProcessPath
P1
T1T2
P2
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State Postulate
The state of a simple compressiblesystem is completely specified bytwo independent, intensive
properties
Remember that during a phase change,
Temperature and Pressure are notindependent
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Iso
Many times we will talk about process paths
where one property is kept constant
Isothermal Constant Temperature
Isobaric Constant Pressure
Isochoric Constant Volume
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Pressure
The force exerted by a fluid per unit
area
Only meaningful for a gas or a liquid In solids we talk about stress
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A
FP=
2m
NPa =
One Pascal isnt verymuch!!
1 atm =101,325 Pa
Units of Pressure
1 atm = 101.325 kPa = 1.01325 bar= 14.7 psi
Note: psi is lbf / in2
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Absolute vs. Gage Pressure
Absolute pressures are measured
relative to a vacuumUsually we will talk about absolute
pressure, and will use absolute pressure in
our calculations
Gage pressures are measured relative
to the surroundings
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Absolute vs. Gage Pressure
Vacuum
Surroundings
Pressure tomeasure
psia
psig
Pat
m
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Devices to measure pressure
Bourdon tube
Barometer
Manometer
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Barometer
h
A
A
FP=
mgF=
Vm=
hAV =
hA=
ghA=
gh=
Note: Thepressuremeasured by abarometer isindependent of
cross sectional
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Barometer
P=ghFor a given barometer the density and
the acceleration due to gravity are
constants, soPressure is directly proportional to
height
Pressure is often measured in mmHg1 atm = 760 mmHg
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Manometer
Used to compare pressures
gas
Atmosphericpressure
2P ghPP atm +=2
ghPPP atm == 2
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Zeroth Law of
Thermodynamics
If two bodies are in equilibrium with a
third body, they are also in equilibriumwith each other
Basis for thermometers
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