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Chapter Two: Basic Concepts of
Thermodynamics
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• System is defined as a quantity of matter or aregion in space chosen for study.
• The mass or region outside the system is calledthe surroundings. They are separated byBoundary.
System
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• Closed system (also known as a control mass)consists of a fixed amount of mass, and no masscan cross its boundary.
• Energy may cross the boundary of closed system
Closed System
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In isolated system, even energy is not allowed tocross the boundary
Isolated System
NO
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In Open system (also called control volume) Bothmass and energy can cross the boundary of acontrol volume.
Open System
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System Properties
Any characteristic of a system is called a property. Somefamiliar properties are
– Pressure, P,
– Temperature, T,
– Volume, V,
– Mass, m.
– Density, specific gravity, or relative density, andSpecific volume
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Types of PropertiesSpecific properties. Some examples of specific properties are
specific volume (v=V/m) and specific total energy (e=E/m).
Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density.
Extensive properties are those whose values depend on the mass of the system such as : volume V, and total energy E.
To determine if a property is intensive or extensive, divide the system into two equal parts with a partition, each part will have the same value of intensive properties as the original system, but half the value of the extensive properties.
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State and Equilibrium
• Set of properties that completely describes the condition, orthe state, of the system. Thermodynamics deals withequilibrium states.
• Equilibrium implies a state of balance.
• Thermal Equilibrium (temperature is the same through thesystem, Mechanical Equilibrium (no change in pressurethroughout the system with time).
• Phase equilibrium when the mass of each phase reaches anequilibrium level and stays there.
• Chemical equilibrium if its chemical composition does notchange with time.
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99
Properties- is the state of a simple compressible system is
completely specified by two independent, intensiveproperties. A process is a system undergoes fromone equilibrium state to another. The path is theseries of states through which a system passes duringa process.
Volume
Pressure
initial
final
path
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Processes
- The prefix iso- is often used to designate a processfor which a particular property remains constant.
- An isothermal process, is a process during which thetemperature T remains constant;
- An isobaric process is a process during which thepressure P remains constant; and
- An isochoric (or isometric) process is a processduring which the specific volume v remains constant.
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Steady and Unsteady States
• The term steady implies nochange with time.
• The opposite of steady isunsteady, or transient. The termuniform, however, implies nochange with location over aspecified region.
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Forms of Energy:
The total energy of a system is divided into two forms:
1. The macroscopic forms of energy are those whicha system possesses as a whole with respect tosome outside reference frame, such as kinetic andpotential energies .
2. The microscopic forms of energy are those which arerelated to the molecular structure of a system andthe degree of the molecular activity. The sum of allmicroscopic forms of energy is called the internalenergy of a system and is denoted by U.
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Forms of Energy:
• kinetic energy: The energy that a system possessesas a result of its motion relative to some referenceKE = ½ m v2 (J),
• potential energy: The energy that a systempossesses as a result of its elevation in agravitational field and is PE = mgz (J)
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Energy Balance
The total energy of a system consists of the kinetic,potential, and internal energies:
E = U + KE + PE .
Most closed systems remain stationary (no change intheir kinetic and potential energies). For stationarysystems E=U
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Types of associated energies with U
1. Portion of the internal energy of a systemassociated with the kinetic energies of themolecules is called the sensible energy.
2. The internal energy associated with the phase of asystem is called the latent energy
3. The internal energy associated with the atomicbonds in a molecule is called chemical energy,
4. The energy associated with the strong bondswithin the nucleus of the atom itself is callednuclear energy.
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The largest source of air pollution is the motorvehicles, and the pollutants released by the vehiclesare usually grouped as hydrocarbons (HC), nitrogenoxides (NOx), and carbon monoxide (CO). There isalso (SOx) emitted to air from some ChemicalIndustries.
Acid Rain and Green house Effect.
Energy and Environment
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Temperature and the Zeroth Law of Thermodynamics:
when an object is in contact with another object that isat a different temperature, heat is transferred fromthe object at higher temperature to the one at lowertemperature until both objects attain the sametemperature (Thermal Equilibrium). The zeroth lawcan be restated as two objects are in thermalequilibrium if both have the same temperaturereading even if they are not in contact.
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Absolute Temperature
-273.15
extrapolation
Experimental dataGas (A)
Gas (B)
Gas (C)
Gas (D)
P
T ( C )
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Temperature• We use four different measures of temperature:
Celsius (°C), Fahrenheit (°F), Kelvin (K),
Rankine (R).
• Kelvin and Rankine are absolute temperature
scales. Both are zero at absolute zero.
0 K=0 R
• Celsius and Fahrenheit are relative temperature
scales, and
0°C=273.15 K 0°C=32°F
0°F=460 R 100°C=212°F
2020
Temperature Conversion
Celcius
kelvin
1 DoC = 1 DK
Fahrenheit
Rankine
1 DoF = 1 DR
1 DoC = (1.8) DoF
1 DK = (1.8) DR
In ordinary usage the D symbol for the unit temperature
difference is omitted. Consequently, you have to infer from
the text whether oF, oC, oR, and K mean temperature or unit
temperature difference.
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Temperature Conversion
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Example 1
Solution
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2323
Example 2
Convert 18 oC to oF
Convert 550 R to oC
4.64
32188.1
328.10
CTFT
2.328.1
32900
09046004.640
CT
FFT
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Pressure
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Pressure
• Pressure is the force per unit area.
• Pressure has units of kPa, psi, atm, inches of Hg, mm of Hg, ft of H2O, and in of H2O, etc.
• Pressure is expressed as absolute pressure or relative pressure (i.e., the pressure above atmospheric pressure)
• Vacuum pressure is expressed in reference to atmospheric pressure.
• Differential pressure is the difference between two pressures.
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Pressure Conversion Factors
2
14.7 psi=1 atm
33.9 ft of H O=1 atm
29.92 of Hg=1 atm
101.3 kPa=1 atm
1.013 bar=1 atm
guage pressure + barometric pressure= absolute pressure
29.92 in. of Hg = 1 atm
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• Pgage = Pabs - Patm (for pressures above Patm)
• Pvac = Patm - Pabs (for pressures below Patm)
P abs
P atm
P gage
P atm
P abs = 0
P atm
P abs
P vac
Absolute vacuum
Gage and Vacuum Pressure
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• Pressure in Depth : pressure in a fluid does not change in the horizontal direction. Pressure in a fluid increases with depth because more fluid rests on deeper layers, and the effect
of this “extra weight” on a deeper layer is balanced by an increase in pressure
• ∆P = P1 – P2 = ρgh = Pgage (h = pressure head)
• Manometers:
• P1 = P2 (horizontally) , But P2 = Patm + ρgh Figure (1)
• Example: A manometer is used to measure the pressure in a tank. The fluid used has a specific gravity of 0.85, and the manometer column height is 55 cm. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.
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Pressure Calculation Example
• Determine the pressure at the bottom of a 10 inch column of Hg (sp. g.=13.55).
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Assignment One.
Problems:
2-59
2-67
2-69
2-75
2-103
Due date: 8/3/2011 (during lecture). Late submission will not be marked and student will get zero.
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Quiz One.
There will be quiz on next Tuesday, 8/3/2011. The quiz will be from Chapters 1 and 2.
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Pressure Example
• What is the absolute pressure in psi of a vacuum of 15 inch of Hg?
vacPatmPabsP
absP
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3333
Class Exercise
• Consider a diver swimming at 50 ft below the surface of the ocean. Determine the gauge and absolute pressure on diver in psi.
gage
atmPgPabsP
gP
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The Manometer
• The manometer is an instrument used to measure the pressure differences of liquids and/or gases.
• Small and moderate pressure differences can be measured by the manometer.
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• SG = Density substance / water density
• Density of substance = 0.85 (1000kg/m3) =
850 kg/m3
• P = Patm + ρgh = 96 kPa + 850 kg/m3 * 9.81 m2/s *0.55m (1 kPa/1000N/m2) * ( 1 N / 1kg. m/s2) =100.6 kPa
Patm = 96kPa
h = 55 cm
SG = 0.85
P = ??
Example
Gas
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P1=Patm + ρ1gh1 + ρ2gh2 + ρ3gh3
P2= P1+ ρ1g(a + h) - ρ2gh – ρ1gaFluid
ρ2
a
h ρ1
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h1
h2
h3
Fluid 1
Fluid 2
Fluid 3
P atm
1
A B
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Example
P1 + ρwater gh1 + ρoil gh2 - ρmercury gh3 = Patm
ρwater=1000 kg/m3, ρmercury=13600 kg/m3 , ρoil=850 kg/m3
h1=0.1 m, h2=0.2 m , h3=0.35 m, Patm=85.6 kPa
Solving for P1 and substituting,
85.6 kPa + (9.81 m/s2)[(13,600 kg/m3) (0.35 m) -(1000 kg/m3) (0.1 m) – (850
kg/m3) (0.2 m)] [(1 N/1 kg · m/s2)( 1 kPa/1000 N/m2)] = 129.6 kPa
h3 h2
h1
1
2Water
Oil
Mercury
ρwater= 1000 kg/m3
ρmercury= 13600 kg/m3
ρoil= 850 kg/m3
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Example
P = Patm + W/A
= 0.97 bar + (60) (9.81)/(0.04) Pa[(1bar/105 Pa)] = 1.12 bar
Patm
W = mg
Patm = 0.97 bar
m = 60 kg
A = 0.04 m2
P=?
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The atmospheric pressure (barometric pressure) is measured by a device called a barometer. As Torricelli measured the atmospheric pressure by inverting a mercury-filled tube into a mercury container that is open to the atmosphere. Note that the cross-sectional area of the tube have no effect on the height of the fluid column of a barometer
Patm = ρgh
Barometer and the Atmospheric Pressure
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• standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0 °C
(ρ= 13,595 kg/m3) and (g = 9.807 m/s2). Called 760mmHg.
1 mmHg = 1 torr = 133.3 Pa
• 1 atm = 760 mm Hg = 760 mm torr
• If water instead of mercury were used to measure the standard atmospheric pressure, a water column will be about 10.3 m. How?
• As we go up atmospheric pressure drops. Why?
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End of Chapter Two
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