Download - Chap 1 Intro to Thermodynamic
-
8/13/2019 Chap 1 Intro to Thermodynamic
1/21
Chapter 1
INTRODUCTION ANDBASIC CONCEPTS
-
8/13/2019 Chap 1 Intro to Thermodynamic
2/21
2
Objectives
Review the metric SI and the English unit systems.
Explain the basic concepts of thermodynamics such
as system, state, state postulate, equilibrium,
process, and cycle.
Review concepts of temperature, temperature scales,
pressure, and absolute and gage pressure.
-
8/13/2019 Chap 1 Intro to Thermodynamic
3/21
3
THERMODYNAMICS AND ENERGY
Thermodynamics:The science of
energy. Energy:The ability to cause changes.
The name thermodynamicsstems fromthe Greek words therme (heat) anddynamis (power).
Conservation of energy principle:During an interaction, energy can changefrom one form to another but the totalamount of energy remains constant.
Energy cannot be created or destroyed.
Thermodynamics gives the criteria for aprocess to take place; it does NOT dealwith the rates of chemical and physicalproperties. Energy cannot be created
or destroyed; it can only
change forms (the first law).
-
8/13/2019 Chap 1 Intro to Thermodynamic
4/21
4
SYSTEMS AND CONTROL VOLUMES System:A quantity of matter or a region
in space chosen for study.
Surroundings:The mass or regionoutside the system
Boundary:The real or imaginary surfacethat separates the system from itssurroundings.
The boundary of a system can be fixed ormovable.
Systems may be considered to be closedor open.
Closed system
(Control mass):A fixed amountof mass, and nomass can crossits boundary.
-
8/13/2019 Chap 1 Intro to Thermodynamic
5/21
5
Open system(control volume):A properlyselected region in space.
It usually encloses a device that involvesmass flow such as a compressor, turbine, or
nozzle.
Both mass and energy can cross theboundary of a control volume.
Control surface:The boundaries of a controlvolume. It can be real or imaginary.
Isolated system: No mass and energyexchanged
An open system (a
control volume) with one
inlet and one exit.
-
8/13/2019 Chap 1 Intro to Thermodynamic
6/21
6
PROPERTIES
OF A SYSTEM
Property:Any characteristic of asystem.
Some familiar properties arepressure P, temperature T, volumeV, and mass m.
Properties are considered to beeither intensive or extensive.
Intensive properties:Those thatare independent of the mass of asystem, such as temperature,pressure, and density.
Extensive properties:Thosewhose values depend on the sizeor extentof the system.
Specific properties: Extensiveproperties per unit mass.
Criterion to differentiate intensive
and extensive properties.
-
8/13/2019 Chap 1 Intro to Thermodynamic
7/21
7
STATE AND EQUILIBRIUM
Thermodynamics deals withequilibrium states.
Equilibrium:A state of balance.
In an equilibrium state there are nounbalanced potentials (or drivingforces) within the system.
Thermal equilibrium:If thetemperature is the same throughoutthe entire system.
Mechanical equilibrium:If there isno change in pressure at any pointof the system with time.
Phase equilibrium:If a systeminvolves two phases and when themass of each phase reaches anequilibrium level and stays there.
Chemical equilibrium:If thechemical composition of a systemdoes not change with time, that is,no chemical reactions occur. A closed system reaching thermal
equilibrium.
A system at two different states.
-
8/13/2019 Chap 1 Intro to Thermodynamic
8/21
Systems of Units
8
Systems of
Units
StandardInternationalSystem
AmericanEngineeringSystem
CentimetreGram SecondSystem
-
8/13/2019 Chap 1 Intro to Thermodynamic
9/21
Systems of units
9
System/
Dimension
Length
Mass
Moles
Time
Temperature
SI
meter, m
kilogram, kg
gram-mole,
mole
second, s
Kelvin, k
American
foot, ft
pound-mass,lbm
pound mole,
lb-mole
second, s
Rankine, R
CGS
centimeter, cm
gram, g
gram-mole,
mole
second, s
Kelvin, K
-
8/13/2019 Chap 1 Intro to Thermodynamic
10/21
Multiple SI Units
10
-
8/13/2019 Chap 1 Intro to Thermodynamic
11/21
Derived SI Units
11
-
8/13/2019 Chap 1 Intro to Thermodynamic
12/21
12
Temperature Scales
All temperature scales are based on some easily reproducible statessuch as the freezing and boiling points of water: the ice point and thesteam point.
Ice point:A mixture of ice and water that is in equilibrium with airsaturated with vapor at 1 atm pressure (0C or 32F).
Steam point:A mixture of liquid water and water vapor (with no air) inequilibrium at 1 atm pressure (100C or 212F).
The reference temperature in the original Kelvin scale was the icepo in t,273.15 K, which is the temperature at which water freezes (or
ice melts).
The reference point was changed to a much more precisely
reproducible point, the t r ip le poin tof water (the state at which all
three phases of water coexist in equilibrium), which is assigned thevalue 273.16 K.
-
8/13/2019 Chap 1 Intro to Thermodynamic
13/21
Temperature Scales
13
-
8/13/2019 Chap 1 Intro to Thermodynamic
14/21
Example 1
Water boils at 212oF at one atmosphere
pressure. At what temperature does water boil
in oC.
14
-
8/13/2019 Chap 1 Intro to Thermodynamic
15/21
Force (F)
Definition-the force acting on a body is proportional tothe product of the mass and the acceleration in the
direction of the force.
Newtons Law: F=ma
SI unit- newton (N)
1 N-the force applied to a mass of 1 kg to produce an
acceleration of 1 ms-2
English unit-pound force (lbf)
1 lbfthe force applied to a mass of 1 pound mass (lbm)
to produce an acceleration of 32.174 ft/s215
-
8/13/2019 Chap 1 Intro to Thermodynamic
16/21
16
PRESSURE
The normal stress (or pressure) on the
feet of a chubby person is much greaterthan on the feet of a slim person.
Pressure:A normal force exerted
by 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
F
A
P=F/A
SI unit: [P]=[F]/[A]=N/m2=1 Pa
English unit: [P]=[F]/[A]=lbf/in2
=1 psi
-
8/13/2019 Chap 1 Intro to Thermodynamic
17/21
PRESSURE Additional units
1 torr- the pressure equivalent to a column of Hg of 1 mmat T=0oC in a standard gravitational field
1 atm- the approximate average pressure exerted by the
earths atmosphere at sea level 1 psig Pgauge=PabsolutePbarometric
1 bar =105Pa = 0.986923 atm
1 mmHg =1 torr =133.322 Pa
1atm = 760 torr
1 bar =750.061 torr =14.5 psia17
-
8/13/2019 Chap 1 Intro to Thermodynamic
18/21
18
Absolute pressure:The actual pressure at a given position. It ismeasured relative to absolute vacuum (i.e., absolute zero pressure).
Gage pressure:The difference between the absolute pressure andthe local atmospheric pressure. Most pressure-measuring devices are
calibrated to read zero in the atmosphere, and so they indicate gagepressure.
Vacuum pressures:Pressures below atmospheric pressure.
Throughout
this text, the
pressure P
will denote
absolute
pressure
unless
specifiedotherwise.
-
8/13/2019 Chap 1 Intro to Thermodynamic
19/21
Example 2
An object at sea level has a mass of 400 kg
a) Find the weight of this object on earth
b) Find the weight of this object on the moonwhere the local gravitational acceleration is
one-sixth that of earth.
19
-
8/13/2019 Chap 1 Intro to Thermodynamic
20/21
Example 3
A vacuum gage connected to a tank reads 30
kPa at a loation where the atmospheric
pressure is 98 kPa. What is the absolute
pressure in the tank?
20
-
8/13/2019 Chap 1 Intro to Thermodynamic
21/21
21
Summary
Importance of dimensions and units
Some SI and English units Systems and control volumes
Properties of a system
TemperatureTemperature scales
Pressure