CHAPTER 1
INTRODUCTION TO THERMODYNAMICS
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What is Thermodynamics?
Thermodynamics: The science of energy.
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What is Thermodynamics?
• Energy: The ability to cause changes.
• Conservation of energy principle: During an
interaction, energy can change from one form
to another, but the total amount of energy
remains constant.
• Energy cannot be created or destroyed !
3
What is Thermodynamics?
(熱力學第一定律)
The first law of thermodynamics: An expression of the conservation of
energy principle.
(熱力學第二定律)
The second law of thermodynamics: It asserts that energy
has quality as well as quantity, and actual processes occur in
the direction of decreasing quality of energy.
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What is Thermodynamics?
First law
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What is Thermodynamics?
First law
Second law
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Application Areas of Thermodynamics
•
All activities in nature involve some interaction
between energy and matter.
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Importance of Dimensions
•
Any physical quantity can be characterized by dimensions.
•
The magnitudes assigned to the dimensions are called units.
•
Some basic dimensions such as mass m, length L, time t, and
temperature T are selected as fundamental dimensions
•
Others such as velocity v, energy E, and volume V are expressed
in terms of the primary dimensions and are called secondary
dimensions, or derived dimensions.
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Importance of Dimensions
•
Metric SI system: A simple and logical system based on a
decimal relationship between the various units. (International
System of Units)
•
English system: It has no apparent systematic numerical base,
and various units in this system are related to each other rather
arbitrarily. (English Units)
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Some SI and
English Units
English Unit
SI Unit
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Dimensional homogeneity
All equations must be dimensionally homogeneous.
Unity Conversion Ratios
All nonprimary units (secondary units) can be
formed by combinations of primary units.
They can also be expressed more conveniently
as unity conversion ratios as
11
Examples:
A school is paying $0.12/kWh for electric power. To reduce its power bill,
the school installs a wind turbine (Fig. 1–12) with a rated power of 30 kW. If
the turbine operates 2200 hours per year at the rated power, determine the
amount of electric power generated by the wind turbine and the money
saved by the school per year.
Answer:
The total amount of electric energy generated per year:
Total energy = ( Energy per unit time ) ( Time interval )
= ( 30 kW ) ( 2200 h)
= 66,000 kWh
Money saved = ( Total energy ) ( Unit cost of energy )
= ( 66,000 kWh) ( $0.12 / kWh)
= $7920
12
System and Control Volumes
System: A quantity of matter or a region in space chosen for study.
Surroundings: The mass or region outside the system
Boundary: The real or imaginary surface that separates the system
from its surroundings.
The boundary of a system can be fixed or movable.
Systems may be considered to be closed or open.
Closed system (Control mass): A fixed amount of mass, and no
mass can cross its boundary.
Open system (control volume): A properly selected region in space.
It usually encloses a device that involves mass flow such as a
compressor, turbine, or nozzle. Both mass and energy can cross the
boundary of a control volume.
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System and Control Volumes
System: A quantity of matter or a region in space chosen for study.
Surroundings: The mass or region outside the system
Boundary: The real or imaginary surface that separates the system
from its surroundings.
The boundary of a system can be fixed or movable.
Systems may be considered to be closed or open.
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System and Control Volumes
Closed system (Control mass): A fixed amount of mass, and no
mass can cross its boundary.
Open system (control volume): A properly selected region in space.
It usually encloses a device that involves mass flow such as a
compressor, turbine, or nozzle. Both mass and energy can cross the
boundary of a control volume.
Closed
Open
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Properties of a system
Property: Any characteristic of a system.
•
Some familiar properties are pressure P, temperature T, volume
V, and mass m.
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Properties of a system
Properties are considered to be either intensive or extensive.
Intensive properties: Those that are independent of the mass of a
system, such as temperature, pressure, and density.
Extensive properties: Those whose values depend on the size—
or extent—of the system.
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Properties of a system
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Properties of a system
Specific properties: Extensive properties per unit mass.
For example:
•
Specific volume, n
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21
State of Equilibrium
Equilibrium: A state of balance.
• In an equilibrium state there are no unbalanced potentials (or driving
forces) within the system.
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State of Equilibrium
• Thermal equilibrium: If the temperature is the same
throughout the entire system.
• Mechanical equilibrium: If there is no change in pressure
at any point of the system with time.
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No equilibrium
Thermal equilibrium
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The state Postulate
The number of properties required to fix the state
of a system is given by the state postulate:
•
The state of a simple compressible system is
completely specified by two independent,
intensive properties.
Simple compressible system: If a system involves no
electrical, magnetic, gravitational, motion, and surface
tension effects.
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Process and Cycles
•
Process: Any change that a
system undergoes from one
equilibrium state to another.
•
Path: The series of states
through which a system passes
during a process.
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• Process
diagrams
plotted
by
employing thermodynamic properties
as coordinates are very useful in
visualizing the processes.
• Some common properties that are
used as coordinates are: temperature
T, pressure P, and volume V,
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• The prefix iso- is often used to
designate a process for which a
particular property remains constant.
• Isothermal process: A process during
which the temperature T remains
constant.
• Isobaric process: A process during
which the pressure P remains constant.
• Cycle: A process during which the initial
and final states are identical.
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The Steady-Flow Process
• The term steady implies no change with time.
The opposite of steady is unsteady, or transient.
• A large number of engineering devices operate
for long periods of time under the same
conditions, and they are classified as steadyflow devices.
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The Steady-Flow Process
Steady-flow process:
A process during which a fluid flows
through a control volume steadily.
• Steady-flow conditions can be closely
approximated by devices that are intended
for continuous operation such as turbines,
pumps, boilers, condensers, and heat
exchangers or power plants or refrigeration
systems.
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The Zeroth Law of Thermodynamics
(熱力學第零定律)
The zeroth law of thermodynamics: If two bodies are in thermal
equilibrium with a third body, they are also in thermal equilibrium with
each other.
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Temperature Scales
All temperature scales are based on some easily reproducible states
such as the freezing and boiling points of water: the ice point and
the steam point.
•
Ice point: A mixture of ice and water in equilibrium at 1 atm
pressure (0°C or 32°F).
•
Steam point: A mixture of liquid water and water vapor in
equilibrium at 1 atm pressure (100°C or 212°F)
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34
Pressure
Pressure: A normal force exerted by a fluid per unit area
Some basic
pressure gages.
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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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Absolute pressure: The actual pressure at a given position. It is measured
relative to absolute vacuum (i.e., absolute zero pressure).
Gage pressure: The difference between the absolute pressure and the local
atmospheric pressure. Most pressure-measuring devices are calibrated to read
zero in the atmosphere, and so they indicate gage pressure.
Vacuum pressures: Pressures below atmospheric pressure.
In this text, the pressure P will denote absolute pressure unless specified
otherwise.
This is what we care
about
This is what we care
about
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Variation of Pressure with Depth
When the variation of density
with elevation is known
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Variation of Pressure with Depth
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Pascal’s law: The pressure
applied to a confined fluid increases
the pressure throughout by the
same amount.
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Pressure Measurement Device
The Barometer (氣壓計)
Atmospheric pressure is measured by a
device called a barometer; thus, the
atmospheric pressure is often referred
to as the barometric pressure.
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Examples:
Determine the atmospheric pressure at a location where the barometric
reading is 740 mmHg and the gravitational acceleration is g=9.805 m/s2.
Assume the temperature of mercury to be 10°C, at which its density is
13,570 kg/m3.
Answer:
h = 740 mm = 0.74 m
Patm – ρgh = 0
Patm = ρgh
= (13,570 kg/m3 )(9.805 m / s2 )(0.740 m)
= 98500 Pa
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Examples:
Solar ponds are small artificial lakes a few meters deep that are used to
store solar energy. The rise of heated (and thus less dense) water to the
surface is prevented by adding salt at the pond bottom. In a typical salt
gradient solar pond, the density of water increases in the gradient zone, as
shown in Fig. 1–52, and the density can be expressed as
 s 

4 H
 = 0 1 + tan 2 
where ρ0 is the density on the water surface, s is the vertical distance measured
downward from the top of the gradient zone (s = −z), and H is the thickness of
the gradient zone. For H = 4 m, ρ0 = 1040 kg/m3, and a thickness of 0.8 m for
the surface zone, calculate the gage pressure at the bottom of the gradient
zone.
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Examples:
Solar ponds are small artificial lakes a few meters deep that are used to
store solar energy. The rise of heated (and thus less dense) water to the
surface is prevented by adding salt at the pond bottom. In a typical salt
gradient solar pond, the density of water increases in the gradient zone, as
shown in Fig. 1–52, and the density can be expressed as
P1,gage = ρgh1 = (1040 kg / m3 )(9.81 m / s 2 )(0.8 m) = 8160 Pa = 8.16 kPa
dP = g ds
s =4
 s 
P2,gage = P1,gage +   g ds = P1,gage +  0 1 + tan 2 
 g ds
4
H


0
0
s
P2,gage = P1,gage + 0 g
4 4
 4
 s 

−1 
sinh −1  tan
 = 8.16 + 1040  9.81  sinh  tan 4 4  = 54000 Pa = 54 kPa

4H

4H
Patm
P1, gage
P2, gage
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The Manometer (流體壓力計)
It is commonly used to measure small
and moderate pressure differences. A
manometer contains one or more fluids
such as mercury, water, alcohol, or oil.
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P1  P2 Because the fluid is moving
some pressure drop exists!
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Examples:
The water in a tank is pressurized by air, and the pressure is measured by
a multifluid manometer. The tank is located on a mountain at an altitude of
1400 m where the atmospheric pressure is 85.6 kPa. Determine the air
pressure in the tank if h1 = 0.1 m, h2 = 0.2 m, and h3 = 0.35 m. Take the
densities of water, oil, and mercury to be 1000 kg/m3, 850 kg/m3, and
13,600 kg/m3, respectively.
Answer:
P1 + ρwater g h1 + ρoil g h2 − ρmercury g h3 = P2 = Patm
P1 = Patm − ρwater g h1 − ρoil g h2 + ρmercury g h3
= Patm + g ( ρmercury h3 − ρwater h1 − ρoil h2 )
= 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)]
= 130000 Pa = 130 kPa
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Summary
•
•
•
•
•
•
•
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Thermodynamics and energy
Importance of dimensions and units
Systems and control volumes
Properties of a system
State and equilibrium
Processes and cycles
Temperature and the zeroth law of thermodynamics
Pressure
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