ME314
THERMODYNAMICS 1
BASIC PRINCIPLES, CONCEPTS AND
DEFINITIONS (1 of 2)
Objectives
• Identify the unique vocabulary associated with
thermodynamics through the precise definition of
basic concepts to form a sound foundation for the
development of the principles of thermodynamics.
• Review the metric SI and the English unit systems.
• Review concepts of temperature, temperature scales,
pressure, and absolute and gage pressure.
• Explain the basic concepts of thermodynamics such
as system, state, state postulate, equilibrium,
process, and cycle.
2
THERMODYNAMICS AND ENERGY
•
Thermodynamics: the science that deals
with the concept of energy, the laws
governing conversion of one form of energy
into another, and the various media employed
to effect the transformation
•
Energy: The ability to cause changes.
•
The name thermodynamics stems from the
Greek words therme (heat) and dynamis
(power).
•
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.
•
The first law of thermodynamics: An
expression of the conservation of energy
principle.
•
The first law asserts that energy is a
thermodynamic property.
Energy cannot be created
or destroyed; it can only
change forms (the first law).
3
•
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.
•
Classical thermodynamics: A
macroscopic approach to the study of
thermodynamics that does 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
microscopic approach, based on the
average behavior of large groups of
individual particles.
• It is used in this text only in the
supporting role.
Conservation of energy
principle for the human body.
Heat flows in the direction of
decreasing temperature.
4
HISTORY
•
Although the principles of thermodynamics have been in
existence since the creation of the universe, thermodynamics
did not emerge as a science until the construction of the first
successful atmospheric steam engines in England by Thomas
Savery in 1697 and Thomas Newcomen in 1712. These engines
were very slow and inefficient, but they opened the way for the
development of a new science.
• The first and second laws of thermodynamics emerged
simultaneously in the 1850s, primarily out of the works of
William Rankine, Rudolph Clausius, and Lord Kelvin (formerly
William Thomson).
•
The term thermodynamics first used in a publication by Lord
Kelvin in 1849.
•
The first thermodynamic textbook was written in 1859 by
William Rankine, a professor at the University of Glasgow.
Application Areas of Thermodynamics
6
Application Areas of
Thermodynamics
• Automobile engines
• Turbines
• Compressors, pumps
• Fossil- and nuclear-fueled power stations
• Propulsion systems for aircraft and rockets
• Combustion systems
• Cryogenic systems, gas separation, and liquefaction
• Heating, ventilating, and air-conditioning systems
• Vapor compression and absorption refrigeration
• Heat pumps
(CONT.) Application Areas of
Thermodynamics
•Cooling of electronic equipment
•Alternative energy systems
•Fuel cells
•Thermoelectric and thermionic devices
•Magnetohydrodynamic (MHD) converters
•Solar-activated heating, cooling, and power generation
•Geothermal systems
•Ocean thermal, wave, and tidal power generation
•Wind power
•Biomedical applications
IMPORTANCE OF DIMENSIONS AND UNITS
• Dimensions - characterize any physical
quantity.
• Units -the magnitudes assigned to the
dimensions.
• Primary or fundamental dimensions
-some basic dimensions such as
mass m, length L, time t, and
temperature T.
• Secondary dimensions, or derived
dimensions.
- are expressed in terms of the
primary dimension, such as velocity
V, energy E, and volume V.
9
2 sets of units are still in common use today:
Metric SI system: A simple and logical system based on a
decimal relationship between the various units.
English system: It has no apparent systematic numerical
base, and various units in this system are related to each
other rather arbitrarily.
1. English system, which is also known as the United
States Customary System (USCS)
fps system: (foot-pound-second)
2. Metric SI (from Le Système International d’ Unités),
which is also known as the International System.
mks system: (meter-kilogram-second)
cgs system: (centimeter-gram-second)
MASS, FORCE AND WEIGHT
Mass – the aggregation of matter in a body/substance
Force – the interaction which causes the body with
mass to change its velocity (which causes the mass to
accelerate)
Weight – It is the gravitational force applied to a body,
and its magnitude is determined from Newton’s second
law
Newton’s Second Law of Motion
“The acceleration of a body is parallel, and is
directly proportional to the net force applied, and
inversely to its mass.”
GRAVITATIONAL
ACCELERATION
acceleration
(rate of change
of velocity)
net force
weight (towards the
center of the earth)
EARTH
UNITS OF MASS AND FORCE
System
mass
force
SI
mks
cgs
fps
Imperial
kilogram
kilogram
gram
pound
slug
(kg)
(kg)
(g)
(lbm)
(slug)
newton
newton
dyne
pound
pound
(N)
(N)
(dyn)
(lbf)
(lbf)
kilogram-force
gram-force
poundal
(kgf)
(gf)
(pdl)
GRAPHIC ILLUSTRATION OF FORCES
GRAPHIC ILLUSTRATION OF WEIGHT
9.81
981
• Weight , W = mg
where: m = mass
g = local gravitational accelaration
= 32.2 ft/s² or 9.81 m/s²
32.2
Some SI and English Units
The SI unit prefixes are used in all
branches of engineering.
32. 2
32. 2
The definition of the force units.
16
A body weighing 60 kgf on earth
will weigh only 10 kgf on the
moon.
*g of moon = 1.625 m/s2
W weight
m mass
g gravitational
acceleration
The relative magnitudes of the
force units newton (N), kilogramforce (kgf), and pound-force (lbf).
17
The weight of a unit mass at sea level.
9.81
9.81
9.81
32. 2
32. 2
Prove that: 1 kgf = 1 kgm and 1lbf = 1lbm at
sea level
Work
can simply be defined as force times
distance;
therefore, it has the unit “newton-meter
(N·m),” which is called a joule (J). That is,
1 N·m = 1 J
Work
SI units: kilojoule (1 kJ = 10³ J).
1 calorie (cal.) - the amount of energy needed to raise the
temperature of 1 g of water at 14.5°C by 1°C.
1 cal = 4.1868 J
English system: Btu (British thermal unit)
1 Btu - energy required to raise the temperature of 1 lbm of
water at 68°F by 1°F.
1 Btu = 1.055 kJ.
Here is a good way to get a feel for these units: If you light a typical match and let
it burn itself out, it yields approximately one Btu (or one kJ) of energy.
Power
Power is defined as the time rate of energy
•unit of Power:
joule/second = Watt
1 J/s = 1 W
1 horsepower = 746 watts
1 hp = 746 W
**Electrical energy typically is expressed in the unit kilowatt-hour
(kWh), which is equivalent to 3600 kJ. An electric appliance with a
rated power of 1 kW consumes 1 kWh of electricity when running
continuously for one hour. (1 kWh = 3600 kJ)
Example: Electric Power Generation
by a Wind Turbine
• A school is paying $0.12/kWh for electric power. To reduce its
power bill, the school installs a wind turbine 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.
• SOLUTION A wind turbine is installed to generate electricity. The amount
of electric energy generated and the money saved per year are to be
determined.
•
Analysis The wind turbine generates electric energy at a rate of 30 kW or
30 kJ/s.
Then the total amount of electric energy generated per year becomes:
Total energy = (Energy per unit time)(Time interval)
= (30 kW)(2200 h)
= 66,000 kWh
The money saved per year is the monetary value of this energy determined as:
Money saved = (Total energy)(Unit cost of energy)
= (66,000 kWh)($0.12/kWh)
= $7920
•The annual electric energy production also could be determined in kJ by unit
manipulations as
Total energy = (30 kW)(2200 h) (3600 s/1 h)(1 kJ/s / 1 kW) = 2.38 x 108 kJ
which is equivalent to 66,000 kWh (1 kWh = 3600 kJ).
Unity Conversion Ratios
All nonprimary units (secondary units) can be formed 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
to properly convert units.
24
Some of commonly used Unity
Conversion
32.2
DENSITY AND SPECIFIC GRAVITY
1. Density:
mass per unit volume
2. Specific gravity: The
ratio of the density of a
substance to the density of
some standard substance
at a specified temperature
(usually water at 4°C).
3. Specific volume:
-reciprocal of density
- volume per unit mass
4. Specific weight:
The weight of a unit
volume of a substance.
Density is mass per unit
volume; specific volume
is volume per unit mass.
26
SPECIFIC GRAVITY
TEMPERATURE AND THE ZEROTH LAW OF
THERMODYNAMICS
• 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.”
• 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
same temperature reading even if they are not in contact.
• Temperature - a measure of “hotness” or “coldness”
- a physical property that determines whether the bodies will be
in thermal equilibrium. We may postulate that when the two blocks
are in thermal equilibrium, their temperatures are equal.
Two bodies reaching
thermal equilibrium
after being brought
into contact in an
isolated enclosure.
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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:(0°C or 32°F)
A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm
pressure .
•Steam point: (100°C or 212°F)
A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure .
•Celsius scale: in SI unit system
•Fahrenheit scale: in English unit system
THERMODYNAMIC TEMPERATURE SCALE
A temperature scale that is independent of the properties of any substance.
•Kelvin scale (SI) and Rankine scale (English)
29
273
460
Comparison of
temperature
scales.
•
•
The reference temperature in the original Kelvin scale was the ice point,
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 triple point of water (the state at which all three phases of water
coexist in equilibrium), which is assigned the value 273.16 K.
30
Example: Expressing Temperature
Rise in Different Units
During a heating process, the temperature of a system rises by
10°C. Express this rise in temperature in K, °F, and R.
SOLUTION:
ΔT(°C) = ΔT(K) = 10 K
ΔT(R) = 1.8 ΔT(K) = (1.8)(10) = 18 R
ΔT(°F) = ΔT(R) = 18°F
The units °C and K are interchangeable when dealing with
temperature differences.
PRESSURE
A normal force exerted by a fluid
per unit area
Some basic pressure
gages.
The normal stress (or “pressure”) on the feet of a chubby person
is much greater than on the feet of a slim person.
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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.
Throughout
this text, the
pressure P
will denote
absolute
pressure
unless
specified
otherwise.
33
EXAMPLE
• Express a pressure gage reading of 35 psi in absolute
pascals.
 101.325 kPa 
 = 241 kPa
35 psi 
 14.7 psi 
P = ( 241 + 101.325) kPa
P = 341.325 kPa
PRESSURE CONVERSION TABLE
Variation of Pressure with Depth
The pressure of a fluid at rest
increases with depth (as a
result of added weight).
37
In a room filled with
a gas, the variation
of pressure with
height is negligible.
Pressure in a liquid
at rest increases
linearly with
distance from the
free surface.
The pressure is the
same at all points on
a horizontal plane in
a given fluid
regardless of
geometry, provided
that the points are
interconnected by
the same fluid. 38
Pascal’s law: The pressure applied to a confined fluid increases the
pressure throughout by the same amount.
where: A2/A1 is the ideal
mechanical advantage of the
hydraulic lift.
Lifting of a large weight by a
small force by the application of
Pascal’s law.
39
The Manometer
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.
The basic
manometer
.
Measuring the pressure
drop across a flow section
or a flow device by a
differential manometer.
40
In stacked-up fluid layers, the pressure change across a fluid
layer of density  and height h is gh.
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 pressure effect to an electrical
effect such as a change in voltage, resistance,
or capacitance.
•
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 deflect between two
chambers open to the pressure inputs.
•
Piezoelectric transducers: Also called solidstate pressure transducers, work on the
principle that an electric potential is generated in
a crystalline substance when it is subjected to
mechanical pressure.
Various types of Bourdon tubes used
to measure pressure.
42
THE BAROMETER AND ATMOSPHERIC PRESSURE
•
Atmospheric pressure is measured by a device called a barometer; thus, the
atmospheric pressure is often referred to as the barometric pressure.
•
A frequently used pressure unit is the standard atmosphere, which is defined as
the pressure produced by a column of mercury 760 mm in height at 0°C (Hg =
13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2).
The length or the
cross-sectional area
of the tube has no
effect on the height
of the fluid column of
a barometer,
provided that the
tube diameter is
large enough to
avoid surface tension
(capillary) effects.
The basic barometer.
43
Example: Measuring Atmospheric
Pressure with a Barometer
• 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.
SOLUTION
Patm = ρgh
= (13,570 kg/m³)(9.805 m/s²)(0.740 m)(1 N / 1 kg·m/s²)(1 kPa / 1000
N/m²)
= 98.5 kPa
EXAMPLE
Calculate the force due to the pressure acting on
the 1-m-diameter horizontal hatch of a
submarine submerged 600 m below the surface.
Solution:
END OF PART 1