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Chapter 2Energy, Work and Heat
(Tenaga, Kerja & Haba)
OlehLt Kol Prof Madya Ir Khalid bin Abd Jalil TUDM
Jabatan Kejuruteraan Mekanikal
Universiti Pertahanan Nasional Malsysia.
012-2094234
Duty, Honor and
Integrity
Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
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Duty, Honor and
Integrity
Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
Objectives
Introduce the concept of energy and define its various forms. Discuss the nature of internal energy.
Define the concept of heat and the terminology associated with energy
transfer by heat.
Discuss the three mechanisms of heat transfer: conduction, convection,
and radiation.
Define the concept of work, including electrical work and several forms of
mechanical work.
Introduce the first law of thermodynamics, energy balances, and
mechanisms of energy transfer to or from a system.
Determine that a fluid flowing across a control surface of a control volumecarries energy across the control surface in addition to any energy transfer
across the control surface that may be in the form of heat and/or work.
Define energy conversion efficiencies.
Discuss the implications of energy conversion on the environment.
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Introduction to Energy
Define as capacity to do work and an important part of daily life.
is an extensive properties which have an ability to change the state of systemand it surrounding.
1st Law of Thermodynamics , energy conservation principle (Prinsipkeabadian tenaga).
Under the 1st law,, energy cannot be created or destroyed during theprocess; it can only changed from one form to another form (work is done).
Forms of energy i.e. Electrical energy, potential , chemical & kinetic.
Example: place a refrigerator in a well insulated room with its open. What willhappen to the room temperature?? Increasing or decreasing??
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
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INTRODUCTION If we take the entire roomincluding the air and the refrigerator (or fan)as
the system, which is an adiabatic closed system since the room is well-sealed
and well-insulated, the only energy interaction involved is the electrical energycrossing the system boundary and entering the room.
As a result of the conversion of electric energy consumed by the device to
heat, the room temperature will rise.
A refrigerator
operating with its
door open in a well-
sealed and well-
insulated room
A fan running in awell-sealed and
well-insulated room
will raise the
temperature of air in
the room.
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5
FORMS OF ENERGY
Energy can exist in numerous forms such as thermal, mechanical, kinetic,
potential, electric, magnetic, chemical, and nuclear, and their sum
constitutes the total energy, Eof a system. Thermodynamics deals only with the changeof the total energy.
Macroscopic forms of energy: Those a system possesses as a whole
with respect to some outside reference frame, such as kinetic and potential
energies.
Microscopic forms of energy: Those related to the molecular structure ofa system and the degree of the molecular activity.
Internal energy, U:The sum of all the microscopic forms of energy.
The macroscopic energy of
an object changes withvelocity and elevation.
Kinetic energy, KE: The energy that
a system possesses as a result of itsmotion relative to some reference
frame.
Potential energy, PE: The energy
that a system possesses as a result
of its elevation in a gravitational field.
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Forms of Energy conts.Total energy of a system
Kinetic energy (KE) related to motion and the influence ofsome external effects such as gravity, magnetism, electricity and
surface tension
Unit mass basis (kJ/kg)
(kJ)
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Chapter 2- Energy, Work and Heat
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Forms of Energy cont.
Potential Energy (PE)a result of its elevation in agravitational field.
Unit mass
basis
(kJ)
(kJ/kg)
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Chapter 2- Energy, Work and Heat
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Forms of Energy cont.
Total energy system is expressed as
E = U + +
e = u + +
Note: most of closed system remain stationary during process and thusexperience no change on their kinetic and potential energies.
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Chapter 2- Energy, Work and Heat
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Total energy
of a system
Energy of a system
per unit mass
Potential energy
per unit mass
Kinetic energyper unit mass
Potential energy
Total energy
per unit mass
Kinetic energy
Mass flow rate
Energy flow rate
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Mechanical EnergyMechanical energy:The form of energy that can be converted to
mechanical work completely and directly by an ideal mechanical device such
as an ideal turbine.Kinetic and potential energies: The familiar forms of mechanical energy.
Mechanical energy of a
flowing fluid per unit mass
Rate of mechanical
energy of a flowing fluid
Mechanical energy change of a fluid during incompressible flow per unit mass
Rate of mechanical energy change of a fluid during incompressible flow
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Internal energy U (KJ)the sum of all microscopic forms of energy of asystem. It is related to the molecular structure and the degree of molecular activity
and may be viewed as the sum of kinetic and potential energies of the molecules; it is
comprised of the following types of energies
Forms of Energy cont.
Type Composition of Internal Energy (U)
Sensible energy (haba
rasa)
the portion of the internal energy of a system associated with kinetic
energies (molecular translation, rotation, and vibration; electron translation
and spin; and nuclear spin) of the molecules.
Latent energy (haba
pendam)the internal energy associated with the phase of a system.
Chemical energy the internal energy associated with the atomic bonds in a molecule.
Nuclear energythe very large amount of energy associated with the strong bonds within the
nucleus of the atom itself.
Energy interactions
those types of energies not stored in the system (e.g. heat transfer, mass
transfer, and work), but which are recognized at the system boundary as they
cross it, which represent gains or losses by a system during a process.
Thermal energy the sum of sensible and latent forms of internal energy.(cannot be convertedto work directly and completely)
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Chapter 2- Energy, Work and Heat
http://en.wikipedia.org/wiki/States_of_matterhttp://en.wikipedia.org/wiki/Chemical_bondshttp://en.wikipedia.org/wiki/Nuclear_energyhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Mass_transferhttp://en.wikipedia.org/wiki/Mass_transferhttp://en.wikipedia.org/wiki/Work_(thermodynamics)http://en.wikipedia.org/wiki/Thermodynamic_systemhttp://en.wikipedia.org/wiki/Thermodynamic_systemhttp://en.wikipedia.org/wiki/Work_(thermodynamics)http://en.wikipedia.org/wiki/Mass_transferhttp://en.wikipedia.org/wiki/Mass_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Nuclear_energyhttp://en.wikipedia.org/wiki/Chemical_bondshttp://en.wikipedia.org/wiki/States_of_matter7/29/2019 Chapter2 Energy
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Some Physical Insight to Internal Energy
The internal energy of a
system is the sum of all forms
of the microscopic energies.
The various forms of
microscopic
energies that make
up sensible energy.
Sensible energy:The portion
of the internal energy of a
system associated with the
kinetic energies of the
molecules.
Latent energy: The internal
energy associated with the
phase of a system.Chemical energy: The internal
energy associated with the
atomic bonds in a molecule.
Nuclear energy: The
tremendous amount of energy
associated with the strongbonds within the nucleus of the
atom itself.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + Latent
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Forms of Energycont.
The internal energy U (KJ) is essentially defined by the first law ofthermodynamics which states that energy is conserved:
Where
U= Q + W + W
Uis the change in internal energy of a system during a process.
Q is heatadded to a system (measured injoules in SI); that is, a positivevalue forQ represents heat flow into a system while a negative value
denotes heat flow out ofa system.
Wis the mechanical workdone on a system (measured in joules in SI)
W'is energy added by all other processes
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Chapter 2- Energy, Work and Heat
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Chapter 2- Energy, Work and Heat
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Chapter 2- Energy, Work and Heat
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Energy can cross the boundary of closed system in 2 distinct
forms:
Heat & WorkHeat energy (Q) crosses the boundary due to the
temperature difference between a system and its
surroundings.
Work energy (W) crosses the boundary under the
action of a forces. Work can be thought of as the
energy expended to lift a weight.
Sign Convention
Heat transfertoa system and work done bysystem are positive.
Heat transferfroma system and Work done on a system are negative.
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Heat Q [J, kJ] (Haba)Heat defined as the form of energy that transferred
between two system by virtue of a temperature
difference.
heat is energy transition (crosses the
boundary) heat is not a properties.
heat is about the state of process.
(depend on process path between end of
process)
Haba dibekalkan kpd
sistem oleh sekitaran (Q>0)
Haba yang disingkirkan oleh
sistem kpd sekitaran. (Q
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
Heat transfer mechanisms:
Conduction:The transfer of energy from the more energetic particles of a
substance to the adjacent less energetic ones as a result of interaction between
particles.
Convection:The transfer of energy between a solid surface and the adjacent fluid
that is in motion, and it involves the combined effects of conduction and fluid
motion.
Radiation:The transfer of energy due to the emission of electromagnetic waves
(or photons).
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Mechanisms of Heat Transfer
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
1. Conduction through Plane Walls
- Conduction heat transfer is progressive exchange or
energy between the molecules of substance
Note: thermal conductivity value can be refer in table 2-3 page 98
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Mechanisms of Heat Transferconts.
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
2. Convection Heat Transfer- Convection heat transfer is a mode of energy transfer
between a surface and the adjacent liquid or gas that is
in motion and it involves the combined effects of
conduction and fluid motion.
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Mechanisms of Heat Transferconts.
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Chapter 2- Energy, Work and Heat
3. Radiation Heat Transfer
- Radiative heat transfer is energy in transition from the
surface of one body to the surface of another due to
electromagnetic radiation.
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Adiabatic Process a process during which there is no
heat transfer (Q = 0)
well insulated (negligible amount of heat can passthrough the boundary)
Tsystem= Tsurrounding
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
Adiabatic System although there is no
heat transfer during the process, energy
content and temp. of the system can stillchanged by other mean such as work.
Example: see figure 2-40 Page71
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Heat transfer perunit mass (kJ/kg) of
a system is donated q :
(kJ/kg)
Heat transfer perunit time
Amount of heat transferred during
the processes (dependent on the
process path)
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Chapter 2- Energy, Work and Heat
Th d i I (EMM2503)
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
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Chapter 2- Energy, Work and Heat
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ENERGY TRANSFER BY WORK Work:The energy transfer associated with a force acting through a distance.
A rising piston, a rotating shaft,andan electric wire crossing the
system boundaries are all associated with work interactions Formal sign convention:Heat transfer to a system and work done by a
system are positive; heat transfer from a system and work done on a systemare negative.
Alternative to sign convention is to use the subscripts inand outto indicatedirection. This is the primary approach in this text.
Specifying the directions
of heat and work.
Work done
per unit mass
Power is the
work done per
unit time (kW)
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Heat vs. Work Both are recognized at the boundaries of
a system as they cross the boundaries.
That is, both heat and work are boundary
phenomena.
Systems possess energy, but not heat or
work.
Both are associated with aprocess, not a
state.
Unlike properties, heat or work has nomeaning at a state.
Both arepath functions(i.e., their
magnitudes depend on the path followed
during a process as well as the end
states).
Properties are point functions
have exact differentials (d).
Path functions
have inexact
differentials ()
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Electrical Work (Kerja Elektrik)
- in electric field, electrons in a wire move under the
effect of electromotive forces, doing work.
Electrical power W = VI = IR = V/R
-Unit : watt/ kW.
V = voltage
I = current
t = time interval
(kJ)Electrical work
done
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Note: (W0) kerja dilakukan oleh sistem
Mechanical work (Joule / Nm)
- Related to a force acting through a distance
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Chapter 2- Energy, Work and Heat
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Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
Step 1: A car is to climb a hill in 12 s. The power needed is to be determined for three different cases.
Step 2: Draw the diagram.
Step 3 : Assumptions: Air drag, friction, and rolling resistance are negligible.
Step 4 Analysis The total power required for each case is the sum of the rates of changes inpotential and kinetic energies. That is,
Step 5 Properties g=9.81m/s
Step 6 Calculation:
(a) =0 since the velocity is constant. Also, the vertical rise is h = (100 m)(sin 30) = 50 m. Thus, &Wa=0
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Chapter 2- Energy, Work and Heat
Step7 Discussion:
The car required 90.1 kW of power to climb the 30 hill in 12s and need to
reduced their power to -57.5 kW to achieve velocity 5 m/s.
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Shaft
Work
A force Facting through
a moment arm r
generates a torque T
This force acts through a distance s
The power transmitted through the shaft
is the shaft work done per unit time
Shaft
work
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Spring Work
Elongation
of a spring
under the
influence of
a force.
When the length of the spring changes by
a differential amount dxunder the influence
of a force F, the work done is
For linear elastic springs, the displacement
xis proportional to the force applied
k: spring constant (kN/m)
Substituting and integrating yield
x1 andx2: the initial and the final
displacements
The
displacement
of a linear
spring doubles
when the force
is doubled.
Work Done on Elastic Solid Bars
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Work Done on Elastic Solid Bars
Work Associated with the Stretching of a Liquid Film
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GravitationalWork
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y ( )
Chapter 2- Energy, Work and Heat
W k D t R i t A l t B d
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Work Done to Raise or to Accelerate a Body
Nonmechanical Forms of Work
1. The work transfer needed to raise a body is equal to
the change in the potential energy of the body.
2. The work transfer needed to accelerate a body is
equal to the change in the kinetic energy of the body.
Electrical work: The generalized force is
the voltage(the electrical potential) and thegeneralized displacement is the electrical
charge.
Magnetic work: The generalized force is
the magnetic field strengthand the
generalized displacement is the total
magnetic dipole moment.
Electrical polarization work: The
generalized force is the electric field
strengthand the generalized displacement
is thepolarization of the medium.
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y ( )
Chapter 2- Energy, Work and Heat
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Thermodynamics I (EMM2503)
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The 1st Law Thermodynamics
Before.. Learned about heat Q, work W, and total
energyE,
No attempt is made to relate them to each other during
the process. Under the 1st law thermodynamics, its cover the
relationships among the various form of energy and
energy interactions.
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The 1st Law Thermodynamics conts.
known as the conservation of energy principle,
also called energy balance.
Under the 1st Law of Thermodynamics, energy
can be either created nor destroyed during a
process; it can only change forms. Thereforeevery bit of energy should be accounted for
during a process.
Such as falling rock (combination of potential
energy and kinetic energy)
1st law statement is the existence of theproperty total energyE.
Example, potato baked, blowing water, well
insulated room heated by heater.
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The 1st Law Thermodynamics conts.
1st law statement is the existence of the property total
energyE.
Example, potato baked, blowing water, well insulated
room heated by heater.
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Chapter 2- Energy, Work and Heat
Potato
E=5kJ
Qin=5kJ
The increase in the energy of a
potato in a oven is equal to the
amount of heat transferred to it.
E=Q net= 12kJ
In the absence if any work
interactions, the energy change
of a system is equal to net the
heat transfer.
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Thermodynamics I (EMM2503)
Ch t 2 E W k d H t
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Chapter 2- Energy, Work and Heat
Energy balance:
The net change in total energy of the system during a process is equal to
the difference between the total energy entering and the total energy
leaving the system during that process.
Total energy entering
the system
Total energy leaving
the system
Change in the total
energy of the system=-
Esystem = Ein - Eout
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Chapter 2- Energy, Work and Heat
Energy Change of a System:
Esystem
(perubahan Tenaga Sistem)
Energy change of a system during a process involves the evaluation of
the system at the beginning and at the end of the process.
Energy at final state Energy at initial state Energy Change=-Esystem = Efinal - Einitial
Total energy of a system
Net energy transfer by heat,
work & mass
Change in internal,
kinetic, potential, etc
energies
For stationary system, the change in kinetic and potential
energy is zero.
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Mechanisms of Energy Transfer, Ein and Eout(Mekanisme Pemindahan Tenaga)
Energy can transfer in 3 forms, heat, work and mass flow.
heat transfer heat transfer to a system (heat gain) will increases the
energy and heat transfer from the system will decreases (heat loss) the
energy.
Work transfer work transfer to the system (work done on the system) will
increases the energy and work transfer from the system (work done by the
system) will decreases the energy of the system. For example car engine,
steam, turbine.
Mass flow an additional mechanisms of energy transfer.
Mass flow in increases the energy and mass flow out decreases the
energy of the system.
Ein Eout = Qin - Qout + W in - Wout + Emass in Emass out =E system
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Chapter 2- Energy, Work and Heat
Mechanisms of Energy Transfer, Ein and Eout.conts.
Mass in
Mass in
Esystem = Ein - EoutNet energy transfer by heat,
work & mass
Change in internal,
kinetic, potential, etc
energies
E system = Ein Eout
= Qin - Qout + W in - Wout + Emass in Emass out
E system
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See problem No 2-50 page 101
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Chapter 2- Energy, Work and Heat
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Chapter 2 Energy Work and Heat
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See problem No 2-51 page 101
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Chapter 2- Energy, Work and Heat
Thermodynamics I (EMM2503)
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Chapter 2- Energy, Work and Heat
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Energy conversion efficienciesEfficiencies is the ratio between the useful output of an energy conversion machine
and the input, in energy terms.
The useful output may be electric power, mechanical work, or heat.
Energy conversion efficiency is not defined uniquely, but instead depends on the
usefulness of the output. All or part of the heat produced from burning a fuel may
become rejected waste heat if, for example, work is the desired output from a
thermodynamic cycle.Indicate how well the energy transfer process is accomplished.
Performance =Desired output
Required input
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Efficiencies of Mechanical and Electrical Devices
Transfer of mechanical energy through
a rotating shaft, often refer to as shaft
work, ie water pump, fan, turbine.
Performance =
Mechanical energy output
Mechanical energy input
Transfer of electrical energy is commonly
converted to a rotating mechanical energy by
electrical motor to drive fans, compressors, robot
arms, car starters and so forth.
Performance =Mechanical energy output
Electrical power input
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See problem No 2-53 page 101
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Chapter 2- Energy, Work and Heat
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See problem No 2-53 page 101
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Chapter 2 Energy, Work and Heat
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Chapter 2 Energy, Work and Heat
Energy and Environment
The conversion of energy from one form to another often affects the environment.
For example fossil fuel such as coal, oil and natural gas have been used to
powering the industrial development since 1700s.
Pollutants emitted during the combustion of the fossil fuel will produced
environment pollution such as smog, acid rain and global warming and climatechange.
The air pollution has been the cause of human health problem such as lung and
heart diseases.
To control the environment pollution, the Clean Air Act of 1970 (USA) have been
introduced. Under the act, emission limits for hydrocarbons (HA), nitrogen oxides
(NO3) and carbon monoxide (CO) have been reduced form 5 gpkm (grams per
kilometer) in 1970 to 0.25 gpkm in 1980 and about 0.06 gpkm in 1999.
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Chapter 2 Energy, Work and Heat
Smog in Schanghai, Dezember 1993Smog in So Paulo
Smog is a kind of air pollution; the word "smog" is
a portmanteau of smoke and fog.
The dark yellow or brown haze that builds up in alarge stagnant air mass and hangs over populated
area on calm hot summer day.
Beijing air on a day after rain (left) and a smoggy day (right)
Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
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Integrity
Chapter 2 Energy, Work and Heat
Ozone
ground level layer in the
stratosphere that protects the
earth from suns harmful
ultraviolet's rays.
- Cause irritates eyes, headaches,fatigue, shortness of breath and
damages the air sacs in the lungs
where oxygen and carbon dioxide
are exchanged (hardening the soft
and spongy tissue in the lungs)
-
Thermodynamics I (EMM2503)
Chapter 2- Energy, Work and Heat
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Duty, Honor and
Integrity
p gy,
Greenhouse Effect