Chapter2 Energy

7/29/2019 Chapter2 Energy

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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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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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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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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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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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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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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


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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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_matter


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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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http://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Joulehttp://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/Positivehttp://en.wikipedia.org/wiki/Negativehttp://en.wikipedia.org/wiki/Mechanical_workhttp://en.wikipedia.org/wiki/Mechanical_workhttp://en.wikipedia.org/wiki/Negativehttp://en.wikipedia.org/wiki/Positivehttp://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/Joulehttp://en.wikipedia.org/wiki/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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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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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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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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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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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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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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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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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 ( )


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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WorkDuty, Honor and

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y ( )




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http://upload.wikimedia.org/wikipedia/commons/d/dc/John_Deere_3350_tractor_cut_transmission.JPGhttp://upload.wikimedia.org/wikipedia/commons/d/dc/John_Deere_3350_tractor_cut_transmission.JPG


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http://upload.wikimedia.org/wikipedia/commons/b/b5/Helicopter_Bristol_171_Sycamore_main_gear_box_and_rotor_head.jpg


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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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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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Ch t 2 E W k d H t


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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


Ch t 2 E W k d H t


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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.


Ch t 2 E W k d H t


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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


Ch t 2 E W k d H t


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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


Chapter 2 Energy Work and Heat


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See problem No 2-50 page 101

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See problem No 2-53 page 101Duty, Honor and

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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


Chapter 2- Energy Work and Heat
http://upload.wikimedia.org/wikipedia/commons/0/09/Efficiency_diagram_by_Zureks.svg


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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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Chapter 2 Energy, Work and Heat




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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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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)


http://upload.wikimedia.org/wikipedia/en/a/a4/Beijing_smog_comparison_August_2005.pnghttp://upload.wikimedia.org/wikipedia/en/a/a4/Beijing_smog_comparison_August_2005.pnghttp://upload.wikimedia.org/wikipedia/en/a/a4/Beijing_smog_comparison_August_2005.pnghttp://upload.wikimedia.org/wikipedia/commons/4/4a/Sha1993_smog_wkpd.jpghttp://upload.wikimedia.org/wikipedia/commons/6/6b/Zona_Leste_-_S%C3%A3o_Paulo-Brasil.jpg


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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)

-




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p gy,

Greenhouse Effect

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Chapter2 Energy