thermo chapter 3.pptx

8/17/2019 thermo chapter 3.pptx

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Chapter 3:ENERGY, ENERGY

TRANSFER AND GENERALENERGY ANALYSIS

CHE 433 THERMODYNAMICS


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FORMS OF ENERGY

Energy can exist in numerous forms suc as terma!"mecanica!" #inetic" $otentia!" e!ectric" magnetic" cemica!"an% nuc!ear" an% teir sum constitutes te total energy, E of a system&

Termo%ynamics %ea!s on!y 'it te change of te tota!energy&

Macrocop!c forms of energy( Tose re!ate% to motion an%te in)uence of some externa! e*ects suc as gra+ity"magnetism" e!ectricity an% surface tension& E&g& #inetic an%$otentia! energies&

M!crocop!c forms of energy( Tose re!ate% to te

mo!ecu!ar structure of a system an% te %egree of temo!ecu!ar acti+ity& Internal energy, U( Te sum of a!! te microsco$ic forms

of energy&

, Te macrosco$ic energy of ano-.ect canges 'it +e!ocity an%


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"!net!c energy, "E( Te energy tat a system$ossesses as a resu!t of its motion re!ati+e tosome reference frame&

#otent!al energy, #E: Te energy tat a system$ossesses as a resu!t of its e!e+ation in agra+itationa! /e!%&

3

0inetic energy $er unitmass

0inetic energy

1otentia! energy $er unitmass

1otentia!energy


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4

Tota! energyof a system

Energy of a

system $er unitmass

Tota! energy$er unit mass

Mass )o'rate

Energy )o' rate

Cloe$ yte% remain stationary%uring a $rocess an% tus no cangein 1E an% 0E 2tat!onary yte%&

∆E ∆5Control &ol'%e in+o!+e )ui% )o' for!ong $erio%s of time& It is con+enientto ex$ress te energy )o' in te rate

form -y incor$orating te mass )o'rate (the amount of mass owing

agnetic" e!ectric an% surface tension e*ects are usua!!y ignore%&


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ENERGY TRANSFER (Y )EAT

6

Energy can cross te-oun%aries of a c!ose%system in te form ofheat an% *or+ &

Tem$erature %i*erence is te%ri+ing force for eat transfer& Te!arger te tem$erature %i*erence"te iger is te rate of eattransfer&

)eat( Energy tat istransferre% -et'een t'o

systems 2or a system an% itssurroun%ings -ecause of a


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7

During an a%ia-atic $rocess" asystem excanges no eat 'it itssurroun%ings&

Amount of eat transfer'en eat transfer ratecanges 'it time

Amount of eat transfer'en eat transfer rate is

constant

Heat transfer $er unitmass

Tere are t'o 'ays a $rocess can -ea%ia-atic(8 Te system is 'e!! insu!ate% so taton!y a neg!igi-!e amount of eat can

$ass troug te -oun%ary8 9ot system an% surroun%ings are atte same tem$erature 2no %ri+ing forcefor eat transfer&


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ENERGY TRANSFER (Y OR"

or+ ( Te energy transfer associate% 'it a force acting troug a%istance& A r!!ng p!ton, a rotat!ng ha-t, an% an electr!c *!re

cro!ng the yte% .o'n$ar!e are a!! associate% 'it 'or#interactions

For%al !gn con&ent!on( Heat transfer to a system and work doneby a system are positive; heat transfer from a system and work doneon a system are negative&

A!ternati+e to sign con+ention is to use te su-scri$ts in an% out toin%icate %irection&

:S$ecifying te%irections of eat an%

;or# %one$er unitmass

1o'er is te'or# %one$er unit time


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)eat &/ or+ 9ot are recogni=e% at te

-oun%aries of a system as teycross te -oun%aries& Tat is" -ot

eat an% 'or# are boundary$enomena& Systems $ossess energy" -ut not

eat or 'or#& 9ot are associate% 'it a

process, not a state& 5n!i#e$ro$erties" eat or 'or# as no

meaning at a state& 9ot are path functions 2i&e&" teir

magnitu%es %e$en% on te $atfo!!o'e% %uring a $rocess as 'e!!as te en% states&

1ro$erties are $oint functions>-ut eat an% 'or# are $atfunctions 2teir magnitu%es%e$en% on te $at fo!!o'e%&

1ro$erties are $oint functions a+eexact %i*erentia!s 2d &

1at functions a+e inexact%i*erentia!s 2δ


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MEC)ANICAL FORMS OF OR"

Tere are t'o re?uirements for a 'or# interaction-et'een a system an% its surroun%ings to exist( tere must -e a force acting on te -oun%ary& te -oun%ary must move.

@ Te 'or# %one is $ro$ortiona! to teforce a$$!ie% 2F an% te %istance

;or# orce × Distance

;en force is notconstant

If tere is nomo+ement" no 'or# is%one&


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Sha-tor+

Energy transmission troug rotating

safts is common!y encountere% in$ractice&

Saft 'or# is $ro$ortiona! tote tor?ue a$$!ie% an% tenum-er of re+o!utions of te

A force F actingtroug a momentarm r generates a

tor?ue T Tis force acts troug a %istance

s

Te $o'er transmitte% troug te

saft is te saft 'or# %one $erunit time

Saft'or#


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E0a%ple 1

Determine te $o'er transmitte% troug tesaft of a car 'en te tor?ue a$$!ie% is , N&man% te saft rotates at a rate of 4

re+o!utions $er minute 2r$m&

BB

( ) ( )4000 1min 1

2 2 200min 60 1000

83.8

sh

kJ W nT N m

s N m

kW

π π = = × ÷ ÷ ÷×

=


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pr ngor+

E!ongationof a s$ringun%er tein)uence

of a force&

;en a force is a$$!ie% on a s$ring"te !engt of te s$ring canges&

or !inear e!astic s$rings" te%is$!acement x is $ro$ortiona! to te

force a$$!ie% k s$ring constant2#Nm

Su-stituting an% integratingyie!%

x B an% x ,( te initia! an% te

/na! %is$!acements

Te %is$!acement of a!inear s$ring %ou-!es'en te force is

;en te !engt of te s$ringcanges -y

a %i*erentia! amount dx un%er tein)uence of a force F " te 'or# %oneis


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E0a%ple 2

Determine 'or# re?uire% 2in #F to com$ress as$ring 'ose s$ring constant is 3 #Nm a%istance of 3 cm from its un!oa%e% !engt&

B3

( )

( )

2 2 2 2 2

2 1

1 1 1

2 2

2 2

3000.03 0

20.135

10.135 0.135

1

x xW Fds kxdx k xdx k

kN mm

kN m

kJ kN m kJ

kN m

= = = = − ÷

= − = ×

= × = ÷×

∫ ∫ ∫

+ D


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or+ Done onElat!c Sol!$(ar

So!i% -ars-ea+e ass$ringsun%er tein)uence of

a force& Stretcing a !i?ui% /!m 'it amo+a-!e 'ire&

A cross sectiona! areaσ norma! stress 2as $ressureunits

orAoc!ate$ *!ththe Stretch!ng

o- a L!'!$ F!l%

2

2

surface tension force per unit length

s

s

dA b dx

F bσ

σ

==

=

+ D t R ! t


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or+ Done to Ra!e or toAccelerate a (o$yB& Te 'or# transfer nee%e%

to raise a -o%y is e?ua! tote cange in te$otentia! energy of te-o%y&

,& Te 'or# transfer nee%e%

to acce!erate a -o%y ise?ua! to te cange in te#inetic energy of te -o%y&

Te energy

transferre% to a-o%y 'i!e -eingraise% is e?ua! tote cange in its$otentia! energy&


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E0a%ple 3

Determine te energy re?uire% to acce!erate an


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E0a%ple 4

A %amage% B, #g car is -eing to'e% -y atruc#& Neg!ecting te friction" air %rag" an% ro!!ingresistance" %etermine te extra $o'er re?uire%

2a for constant +e!ocity on a !e+e! roa%2- for constant +e!ocity of 6 #m on a 3o 2from ori=onta! u$i!! roa%2c to acce!erate on a !e+e! roa% from sto$ to @#m in B, s&

B:


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B<

( )

( ) ( )

2 1

2

2 2

Power require is the sum of the rates of changes in potential an !inetic energies

" #

" # 0

sin30

50000 11200 9.81

3600 1000

total a g

a

o

total g

W W W

a zero

b W z

W W mg z z t mg mgV t

m kJ kg kg m s

s m s

= +

=∆

= = − ∆ = =∆

= ÷

( )

( ) ( ) ( )2

2 2

2 1 2 2

0.5 81.$

" # 0

1 1 90000 11200 0 12 31.3

2 2 3600 1000

g

total a

kW

c W

m kJ kg W W m V V t kg s kW

s m s

=÷

= = = − ∆ = − = ÷ ÷ ÷ ÷


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Non5%echan!cal For%o- or+

Electr!cal *or+ ( Te genera!i=e% force is tevo!tage 2te e!ectrica! $otentia! an% tegenera!i=e% %is$!acement is te e!ectrica! charge.

Magnet!c *or+ ( Te genera!i=e% force is temagnetic "e!d strength an% te genera!i=e%%is$!acement is te tota! magnetic dipo!emoment .

Electr!cal polar!6at!on *or+ ( Te genera!i=e%force is te e!ectric "e!d strength an% tegenera!i=e% %is$!acement is te po!ari#ation ofthe medium.

B@


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,

Electr!cal or+

E!ectrica! $o'er in terms ofresistance $" current %" an%$otentia! %i*erence 7&

E!ectrica! 'or#

E!ectrica!

$o'er

;en $otentia! %i*erencean% current cange 'ittime

;en $otentia!%i*erence an% current

remain constant

N e!ectrica!carge )o'


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T)E FIRST LA OFT)ERMODYNAMICS The rst law of thermodynamics 8the conservation of energy

principle9 $ro+i%es a -asic to stu%y te re!ationsi$s among +ariousforms of energy an% energy interactions&

Te /rst !a' states tat energy can be neither created nordestroyed during a process; it can only change forms.

The First Law or a!! a%ia-atic $rocesses -et'een t'o s$eci/e%

states of a c!ose% system" te net 'or# %one is te same regar%!essof te nature of te c!ose% system an% te %etai!s of te $rocess&

,B

Energycannot -ecreate% or%estroye%>it can on!ycangeforms&

Te increase in te energy of a$otato in an o+en is e?ua! to teamount of eat transferre% to it&


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,,

In te a-sence of any'or# interactions" teenergy cange of asystem is e?ua! to tenet eat transfer&

Te 'or#2e!ectrica!%one on ana%ia-atic

system is e?ua!to te increasein te energyof te system&

Te 'or#2saft %oneon ana%ia-aticsystem ise?ua! to teincrease in teenergy of tesystem&

Te energy cange of asystem %uring a $rocess ise?ua! to te net 'or# an%eat transfer -et'een te


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Energy (alance

&he net change (increase or decrease) in the tota! energy ofthe system during a process is e'ua! to the dierencebetween the tota! energy entering and the tota! energy!eaving the system during that process.

,3


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Energy Change o- a Syte%,Eyte%

,4

Energy can exist in numerous

forms suc as interna!" #inetic"$otentia!" e!ectric" magnetican% te sum constitute tetota! energy of te system" E&In te a-sence of e!ectric an%

magnetic e*ects"


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Mechan!% o- EnergyTran-er, E!n an$ Eo't

,6

Heat transfer"G

;or#transfer" ;

Mass )o'" m

or constant rates"

In te rate form"

Energy

-a!ance $erunit mass


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,7

Te energy content of a

contro! +o!ume can -ecange% -y mass )o' as'e!! as eat an% 'or#interactions&

or a cyc!e * " tus + * &

A c!ose% system %oesnot in+o!+e any mass

)o'" on!y eat transferan% 'or#&

Energy -a!ance for a cyc!e 2c!ose%system

;net"out ;out 8 ;in Gnet"in Gin 8 Gout


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1RE1ARED 9Y(NORASMAH MOHAMMED MANSHORAC5TY O CHEMICA ENJINEERINJ" 5iTM SHAH AAM&B@,37

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