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MODULE THREE: SECOND LAWOF THERMODYNAMICS

Objectives

1. To introduce second law ofthermodynamics

2. To consider corollaries of the second

law of thermodynamics3. To consider applications of the

second law of thermodynamics

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Introdction

It is ! "!tter o# ever$d!$e%&erience t'!t t'ere is ! de(nitedirection #or spontaneous

processes

Spontaneous processes arenatural

occurrence processes with thetendency

of attaining equilibrium

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%!"& es o &on !neos*rocesses

An hot object at temperature Ti placed in an

environment of temperature To get cools to aTemperature T and eventually has a temperatureTo.

n conformity with the conservation of energy

principle! internal energy lost by the object e"ualsthe internal energy gained by the environment.

Inverse process would not ta#e placespontaneously

i.e nternal energy of the

environment would not decrease

spontaneously! while the object

warmed from TotoTi

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E%!"&)es o# S&ont!neos *rocesses

Air having high pressure $iin a closed

container would %ow spontaneously tothe lower pressure $oof the surrounding!once the container is opened.

&low of air ceases once the air pressuree"uals the pressure of the surroundings.

Inverseprocess would not ta#e place

spontaneouslyi.e air would not %ow spontaneously

from the

surrounding at $ointo the

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%!"& es o &on !neos*rocesses

A mass suspended by a cable at

elevation 'iwould fall when released. (hen it comes to rest! initial potential

energy lost by the mass e"uals internal

energy gained by both the mass and thesurroundings. Inverseprocess would not ta#e placespontaneously

i.e the mass would not returnspontaneously to its initial elevation! while its internal

energy or that

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E%!"&)es o# S&ont!neos*rocesses

n each case described above! initialcondition of the system could berestored! but not in a spontaneous

process.

Auxiliary devices would be re"uired.

These devices would re"uired energyinput to perform! resulting into

permanentchange in the condition of

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Introdction Spontaneous processes can proceed only in

a particular direction. The rst law ofthermodynamics gives no information

about direction; it states only that whenone form of energy is converted into

another, identical quantities of energy areinvolved regardless of the feasibility of the

process.

First law fails to address both the directionof the process and the extent of change ofenergy from one form to another !ence,

another general principle is required

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Introdction

"an one form of energy becompletely converted to anotherform# n practice! when energy istransferred from one form to

another! there is often adegradation of the supplied energyinto a less useful form.

)econd Law of Thermodynamicsaddresses some of these

di*culties

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Co""on)$ Used Ter"s Heat (thermal) reservoir is a su*ciently

large system in stable e"uilibrium to whichand from which +nite amounts of heat can betransferred without any change in itstemperature

A high temperature heat reservoir from whichheat is transferred is sometimes called a

heat source $eg furnace%.

A low temperature heat reservoir

from which heat is transferred is

sometimes called a heat sin& $eg

atmosphere%.

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Co""on)$ Used Ter"s

Work reservoir is a su*ciently largesystem in stable e"uilibrium to which andfrom which +nite amounts of wor# can betransferred adiabatically without any

change in its pressure

Thermodynamic cycle: A system hascompleted a thermodynamic cycle when the

system undergoes a series of processes andthen returns to its original state! so that theproperties of the system at the end of thecycle are the same as at its beginning.

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Co""on)$ Used Ter"s A reversib)e &rocessis one in which both the

system and its environment can be returned toexactly the states they were in before theprocess occurred.

He!t En+ine: A device or machine thatproduce wor# from heat in a cyclic process. ,.g.)team power plant in which the wor#ing %uid-2/0 periodically returns to its original state.

A thermodynamic system operating in athermodynamic cycle to which net heat istransferred and from which net wor# is delivered.

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Co""on)$ Used Ter"s

'or&ing substance of the engine isthe material within the engine thatactually does the wor#.

,xamples include steam in steampower plant and the gasolineairmixture in an automobile engine.

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St!te"ent , o apparatuscan operate insuch a way that

its only eect -insystem andsurroundings0 isto convert heatabsorbed by asystemcompletely into

wor& done by

St!te"ent -

o process ispossible solely inthe transfer ofheat from onetemperature

level to a higherone

St!te"ents o# t'e Second L!. o#r"od$n!"ics

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St!te"ent , c!n be e%&ressed in!not'er #or" !s:

St!te"ent ,!t is impossible by a cyclic process

to convert the heat absorbed by

a system completelyinto wor#done by the system

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C)!sis s!$s 4t will arouse other changeswhile the heat is transferred from

the low temperature object to thehigh temperature one5

6elvin says4t will arouse other changes while

the heat from the single thermalsource is ta#en out and is totally

changed into wor#.5

)econd Law of Thermodynamics

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)econd Law of Thermodynamics

/e)vin0*)!nc1 St!te"ent: t is impossible for any device that

operates on a cycle to receive heatfrom a single reservoir and

produce a net amount of wor#. o heat engine can have a

thermal e*ciency of 1778.

&or a power plant to operate! thewor#ing %uid must exchange heatwith the environment as well asthe furnace.

t , i

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eat ,ngine (or# can be easily converted completely to other forms

of energy

9onverting other forms of energy to wor# is not thateasy

(or# can be converted to heat directly and completely

eat can be converted to wor# directly! but not

completely 9onverting heat to wor# re"uires the use of a device

called a heat engine

eat engines come in many forms! pure heat engines

-steam power plants0 and semi heat engines -gas turbines0

,ach heat engine operates by using

a wor&ing substance $(uid%

i

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eat ,ngines They receive heat from a high

temperature source -eg. &urnace0.

They convert part of this heat

to wor# -mostly through

rotating shaft0. They reject the waste heat to

lowtemperature sin#.

-eg. Atmosphere! :iver0. They operate on a cycle

$urposely built to convert heat

to wor#.

i

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eat ,ngine;in < amount of heat

supplies to steam inboiler from hightemperature source-furnace0

;out < amount of heatrejected from steam incondenser to a lowtemperature sin#

(out < amount of wor#delivered by steam as itexpands in turbine(in < amount of wor#

re"uired to compresswater to boiler ressure

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eat ,ngine ,*ciency

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C)!ss Wor1

1. (hich of the following heat engineconditions

is most preferable>

a0 absorbed heat of 377 = and discarded?7 = of heat

b0 absorbed heat of 3?7 = and 377 = ofwor# done

c0 @77 = of wor# done and 177 = of heatdiscarded.

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C)!ss Wor1

2. A Carnot engine receives 250 kJ/s of heat

from a heat-source reservour at 525 C and

rejects heat to a heat-sink reservour at 50 C.

What are the poer deve!oped and the heat

rejected"

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)olution

,*ciency of 9arnot eat ,ngine!

< where

< ?7 C # $50 % 2&'( ) # '2' ) and< 2?2 C # $252 % 2&'( ) # 525 )

*o < < < 7.3@B

E2cienc$ o# C!rnot He!t En+ine3 4567898

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E%ercises

1. Describe a process that would satisfythe conservation of energy principle!but does not actually occur in nature.

2. To increase the thermal e*ciency of areversible power cycle operatingbetween thermal reservours attemperatures Tand T9! would it bebetter to increase Tor decrease T9bye"ual amountsE

3. (hat is the thermal e*ciency of anheat en ine that absorbed 3B@ # and

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E%ercises

@.

/btain and

R # i t d H t

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Re#ri+er!tors !nd He!t*"&s

eat moves in nature from hightemperatures to lower temperatures! nodevices re"uired.

The reverse process! heat from lowtemperatures to high temperature! re"uires

special devices called refrigerators

or heat pumps

(or#ing %uid used in refrigeration cycle is called a refrigerantT'e objective o# re#ri+er!tor is to

re"ove 'e!t #ro" t'e re#ri+er!ted s&!ce

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*"&s

The refrigerant enters the compressor as

vapour and compressed to condenser pressure t condenses as it %ow through the coils of the

condenser by rejecting heat to

the surrounding medium ts pressure and temp.

drop drastically as it

expands in capillary tube.

t evaporates in evapo

rator by absorbing heat

from the refrigerated space.

e r +er! ors !n e!

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e r +er! ors !n e!*"&s

n household refrigerator! the freeGer

compartment -where heat is absorbed bythe refrigerant0 is the evaporator.

9oils behind the refrigerator -where heat

is dissipated to surroundings0 is thecondenser.

n a refrigerator,the interiorof the unit is the cold reservour!

while the warmer exterior

is the hot reservour.

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*"&

n the refrigeration process! wor# 'is usedto remove heat )9from the cold reservoir

and de osit heat into the hot reservoir.

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Coe2cient o# *er#or"!nce CO*;

"oe*cient of +erformance!

< a0 < 1?77 #=! < 1?7 #=.b0 < 1@77 #=! < 1J77 #=.

c0 < 1J77 #=! < @77 #=.

d0 < ?.

) l ti

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)olution / '20 )

# 2+0 )

t is reversible! if