Laws Thermodynamics

7/25/2019 Laws Thermodynamics

1/77

What is Thermodynamics?

Thermodynamics is the branch of science

which deals with energy transfer and its

effect on the state or condition of a system.Essentially, thermodynamics pertains to the

study of :

Interaction of system and surroundings; it

relates the changes which the system

undergoes to the influences to which it is

put.


2/77

Energy and its transformation; energyintercon!ersions in the form of heat and

wor". #ames a #oule has pro!ed with his

well"nown e$periments with churnerthat mechanical wor" can be con!erted

into heat energy.


3/77

%elationship between heat, wor", and

physical properties such as pressure,

!olume and temperature of the wor"ingsubstance employed to obtain energy

con!ersion

&easibility of a process and concept ofe'uilibrium processes.


4/77

Thermodynamics the study of heat and related 'uantities

system is some part of the world in which we are

interested and plan to describethermodynamically.E$. a piece of matter, a reaction !essel and its contents,or an engine.

surroundings is the rest of the world outside the system.

state of the system is determined when we specify!aluesfor the minimum number of intensi!e !ariables

( temperature, pressure, etc. ) so that all other properties(dependent !ariables ) are fi$ed.

state function - a physical property that has a specific

!alue once the state is defined; including the !olume *(T,+), whose relationship to T and + was gi!en by thee'uation of state


5/77

&orms of Energy

Energy comes in a !ariety of forms

+otential

-echanical hemical Electrical

Internal /inetic

0eat


6/77

Thermodynamics is based on a few statements

called laws that ha!e broad applications to physical

and chemical systems. Three such statements thatwe will e!entually discuss are the first, second, and

third laws of thermodynamics (profound

discussions on the last two laws will be discussed

in the sub1ect Thermodynamics).


7/77

The 2eroth 3aw of Thermodynamics

4 more fundamental idea that is usually

assumed but rarely stated because it is too

ob!ious. 5ccasionally, this idea is referred

to as thezeroth law of thermodynamics.

Thermal e'uilibrium is the sub1ect of the

2eroth 3aw of Thermodynamics. The

67eroth law6 states that if two systems are at

the same time in thermal e'uilibrium with athird system, they are in thermal e'uilibrium

with each other.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thereq.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thereq.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thereq.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thereq.html


8/77

If 4 and are in thermal e'uilibrium with 8,then 4 is in thermal e'uilibrium with .

+ractically this means that all three are at the

same temperature, and it forms the basis for

comparison of temperatures. 9 2eroth law is

so named because it logically precedes the

&irst and econd 3aws of Thermodynamics.


9/77

Energy onser!ation

The First Law of Thermodynamicsstates that total energy is conser!ed for any

thermodynamic system < energy can not be created nor destroyed< energy can only change from one form to another

constant)( =EEnergy

constantelectricalchemicalheat

mechanicalpotentialkineticinternal

=++

++++

EEE

EEEE


10/77

The First Law of Thermodynamics

The first law of thermodynamics is a statement of

the conservation of energy.

If a systems volume is constant, and heat is

added, its internal energy increases.


11/77


If a system does work on the external world, and

no heat is added, its internal energy decreases.


12/77


Comining these gives the first law of

thermodynamics. The change in a systems

internal energy is related to the heat Qand the

work Was follows!

It is vital to kee" track of the signs of Qand W.


13/77


The internal energy of the system de"ends only

on its tem"erature. The work done and the heatadded, however, de"end on the details of the

"rocess involved.


14/77

The oncept of Wor"

#orkis a $echanicalform of %nergy!

DistanceForceWork =

xFdW =

ForceForce

&istance&istancexx


15/77

The oncept of Wor"

#orkis a $echanicalform of %nergy!

%ecall the definition of pressure:

We can thus define wor" as:

DistanceForceWork =xFdW =

( ) 2Area

Forcep

x

F

==

pdVdW=


16/77

The oncept of Wor"

Changes in 'olume Cause #ork!

Wor" is performed when air e$pands

Wor" of E$pansion:

5ccurs when a system performs wor"

(or e$erts a force) on its en!ironment

Is positi!e:

%ising air parcels (or balloons) undergo e$pansion wor"ince the en!ironmental pressure decreases with height,

with height a rising parcel must e$pand

to maintain an e'ui!alent pressure

0dW>

F


17/77

The oncept of Wor"


imilar to a piston in a car engine

FF


18/77

The oncept of Wor"


Wor" is performed when air contracts

Wor" of ontraction:

5ccurs when an en!ironment performs wor"

(or e$erts a force) on a system

Is negati!e:

in"ing air parcels (or balloons) undergo contraction wor"ince the en!ironmental pressure decreases with height,

with height a sin"ing parcel must contract

to maintain an e'ui!alent pressure

0dW+otential Energy

(of the molecules in the system)epends onlyon the current system state (p,*,T)oes notdepend on past statesoes notdepend on how state changes occur

hanges are the result of e$ternal forcing on the system (in the form of workor heat)

&irst 3aw of Thermodynamics

tenvironmentenvironmeninternal HeatWorkE +=

dQdWdU +=

dQpdVdU +=


20/77Thermodynamics

+ressure*olume (+*) iagrams

(nother #ay of &e"icting Thermodynamic )rocesses!

onsider the transformation: i f

"

''f'i

"i

"f

i

f


21/77Thermodynamics

(nother #ay of &e"icting #ork!

onsider the transformation: i f

"

'

pdVdW=

= f

ipdVW

'f'i

"i

"f

i

f The wor" done is the areaunder the i < f cur!e

(or gray area)

+ressure*olume (+*) iagrams


22/77Thermodynamics

#oules 3aw

'alve

Closed

(ir(ir'acuum'acuum

Thermally Insulated ystem


23/77

Thermodynamics

#oules 3aw

Thermally Insulated ystem

'alve

*"en

(ir(ir(ir(ir


24/77

Thermodynamics

#oules 3aw

dQpdVdU +=

'alve

*"en

(ir(ir(ir(ir

(ir ex"anded to fill the container

hange in !olumehange in pressure

+o external work was done4ir e$panded into a !acuum

within the system

+o heat was added or sutractThermally insulated system

+o change in internal energy+o change in tem"erature

#hat does this mean

0dU=


25/77

Thermodynamics

#oules 3aw

dQpdVdU +=

'alve

*"en

(ir(ir(ir(ir

(ir ex"anded to fill the container

hange in !olumehange in pressure

+o external work was done4ir e$panded into a !acuum

within the system

+o heat was added or sutractThermally insulated system

+o change in internal energy+o change in tem"erature

Internal %nergy is only a function oftem"erature

0dU=U()U=


26/77

Thermodynamics

Thermal apacities (pecific 0eats)

(ssume! 4 small 'uantity of heat (d@) is gi!en to a parcel

The parcel responds by e$periencing a small temperature increase (dT)

"ecific eat /c0!

Two Ty"es of "ecific eats!

epends on how the material changes as it recei!es the heat

Constant 'olume!

Constant )ressure!

vol!meconstant

vd

dQc

= +arcel e$periences no

change in !olume

+arcel e$periences no

change in pressure

press!reconstant

pd

dQc

=

dT

dQ=c


27/77

Thermodynamics


"ecific eat at Constant 'olume!

tarting with:

If the !olume is constant (d* = A), we can rewrite the first law as:

4nd substitute this into our specific heat e'uation as

vol!meconstant

vd

dQc

=

dQpdVdU += dQdU=1

=d

dUcv or dcdU v=


28/77


"ecific eat at Constant 'olume!

ince the internal energy is a state !ariable and does not depend on past states or how state changes occur, we can define changes in internal energy as:

4lso, if we substitute our specific heat e'uation into the first law:

We can obtain an alternative formof the First Law of Thermodynamics:

=2

"

dcU vT

T

pdVdcdQ v =

dQpdVdU +=1dcdU v=


29/77

Thermodynamics


"ecific eat at Constant )ressure!

tarting with

and recogni7ing that,

we can obtain another alternative formof the First Law of Thermodynamics:

4lso,

press!reconstant

pd

dQc

=

pdVdcdQ v =

VdppdVd(pV) +=

VdpdcdQ p =

#

vp n$cc +=

n$pV #=


30/77

Thermodynamics

oncept of Enthalpy

(ssume! 0eat (d@) is added to a system at constant pressure

Im"act! 20 The systemBs !olume increases (*C


31/77


32/77

Thermodynamics

&orms of the &irst 3aw of Thermodynamics

For a gas of mass m For unit mass

dWdUdQ +=pdVdUdQ +=

pdVdcdQ v +=

VdpdcdQ p =

d&d!d' +=pdd!d' +=

pddcd' v +=

dpdcd' p =

where! " 5 "ressure 4 5 internal energy

' 5 volume # 5 workT 5 tem"erature 6 5 heat energy

7 5 s"ecific volume n 5 numer of moles

cv5 s"ecific heat at constant volume /828 9 kg-2:-20

c"5 s"ecific heat at constant "ressure /2;;< 9 kg-2:-20

=d5 gas constant for dry air /3>8 9 kg-2:-20

=? 5 universal gas constant />.@2


33/77

Thermal )rocesses

#e will assume that all "rocesses we discuss

are Auasi-static B they are slow enough that thesystem is always in eAuilirium.

#e also assume they are reversile!

For a process to be reversible, it must be possible to

return both the system and its surroundings to exactly

the same states they were in before the process began.


34/77

Thermal )rocesses

This is an idealied reversile "rocess. The gas

is com"ressedD the tem"erature is constant, soheat leaves the gas. (s the gas ex"ands, it

draws heat from the reservoir, returning the gas

and the reservoir to their initial states. The

"iston is assumed frictionless.


35/77

Thermal )rocesses

#ork done y an ex"anding gas, constant

"ressure!


36/77

Thermal )rocesses

If the volume stays constant, nothing moves and

no work is done.


37/77

Thermal )rocesses

If the tem"erature is constant, the"ressure varies inversely with the

volume.


38/77

Thermal )rocesses

The work done is the area under the curve!


39/77

Thermal )rocesses

4n adiaaticprocess is one in which no heat

flows into or out of the system. The adiabatic PV

cur!e is similar to the isothermal one, but is

steeper. 5ne way to ensure that a process is

adiabatic is to insulate the system.


40/77

(nother way to ensurethat a "rocess is

effectively adiaatic is

to have the volume

change occur very

Auickly. In this case,

heat has no time to

flow in or out of thesystem.

Thermal )rocesses


41/77

Thermal )rocesses

ere is a summary of the different ty"es ofthermal "rocesses!

"ecific eats for an Ideal Eas! Constant


42/77


)ressure, Constant 'olume

"ecific heats for ideal gases must e Auoted

either at constant "ressure or at constant

volume. For a constant-volume "rocess,



43/77



(t constant "ressure,



44/77



oth CVand CPcan e calculated for amonatomic ideal gas using the first law of

thermodynamics.



45/77



(lthough this calculation was done for an ideal,monatomic gas, it works well for real gases.



46/77



The )-' curve for an adiaat isgiven y

where

The econd Law of Thermodynamics


47/77

The econd Law of Thermodynamics

We obser!e that heat always flows spontaneously

from a warmer ob1ect to a cooler one, although the

opposite would not !iolate the conser!ation ofenergy. This direction of heat flow is one of the

ways of e$pressing the second law of

thermodynamics:

When o()ects o* di**erent temperat!res are (ro!+ht into

thermal contact, the spontaneo!s *lo& o* heat that

res!lts is al&a-s *rom the hi+h temperat!re o()ect to

the lo& temperat!re o()ect. /pontaneo!s heat *lo&never proceeds in the reverse direction.

eat %ngines and the Carnot Cycle


48/77


( heat engine is a device that converts heat into

work. ( classic exam"le is the steam engine.

Fuel heats the waterD the va"or ex"ands and

does work against the "istonD the va"orcondenses ack

into water againand the cycle

re"eats.



49/77


(ll heat engines have!

a high-tem"erature reservoir

a low-tem"erature reservoira cyclical engine

These are illustrated

schematically here.



50/77


(n amount of heat Qhis su""lied from the hot

reservoir to the engine during each cycle. *f thatheat, some a""ears as work, and the rest, Qc, is

given off as waste heat to the cold reservoir.

The efficiency is the fraction of the heat

su""lied to the engine that a""ears as work.



51/77


The efficiency can also e written!

In order for the engine to run, there must

e a tem"erature differenceD otherwiseheat will not e transferred.



52/77


The maximum-efficiency heat engine is

descried in Carnots theorem!

If an engine operating between two constant-

temperature reservoirs is to have maximum

efficiency, it must be an engine in which all processesare reversible. In addition, all reversible engines

operating between the same two temperatures, Tc

and Th, have the same efficiency.

This is an idealiationD no real engine can e

"erfectly reversile.



53/77


If the efficiency de"ends only on the two

tem"eratures, the ratio of the tem"eratures muste the same as the ratio of the transferred heats.

Therefore, the maximum efficiency of a heat

engine can e written!



54/77


The maximum work a heat engine can do is

then!

If the two reservoirs are at the same

tem"erature, the efficiency is eroD the

smaller the ratio of the cold tem"erature to

the hot tem"erature, the closer the efficiencywill e to 2.

=efrigerators, (ir Conditioners, and eat


55/77


)um"s

#hile heat will flow s"ontaneously only from a

higher tem"erature to a lower one, it can e

made to flow the other way if work is done on

the system. =efrigerators, air conditioners,

and heat "um"s all use work to transfer heat

from a cold oGect to a hot oGect.



56/77

g , ,

)um"s

If we com"are the

heat engine and therefrigerator, we see

that the refrigerator

is asically a heat

engine runningackwards B it uses

work to extract heat

from the coldreservoir /the inside of the refrigerator0 and

exhausts to the kitchen. +ote that- more heat is exhausted to the kitchen than is

removed from the refrigerator.



57/77

g , ,

)um"s

(n ideal refrigerator would remove the mostheat from the interior while reAuiring the

smallest amount of work. This ratio is called the

coefficient of "erformance, C*)!

Ty"ical refrigerators have C*) values etween

3 and H. igger is etter



58/77

g , ,

)um"s

(n air conditioner is

essentially identical to a

refrigeratorD the cold reservoir

is the interior of the house or

other s"ace eing cooled, andthe hot reservoir is outdoors.

%xhausting an air conditioner

within the house will result in

the house ecoming warmer,Gust as kee"ing the refrigerator

door o"en will result in the

kitchen ecoming warmer.



59/77

g , ,

)um"s

Finally, a heat "um" is the

same as an air conditioner,

exce"t with the reservoirs

reversed. eat is removed

from the cold reservoiroutside, and exhausted

into the house, kee"ing it

warm. +ote that the work

the "um" does actuallycontriutes to the desired

result /a warmer house0 in

this case.



60/77

g , ,

)um"s

In an ideal heat "um" with two o"erating

tem"eratures /cold and hot0, the Carnot relationshi"holdsD the work needed to add heat Qhto a room is!

The C*) for a heat "um"!

%ntro"y


61/77

"y

( reversile engine has the following relation

etween the heat transferred and the reservoir

tem"eratures!

=ewriting,

This Auantity, QJT, is the same for oth reservoirs,

and is defined as the change in entro"y.

%ntro"y


62/77

"y

For this definition to e valid, the heat transfer

must e reversile.

In a reversile heat engine, it can e shown

that the entro"y does not change.

%ntro"y


63/77

"y

( real engine will o"erate at a lower efficiency

than a reversile engineD this means that less

heat is converted to work. Therefore,

(ny irreversile "rocess results in an

increase of entro"y.

%ntro"y


64/77

"y

To generalie!

The total entropy of the universe increases whenever

an irreversible process occurs.

The total entropy of the universe is unchanged

whenever a reversible process occurs.

ince all real "rocesses are irreversile, the

entro"y of the universe continually increases. If

entro"y decreases in a system due to workeing done on it, a greater increase in entro"y

occurs outside the system.

%ntro"y


65/77

y

(s the total entro"y of the universe

increases, its aility to do work decreases.

The excess heat exhausted during an

irreversile "rocess cannot e recoveredD

doing that would reAuire a decrease in

entro"y, which is not "ossile.

*rder, &isorder, and %ntro"y


66/77

%ntro"y can e thought of as the increase in

disorder in the universe. In this diagram, the

end state is less ordered than the initial state B

the se"aration etween low and high

tem"erature areas has een lost.



67/77

If we look at the ultimate fate of the universe in

light of the continual increase in entro"y, we

might envision a future in which the entire

universe would have come to the same

tem"erature. (t this "oint, it would no longer e"ossile to do any work, nor would any ty"e of

life e "ossile. This is referred to as the Kheat

death of the universe.



68/77

o if entro"y is continually increasing, how is

life "ossile ow is it that s"ecies can evolveinto ever more com"lex forms &oesnt this

violate the second law of thermodynamics

+o B life and increasing com"lexity can exist

ecause they use energy to drive their

functioning. The overall entro"y of the universe

is still increasing. #hen a living entity sto"s

using energy, it dies, and its entro"y canincrease rather Auickly.

The Third Law of Thermodynamics


69/77

(solute ero is a tem"erature that an oGect

can get aritrarily close to, ut never attain.Tem"eratures as low as 3.; x 2;->: have een

achieved in the laoratory, ut asolute ero will

remain ever elusive B there is sim"ly nowhere to

K"ut that last little it of energy.

This is the third law of thermodynamics!

It is impossible to lower the temperature of an obect

to absolute !ero in a finite number of steps.

ummary of


70/77

#hen two oGects have the same tem"erature,

they are in thermal eAuilirium.

The first law of thermodynamics is a statement

of energy conservation that includes heat.

The internal energy of a system de"ends only

on its tem"erature, "ressure, and volume.

( Auasi-static "rocess is one in which thesystem may e considered to e in eAuilirium

at all times.

ummary


71/77

In a reversile "rocess it is "ossile to return

the system and its surroundings to their initialstates.

Irreversile "rocesses cannot e undone.

The work done during a "rocess is eAual to thearea under the curve in the PV"lot.

The work done at constant "ressure is

The work done at constant volume is ero.

The work done in an isothermal ex"ansion is

ummary


72/77

(n adiaatic "rocess is one where no heat

transfer occurs.

The value of the s"ecific heat de"ends on

whether it is at constant "ressure or at constant

volume.

$olar s"ecific heat is defined y!

For a monatomic gas at constant volume!

For a monatomic gas at constant "ressure!

ummary


73/77

In a )' "lot, is constant, where

For a monatomic ideal gas,

The s"ontaneous flow of heat etween oGects

in thermal eAuilirium is always from the hotter

one to the colder one.( heat engine converts heat into work.

%fficiency of a heat engine!

ummary


74/77

( reversile engine has the maximum "ossile

efficiency,

The maximum "ossile work!

=efrigerators, air conditioners, and heat "um"s

use work to transfer heat from a cold region to a

hot region.

ummary


75/77

Coefficient of "erformance of a refrigerator!

#ork done y an ideal heat "um"!

Coefficient of "erformance for a heat "um"!

ummary


76/77

Change of entro"y during a reversile heat

exchange!

Total entro"y of the universe increases whenever an

irreversile "rocess occursD total entro"y is

unchanged after an ideal reversile "rocess.

%ntro"y is a measure of disorder.

The heat death of the universe will occur when

everything is the same tem"erature and no more workcan e done.

It is im"ossile to lower the tem"erature of an oGect

to asolute ero in a finite numer of ste"s.


77/77

=eferences!8all, a!id W. (DACD). Physical Chemistry. DndEdition. 4: engage 3earning.

Winnic", #ac" (CFFG). Chemical Engineering Thermodynamics. anada: #ohn Wiley and

ons, Inc.mith, #. -., *anness, 0. ., et al. (DAAH). Introduction to Chemical Engineering.Gth

Edition. ingapore: -craw0ill 8oo". o.

/umar, . . (DACD). Engineering Thermodynamics (+rinciples and +ractices). Jew elhi:

. /. /ataria K ons.

-oran, -ichael #. hapiro, 0oward J., et al. (DACC). Principles of Engineering

Thermodynamics. GthEdition.anada: Wiley K ons, Inc.

enapati, -.(DAAL). Advanced Engineering Chemistry. econd Edition. Jew elhi:

Infinity cience +ress 33.

+erryBs hemicalEnigneersB 0andboo" ( CFFG). Gth

Edition. Jew Mor": -craw0ill 8oo"o.

Laws Thermodynamics

Documents

Transcript of Laws Thermodynamics