6b Thermodynamics

7/25/2019 6b Thermodynamics

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1. Design and Use of

Thermometers

Teacher: Mr B. Burnett

Class: 6B

Date: 12/03/13



What is Temperature?

• Temperature is that property which determines whether

or not one ody is in thermal e!uilirium with another.

• "# $nits: %el&in '%(.

•

Thermal e!uilirium) the point attained where the net*low o* heat etween two odies+ , and B+ is constant.



Temperature scales

• #n de*inin- temperature scales+ the thermometric

properties o* particular sustances are *irst scrutinied. ,

thermometric property’ is that property tells you how a

sustance/!uantity &aries with temperature. amples o*

such sustances are:

• ) those which are accurately measurale o&er a wideran-e o* temperatures e.- mercury used in

'li!uid/mercury)in)-lass thermometers(.



Temperature Scales cont…

• a&in- selected a particular thermometric property+ a

temperature scale is then de*ined y means o* two

fixed points ' accurately reproducile sin-letemperatures at which a speci*ic physical e&ent

always occurs(. The inter&al etween the two *ied

points is called the fundamental interval . amples

o* *ied points are:• i) Ice point

• ii) Steam point

• iii) Triple point



Fixed Points

• Ice point (00C)- That temp at which pure ice can eist in

e!uilirium with water at standard atmospheric pressure 's.a.p(

• Steam point (1000C) ) That temperature at which steam and oilin- water are in e!uilirium at standard atmospheric

pressure 's.a.p(.

• Triple point) That temperature at which the solid+ li!uid and

&apour phases o* a particular sustance eist in thermal

e!uilirium. or water it is the temperature at which pure ice+

water and water &apour eists in e!uilirium.

• N.B The value for s.a.p is 1.013 x 105 Pa (N/m2 . !t is the

pressure exerte" at the #ase of a colum$ of mercury %&0mm tall.



Graph of Thermometric

Property vs. Temp !"#

Xθ4 the thermometric property at un5nown

temperature t

X0 4 the thermometric property at the ice point

X100 4 the thermometric property at the steam

point.lθ = len-th o* mercury at a particular

temperature.

l0 = len-th o* mercury at lower *ied point

'*reein- point(

l100

o = len-th o* mercury at the upper *ied

point 'oilin- point(



$nalysis of Graph

• The -raph indicates that the relationship etween

the temperature and the thermometric property o*

the sustance 'e.- le$'th of mercury i$ the

mercuryi$'lass thermometer ( is ‘linear’.

•

*N.B This ‘linear’ relationship also holds true for the thermocouple and constant volume gas

thermometers as !e !ill see later on).



%xample 1

• , student places a mercury)in)-lass thermometer in a

ea5er o* water and measures the len-th o* mercury to e30cm at the ice point and 0cm at the steam point. "he

remo&es the thermometer *rom the water and notices that

the readin- starts to drop. 7hat is the temperature o* the

water when the mercury is at 80cm9

• ,ns: 4 ;00C



&i'uid()ercury*in*glass

Thermometer



)ercury*in*glass +pecs

• Thermometric property: The len-th o* the mercury or li!uid 'e.-.

alcohol( in the capillary tue o* the thermometer &aries with chan-es in

temperature.

• Range: 'mercury( )3<0C to =8000C > ? ')1180C to =@0C( 'alcohol(

• Uses: used in measurin- *airly steady temperatures+ weather recordin-+

clinical eercises.

• A!antages: "imple to use+ direct readin-+ cheap+ portale+ compact

•

"isa!antages: limited temp. ran-e+ *ra-ile+ relati&ely lar-e heatcapacity which means that it is unale to measure rapidly chan-in-

temperatures 'slow response(.

• #rrors: non)uni*ormity o* capillary ore+ paralla error+ epansion o*

-lass ul and stem.

θ (in 0C) = l θ - l0 x 100

l100 – l0



"onstant*volume gas

thermometer

,tmospheric pressure+ or p,

p 4 h = p,



",G thermometer +pecs

• Thermometric propert": The pressure o* a *ied mass o* -as at

constant &olume.

• Range: )2@00C to 18000C• Uses: used as a standard to calirate other more practical types

• A!antages: &ery accurateA &ery sensiti&eA wide temperature ran-e.

• "isa!antages: ul5yA must manually calculate temp'does not -i&e

a direct readin-(A has a lar-e heat capacity which means that it is

unale to detect rapid chan-es in temperature 'slow response(

• #rrors: paralla error in readin- li!uid le&elsA epansion o* the

ul implies &olume is not constantA air is not an ideal -as.



Thermistor

Sym$ol



Thermistor in Wheatstone

-ridge



Thermistor +pecs

• Thermometric property: lectrical esistance o* a

"emiconductor. The thermistor is a de&ice whose resistance &aries

mar5edly with temperature.

• Range: )800 C to =3000 C

• Uses% in olometers to detect chan-es in in*rared radiation and in

industrial monitorin- and temperature control.

• A!antages: &ery sensiti&eA cheapA roustA small heat capacity

which means that it can detect rapidly chan-in- temperatures.• "isa!antages: ul5y '7heatstone rid-e arran-ement( and

there*ore less stale and less stale than platinum resistance

thermometers. #t is non)linear .



Thermistor

• "ome thermistors ha&e either a positi!e temperat&re

coe''icient (TC) 'its resistance increases with an

increase in temperature( or a negati!e temperat&re

coe''icient (TC) 'its resistance decreases with an

increase in temperature.



T" / PT" thermistor plot

TC e-ati&e temp.

coe**icient ' as temp rises+

resistance decreases(.

TC * TC+ where ETC 4 positi&etemperature coe**icient ' as temp rises

its resistance increases(.



Thermocouple



Thermocouple +pecs

• Thermometric property: The electrical potential di**erence

'milli&olts+ mF( etween two metals in contact.

•Uses: used to measure temperature at points and to *ollow rapidly&aryin- temperatures.

• Range: )2800 C ) =18000 C

• A!antages: 7ide temperature ran-eA !uite accurateA compact and

roustA can measure the temp. o* small oGects and its small heat

capacity -i&es it the aility to *ollow rapidly chan-in- temperatures.• "isa!antages: Because the em* produced is small+ sensiti&e

electrical e!uipment has to e used to ensure accurate measurements.

θ = #θ + #

0, 100

#100 + #0



Thermocouple used 0ith a

datalogger

datalo--er



&inear vs on*linear

relationships 0ith temp.

&inear

•

ength o' merc&ry inmerc&ry-in-glass

thermometers

• latin&m-Resistance

thermometer

• Constant !ol&me gas

thermometer.

• Thermoco&ple

on*linear

•

Thermistor



Tip to note

• or the %el&in scale+ the standard thermometer a-ainst

which all other thermometers are calirated is the

constant !ol&me gas thermometer.



Temperature scale revie0

#n summary+ each thermometer has a particular temperature

scale accordin- to its thermometric property . Di**erentthermometric properties &ary di**erently with temperature

with the eception at the 'i,e points where they a-ree. or

eample+ a mercury)in)-lass thermometer may -i&e a &alue

o* 280

C in a room while a constant &olume -as thermometermay read otherwise. owe&er+ oth thermometers are

correctA accordin- to their own scale.



$-solute thermodynamic scale

• ,s a result o* these &ariations in temperature readin-s

*rom one thermometer to the net+ a standard

temperature scale has een adopted that is independent o*

any thermometric property called the a$sol&te

temperat&re scale. #t is the fu$"ame$tal temperature

scale used in science and has the ".# $nit: the /el!in .

#ts temperature is -i&en y:• T 4 ET [email protected]

Etr



$-solute thermo. scale cont

• where+

• # T is the ideal -as pressure at temperature T

• tr is the ideal -as at the triple point o* water ' the *ied

point o* oth the asolute scale and the ideal -as scale(

The ero o* this scale is called a$sol&te 2ero (0)

'which is impossile to achie&e in practiceH(.



$-solute thermo scale cont...

•

The other unit o* temperature which is commonly used isthe de-ree Celsius '0C( which is the unit o* the Celsius

scale and is de*ined y:

• 'in 0C( 4 T 'in %( [email protected]



$-solute thermodynamic +cale

• Eoints to note:

• ) , temp chan-e o* 10C 4 a temp chan-e o* 1%

• ) #ce point '00C( is [email protected]% and the steam point '1000C(

is [email protected]%.



Different Temperature scales



2. Thermal

Properties



3eat / 4nternal %nergy

• 3eat is de*ined as the trans*er o* ener-y *rom a ody at a

hi-her temperature to one that is at a lower temperature y conduction+ con&ection and radiation.

• 7hen the atoms o* a soli are heated they &irate to and

*ro their e!uilirium positions alternately attractin- and

repellin- each other. ,s a result+ the particles contain

ener-y that is partly due to their 5inetic and potential

ener-ies+ constitutin- what is 5nown as the internal

energ"’ o* the system.



4nternal %nergy cont

• owe&er+ in the case o* the gas+ the intermolecular *orces

are wea5 resultin- in the internal ener-y to e almost

entirely 5inetic.



+olids vs gases

Ias



4nternal %nergy cont..5

Potential %nergy P%#

•

store in the interatomic$ons that are

contin&o&sly stretche

an compresse as the

atoms !i$rate an it

epens on the si2e o' the$oning 'orces as 4ell as

the particle separation

6inetic %nergy 6%#

•

Due to the vi-ratorymotion of the atoms and

according to the 7inetic

theory depends on the

temperature.



4nternal %nergy cont

• There are two '2( ways in which the internal energy o*

a ody can e chan-ed:

• 1( y doin- wor5 the more wor5 done on a system+ the-reater the internal ener-y o* the system. .- ruin-

hands to-ether and the -as compression in a icycle

pump.

•2( y heat trans*er ) causes a rise in the internal ener-y o*the ody recei&in- the heat which in turn increases the

temperature o* the ody.



%ffect of temperature on

4nternal %nergy



3eat "apacity

• "ymol: C

• ".# $nit: Joule per 5el&in 'or de-ree Celsius( J/% or J/0C

• De*inition: The heat capacity C o* a ody is the amount

o* heat ener-y 'KL( which must *low into a ody to

produce unit temperature rise o* one 5el&in '1%( in the

ody.

KL 4 amount o* heat ener-y asored or remo&ed 'J(KT 4 chan-e in temperature '%(



3eat "apacity cont

• The amount o* heat a ody recei&es is dependent on two

properties o* the ody:• 1( its mass ' the smaller the mass+ the -reater the

temperature(

• 2( the material *rom which it is made+

• ) oGects o* the same mass ut di**erent materials willunder-o di**erent temperature rises when supplied with

the same amount o* heat.



+pecific heat capacity

• "ymol: c

• ".#. $nit: Joule per 5ilo-ram per 5el&in+ J 5-)1% )1 'orde-ree Celsius+ J 5-)1 0C)1(

De*inition: The speci'ic heat capacity (c) o* a sustance is

the amount o* heat ener-y needed to produce a temperaturerise o* one de-ree in 15- o* a sustance.

c 4 KL

mKT

But "#HH

KL 4 C+ thus C = mc

KT



Tip to note

• The greater the speci*ic heat capacity o* a sustance+ the

more energ" needs to e trans*erred to the sustance i*

the other conditions 'KT+ KL and the mass+ m( are the

same.



&ist of +pecific heat"apacities



P81

• , hot water tan5 contains 1605- o* water at 200C.

Calculate:• a( The !uantity o* heat ener-y re!uired to raise the

temperature o* the water to 600C and

• ( The time this will ta5e usin- a 857 electric immersion

heater. 'cwater 4 ;200J5-)1

% )1

(

• ,ns: 2@MJA 8.; 103s



P82

• a( ow much heat input is needed to raise the

temperature o* an empty 20)5- &at made o* iron *rom

100C to <00C9 '( 7hat i* the &at is *illed with 205- o*

water9 'Ii&en: speci*ic heat capacity o* iron4 ;80 J/5-0C+ spec. heat. Capacity o* water 4 ;16 J/5- 0C(

• ,ns: KL 4 @;205J



P89

• #* 200cm3 o* tea at <80C is poured into a 180)- -lass cup

initially at 280

C+ what will e the common *inaltemperature T o* the tea and cup when e!uilirium is

reached+ assumin- no heat *lows to the surroundin-s9

ence deduce the drop in temperature o* the tea. 'ccup4

;0 J/5-

0

C '-lass cup(+ ctea4 cwater 4 ;16J/5-

0

C(

• ,ns: 8.0CA <.20C



P8:

• ,n en-ineer wishes to determine the speci*ic heat

capacity o* a new metal alloy. , 0.1805- sample o* the

allow is heated to 8;00C. #t is then !uic5ly placed in ;00-

o* water at 100C+ which is contained in a 200)-

aluminium calorimeter cup. The *inal temperature o* the

system is 30.80C. Calculate the speci*ic heat capacity o*

the alloy. '"peci*ic heat capacity o* water 4 ;16J 5-)1 0C)

1 and spec. heat capacity o* the cup 4 <00 J 5- )1 0C)1.

• ,ns: calloy4 ;<@.; J 5-)1 0C)1



P8;

• , piece o* copper o* mass 100- is heated to 1000C and

then trans*erred to a well)la--ed copper can o* mass80.0- containin- 200- o* water at 100C. e-lectin- heat

loss+ calculate the *inal steady temperature o* the water

a*ter it has een well stirred. Ta5e the speci*ic heat

capacity o* copper to e ;.00 10

2

J 5-

)1

0

C

)1

and waterto e ;.2 103 J 5-)1 0C)1 .

• ,ns: T*inal 4 1;0C.



"hange of state



&atent heat

• #* a ea5er o* water is heated+ e&entually the water starts

to oil at 100 0C. urther applyin- heat to the system+ itwill e noticed that there is no increase in the temperature

o* the system+ howe&er there is a change in state (li5&i-

to-gas). The heat ener-y which a ody asors or

releases durin- meltin-+ e&aporation or sulimation(phase cha$'e and which it -i&es out durin- *reein- or

condensation is called the latent (6hien7) heat.



"hange of +tate

0

)80

80

100

eat ein-

asored 'added(

eat etracted

'e&erse process(

0C

t/s



$nalysis of temp vs timegraph

, B The temperature o* the solid is risin- approachin- the meltin- point

o* water '00C(.

B C The solid starts meltin- at a constant temperature 'under-oes a phase

chan-e *rom solid to li!uid(.

C D The li!uid is now heated up causin- its temperature to increase to

point D where it starts to oil '1000C(.

D Boilin- point reached and -radually under-oes a chan-e o* state

'li!uid to -as( at constant temperature.

The li!uid has &apourised and the temperature o* the -as increases.

urther heatin- o* the -as will cause the temperature o* the -as to rise ao&e

.



&atent heat of fusion / vapourisation

• 7hen the meltin- point o* water is reached+ the heat

ener-y supplied to cause a phase chan-e *rom a solid toli!uid at a constant temperature is called the latent heat

of fusion’.

• 7hen its oilin- point is reached+ the heat ener-y

supplied to cause the phase chan-e *rom li!uid to &apourat a constant temperature is called the latent of

vapori$ation’.



4nternal %nergy

eatin- a solid+ e.- ice+ causes oth the 5inetic and

molecular potential ener-y 'epansion o* the solid(components o* its internal ener-y to increase. #t is important

to note howe&er that it is the latent heat that increases the

mean potential energ" component o* the internal ener-y y

increasin- the mean separation o* the molecules. The5inetic ener-y '%.( component increases as a result o* an

increase in temperature.



"ooling -y %vaporation

• rom the chan-e o* phase o* a sustance (li)ui"to'as

to the coolin- o* our odies+ the e**ect o* coolin- ye&aporation can e oser&ed due to ener-y.

• But where does this ener-y come *rom999

• #t comes *rom the latent heat o* the system. or

eample+ when the ody sweats 'cools(+ the water produced is e&aporated *rom the ody as a result o* the

latent heat it recei&es.




• "imilarly+ when a li!uid is chan-in- into a -as the heat

ener-y re!uired to ma5e this chan-e is the latent

hidden) heat.




• ,ccordin- to the 5inetic theory+ the molecules o* a li!uidha&e an a&era-e 5inetic ener-y that increases with

temperature. Those molecules at the sur*ace o* the li!uid

-ets ecited and Gump out o* the li!uid lea&in- ehind

the less ener-etic molecules+ that is+ it e&aporates.Conse!uently+ there will e a decrease in temperature.



+pecific latent heat

• "ymol: l • "# $nit: Joule per 5ilo-ram 'J 5-)1(

• The speci*ic latent heat (l o* *usion+ l f 'or &aporiation+ l &

or sulimation+ l s( o* a sustance is the ener-y needed to

chan-e 15- o* the sustance *rom solid to li!uid 'orli!uid to -as or solid to -as( at constant temperat&re.

89 = ml



Tips to note

• ,ll chan-es o* state occur at constant temperat&re

• The latent heat is di**erent *rom the heat that is supplied to cause a

chan-e in temperature o* the sustance.

• e&ersin- the process '*reein- or condensation( the latent heat is

remo!e *rom the sustance.

• The heat ener-y re!uired *or a chan-e o* state *rom solid to li!uid

'meltin-( is e!ual to the ener-y re!uired to chan-e *rom li!uid to

solid '*reein-(



Tip to note

• ecall:

• eat ener-y needed to chan-e the temperature o* a unit mass y 1% is+

• KL 4 mcK T• eat ener-y needed *or a chan-e o* state+

• KL 4 ml ,• m 4 mass o* sustance

•c 4 speci*ic heat capacity o* sustance

• KL4 eat ener-y asored y or remo&ed *rom sustance

• KT 4 chan-e in temperature

• l 4 latent heat o* *usion 'l ( or &aporiation 'l &(



P81

• ow much ener-y does a *reeer ha&e to remo&e *rom

1.85- o* water at 200C to ma5e ice at )120C9 '"pec. latent

heat o* water 4 3.33 108 J/5-+ spec. heat capacity o*

water4 ;16 J/5- 0C+ spec. heat capacity o* ice 4

2100J.5- 0C(.

• ,ns: 6605J



P82

• Calculate the mass o* ice which melts when 2.85- o*

caa-e at a temperature o* 200C is mied with ice and

water at 00C in a well)insulated container. Nou may

assume that heat loss to the surroundin-s is ne-li-ile

and that all the caa-e is cooled to 00C. '"peci*ic heat

capacity o* caa-e 4 2.0 103 J /5- 0 CA speci*ic latentheat o* *usion o* ice 4 3.; 108 J /5-



P89

• Calculate the total amount o* heat which is -i&en out

when 200- o* steam condenses to water at ;00C. '"peci*ic

heat capacity o* water 4 ;.2 103 J 5-)1 0C)1 and speci*ic

latent heat o* steam 4 2.26 106

J 5-)1

.(



P8:

• ,t a reception+ a 0.805- chun5 o* ice at )100C is placed in

3.05- o* iced tea at 200C. ,t what temperature and inwhat phase will the *inal miture e9 The tea can e

considered as water. #-nore any heat *low to the

surroundin-s+ includin- the container. 'speci*ic latent

heat o* *usion o* ice 4 3335J/5-+ speci*ic heat capacity o*water 4 ;16 J 5-)1 0C)1 + speci*ic heat capacity o* ice 4

2100 J 5-)1 0 C)1(



P8;

• The speci*ic heat capacity o* li!uid mercury is 1;0J/5-0C.

7hen 1.05- o* solid mercury at its meltin- point o* )3<0C

is placed in a 0.805- aluminum calorimeter can *illedwith 1.25- o* water at 20.00C+ the *inal temperature o* the

comination is *ound to e 16.80C. 7hat is the latent heat

o* *usion o* the mercury in J/5-9 'spec. heat capacity o*

can 4 <00J/5-/0C+ speci*ic heat capacity o* water 4

;16J/5-/0C(

• ,ns: 11+;00J/5-

9 3 f



9. 3eat transfer



Thermal "onduction

•Thermal conduction is the process y which heat *lows*rom the hotter to colder re-ions o* the ody without

trans*er o* the material itsel*. There are t!o %)

mechanisms in which thermal conduction occurs:

•

&) 'ree electron diffusion• %) (tomic or molecular coupling



1. Free electron diffusion

• #n this mechanism+ on applyin- heat to a particular re-ion

o* a metal+ the electrons in that area ecome ‘excited’

'-ain ener-y( and mo&e or *"iffuse’ into colder re-ions o*

the metal in a process called thermal con&ction. ,s a

result+ we say metals are good conductors o* heat.



%xample< Free electrondiffusion



2. $tomic (molecularcoupling

• #n the case o* atomic or molecular couplin-+ heat is

applied to a material causin- its atoms to -ain ener-y

'&irational %( and ecome ecited and collide into

nei-hourin- atoms therey trans*errin- ener-y.



%xample of molecularcoupling



Thermal conduction e'uation

• KL/Kt 4 ate o* trans*er o* ener-y 'heat

*low( 'units: 7 or Js)1 (

• 5 thermal conducti&ity o* the material

'constant( '7 m)1 % )1 (

• K/K) temperature -radient'0 C/m or %/m(

• , cross-sectional area 'm2(

• .B the ')( si-n means that the temperature

drops as the heat ener-y *lows throu-h the

material.

x

+,

t

-

∆

∆−=

θ



Thermal conductivity

• De*inition: The thermal conductivit" ) o* a material is

de*ined as the rate o* *low o* heat ener-y per unit area+

per unit temperature -radient when the heat *low is

normal to the *aces o* a thin+ parallel)sided sla o* the

material under steady state conditions.



"ome &alues o* thermal conducti&ity o* materials

'7 m)1% )1(

Diamond 4 <8 2300

"il&er 4 ;20Copper 4 ;00

7ater 4 0.60

,ir 'dry( 4 0.028

uman s5in 4 0.3@

Ilass 4 0. 1.0





)or the per'ectly lagge $ar all heat enterin- one end lea&esat the other end. one lea&es throu-h the sides. This is parallel

heat *low. Temp drop is linear.

e.-.

Calculate the rate o* heat loss throu-h a window o* thic5ness6mm and area 2 m: i* the temperature di**erence etween the

two sides is 200C. ',ssume it is a -lass window(.

,ns 4 8.3 57



Comment on answer: Clearly &ery hi-h.

owe&erA the heat conducted throu-h the -lass warms the air Gust

outside the window. This raises the temp o* the outer -lass+

reducin- the temperature -radient and hence reduces the heat

loss.



P81 = 3eat loss through 0indo0s

• , maGor source o* heat loss *rom a house is throu-h the

windows. Calculate the rate o* heat *low throu-h a -lass

window 2.0m 1.8m in area and 3.2mm thic5+ i* the

temperature at the inner and outer sur*aces are 180C and

1;0C respecti&ely. 'use 5 o* -lass 4 0.; 7 m)1 0C)1(

• ,ns: @<0 J/s or @<07



P82* "onduction

•

Calculate the !uantity o* heat conducted throu-h 2m2

o* a ric5 wall 12cm thic5 in 1 hour i* the temperature on one

side is 0C and on the other side is 20C. 'Thermal

conducti&ity o* ric5 4 0.137m)1 % )1(

• ,ns: 1865J



The composite -ar.

•Consider a ar made o* two '2( materials+ oth o* samecross sectional area ,+ one a -ood conductor and the

other a poor conductor. OThe ar is per*ectly la--edP

• The heat *lowin- into one end 4 heat *lowin- out o* the

other end so the rate o* heat *low in one material 4 rate o*heat *low in the other material.



l

- θθ



• or the -ood conductor:

or the poor conductor:

This means

#* the len-ths are e!ual+ the temp drop throu-h the -oodconductor will e much less than that throu-h the

poor conductor.

1

1

1

l ,+

t

- θ θ −−=

2

2

2

l ,+

t

- θ θ −−=

1

1

1

l ,+ θ θ −

2

2

2

l ,+ θ θ −

=



P81* "omposite -ar

• The dia-ram '-i&en in class( shows a per'ectly lagge

composite $ar QNR which consists o* 0.@8m o* aluminium

Goined to 0.28m o* copper. ach ar has a radius o* 3cm. #*the ends Q and R are 5ept at temperatures o* 1200C and

200C respecti&ely+ calculate:

• a( the temperature where the two ars are Goined

• ( The heat *low throu-h the composite ar. 'thermal

conducti&ity o* ,l 4 210 7m)1% )1 and Cu 4 3<0 7m)1% )1(

• ,ns: a( 30.2% or 38.20CA ( 6@J/s



P82

ow many Joules o* thermalener-y *low throu-h the wall per

second9

'5 o* insulation 4 0.2 J/ 'sm0C(

' 5 o* plywood 4 0. J/ 'sm0C(

,ns: 200 J/s



)easurement of thermal

conductivity Principle only#

"earles ar method) Iood Conductor

la--in-



• "pecimen in the *orm o* a lon-+ thin ar 'why9(+ hi-hly polished'why9(+ hea&ily la--ed and placed in a wooden o.

Apparat&s

• Constant)head apparatus 'rate o* *low will e constant(+

measurin- cylinder+ stop watch+ "earleSs apparatus+ steam-enerator+ *our thermometers T1+ T2+ T3+ T;+ Fernier callipers.

• T1 and T2 measure the temperature at points on the ar+ T3 andT; measure the temperature o* water enterin- and lea&in- thespiral C

;etho



;etho • ,dGust the constant)head de&ice to -i&e a steady *low

o* water throu-h the coiled tue.

• Eass steam *rom the steam -enerator throu-h the steamchest. wait until the thermometers ha&e reached asteady state 'i.e. no si-ni*icant increase or reduction o*temperature *or 10 minutes(.

•

Measure T1+ T2+ T3 and T;.• Measure the rate o* water *low throu-h the spiral y

measurin- the amount o* water 'm( collected in themeasurin- cylinder in a -i&en time 't(. Collectapproimately 1 litre.

• $sin- Fernier callipers+ measure the diameter o* the ar D at di**erent points alon- the tue and the distanced etween the thermometers T1 and T2.



• ,pply the e!uation *or thermal conducti&ity

where L 4 mc'T3 T;(

• <or a oor Con&ctor

<or a R con&ctor



• 7ith a poor conductor+ the rate o* heat *low will e small andso a thin specimen with lar-e cross)sectional area has to e

used.

• "pecimen is on steel plate with steam chest on top.

• Temp at the ase o* the steam chest and the top o* the ase plate is measured

• Thermometers are in -ood thermal conductors and will

e**ecti&ely measure the temp o* the *aces o* the specimen



• 7hen steady state is reached+ the temps are recorded.

• ,ssume heat loss *rom side o* specimen is small '#t is an

insulator(

• Base plate is polished so that heat loss y radiation is small



• .>. %eepin- the area+ ,+ lar-e and small produces a lar-e

rate o* ener-y trans*er across the sample. %eepin- small also

means that the apparatus reaches a steady state 'when

temperatures T1 and T2 are constant( more !uic5ly.• The &alue o* is usually 5nown.



"onvection

• onvection is the process wherey heat *lows y the mass

mo&ement o* molecules '*luid( *rom one place to another.

,s the *luid is heated+ the atoms and molecules o* the *luidmo&es *aster+ spread out and ecomes less dense. This

heated+ less dense *luid will rise *rom a uoyant *orce *rom

the surroundin- cooler+ more dense *luid. The heated *luid

continues to rise until it has een cooled 'y trans*errin-

heat to the rest o* the *luid(+ ecomes more dense than the

surroundin- *luid+ and *alls ac5 down to the heat source

where the process is repeated.



"onvection currents

• onvection currents are a *low o* li!uid or -as caused y

a chan-e in density+ in which the whole medium mo&es

and carries ener-y with it.



%xample< "onvection"urrents



atural "onvection

and and sea reees an- -lidin- use risin- co$vectio$ curre$ts o*

warm air 5nown as thermals to -i&e them etra

hei-ht so they so stay up lon-er



atural< >cean and 0indcurrents



Forced "onvection



Thermal adiation

• This 5ind o* radiation consists o* electroma-netic wa&es

emitted y oGects as a result o* their temperature. $nli5e

conduction and con&ection+ radiation does not re!uire the presence o* matter to transport heat *rom hotter to colder

re-ions. Un the electroma-netic spectrum+ this radiation

ran-es *rom the in'rare raiation to !isi$le light to the

&ltra!iolet region. "ome properties are:

• ) it tra&els in strai-ht lines

• ) it tra&els at the speed o* li-ht 'c 4 3.0 10 m/s(

• ) #t is una**ected y electric or ma-netic *ields



@lac7 -ody adiation

• , ‘+lac +od"’ is one which asors all the radiation that

*alls upon it+ and re*lects and transmits none. ,ny space

which is almost wholly enclosed approimates to a lac5

ody. Blac5 ody radiation is sometimes called

temperature radiation ecause the relati&e intensities o*

di**erent wa&elen-ths present depend only on the

temperature o* the ody. amples o* lac5 odiesinclude pupil o* the eye+ a hole in a can and anythin- that

appears 6$lac/7.



%xamples of -lac7 -odies



@lac7 vs shiny surfaces

•

Shiny s&r'aces asor little o* the radiation incidentupon them 'most is re*lected(. Un the other hand+ $lac/

an !ery ar/ o$?ects@s&r'aces asor nearly all the

radiation that *alls on them and emits e!ually well.

• ( good a+sor+er is a good emitter.

St ' 7 l t t th t th t t hi h G t di t



+tefanAs &a0

• Ste'an7s la4 states that the rate at which an oGect radiates ener-y

'KL/Kt( is proportional to the temperature raised to the *ourth power 'T;(.

• "te*ans aw can e written as:

• 7here # 4 intensity o* radiation

• T4 temperature in 5el&in '%(

• The constant+ V+ is "te*ans constant 4

V 4 8.@ 10) 7 m) 2 % );

The total power radiated y a lac5 ody o* sur*ace area , at temperature T

is then

E 4 , VT

;T ! σ =



P81* +tefanAs la0

•

The tun-sten *ilament o* an electric lamp has a len-th o*0.8m and a diameter 6 10)8 m. The power ratin- o* the

lamp is 60 7. ,ssumin- the radiation *rom the *ilament is

e!ui&alent to 0W that o* a per*ect lac5 ody radiator at

the same temperature+ estimate the steady temperature o*

the *ilament. '"te*an constant 4 8.@ 10) 7 m) 2 % );(.



Greenhouse effect

• This is a process wherey the earth recei&es short)

wa&elen-th radiation 'hi-h ener-y( *rom the sun which it

re)emits as lon- wa&elen-th radiation and is trapped orasored y -reenhouse -ases such as car$on io,ie

(C: ) in the atmosphere. U&er time+ this causes a rise in

temperature in the atmosphere -i&in- rise to the

haardous e**ect 5nown as 6Blo$al 4arming7. , greenhouse which is used to help certain plants -row

operate in a similar way.



Greenhouse effect



,acuum flas7

• More commonly 5nown as the 6thermos7 is an insulatin-

doule)walled container that allows it contents to stay

hot or cold *or lon- periods o* time. #n order to do this+

all three processes o* heat trans*er must e rou-ht to a

minimum.



,acuum Flas7

• onduction is totally pre&ented throu-h the sides o* the

*las5 y the &acuum etween the doule -lass walls o*

the ottle. The cor5 or plastic stopper contains a lot o*trapped air which is a poor conductor o* heat.

• onvection is reduced y the stopper ut upon remo&al+

it cannot e pre&ented.

• #n the case o* Infrared radiation two sil&er ' "ue to its shi$y appeara$ce ma+es it a 'oo" reflector of ra"iatio$(

coatin-s are used on the -lass walls o* the ottle.



,acuum flas7



+olar Water 3eater

• Solar 4ater heaters use solar collectors to heat water

stored in the hot water tan5 *or industrial and a-ricultural production and daily li*e with hot water. #t consists o*

three '3( main components:

• )solar collectors

• )water tan5 • )circulation lines



+olar Water 3eater

6b Thermodynamics

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