a2 chapter 13 thermal properties of material

7/21/2019 A2 Chapter 13 Thermal Properties of Material

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Chapter 13Thermal Properties

of MaterialsA2

Mr. Chong Kwai Kun


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Learning Outcomes

Candidates should be able to:

(a) explain using a simple kinetic model for matter why melting and boiling take place without a change in temperature

the specific latent heat of vaporisation is higher than specific latent heat of

fusion for the same substance

a cooling effect accompanies evaporation

(b) define and use the concept of specific heat capacity, and identify the main

principles of its determination by electrical methods

(c) define and use the concept of specific latent heat, and identify the main

principles of its determination by electrical methods

(d) relate a rise in temperature of a body to an increase in its internal energy

(e) show an understanding that internal energy is determined by the state of the

system and that it can be expressed as the sum of a random distribution ofkinetic and potential energies associated with the molecules of a system

(f) recall and use the first law of thermodynamics expressed in terms of the

increase in internal energy, the heating of the system and the work done on

the system.


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CHAPTER13.1

Specific HeatCapacity


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Heat Capacity (C)

where Q = thermal energy absorbed

= change in temperature

SI unit is J K1or J C1

the amount of heat energy

required to raise thetemperature of a body by 10C

(or 1 K).

Heat Capacity

= QC =CQ


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!uestion 1


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!uestion "


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Speci#c Heat Capacity (c)

where Q = thermal energy absorbed

= change in temperature

m = mass o$ substance

SI unit is J K1%g1or J C1%g1

the amount of heat energy

required to raise the temp of 1g of a substance by 1 K.

!pecific Heat

Capacity

=mQc = mcQ


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!uestion &

"hat is the amount of heat required to raise thetemperature of # g of iron from $00C to %00C

gi&en that the specific heat capacity of iron is '0

*(g oC) +

Q = mc

= 3kg x 460J/(kg oC) x (80 - 20)oC

= 82 800 J


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!uestion '


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!ome materials ha&e a greater capacity for absorbing

heat than others. (for the same mass and rise intemp)

"ater has a specific heat

capacity of '$00 *(g oC),luminium has a specific

heat capacity of -00*(g oC)

Speci#c Heat Capacity (c)


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peci!c Heat Capacit" of somesu#stances

Material Specific eat capacit!/ J /(kg oC)

"l#mi$i#m %00

&ra'' 380gla'' 60

ice 200

iro$ 460

merc#r! 40*ater 4200+i$c 3%0

met!late, 'pirit 2400


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"ater has a high specific heat capacity.

"ater needs a lot of energy to arm it up.

/oss of a large amount of energy only causes small

rise in temperature of ater. hus ater used as circulating liquid in central heating

systems and car engines.

.g. Hot ater bottles to pro&ide armth.

Applications of Highpeci!c Heat Capacit" of

$ater


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easuring speci#c heat capacity

Soli,aggi$g = i$'#lator e.g. *ool

M i i! h t


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Assuming no heat is lost to the surroundings,

Heat suppliedby heater = Heat absorbedby solid

t = mc

Soli,

Measuring speci!c heatcapacit"


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i#i,

calorimeter

easuring speci#c heat capacity

M i i! h t


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i#i, ,ssuming no heat is lost to the surroundings2

Heat suppliedby heater 3 Heat absorbedby

liquid4 heat absorbed by calorimeter

t = ml

cl

1 mc

cc

Measuring speci!c heatcapacit"


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!uestion


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!uestion *


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!uestion +


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CHAPTER13.2

SpecificLatent Heat


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emperature at hich this change of state occurs is

called the melting pointof the substance.

5e&erse process is freezing.

Freezing Point = Melting Point Freezing pointis used to differentiate the process of

free6ing from melting.

elting , -ree.ing a process hereby energy supplied

changes the state of a substance fromsolid state to liquid state ithout a

change in temperature.

7elting

Notes:


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8etermined by plotting cooling cur&e shon belo.

,s liquid naphthalene cools it loses

thermal energy to the surroundings

and its temperature falls (cur&e ,9).

aphthalene begins to solidify.

;ts temperature remains

constant (straight line 9C) until

it completely solidified.

elting /oint


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elting /oint

8uring melting2 the heat energy is not used toincrease the temperature but rather to mo&e the

molecules slightly furtherapart.

his means that the


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emperature at hich this change of state occurs is

called the boiling pointof the substance.

5e&erse process is condensation.

oiling only happen hen boiling point is reached.

0oiling , process hereby energy supplied

changes the state of a substance fromliquid state to gaseous state ithout a

change in temperature.

9oiling

Notes:


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8uring boiling2 the heat energy is not used toincrease the temperature but rather to do or:

1. to separatethe molecules and

$. to pushbac on the surrounding atmosphere

,gain constant temperature means the a&erage K

of the molecules remains unchanged.

0oiling /oint


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emperature ill become constant hen the ater boils.

nergy gained ithout any rise in temperature is calledlatent heat of &aporisation of ater.

Heating Cur%e of $ater


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9oiling point and melting point of ben6ene are %0 C

and C respecti&ely

Cooling Cur%e of &en'ene


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2aporation1. he molecules of a liquid are alays mo&ing randomly

at differentspeeds.

$. ,t the liquid surface2 those molecules hich are more

energeticare able to o&ercomethe attracti&e forces of

other molecules and escapeinto the atmosphere.

#. his process is non as e&aporation.

'. &aporation can happen at any temperature.=. &aporation occurs hen faster>mo&ing molecules

escape from attraction and lea&e surface of liquid.

. ,fter the fastermolecules ha&e left2 the a&erage inetic

energyof the remaining molecules is loered.?. !ince the temperature is directly proportional to the

a&erage K2 therefore2 the temperature drops.

%. &aporation caused cooling effect.


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3i4erences between 2aporation and 0oiling

BOILING EVAPORATION

1 Bubbles are formed No bubbles are formed

2 Occurs withi theli!uid

Occurs at the surface ol"

# Occurs at a fi$edtem%erature

Occurs at a" tem%erature

& Tem%erature remais

costat

Accom%aied b" cooli'

( )ource of eer'"eeded

Eer'" su%%lied b"surroudi'


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!pecific /atent Heat of @usion /f the amount of heat

energy required to change 1 gof the solid to liquid or &ice

&ersa2 ithout any change in

temperature.

!pecific /atent

Heat of @usion

Notes:

or Q = mLf Q = heat m = mass

m

Q=fL


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!pecific /atent Heat of Aaporisation /V

amount of heat energy

required to change 1 g of theliquid to &apour2 or &ice &ersa

ithout any change in

temperature.

!pecific

/atent Heat ofAaporisation

Notes:

e$ce or

SI Unit of Specific Latent Heat is J / g

m

Q=VL VmLQ=


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S56I3 75 6I!8I391. nergy is needed to mo&e the molecules slightly further

apart so that the attraction beteen the molecules is

eaer. Br energy is needed to brea the intermolecular

bondsbeteen the molecules of the solid.

$. o the molecules can roll and slideo&er one another

instead of ust &ibrating about fiDed positions.

Lfof ice =3.4 x 0J/kg


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6I!8I3 75 :;S91. nergy is needed to separate the molecules againsttheir mutual attractions.

$. ;n addition2 eDtra energy needed for the &apour to push

bac on the surrounding atmosphere.

his is hy /&is much largerthan /f

Lvof *ater =2.3 x 06J/kg

62 < 6$


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!uestion


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!uestion >


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!uestion 1?


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!uestion 11

i


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!uestion 1"

! i " (C i d)


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!uestion 1" (Continued)


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CHAPTER13.3

InternalEnergy

; t l


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;nternal nergy is the combination of the total inetic

energy and potential energy of themolecules in the body.

;nternal

nergy

Notes:

;ncludes inetic and potential energy associated iththe random translational2 rotational and &ibrational

motion of the atoms or molecules

,lso includes the intermolecular potential energy

8oes not include macroscopic inetic energy or

eDternal potential energy


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(nternal Energ"

;nternal energy is made up of tocomponents:

1. Kineticenergy E due to the &ibrationofthe particles (directly related to the

temperature)

$.


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7hermal nergyhermal energy:

the total energy of all the particles in a substance.

K.: the energy of a particle due to its motion


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CHAPTER13.)

First Law ofThermodynamics

Thermo* namics


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Thermodynamics is the branch of

physics that is built upon thefundamental las that heat and or

obey.

he collection of obects on hich

attention is being focused is called the

system2 hile e&erything else in the

en&ironment is called the

surroundings.

Thermo*"namics


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he internal energy of a system changes due to heat and or:

!ork is negative when it is done by the system andnegative when it is done on the system.

"eat is positive when the system gains heat and

negative when the system loses heat.

THE +(RT LA$ O+ THERMO,-AM(C

WQU +=

! ti 1"


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;n part aof figure2 the system gains 1=00 of heat and

$$00 of or is done by the system on its surroundings.(a)

!uestion 1"

( ) ( )

J!""

J##""J$%""

=

++=

+=

U

U

WQU

! ti 1&


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(b) ;n part b2 the system also gains 1=00 of

heat2 but $$00 of or is done on the

system.

!uestion 1&

( ) ( )

J&!""

J##""J$%""

+=

+++=

+=

U

U

WQU

! ti 1'


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he temperature of three moles of a monatomic ideal gas is

reduced from ='0K to #=0K as ==00 of heat flos into thegas.

@ind (a) the change in internal energy and (b) the or done

by the gas.

!uestion 1'

nRTU#

&=WQUUUif

+==


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(a)

(b)

( ) ( )( )( ) J!$""'%("'&%"'molJ&$)*mol")&

#&

#&

==

= if nRTnRTU

( ) J$#+""J!$""J%%"" =++== UQW

) t"pes of thermal processes


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,n isobaric processis a process that occurs at

constant pressure.

,n isochoric processis a process that occurs at

constant &olume.

,n isothermal processis a process that occurs at

constant temperature.

,n adiabatic processis a process during hich no

energy is transferred to or from the system as heatat.

) t"pes of thermal processes

iso#aric process


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,n isobaric process is one that occurs at constant pressure.

Isobaric process:

iso#aric process

( ) VPAsPFsW ===

VPW =

!uestion 1


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Bne gram of ater is placed in the

cylinder and the pressure is maintainedat $.0D10=%m#.

@ind the or done and the change in

internal energy.

!uestion 1

olution


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olution

( )( ) J""#")"m$"")$,a$"")#&*% ==

=

VPW

J$&"J""#")"J$&" === WQU

( ) ( )[ ]( ) J$&"-&$-gJ($*+g""$")" === TmcQ

(sochronic


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isochoric: constant &olume

Why is work eual to ! for an isochoric process"

(sochronic

QWQU ==

"=W

(sothermal


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;!BH57,/ F


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o moles of the monatomic gas argon eDpand isothermally

at $-%K from and initial &olume of 0.0$=m#to a final &olume

of 0.0=0m#. ,ssuming that argon is an ideal gas2 find(a) the or done by the gas2

(b) the change in internal energy of the gas2 and

(c) the heat supplied to the gas.

!uestion 1*

(a)

(b)

(c)

Solution

( ) ( ) ( ) J&(""m#%")"

m"%")"ln#.*&$)*")#ln

&

&

=

=

=

i

f

V

VnRTW

"#&

#& == if nRTnRTU

WQU +=

J&(""+== WQ

A*ia#atic


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,8;,9,;C F


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0.10 mol of a monoatomic gas follos the process shon in the

@igure2 "hat is the total change in thermal energy of the gas+

!uestion 1+

!uestion 1


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!uestion 1

!uestion 1


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!uestion 1

+ormul


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,e!nition+ormul

a/nits

Latent

heat offusion

7he energy needed

to change asubstance $romsolid to li@uidwithout a change intemperatureA

Latentheat of%aporisation

7he energy neededto change asubstance $romli@uid to gas

without a change intemperatureApeci!clatentheat of

fusion

7he amount o$energy needed tochange a unit mass

o$ a substance $rom J %g1

,e!nition +ormula /nits


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,e!nition +ormula /nitspeci!clatent

heat offusion

7he amount o$energy needed to

change a unit masso$ a substance $romsolid to li@uidwithout a change intemperatureA

J %g1

peci!clatentheat of%aporisati

on

7he amount o$energy needed tochange a unit masso$ a substance $rom

li@uid to gas withouta change intemperatureA

J %g1

a2 chapter 13 thermal properties of material

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