U.3.1. Thermal Energy

8/8/2019 U.3.1. Thermal Energy

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Unit 3Unit 3

Thermal EnergyThermal Energy
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Energy

neithercreates ordestroys

justtransforms

.


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THERMAL ENERGY

Therm alenergy represents the totalin te rn a l e n e rg y o f a n o b je ct: the

su m o f its m o le cu la r kin e tic a n d

.p o te n tia l e n e rg ie s
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What is temperature?

n The average kineticenergy of the molecules

.in an object or system
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What is heat?

n Heat is defined asthe transfer of

thermal energy thatis due to a

difference of.temperature
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hermal EquilibriumTwo objects are said to be in

thermal equilibrium if and only if.they are at the same temperature


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What is a thermometer?

A thermometer is a divice,that through marked scales,can give an indication of its

.own temperature
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SCALES

Celsius Scale.- developed by Anders.Celsius He assigned the number 0 to

the ice point and the number 100 to thesteam point of water. Each division or

()unit on the scale is called a degree

.-Fahrenheit Scale developed by GabrielDaniel Fehrenheit. The ice point is 32 F

.and the steam point is 212 F


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The absolute temperature scale

Temperature

Volume

100C(steam point)

373 K

0C(ice point)

273 K

AbsoluteZero

-273C0 K

Kelvin Scale Rankine Scale

Specifictemperatures

TK = tC+273 TR = tF + 460

1 K = 1 C 1 R = 1 F

.Absolute zero


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Conversions

( )

( )

325

9

67.4599

5

32

9

5

15.273

0

00

0

+=

+=

=

+=

CF

FK

FC

CK

oo


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roblems)1 . .Body temperature is normal at 98 6 F What is the

( . )corresponding temperature on the Celsius scale? 37 0 C

) .2 A steel rail cools from 70 to 30 C in 1 h What is( )the change in temperature in Fahrenheit scale? 72 F

)3 At what temperature will the Celsius scale and theFahrenheit scale have the same numerical reading?

4) The boiling point of oxygen is -297.35F. Express this temrature

in kelvins and in degrees Celsius. (90.0K, -183C)


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he quantity of heatIs the thermal energy necessary to elevate the temperature of a given

mass. Units in SI: Joules (J)

There are three older units that remain in use today:

The calorie (cal), is the quantity of heat required to change thetemperature of 1 gram of water through 1 Celsius degree. 1cal=4.186J

The kilocalorie (kcal), is the quantity of heat required to change thetemperature of 1 kilogram of water through 1 Celsius degree. (1 kcal =1000 cal).

Thebritish thermal unit (Btu), is the quantity of heat required to change

the tempreature of 1 standard pound (lb) through 1 Fahrenheit degree.1Btu=252 cal


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Specific heat capacityThe heat capacity of a body is the ratio of heat supplied tothe corresponding rise in temperature of the body

T

Q

=capacityheat

C

J

K

J

FBtu

The specific heat of a material is the quantity of heat requiredto raise the temperature of a unit mass through one degree.

Tm

Qc

=

CKg

J

Cg

cal

FlbBtu


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SPECIFIC HEAT CAPACITIES


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)1 How much heat is required to raise the temperatureof 200 g of mercury from 20 to 100 C?

roblems


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)3 / Hot coffee with specific heat of 4186 J kgC is. .poured into a 0 5 kg ceramic cup How much heat is

absorbed by the cup if its temperature increases from20 to 80 C? 126 kJ

) - ( = / )4 A 4 kg chunk of metal c 320 J kg C is initially .at 300 C What will its final temperature be if it

loses 50 kJ of heat energy? 261 C
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The measurement of heatThe principle of thermal equilibrium tells us that whenever

objects are placed together in an insulated encolure, theuy willeventually reach the same temperature.

The transfer of thermal energy is given from the warmerbodiesto the coolerbodies.


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If energy is to be conserved, we say that the heat lost by thewarm bodies must equal the heat gained by the cool bodies.

Heat lost = Heat gained Qtotal=0

Conservation of energy

Where the heat lost or gained is calculated by Q = mct

Conservation of thermal energy


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5) A handful of copper shot is heated to 98 C and .thendropped into 160 g of water at 20 C The final

.temperature of the mixture is 25 C What was themass of the shot? 118 g

) - 6 A 400 g copper pipe initially at 200 C is dropped into .a container filled with 3 kg of water at 20 C Ignoring,other heat exchanges what is the equilibrium temperature

of the mixture?. 22 2 C

) - 7 A 450 g chunk of metal is heated to 100 C and then dropped

- .into a 50 g aluminum calorimeter cup containing 100 g of water ,The initial temperature of the cup and water is 10 C and the. .equilibrium temperature is 21 1 C Find the specific heat of

. . / the metal 0 0347 cal gC

) . - 8 A 1 3 kg copper block is heated to 200 C and then

dropped into an insulated container partially filled .with 2 kg of water at 20 C What is the equilibrium. temperature? 30 3 C


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Change of phaseThe states of matter are 4:The states of matter are 4:

LiquidLiquidSolidSolidGasGas

PlasmaPlasma

When a substanceWhen a substance

changes from onechanges from onestate to another westate to another wesay that it hassay that it hasundergone aundergone achangechangeof phaseof phase..
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The change of phase from a solid to a liquid is calledThe change of phase from a solid to a liquid is called fusionfusion, and, and

the temperature at which this change of phase occurs is calledthe temperature at which this change of phase occurs is called

melting point.melting point.

The latent heat of fusion Lf of a

substance is the heat per unit massrequired to change the substancefrom the solid to the liquid phase atits melting temperature

m

QHf =


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The change of phase from a liquid to a vapor is called vaporization, and thetemperature associated with this change is called the boiling point of asubstance..

The latent heat of vaporization Lv of a

substance is the heat per unit massrequired to change the substance from aliquid to a vapor at its boilingtemperature.

m

QHv =
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Change of phase

Quantity of heat,Q

Temperature (C)

100C

Wateronly

0C

-20CIce

Ice andwater

Water and steam Steamonly

0.5cal/gC 80 cal/g

1 cal/gC

540cal/gC

0.48cal/gC


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Heats of fusion and heats of vaporization forvarious substances.


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1) What quantity of heat is required to change 100 g of ice at -15Cto water at 0C?

Two processes

T1

=-150 C

T2=0

=t 0-(- )=15 15m=100grm=100gr

C=C=0.50 cal/gC= =(. )( )Q c m t 5 100( )15

= 750 cal

Hf=80 cal/gr

=Q mHf =( )( )100 80

=8000cal

Qf = +750 8000

=8750cal

roblems


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Five processes.

T1=-100 C

T2=0=t 0-(- )=10 10

m=100grm=100gr

C=C=0.50 cal/gC

Q1= =(. )c m t 5( )( )100 10

= 500 cal

Lf=80 cal/gr

Q2 = mHf =( )( )100 80

=8000cal

2) What quantity of heat is required to change 100 g of ice at -10C2) What quantity of heat is required to change 100 g of ice at -10C

to steam at 130C.to steam at 130C.

T1=00 C

T2=1000 C

=t 100-( )=0 100m=100grm=100gr

C=C=1 cal/gCQ3= =( )( )c m t 1 100( )100

= 10000 cal


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=m 100 gr

Hf=540 cal/gr

Q4 = mHv =( )100( )540

=54000cal

T1=1000 C

T2=1300 C

=t 130-( )=100 30m=100grm=100gr

C=C=.46 cal/gC

Q5= =(. )c m t 46

( )( )100 30= 1380 cal

= + + + +QT Q1 Q2 Q3 Q4 Q5=QT + + + +00 cal 8000 cal 10000 cal 54000 cal 1380= .al 73880 cal


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THERMAL EXPANSION

The most common effect produced by temperature changes is a change insize

, , ,In a solid liquid or gas as the temperature increases the atoms; ,oscillate with greater amplitudes as a result the separation

.between them increases

Example: opening a jar with the aid of warm water.Example: opening a jar with the aid of warm water.


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LINEAR EXPANSION

A change in any one dimension of a solid is called linearA change in any one dimension of a solid is called linearexpansion. In which we need data such as theexpansion. In which we need data such as the initial lengthinitial length,,change of temperaturechange of temperature, and the, and the average coefficient of linearaverage coefficient of linearexpansionexpansion..

i.e. A steel rod.i.e. A steel rod.

tLL i=

)( ifiif ttLLL =

L0

L

Lt0

t t


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Exercise: A 300 m. iron tube at 20C will be used to transfer hotExercise: A 300 m. iron tube at 20C will be used to transfer hotvapor. What will be the total length of the tube once it isvapor. What will be the total length of the tube once it isoperating? =1.2 x 10operating? =1.2 x 10-5-5 / C/ C

tLL i=

)20100)(300)(/102.1( 5 CCmCx

mL 288.0=mLf 29.300=


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AREA EXPANSION

Every linear dimension increases by the same percentage with a changeEvery linear dimension increases by the same percentage with a changein temperature, including holes.in temperature, including holes.

Thermal ExpansionThermal Expansion of anof an areaarea can be calculated from thecan be calculated from the linearlinearexpansion coefficient.expansion coefficient.

tAA i=

2=


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brass disk has a hole 80 mm in diameter punched in. ,ts center Later the disk at 23C is placed into boiling

water for a few minutes. What will be the new area of the hole?

= .A 5040 9 mm2tAA i=

2=


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VOLUME EXPANSIONOver small temperature ranges, theOver small temperature ranges, the thermal expansionthermal expansion is describedis described

by the coefficient ofby the coefficient of linear expressionlinear expression. If the linear expansion is put. If the linear expansion is putin the formin the form

the expanded volumethe expanded volume isis

V V t= 0

)1( tLL if +=

V0 V V V


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A pyrex glass bulb is filled with 50 cm 3 of mercury at 20C. Whatvolume will overflow if the system is heated uniformly to a temperature of60C?

= .V 0 342cm3

Hg = 1.8 x 10-4/C

pyrex = 0.3 x 10-5/C)1( tLL if +=

V V t= 0

The Unusual Expansion of


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TheThe densitydensity, and therefore the, and therefore the volumevolume, of water expand with changes in, of water expand with changes intemperature, bothtemperature, both above and below 4above and below 4CC..

The Unusual Expansion ofWater


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( ).An iron pipe is 60 m long at room temperature 20 C If,this pipe is to be used as a steam pipe how much

,allowance must be made for expansion and what will bethe new length of pipe after steam has been flowing fora while?


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U.3.1. Thermal Energy

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