HEAT 1. Heat cont… Heat Consist of the following (i) Thermometry (Thermometers) (ii) Heat Transfer...

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HEAT

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Heat cont…

Heat Consist of the following(i) Thermometry (Thermometers)(ii) Heat Transfer -Thermal Conduction -Thermal Convection -Thermal Radiation(iii) Thermodynamics

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Heat cont…• Definition: Heat is the form of energy which can flow from one point to another

depending to the change of temperature.• INTRODUCTION Heat is the form of energy or we simply call thermal energy which can flow

in a given system depending to fundamental quantity called Temperature• TEMPERATURE The temperature is a quantity of the degree of hotness or coldness being

common to two systems being in thermal dynamics equilibrium. The dimension of temperature is Q

• Temperature is described by a number (chosen Scale such that when two bodies are in Contact heat flows from higher temperature to lower temperature

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Temperature measurements

• The only apparatus used to measure temperature is the thermometer. A Thermometer is a chosen system or body which has some properties continuously varying degree of hotness. The properties are called Thermometric properties.

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Temperature measurements cont…

• Thermometer

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Thermometric properties

• Thermometric property is the property of the substance which varies linearly with the temperature.

ality.proportion ofconstant K where

.X

X

T.re, temperatuat theproperty icthermometrXlet

T

T

T

kT

T

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Thermometric property examples

i. The length of liquid column in a glass to be ( This is for mercury in –glass Thermometer)

ii. The e.m.f of a Thermo couple (for thermocouple Thermometer)

iii. The electrical resistance of platinum wire wound in to a Coil.(for platinum resistance thermometer)

iv. The pressure of a gas whose volume is kept constant (for constant volume gas Thermometer)

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Thermometric property examples cont…

(v) Volume of the gas at constant pressure. (for constant pressure thermometer)

(vi) Colour of radiation emitted by hot body

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Thermometric Property Examples cont…

(i) Length of a liquid column L• Mercury in glass thermometer• Alcohol in glass thermometer• Laboratory thermometer• Clinical thermometer• (Barometer) / Thermometer

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(ii) Electromotive force (e.m.f) of a thermocouple.

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Thermometric Prop. Examples cont…(iii) resistance of the wire

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Thermometric Prop. Examples cont…

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THE FIXED POINTS OF A THERMOMETER.

• Fixed point is a single temperature at which it can confidently be expected that a particular physical events always takes place.

• The two fixed temperatures point must be known and required for setting the temperature range of any thermometer.

• Examples of fixed points: (i) Melting point of pure ice (ice point) or lower fixed

point (ii) upper fixed point (steam point)

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THE FIXED POINTS OF A THERMOMETER. cont…

i. The lower fixed point, or ice point, is the temperature of pure melting ice at normal atmospheric pressure.

ii. The upper fixed point, or steam point, is the temperature of pure boiling water at normal atmospheric pressure.

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.

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.

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HOW TO DEFINE A TEMPERETURE SCALE.

• Essential steps are needed.a) Choose the thermometric materialb) Select the thermometric property of the

materialc) Select the two fixed point (lower and upper)d) To allocate a value such as tm to degree of

material.

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Determination of temperature by using thermometric property.

.

m

m

L

L

u

u

ttureat tempera

material a ofproperty ric thermometbe x

tis re temperatuepoint whos fixedlower

at theproperty ric thermometbex

. tis

re temperatuepoint whos fixedupper

at theproperty ric thermometbe let x

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Determination of temperature by using thermometric property. cont…

lu

lm

lu

lm

lmlm

lulu

uullmm

m

tt

tt

xx

xx

iittkxx

ittkxx

ktxktxktx

tfrom

(i)by (ii)equation divide

).......(

)........(

,,

then, x; m

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.

interval lfundamenta

N where,

or

N

ttNxx

xxtt

ttxx

xxtt

lulu

lmlm

lulu

lmlm

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100

1000

1000100

100 ,0

C scale centigradeIn

100

100

0

0

o

Cxx

xxt

Cxx

xxCt

CCCttN

CtCt

o

l

lmm

o

l

lmom

oolu

oul

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.

Fxx

xxFt

FN

FtFt

F

o

l

lmom

o

ou

ol

18032

180

212,32

)( scale FahrenheitIn

212

o

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Examples of thermometric property

.

CPP

PPt

tP

CEE

EEt

tE

CRR

RRt

tR

o

l

lmm

o

l

lmm

o

l

lmm

mm

100

er thermometgas olumeConstant v.3

100

er thermometleThermocoup.2

100

er thermometresistance platinum .1

100

mm

100

mm

100

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Examples of thermometric property. Cont…

.

CLL

LLt

tL

CVV

VVt

t

o

l

lmm

mm

o

l

lmm

m

100

columnmercury theoflength er, thermometLiquid.5

100

Ver thermometgas pressureConstant .4

100

100

m

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Example 1

• A given platinum has resistance Ro and R100 at the ice point and steam point respectively

i. What is its fundamental interval?ii. If it has resistance R at unknown

temperature tp. what is the expression for tp?

• Solution

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Solution

i. Fundamental internal is the difference between the thermometric value of upper fixed point and lower fixed point.

ii. CRR

RRt p

0

0100

0 100

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Example 2

• Write down an expression which calibrates a thermometer using a thermometric property X to read temperature in 0C is . Hence find the temperature of system when its pressure is 4.5 Pa given that at lower and upper fixed point the pressure is respectively 1.5 Pa and 3.0 Pa.

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Solution

.

NB: If the scale of the degree of hotness is to be in k, then tm =T, t =Ok, N=273. 16k.

100

CXX

XX

LU

Lm 0100

5.13

5.15.4

Co200

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Example3

• The resistance of a resistance thermometer at Celsius temperature as measured by a constant volume gas thermometer is given by

Calculate the temperature as measured on the

scale of the resistance thermometer which corresponds to a temperature of 60 0C on the gas thermometer

2

0 baRR

24100.317.00.50 R

R

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Solution

.

50

0100.3017.050

then ,

be wellascan Rfor equation general thesince

100

0

240

20

0100

0

R

R

baRR

CRR

RR o

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Solution. Cont…

.

C

RR

RRso

R

R

o4.56

1005010010310017.050

50601036017.050

100 ,

10010310017.050

601036017.050

24

24

0100

060

2460

2460

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Thermometers comparedTypes of thermometer

Thermometric property

Advantages Disadvantages

Mercury in glass thermometer

Length of mercury column in capiraly tube

-portable -we read directly

-relatively small-Not very accurate-its error is larger compared to others

Constant volume gas thermometer

pressure -very wide range of temperature-very accurate -very sensitive

-very cumbersome a bulb may have a capacity ofVery slow to use

3310 m

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Thermometers compared. Cont…Types of thermometer

Thermometric property

Advantages Disadvantages

Platinum resistance Electrical resistance -wide range -the most accurate

-not suitable for varying temperature

thermocouple -emf of two wires of different metals

-wide range -very small and used for both local and varying temperature-is the most accurate.

Not as accurate as platinum resistance.

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HEAT TRANSFER

• Heat may be transferred from one point to another depending in the difference in temperature between the two points.

• When heat travels it involves some molecular vibrations or complete movement

• Therefore Conduction of heat can be explained in terms of the kinetic theory of matter.

•

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CONDUCTION

• In conduction a temperature difference causes the transfer of energy from one region hot body to another region of the same body which is at a lower temperature. The flow tends to equalize the temperature within the body.

• Conduction takes place in solids, liquids and gases.

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In solids and Liquids

• Free electrons in the lattice structure of solids or liquids move randomly as molecules in the gas. These transfer energy from the hot region to the cold region of the body.

• In solids the ions are coupled (joined together to form a shape of solid or liquids. The atoms in a heated region vibrate with high K.E to the coupling of atoms they easily transfer this energy to neighboring atoms.

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In gases

• It is are suit of collision of fast moving molecules and slow moving molecules and slow moving molecules during a collision gain K.E

• From kinetic theory the fast moving molecules are from the heated point.

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Temperature distribution in heated conductor

• Consider a cylindrical bar of uniform cross section area A which is heated at one end.

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1x 2x

12

12

gradient etemperetur

above figure the

xx

dx

d

from

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Temperature distribution in heated conductor cont…

NB:• Temperature gradient may be constant or

continuously changing depending on the heat loss to the surrounding before it reaches the other end.

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x

laggedperfect for

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x

bar unlaggedfor

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• If the bar is lagged no loss of heat by any way –conduction, convection or radiation.

• Temperature gradient determines the rate of flow of heat in a body. This indicates that the rate of flow of heat is constant the lines of flow of heat are parallel.

• The temperature gradient along the un lagged bar is not a constant, it decreases with distance from the heated end

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x

dx

d-constant a isbar lagged ain

gradient re temperatu theWhile

. when than that

greater is at slope The

2

1

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NB:• In both cases, the temperature gradient is

negative indicating that the heat flows in the direction of increasing distance between decreasing temp.

• For un lagged bar the lines of flow of heat are diverged

dx

d

x

0

lim

x

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THERMAL CONDUCTIVITY

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HEAT 1. Heat cont… Heat Consist of the following (i) Thermometry (Thermometers) (ii) Heat Transfer...

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