HEAT 1. Heat cont… Heat Consist of the following (i) Thermometry (Thermometers) (ii) Heat Transfer...
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Transcript of 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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Thermometric Property Examples cont…
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Thermometric Property Examples cont…
(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 Property Examples cont…
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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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THE FIXED POINTS OF A THERMOMETER. cont…
.
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THE FIXED POINTS OF A THERMOMETER. cont…
.
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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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Determination of temperature by using thermometric property. cont…
.
interval lfundamenta
N where,
or
N
ttNxx
xxtt
ttxx
xxtt
lulu
lmlm
lulu
lmlm
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Determination of temperature by using thermometric property. cont…
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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Determination of temperature by using thermometric property. cont…
.
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.
21
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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Temperature distribution in heated conductor cont…
x
laggedperfect for
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Temperature distribution in heated conductor cont…
x
bar unlaggedfor
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Temperature distribution in heated conductor cont…
• 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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Temperature distribution in heated conductor cont…
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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Temperature distribution in heated conductor cont…
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