Chapter 13 The Transfer of Heat. 13.1 Convection CONVECTION Convection is the process in which heat...
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Transcript of Chapter 13 The Transfer of Heat. 13.1 Convection CONVECTION Convection is the process in which heat...
![Page 1: Chapter 13 The Transfer of Heat. 13.1 Convection CONVECTION Convection is the process in which heat is carried from one place to another by the bulk movement.](https://reader036.fdocuments.us/reader036/viewer/2022062321/56649eb05503460f94bb6512/html5/thumbnails/1.jpg)
Chapter 13
The Transfer of Heat
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13.1 Convection
CONVECTION
Convection is the process in which heat is carried from one placeto another by the bulk movement of a fluid.
convection currents
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13.1 Convection
Conceptual Example 1 Hot Water Baseboard Heating and Refrigerators
Hot water baseboard heating units are mounted on the wall next tothe floor. The cooling coil in a refrigerator is mounted near the top ofthe refrigerator. Each location is designed to maximize the production of convection currents. Explain how.
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13.1 Convection
“Thermals” can be used by glider pilots to gain considerablealtitude.
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13.1 Convection
Forced Convection
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13.2 Conduction
CONDUCTION
Conduction is the process whereby heat is transferred directly througha material, with any bulk motion of the material playing no role in the transfer.
One mechanism for conduction occurs when the atoms or moleculesin a hotter part of the material vibrate or move with greater energy thanthose in a cooler part.
By means of collisions, the more energetic molecules pass on some oftheir energy to their less energetic neighbors.
Materials that conduct heat well are called thermal conductors, and thosethat conduct heat poorly are called thermal insulators.
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13.2 Conduction
The amount of heat Q that is conducted through the bar depends on a number of factors:
1. The time during which conduction takes place.2. The temperature difference between the ends of the bar.3. The cross sectional area of the bar.4. The length of the bar.
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13.2 Conduction
CONDUCTION OF HEAT THROUGH A MATERIAL
The heat Q conducted during a time t through a bar of lengthL and cross-sectional area A is
L
tTkAQ
SI Units of Thermal Conductivity: J/(s·m·Co)
thermalconductivity
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13.2 Conduction
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13.2 Conduction
Materials with dead air spaces are usually excellent thermalinsulators.
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13.2 Conduction
Example 4 Layered insulation
One wall of a house consists of plywood backed by insulation. The thermal conductivities ofthe insulation and plywood are, respectively,0.030 and 0.080 J/(s·m·Co), and thearea of the wall is 35m2.
Find the amount of heat conducted through the wall in one hour.
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13.2 Conduction
plywoodinsulation
L
tTkA
L
tTkA
But first we must solve for the interface temperature.
plywoodinsulation QQQ
m 019.0
C0.4CmsJ080.0
m 076.0
C0.25CmsJ030.0 tTAtTA
C8.5 T
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13.2 Conduction
J105.9
m 076.0
s 3600C8.5C0.25m 35CmsJ030.0
5
2
insulation
Q
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13.2 Conduction
Conceptual Example 5 An Iced-Up Refrigerator
In a refrigerator, heat is removed by a cold refrigerant fluid that circulates within a tubular space embedded within a metalplate. Decide whether the plate shouldbe made from aluminum or stainless steeland whether the arrangement works betteror worse when it becomes coated witha layer of ice.
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13.3 Radiation
RADIATION
Radiation is the process in whichenergy is transferred by means ofelectromagnetic waves.
A material that is a good absorber is also a good emitter.
A material that absorbs completelyis called a perfect blackbody.
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13.3 Radiation
THE STEFAN-BOLTZMANN LAW OF RADIATION
The radiant energy Q, emitted in a time t by an object that has aKelvin temperature T, a surface area A, and an emissivity e, is givenby
AtTeQ 4
The emissivity e is a dimensionless number between zero and one. It is the ratio of what an object radiates to what the object would radiate ifit were a perfect emitter.
428 KmsJ1067.5
Stefan-Boltzmann constant
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13.3 Radiation
Example 6 A Supergiant Star
The supergiant star Betelgeuse has a surface temperature ofabout 2900 K and emits a power of approximately 4x1030W.
Assuming that Betelgeuse is a perfect emitter and spherical,find its radius.
AtTeQ 4
24 r
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13.3 Radiation
trTeQ 24 4
m103
K 2900KmsJ1067.514
W104
4
11
4428
30
4
Te
tQr
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13.4 Applications
A thermos bottle minimizes heattransfer via conduction, convection,and radiation.
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13.4 Applications
The halogen cooktop stove createselectromagnetic energy that passesthrough the ceramic top and is absorbeddirectly by the bottom of the pot.