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Lecture 33 Temperature and Heat 1
The Transfer of HeatAdapted from Dr. Al-Kofahi. Picture ref’s:Cutnell & Johnson
PC225, Dec 3rd
Convection
CONVECTION
Convection is the process in which heat is carried from one placeto another by the bulk movement of a fluid.
convection currents
Lecture 33 2Temperature and Heat
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.
Lecture 33 3Temperature and Heat
13.1 Convection
“Thermals” can be used by glider pilots to gain considerablealtitude.
Lecture 33 4Temperature and Heat
13.1 Convection
Forced Convection
Lecture 33 5Temperature and Heat
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.
Lecture 33 6Temperature and Heat
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.
Lecture 33 7Temperature and Heat
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
Lecture 33 8Temperature and Heat
13.2 Conduction
Lecture 33 9Temperature and Heat
13.2 Conduction
Materials with dead air spaces are usually excellent thermalinsulators.
Lecture 33 10Temperature and Heat
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.
Lecture 33 11Temperature and Heat
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
Lecture 33 12Temperature and Heat
13.2 Conduction
J105.9
m 076.0
s 3600C8.5C0.25m 35CmsJ030.0
5
2
insulation
Q
Lecture 33 13Temperature and Heat
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.
Lecture 33 14Temperature and Heat
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.
Lecture 33 15Temperature and Heat
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
Lecture 33 16Temperature and Heat
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
Lecture 33 17Temperature and Heat
13.3 Radiation
trTeQ 24 4
m103
K 2900KmsJ1067.514
W104
4
11
4428
30
4
Te
tQr
Lecture 33 18Temperature and Heat
13.4 Applications
A thermos bottle minimizes heattransfer via conduction, convection,and radiation.
Lecture 33 19Temperature and Heat
13.4 Applications
The halogen cooktop stove createselectromagnetic energy that passesthrough the ceramic top and is absorbeddirectly by the bottom of the pot.
Lecture 33 20Temperature and Heat