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Chapter 13
The Transfer of Heat
1) Convection
2) Conduction
3) Radiation
Transfer of heat from one point to another
13.1 Convection
The process in which heat is carried from one placeto another by the bulk movement of a fluid.
convection currents
CONVECTION
13.1 Convection
Conceptual Example: 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.
13.1 Convection
Forced Convection
13.2 Conduction
The process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer.
By means of collisions, the more energetic molecules pass on some oftheir energy to their less energetic neighbors.
Materials that conduct heat poorly
CONDUCTION
No bulk motion of medium
One mechanism for conduction occurs when the atoms or molecules in a hotter part of the material vibrate or move with greater energy than those in a cooler part.
Thermal Conductors: Materials that conduct heat well
Thermal Insulators:
13.2 Conduction
5. Thermal conductivity of the material, k.
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, t.
2. The temperature difference between the ends of the bar, ΔT.
3. The cross sectional area of the bar, A.4. The length of the bar, L.
13.2 Conduction
CONDUCTION OF HEAT THROUGH A MATERIALThe 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?
thermalconductivity
J/(s·m·Co)
13.2 Conduction
13.2 Conduction
Materials with dead air spaces are usually excellent thermal insulators.
13.2 Conduction
Example: Layered insulation
One wall of a house consists of 0.076 m thick insulation. The thermal conductivities of the insulation 0.030 J/(s·m·Co), and the area of the wall is 35 m2. The temperature inside the house is 25.0 oC, while outside temperature is 4.0 oC. Find the amount of heat conducted through the wall in one hour.
( )L
tTkAQ ∆=
0.076 m
25.0 oC4.0 oC
Inside temperature
Outside temperature
1.044 × 106𝐽𝐽
13.2 Conduction
Conceptual Example: 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.
Higher the thermal conductivity, higher the conduction (See table 13.1)
Aluminum
Need better conduction
Ice-layer is worst
The Transfer of Heat
Convection and Conduction both need medium
Convection : Bulk motion of fluid (medium)
Conduction : No bulk motion of fluid (medium)
( )L
tTkAQ ∆=
What if there is no medium?
13.3 Radiation
A material that absorbs completelyis called a perfect blackbody.
RADIATION
Radiation is the process in which energy is transferred by means of electromagnetic waves.
No medium is required
A material that is a good absorber is also a good emitter of electromagnetic waves.
13.3 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 given by
AtTeQ 4σ=
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 ⋅⋅×= −σ
Stefan-Boltzmann constant
THE STEFAN-BOLTZMANN LAW OF RADIATION
For perfect emitter (An Ideal blackbody), e = 1
13.3 Radiation
Example: A Supergiant StarA supergiant star has a surface temperature of about 2900 K and emits a power of approximately 4x1030 W. Assuming that this superstar is a perfect emitter and spherical, find its radius.
AtTeQ 4σ=24 rπ
Power = Q/t44 Te
tQrσπ
=
e = 1
( ) ( )[ ]( )4428
30
K 2900KmsJ1067.514W104⋅⋅×
×=
−π
m103 11×=r
13.4 Applications
A thermos bottle minimizes heattransfer via conduction, convection,and radiation.
13.4 Applications
The halogen cooktop stove createselectromagnetic energy that passesthrough the ceramic top and is absorbeddirectly by the bottom of the pot.
1) Convection
2) Conduction
3) Radiation
Transfer of heat from one point to another
Convection and Conduction both need medium
( )L
tTkAQ ∆=
AtTeQ 4σ=
Q: Several days after a snowstorm, the outdoor temperature remains below freezing. The roof of house A is uniformly covered with snow. On house B, however, the snow on the roof has completely melted. Which house is better insulated?1) House A.2) House B
Q: Concrete walls often contain steel reinforcement bars. Does the steel 1) enhance2) degrade or3) have no effectOn the insulating value of the concrete wall ?
Thermal conductivity of steel is higher than that of concrete
Q: One way that heat is transferred from place to place inside human body is by the flow of blood. Which one of the following heat transfer process best describes this action of blood?1) Convection2) Conduction3) Radiation
forced
HWAssignment#4 on WileyPlus is due by Thursday, June 8th
For PracticeCh. 13
FOC: 1, 2, 8 & 11. Problems 10, 20, & 42.
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