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.