Chapter 13The Transfer of Heat

13.1 ConvectionCONVECTION

Convection is the process in which heat is carried from one placeto another by the bulk movement of a fluid.convection currents

13.1 ConvectionConceptual 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.

13.1 ConvectionThermals can be used by glider pilots to gain considerablealtitude.

13.1 ConvectionForced Convection

13.2 ConductionCONDUCTION

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.

13.2 ConductionThe amount of heat Q that is conducted through the bar depends on a number of factors:

The time during which conduction takes place.The temperature difference between the ends of the bar.The cross sectional area of the bar.The length of the bar.

13.2 ConductionCONDUCTION OF HEAT THROUGH A MATERIAL

The heat Q conducted during a time t through a bar of lengthL and cross-sectional area A isSI Units of Thermal Conductivity: J/(smCo)thermalconductivity

13.2 Conduction

13.2 ConductionMaterials with dead air spaces are usually excellent thermalinsulators.

13.2 ConductionExample 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/(smCo), and thearea of the wall is 35m2.

Find the amount of heat conducted through the wall in one hour.

13.2 ConductionBut first we must solve for the interface temperature.

13.2 Conduction

A beverage cooler is in the shape of a cube, 42 cm on each inside edge.Its 3.0 cm thick walls are made of plastic (kT = 0.050 W/m*K). Whenthe outside temperature is 20o C, how much ice will melt inside the cooler each hour?Qcooler = Qice(kAT)t = mL L

13.2 ConductionConceptual 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.

13.3 RadiationRADIATION

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.

Blackbody

Blackbody

QT:13

13.3 RadiationTHE 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 givenbyThe 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.Stefan-Boltzmann constantEmissivity of the human bodyis .65 - .8

13.3 RadiationExample 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.

13.3 Radiation

13.3 Radiation

13.3 RadiationDistance from Earth to Sun = 1.50 x 1011m

In order to cool down a cup of coffee the fastest - do you add thecream right away or do you wait for a few minutes and then add the cream?

In order to cool down a cup of coffee the fastest - do you add the cream right away or do you wait for a few minutes and then add the cream?

A spherical body of 2.0 cm diameter is maintained at 600o C. Assuming that it radiates as if it were a blackbody, at what rate (in watts) is energyradiated from the sphere?

13.4 ApplicationsA thermos bottle minimizes heattransfer via conduction, convection,and radiation.

*NASA photo showing heat areas around Boston and NY. Reddish are giving off most heat - urban areas.****************************