Chapter 13 The Transfer of Heat

Transcription

1 Chapter 13 The Transfer of Heat Transfer of heat from one point to another 1) Convection 2) Conduction 3) Radiation

2 13.1 Convection CONVECTION The process in which heat is carried from one place to another by the bulk movement of a fluid. convection currents

3 13.1 Convection Conceptual Example: Hot Water Baseboard Heating and Refrigerators Hot water baseboard heating units are mounted on the wall next to the floor. The cooling coil in a refrigerator is mounted near the top of the refrigerator. Each location is designed to maximize the production of convection currents.

4 13.1 Convection Forced Convection

5 13.2 Conduction CONDUCTION The process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer. 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. By means of collisions, the more energetic molecules pass on some of their energy to their less energetic neighbors. Thermal Conductors: Thermal Insulators: Materials that conduct heat well Materials that conduct heat poorly

6 13.2 Conduction 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. 5. Thermal conductivity of the material, k.

7 13.2 Conduction CONDUCTION OF HEAT THROUGH A MATERIAL The heat Q conducted during a time t through a bar of length L and cross-sectional area A is Q = ( ka T ) L t thermal conductivity SI Units of Thermal Conductivity? J/(s m C o )

8 13.2 Conduction

9 13.2 Conduction Materials with dead air spaces are usually excellent thermal insulators.

10 13.2 Conduction Example: Layered insulation One wall of a house consists of m thick insulation. The thermal conductivities of the insulation J/(s m C o ), and the area of the wall is 35 m 2. The temperature inside the house is 25.0 o C, while outside temperature is 4.0 o C. Find the amount of heat conducted through the wall in one hour. Q = ( ka T ) L t Inside temperature 25.0 o C Outside temperature 4.0 o C JJ m

11 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 metal plate. Decide whether the plate should be made from aluminum or stainless steel and whether the arrangement works better or worse when it becomes coated with a layer of ice. Need better conduction Higher the thermal conductivity, higher the conduction (See table 13.1) Ice-layer is worst Aluminum

12 The Transfer of Heat Convection : Bulk motion of fluid (medium) Conduction : No bulk motion of fluid (medium) Q = ( ka T ) Convection and Conduction both need medium What if there is no medium? L t

13 13.3 Radiation 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. A material that absorbs completely is called a perfect blackbody.

14 13.3 Radiation THE STEFAN-BOLTZMANN LAW OF RADIATION The radiant energy Q, emitted in a time t by an object that has a Kelvin temperature T, a surface area A, and an emissivity e, is given by Q = e σ T 4 At Stefan-Boltzmann constant σ = 2 4 ( s m K ) J 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 if it were a perfect emitter. For perfect emitter (An Ideal blackbody), e = 1

15 13.3 Radiation Example: A Supergiant Star A supergiant star has a surface temperature of about 2900 K and emits a power of approximately 4x10 30 W. Assuming that this superstar is a perfect emitter and spherical, find its radius. Q = e σ T 4 At e = 1 r = Q t 4πeσT 4 Power = Q/t 4π r 2 = 4π W [ ]( 2900 K) 4 8 ( ) ( J s m K ) r = m

16 13.4 Applications A thermos bottle minimizes heat transfer via conduction, convection, and radiation.

17 13.4 Applications The halogen cooktop stove creates electromagnetic energy that passes through the ceramic top and is absorbed directly by the bottom of the pot.

18 Transfer of heat from one point to another 1) Convection 2) Conduction 3) Radiation Convection and Conduction both need medium Q = ( ka T ) L t Q = e σ T 4 At

19 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) enhance 2) degrade or 3) have no effect On the insulating value of the concrete wall? 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) Convection forced 2) Conduction 3) Radiation Thermal conductivity of steel is higher than that of concrete

20 For Practice Ch. 13 FOC: 1, 2, 8 & 11. Problems 10, 20, & 42.