C H A P T E R   13The Transfer of Heat

Q: Name three methods of heat transfer?

A: Convection, Conduction, and Radiation.

13.1 Convection

Convection is the process in which heat is carried from place to place by the bulk movement of a fluid.

Convection currents are set up when a pan of water is heated.

During a volcanic eruption, plume of smoke rises because of convection.

Q: In the living room, the heating unit is placed in the floor but the the refrigerator has a top-mounted cooling coil. Why?

A: Air warmed by the baseboard heating unit is pushed to the top of the room by the cooler and denser air. Air cooled by the cooling coil sinks to the bottom of the refrigerator.

13.2 Conduction Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer.

Those materials that conduct heat well are called thermal conductors, while those that conduct heat poorly are known as thermal insulators.

Metals are excellent thermal conductors.Wood, glass, and most plastics are thermal insulators.

The free electrons in metals are responsible for the excellent thermal conductivity of metals.

Rate of heat transfer by conduction, Q/t through the length, L across the cross-sectional area, A is given by the following equation, where k is the thermal conductivity and ΔT is the temperature difference between the two ends.

.L

TkA

t

Q

SI Unit of Thermal Conductivity: J/(s · m · C°)

Conduction: Theory

Table 13.1: Thermal Conductivities of Selected Materials

Other Materials  

Asbestos 0.090

Body fat 0.20

Concrete 1.1

Diamond 2450

Glass 0.80

Goose down 0.025

Ice 2.2

Styrofoam 0.010

Water 0.60

Wood (oak) 0.15

Wool 0.040

Metals  

Aluminum 240

Brass 110

Copper 390

Iron 79

Lead 35

Silver 420

Steel (stainless)

14

Gases  

Air 0.0256

Hydrogen 0.180

Nitrogen 0.0258

Oxygen 0.0265

Thermal Conductivity, k [J/(s · m · C°)]

Styrofoam

Styrofoam is an excellent thermal insulator because it contains many small, dead-air spaces. These small spaces inhibit heat transfer by convection currents, and air itself has a very low thermal conductivity.

Fruit growers sometimes spray water to protect their crops against freezing. After a subzero night, these berries are visible in their insulating jackets of ice.

13.3 Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves.

Heat transfer by radiation can take place through vacuum. This is because electromagnetic waves are involved in radiation and they can propagate through empty space.

Suntans

Suntans are produced by radiation, ultraviolet rays.

Black BodyThe temperature of the lampblack block rises faster than the silver block because the black surface absorbs radiant energy from the sun at a greater rate.

Since absorption and emission are balanced, a material that is a good absorber, like lampblack, is also a good emitter, and a material that is a poor absorber, like polished silver, is also a poor emitter.

A perfect blackbody, being a perfect absorber, is also a perfect emitter.

Summer Clothing

Q: People are uncomfortable wearing dark clothes during the summer. Why?

A: Dark clothes absorb a large fraction of the sun's radiation and then reemit it in all directions. About one-half of the emitted radiation is directed inward toward the body and creates the sensation of warmth. Light-colored clothes, in contrast, are cooler to wear, since they absorb and reemit relatively little of the incident radiation.

The Stefan–BOLTZMANN Law Of Radiation

σ is the Stefan-Boltzmann constant: σ = 5.67 10-8 W/m2.K4.

.4eATPt

Q

The rate at which an object emits radiant energy is proportional to the fourth power of its absolute temperature.

e is called the emissivity: a number between 0 and 1. For perfect radiators e = 1.

A is the surface area.

T is the temperature of the radiator in Kelvin.

Net Radiant Power

The net power is the power the object emits minus the power it absorbs. Applying the Stefan-Boltzmann law leads to the following expression for Pnet when the temperature of the

object is T and the temperature of the environment is T0:

).( 40

4 TTeAPt

Qnet

Home Insulation: R Value

The term L/k in the denominator is called the R value of the insulation.

In the US R-values are expressed in British units, ft2.h.F0/Btu.

For example, the 6.0 inch thick fiber-glass batting has an R-value of 19 ft2.h.F0/Btu.

Regulating the temperature of an orbiting satellite

The highly reflective metal foil covering this satellite (the Hubble Space Telescope) minimizes radiation temperature changes.

Thermos Bottle

A thermos bottle minimizes energy transfer due to convection, conduction, and radiation.

Stopper- minimize conduction.

Double-walled glass vessel with the space between the walls is evacuated to minimize energy losses due to conduction and convection.

The silvered surfaces reflect most of the radiant energy that would otherwise enter or leave the liquid in the thermos.

Halogen Cooktop

In a halogen cooktop, quartz-iodine lamps emit a large amount of electromagnetic energy that is absorbed directly by a pot or pan.