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Heating and CoolingIntroduction to Chapter 27

What process does a hot cup of coffee undergo as it cools? How does your bedroombecome warm during the winter? How does the cooling system of a car work?Understanding heat transfer will help you answer these questions. This chapter willlook into three types of heat transfer: convection, conduction and radiation.

Investigations to Chapter 27

In this Investigation you will compare heat conduction in several materials by usingyour sense of touch and then rank their thermal conductivity.

In this Investigation you will observe both natural convection and forcedconvection. A flask of hot water with red dye will be placed in a beaker filled withcool water. The hot red water will rise into the cooler water due to naturalconvection. You are going to observe the process and take temperature data toanalyze how much heat is transferred via convection. You will also blow through astraw to force the red dye out of the flask into the larger beaker to explore forcedconvection.

In this Investigation you will use a 100-watt light bulb as the source of radiation.You will observe and compare the increase in temperature (using a temperatureprobe) in air, water, sand, and soil.

27.1 Conduction How well do common materials conduct heat?

27.2 Convection How much heat is transferred throughconvection?

27.3 Radiation Which materials are good absorbers ofradiation?

Chapter 27Heat

Transfer

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Chapter 27: Heat Transfer

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Learning Goals

In this chapter, you will:

Describe how thermal energy is transferred by conduction.

List various kinds of materials that are heat conductors or insulators.

Explain why thermal and electrical conductivity of a material are related.

Analyze how energy can be transferred through convection.

Describe the motion of liquid because of temperature differences within the system.

Describe applications of convection.

Explain what properties make a good absorber of heat.

Explain the color-temperature relationship.

Vocabulary

absorbers forced convection reflectors thermal insulatorsconduction heat transfer sea breeze ultraviolet lightconvection infrared light thermal conductivityemitters natural (or buoyant) convection thermal conductors

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27.1 Conduction

Figure 27.1: A metal spoon placed in hot water quickly transmits the heat to your hand.

Figure 27.2: When a warmer material, like the hot water in this cup, comes in contact with a cooler material, like the spoon, there are lots of collisions between the atoms and molecules of each.

27.1 ConductionThermal energy travels as heat from a material at a higher temperature to a material at a lowertemperature. This general process is called heat transfer. How is heat transferred from material tomaterial, or from place to place? What materials do we use to keep things warm or cold? It turns outthere are three quite distinct mechanisms of heat transfer. In this section, you will learn aboutconduction, which is the transfer of heat by the direct contact of particles of matter. By comparing theheat conduction of different materials, you can rank their thermal conductivity.

What is conduction?

What isconduction?

Conduction is the transfer of heat by the direct contact of particles of matter.Conduction occurs between two materials at different temperatures when they aretouching each other. Conduction can also occur through one material, if one partof the material is hotter than another part. For example, a metal spoon placed inhot water quickly transmits the heat to your hand.

Conduction is the transfer of heat by the direct contact of particles of matter.

How is thermalenergy

transferred?

Imagine that you are the size of an atom. Things that used to look unmoving like atable or even air now appear as a sea of atoms and molecules in constant motion.In solids the molecules and atoms vibrate in place, in liquids they move over andaround each other, and in gases they shoot around. Collisions are occurringeverywhere as atoms and molecules jiggle and zoom.

All these moving atoms and molecules have kinetic energy. Now imagine whathappens at the atomic level when a warmer material comes in contact with acooler material. The atoms and molecules of the warmer material are movingaround faster than the atoms and molecules of the cooler material. Where the twomaterials are in contact there are lots of collisions between the atoms andmolecules of each.

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What happens asthese collisions

take place?

Imagine the beginning of a giant bumper car rally. Some cars start out fast andothers start out slow. Soon they are all hitting each other. When a fast car bouncesinto a slow car, the fast car slows down a bit and the slow car speeds up a bit. Thecars may even change direction as well! As each car changes direction it then hitsother cars around it. Pretty soon all the cars in the giant rally are bouncing off eachother at about the same average speed. When this happens, they are in equilibrium.

The collisionshappen until

thermalequilibrium is

reached

The same thing happens at the atomic level. As collisions occur, the atoms andmolecules of the warmer material slow down, and the atoms and molecules of thecooler material speed up. Some of the kinetic energy of the hotter material istransferred, one collision at a time, to the cooler material. Soon, both materials areat the same temperature (figure 27.3). This is how two materials reach thermalequilibrium by conduction.

Conductors and insulators

Which state ofmatter conducts

best?

Conduction can take place in solids, liquids and gases. However, the more denselypacked atoms or molecules of a solid can conduct more heat because there aremany more collisions taking place. The low density of gases mean that relativelyfew collisions take place per second and therefore air is a poor conductor of heat.This explains why many things we use to keep things warm or cold, such as foam,fiberglass insulation, and down jackets, contain air pockets that slow down thetransfer of heat.

What are thermalinsulators and

thermalconductors?

In general, materials that conduct heat easily are called thermal conductors andthose that conduct heat poorly are called thermal insulators. For example, metal isa thermal conductor, and foam is a thermal insulator. You may remember that thewords conductor and insulator are also used to describe a material’s ability toconduct electrical current. There is a reason for this common usage. In general,good electrical conductors are also good heat conductors. Remember, metals aregood conductors of current because the metal atoms have electrons that escapeeasily from the atom. When a metal conducts heat, these free electrons alsotransfer kinetic energy easily.

Figure 27.3: In a hot glass of cocoa, the particles in the liquid and air collide with the particles in the glass. Because kinetic energy is transferred by these collisions, cocoa, glass and air eventually reach the same temperature.

Figure 27.4: Styrofoam is a better thermal insulator than glass. Therefore, liquid in a foam container will retain heat longer than it would in a glass container.

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27.1 Conduction

Figure 27.5: Heat flows from hot to cold. On a cold winter day, heat flows through the window from inside to outside.

Heat flows quickly if the temperature changes a lot over a short distance. With single pane glass the temperature changes from inside to outside over the thickness of the glass, resulting in a large heat flow.

Double pane glass spreads the temperature change over a much longer distance, and puts a layer of insulating air between the glass panes. As a result, much less heat is lost through the window.

�Thermal conductivity in the building and manufacturing industries

Differentmaterials conductthermal energy at

different rates

Thermal conductivity is a measure of how well a material conducts heat. Althoughsolids in general are better conductors of heat than liquids or gases, each materialconducts heat at a different rate. We can compare thermal conductivities bymeasuring how fast a certain amount of thermal energy flows through uniformlysized pieces of various materials.

Thermalconductivity in

your home

Measuring the thermal conductivity of different materials is important in thebuilding and manufacturing industries. We don’t want hot air to leave our homewhen it’s cold outside and we also don’t want hot air to enter our home when it’svery hot outside. A home built with good insulators lessens heat transfer in bothdirections.

Thermalconductivity in the

workplace

Sometimes you want to conduct heat away quickly. Did you know that hammeringa penny or paper clip makes it hot? Some of the work done on the penny orpaperclip is converted to thermal energy. Drilling, hammering, and many othermanufacturing processes create unwanted increases in temperature. If the heat hasto be transferred away quickly, structures like metal grills and fins are often used.Metal is a good conductor of heat, and grills and fins add surface area to increasecooling by convection, another form of heat transfer.

�Windows and energy loss

The thermal conductivity of a single-pane glass window is very highand heat is easily transferred through it. Air leakage also occursaround windows, which results in even more heat transfer. This meansthat about one-third of home heat in the United Sates is lost throughwindowpanes and window frames. This energy loss is about equal toall the energy available from the oil flowing through the Alaskanpipeline in an entire year.

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Figure 27.6: The air right above the flame heats up and expands, transferring heat to your hand.

Figure 27.7: Currents caused by convection are responsible for much of our weather. Warm air rises off the surface of the Earth. As it cools, it sinks back down and replaces warmer air.

27.2 ConvectionAnother type of heat transfer that is due to temperature differences is called convection. This type ofheat transfer is responsible for global weather patterns, the heating of our homes and the circulation ofwaters in the oceans.

What is convection?

What isconvection?

Have you ever warmed up your hands by putting them over an open flame? Youcan do this because the air right above the flame heats up and expands. Becausethe expanded air is less dense, it rises, bringing the heat to your hand (figure 27.6).This heat transfer process is called convection. Unlike conduction, which occursmostly in solids, convection occurs only in liquids and gases. Convection comesfrom a Latin word meaning to carry together.

Convection is the transfer of heat by the actual motion of a fluid (liquid or gas) in the form of currents.

Convection can occur in all fluids, whether liquids or gases. Convection occursbecause warmer fluids are less dense, and rise. Cooler fluids are more dense, andsink. This motion of fluids causes currents.

Convection causesthe weather

patterns on Earth

The currents caused by convection occur constantly in our atmosphere and areresponsible for much of our weather. On a global scale, hot air near the equatorrises and is forced toward the poles as shown in figure 27.7. The sinking air forcescold air at the poles toward the equator. Combined with forces due to the rotationof the Earth, convection and unequal heating are the primary causes of weather.

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27.2 Convection

Natural convection

Why does warmerair rise?

There is a natural upward force called buoyancy. This force occurs whenever youhave an object submerged in a denser medium. An example is an inflated ballunder water. The ball is less dense than the water. There is an upward force equalto the weight of the displaced medium that pushes the ball out of the water. Butthis force not only applies to solids, it also applies to fluids and gases. As heatedair or a fluid rises, there are density differences, which act with gravitationalforces to produce natural or buoyant convection.

Sea breezes aredue to convection

On a smaller scale near coastlines, convection is responsible for sea breezes.During the daytime, land is much hotter than the ocean. A sea breeze is createdwhen hot air over the land rises due to convection and is replaced by cooler airfrom the ocean. In the evening, the ground cools rapidly but the ocean remainswarm, due to water’s high specific heat. Warm air rises over the water and isreplaced with cooler air from over the land. This is known as the land breeze.

Heating a room As a clear example of natural convection, we can analyze how a room radiatorheats a room during winter (figure 27.8). As the temperature of the air around theradiator is increased by conduction, it becomes less dense than the cold air in theroom. This warmer air rises and cooler air from the far side of the room replaces it.This air circulation transfers heat from the radiator to the cooler parts of the room.

Figure 27.8: During the day, a sea breeze is created when hot air over the land rises due to convection and is replaced by cooler air from the ocean.At night, temperatures reverse and a land breeze occurs. This happens because the land cools more rapidly than the ocean.

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Convection in thekitchen

Another application of natural convection is cooking on a gas stovetop. As with acandle, the heat from the burning gas rises to boil the water in the pot above it.Even the water in the cooking pot itself experiences convection. The hot water atthe bottom of the pot rises to the top of the pot, replaced by the cooler water. Nextthe cooler water is heated. If this did not happen, we would have to rely on theslower method of conduction to boil a pot of water.

Why wearing asweater keeps you

warm

Through the process of convection, air carries heat away from your body. A woolsweater prevents this from happening by trapping air in many small pockets sothat it cannot flow and carry the heat away. Similarly, in cold weather birds trappockets of air by fluffing their feathers.

Wind chill If you expose bare skin to cold temperatures, natural convection can quicklybecome dangerous. As the air surrounding your body warms up, it rises and carriesheat away. The effect of air carrying heat away is greatly increased on a windy daywhen a steady stream of air flows. The faster the wind speed, the more effectivelyheat is carried away. Antarctic explorers created a commonly used method for“measuring” the chilling effects of the wind, called the Wind Chill EquivalentTemperature or wind chill factor. The wind chill factor was originally based on thetemperature at which plastic jugs of water placed on top of a high pole wouldfreeze, given a certain wind speed.

Why does smokerise up thechimney?

Convection causes the smoke from the fire in a fireplace to rise up the chimneyinstead of entering your home. This is because convection is extremely efficient infocusing the heat in one direction: up! Smoke particles are carried upward by therising hot air. If you have ever toasted marshmallows on a campfire, you may havenoticed that if you hold the marshmallow on the stick right next to the fire, it getsnice and toasty. However, if you hold the marshmallow directly above the fire, itquickly catches fire and burns to a crisp. The air directly above the fire carriesmuch more heat by convection. At the edge of the fire, the heat is mostly carriedby a different kind of heat transfer called radiation.

Figure 27.9: Convection in water. The hot water at the bottom of the pot rises to the top and replaces the cold water.

Figure 27.10: Convection is extremely efficient at focusing heat in one direction: up.

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27.2 Convection

Forced convection

What is forcedconvection?

Another type of convection is the one in which a mechanical device is used toforce the fluid or gas to move, as opposed to the buoyant force. This is calledforced convection. Air or liquids can be forced with fans or pumps. Warm fluidscan carry heat to cooler regions and cool fluids can take heat away from hotregions.

� Applicationsof forced

convection

Most heating systems use a combination of forced and natural convection. Let’sreturn to the example of a radiator for home heating. Water is heated in thebasement and pumped into the rooms of the house. The process of pumping thehot water through the house is forced convection. In a room, the hot water releasesheat to the air surrounding the radiator through conduction. The heat is thencarried away from the radiator by natural convection (figure 27.11).

The opposite occurs in an air-conditioning system, where cool air is blownthrough a room with a fan. This forces the cooler air to replace the warmer air inthe room.

Forced convectionin a car

In the radiator of a car, there are two examples of forced convection. In the coolingsystem of a car, heat is transferred from the engine to the water by conduction.Then the heated water is pumped to the radiator by forced convection. After thewater is inside the radiator, heat from the water is conducted to the radiator fins.The radiator fins are cooled by air blowing over the radiator.

Figure 27.11: Both natural and forced convection help to heat a house.

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Figure 27.12: Most of the Earth’s heat is electromagnetic radiation that comes from the sun.

Where does solar radiation go?

Of the total incoming solarradiation:

• 30% is returned to outerspace.

• 47% is absorbed by theEarth.

• 23% is used to drive thehydrologic cycle.

• 0.2% drives the winds.• .02% is absorbed by

plants to be used inphotosynthesis.

27.3 RadiationHave you ever stood in the sun on a cold day? If it is not too windy, you will feel the sun’s warmth, nomatter how cold it is outside. On a warm day, you will feel even hotter if you stand in the sun. Howdoes the warmth of the sun reach the Earth? In this section, you will learn about another type of heattransfer known as radiation that is responsible for the way the sun warms our planet. Radiation is a typeof heat transfer that does not require matter to travel through.

Electromagnetic radiation

Radiation is heat transfer by electromagnetic waves.

What iselectromagnetic

radiation?

One form of heat transfer due to radiation comes from electromagnetic radiationsuch as light, ultraviolet rays, X rays, and infrared rays. You know that conductionand convection require matter to transfer heat. However, as you learnedpreviously, electromagnetic waves can travel through a vacuum. This is fortunatebecause the Earth receives most of its heat in the form of electromagnetic radiationfrom the sun. Since space is a vacuum, radiation is the primary way we can receiveheat from the sun.

Energy-radiation relationships

What types ofradiation do

objects emit?

All objects emit radiation due to their thermal properties, or because they havesome internal thermal energy. Some objects emit mostly visible light, someultraviolet, and some infrared. The type of radiation an object emits depends on itstemperature. Hotter objects have more energy per molecule than cold objects.Thus hot objects emit light with a higher frequency than cold objects. Ultravioletphotons have more energy than visible light. Visible light has more energy thaninfrared light. You learned previously how the colors of the rainbow, Red, Orange,Yellow, Green, Blue, and Violet (ROYGBV) are related to the energy of thevisible light.

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27.3 Radiation

What is infraredradiation?

Infrared radiation has lower energy than visible light. While human eyes cannotdetect infrared radiation, certain species of snakes can. You may have seen popularspy movies where the hero uses an infrared viewer to see people in the dark. Inaddition, firefighters use infrared equipment to find people in smoke-filled rooms.

Color-temperaturerelationships

You may have noticed that when a light bulb on a dimmer is turned on slowly, thebulb will begin to heat up, then glow in the red, then orange, and then yellow areasof the electromagnetic spectrum. This is because different temperatures cause thefilament in the light bulb to glow at different colors (figure 27.13).

Why do starsappear in different

colors?

Stars also have different colors. The coolest stars are red, such as Antares in theheart of Scorpio the scorpion. The warmest stars are blue. A bright blue star isBetelgeuse, in the knee of Orion the hunter. Astronomers can tell the temperatureof a star by looking at its color. But that does notmean the star only emits light at that color. Thestar emits light in a range of colors; however,the peak color is what we actually detect withour eyes. The light emitted by a star can berepresented using a spectral diagram, like thediagram at the right.

�Light bulbs

If objects have to be incredibly hot in order to glow in the visiblespectrum, why doesn’t the filament in a light bulb burn up?Remember that a fire requires two things, fuel and oxygen. One ofThomas Edison’s contributions to engineering was developing the

tungsten filament, which could withstand high temperatures. He also had theidea of removing the air from the bulb to prevent the filament from reactingwith oxygen too quickly. Edison invented the incandescent light bulb in 1879.

Figure 27.13: As the temperature of the light bulb increases, the light glows in the red, then orange, then yellow areas of the spectrum.

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Figure 27.14: A black road surface is a good absorber and good emitter of radiation. A white sand beach is a poor absorber and poor emitter of radiation.

Absorption and emission

Absorbers andreflectors

Some objects are good absorbers and some objects are good reflectors. Goodreflectors reflect most of the radiation that hits the object. Shiny metallic objectsare great reflectors. Generally, materials that are good conductors of electricityand heat are also good reflectors. White and light colored objects also make goodreflectors.

Color andabsorption

Remember that white objects reflect light of all wavelengths. Black and darkobjects tend to absorb all light that falls upon them. They take the radiation andconvert it into thermal energy, increasing the temperature of the object. Solar cells,which convert sunlight to electricity, are black so that they will absorb as muchlight energy as possible.

Emitters ofradiation

Objects that are good absorbers of radiation are also good emitters of radiation.Thus, after sunset, a black road surface emits radiation and cools quickly, whereasa white sandy surface of the beach would not emit radiation efficiently and wouldcool slowly (figure 27.14).

�Reflection and planets

When we look up in the nighttime sky, we can see stars, planetsand our moon. The stars generate their own light, just like oursun. The planets and our moon only reflect light that the sunemits. The albedo of a planet is the percentage of the sun’s light

reflected from its surface. A planet with little or no atmosphere (such as Mercuryor our moon) reflects very little light because rocks tend to absorb light. Cloudstend to have a high reflectivity, thus Venus, Jupiter, and Saturn have very highalbedos because they have lots of clouds. Ice and snow also have a very highreflectivity. The icy moons of Saturn reflect over 90 percent of the light that hitstheir surface. The albedo of Earth varies with the constantly changing cloud coverand the amount of snow covering the planet’s surface. Given the low albedo of thecloudless moon, why do you think it appears so bright in our nighttime sky?

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Chapter 27 Review

Chapter 27 Review

Vocabulary review

Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms.

Concept review

1. What properties make a material a good thermal conductor?Give three examples of good thermal conductors.

2. What properties make a material a good thermal insulator in asolid? Give three examples of solids that are insulators.

3. Why is air a bad conductor of heat? How can air be used as aninsulator? Give two examples.

4. Why does hot air rise?

5. Why doesn’t convection occur in a solid?

6. What is a sea breeze? When and why does it happen?

7. What is a land breeze? When and why does it happen?

8. What is forced convection?

9. Explain the color-temperature relationship.

10. What properties make a material a good absorber?

11. What properties make a material a good reflector?

Set One Set Two1. heat transfer a. A method of heat transfer by direct contact of

particles of matter1. convection a. The transfer of heat by electromagnetic waves

2. thermal conductors b. When energy in the form of heat travels from a hot object to a cold object

2. forced convection b. The process by which radiant energy raises the temperature of a material

3. conduction c. Materials that conduct heat poorly 3. natural convection c. A process where the buoyancy of warmer air causes a current of air to carry away heat

4. thermal insulators d. Property of a material that describes how well (or poorly) it conducts heat

4. absorption d. A process where a mechanical device is used to move a fluid or gas to transfer heat

5. thermal conductivity e. Heat transfer by the movement of particles

5. radiation e. Heat transfer caused by the actual movement of matter

f. Materials that conduct heat easily f. Heat transfer due to density differences in solid materials