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Transcript
9 Heating and Cooling Introduction to Chapter 27 What process does a hot cup of coffee undergo as it cools? How does your bedroom become warm during the winter? How does the cooling system of a car work? Understanding heat transfer will help you answer these questions. This chapter will look into three types of heat transfer: convection, conduction and radiation. Chapter 27 Heat Transfer Investigations to Chapter 27 27.1 Conduction How well do common materials conduct heat? In this Investigation you will compare heat conduction in several materials by using your sense of touch and then rank their thermal conductivity. 27.2 Convection How much heat is transferred through convection? In this Investigation you will observe both natural convection and forced convection. A flask of hot water with red dye will be placed in a beaker filled with cool water. The hot red water will rise into the cooler water due to natural convection. You are going to observe the process and take temperature data to analyze how much heat is transferred via convection. You will also blow through a straw to force the red dye out of the flask into the larger beaker to explore forced convection. 27.3 Radiation Which materials are good absorbers of radiation? 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 temperature probe) in air, water, sand, and soil. 457 Chapter 27: Heat Transfer Learning Goals In this chapter, you will: D Describe how thermal energy is transferred by conduction. D List various kinds of materials that are heat conductors or insulators. D Explain why thermal and electrical conductivity of a material are related. D Analyze how energy can be transferred through convection. D Describe the motion of liquid because of temperature differences within the system. D Describe applications of convection. D Explain what properties make a good absorber of heat. D Explain the color-temperature relationship. Vocabulary absorbers conduction convection emitters 458 forced convection heat transfer infrared light natural (or buoyant) convection reflectors thermal insulators sea breeze ultraviolet light thermal conductivity thermal conductors Chapter 27 27.1 Conduction Thermal energy travels as heat from a material at a higher temperature to a material at a lower temperature. This general process is called heat transfer. How is heat transferred from material to material, or from place to place? What materials do we use to keep things warm or cold? It turns out there are three quite distinct mechanisms of heat transfer. In this section, you will learn about conduction, which is the transfer of heat by the direct contact of particles of matter. By comparing the heat conduction of different materials, you can rank their thermal conductivity. What is conduction? What is Conduction is the transfer of heat by the direct contact of particles of matter. conduction? Conduction occurs between two materials at different temperatures when they are touching each other. Conduction can also occur through one material, if one part of the material is hotter than another part. For example, a metal spoon placed in hot water quickly transmits the heat to your hand. Figure 27.1: A metal spoon placed in hot water quickly transmits the heat to your hand. Conduction is the transfer of heat by the direct contact of particles of matter. How is thermal Imagine that you are the size of an atom. Things that used to look unmoving like a energy table or even air now appear as a sea of atoms and molecules in constant motion. transferred? In solids the molecules and atoms vibrate in place, in liquids they move over and around each other, and in gases they shoot around. Collisions are occurring everywhere as atoms and molecules jiggle and zoom. All these moving atoms and molecules have kinetic energy. Now imagine what happens at the atomic level when a warmer material comes in contact with a cooler material. The atoms and molecules of the warmer material are moving around faster than the atoms and molecules of the cooler material. Where the two materials are in contact there are lots of collisions between the atoms and molecules of each. 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 Conduction 459 Chapter 27 What happens as Imagine the beginning of a giant bumper car rally. Some cars start out fast and these collisions others start out slow. Soon they are all hitting each other. When a fast car bounces take place? into a slow car, the fast car slows down a bit and the slow car speeds up a bit. The cars may even change direction as well! As each car changes direction it then hits other cars around it. Pretty soon all the cars in the giant rally are bouncing off each other at about the same average speed. When this happens, they are in equilibrium. The collisions happen until thermal equilibrium is reached The same thing happens at the atomic level. As collisions occur, the atoms and molecules of the warmer material slow down, and the atoms and molecules of the cooler material speed up. Some of the kinetic energy of the hotter material is transferred, one collision at a time, to the cooler material. Soon, both materials are at the same temperature (figure 27.3). This is how two materials reach thermal equilibrium by conduction. Conductors and insulators Which state of Conduction can take place in solids, liquids and gases. However, the more densely matter conducts packed atoms or molecules of a solid can conduct more heat because there are best? many more collisions taking place. The low density of gases mean that relatively few 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 the transfer of heat. What are thermal insulators and thermal conductors? 460 In general, materials that conduct heat easily are called thermal conductors and those that conduct heat poorly are called thermal insulators. For example, metal is a thermal conductor, and foam is a thermal insulator. You may remember that the words conductor and insulator are also used to describe a material’s ability to conduct electrical current. There is a reason for this common usage. In general, good electrical conductors are also good heat conductors. Remember, metals are good conductors of current because the metal atoms have electrons that escape easily from the atom. When a metal conducts heat, these free electrons also transfer 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. Chapter 27 Thermal conductivity in the building and manufacturing industries Different materials conduct thermal energy at different rates Thermal conductivity is a measure of how well a material conducts heat. Although solids in general are better conductors of heat than liquids or gases, each material conducts heat at a different rate. We can compare thermal conductivities by measuring how fast a certain amount of thermal energy flows through uniformly sized pieces of various materials. Thermal Measuring the thermal conductivity of different materials is important in the conductivity in building and manufacturing industries. We don’t want hot air to leave our home your home when it’s cold outside and we also don’t want hot air to enter our home when it’s very hot outside. A home built with good insulators lessens heat transfer in both directions. Windows and energy loss The thermal conductivity of a single-pane glass window is very high and heat is easily transferred through it. Air leakage also occurs around windows, which results in even more heat transfer. This means that about one-third of home heat in the United Sates is lost through windowpanes and window frames. This energy loss is about equal to all the energy available from the oil flowing through the Alaskan pipeline in an entire year. Figure 27.5: Heat flows from hot to cold. On a cold winter day, heat flows through the window from inside to outside. Thermal Sometimes you want to conduct heat away quickly. Did you know that hammering conductivity in the a penny or paper clip makes it hot? Some of the work done on the penny or workplace paperclip is converted to thermal energy. Drilling, hammering, and many other manufacturing processes create unwanted increases in temperature. If the heat has to 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 increase cooling by convection, another form of heat transfer. 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. 27.1 Conduction 461 Chapter 27 27.2 Convection Another type of heat transfer that is due to temperature differences is called convection. This type of heat transfer is responsible for global weather patterns, the heating of our homes and the circulation of waters in the oceans. What is convection? What is Have you ever warmed up your hands by putting them over an open flame? You convection? can do this because the air right above the flame heats up and expands. Because the 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 occurs mostly in solids, convection occurs only in liquids and gases. Convection comes from a Latin word meaning to carry together. Figure 27.6: The air right above the flame heats up and expands, transferring heat to your hand. 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 occurs because warmer fluids are less dense, and rise. Cooler fluids are more dense, and sink. This motion of fluids causes currents. Convection causes The currents caused by convection occur constantly in our atmosphere and are the weather responsible for much of our weather. On a global scale, hot air near the equator patterns on Earth rises and is forced toward the poles as shown in figure 27.7. The sinking air forces cold air at the poles toward the equator. Combined with forces due to the rotation of the Earth, convection and unequal heating are the primary causes of weather. 462 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. Chapter 27 Natural convection Why does warmer There is a natural upward force called buoyancy. This force occurs whenever you air rise? have an object submerged in a denser medium. An example is an inflated ball under water. The ball is less dense than the water. There is an upward force equal to the weight of the displaced medium that pushes the ball out of the water. But this force not only applies to solids, it also applies to fluids and gases. As heated air or a fluid rises, there are density differences, which act with gravitational forces to produce natural or buoyant convection. Sea breezes are On a smaller scale near coastlines, convection is responsible for sea breezes. due to convection During the daytime, land is much hotter than the ocean. A sea breeze is created when hot air over the land rises due to convection and is replaced by cooler air from the ocean. In the evening, the ground cools rapidly but the ocean remains warm, due to water’s high specific heat. Warm air rises over the water and is replaced 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 radiator heats a room during winter (figure 27.8). As the temperature of the air around the radiator is increased by conduction, it becomes less dense than the cold air in the room. 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. 27.2 Convection 463 Chapter 27 Convection in the Another application of natural convection is cooking on a gas stovetop. As with a kitchen candle, 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 at the bottom of the pot rises to the top of the pot, replaced by the cooler water. Next the cooler water is heated. If this did not happen, we would have to rely on the slower method of conduction to boil a pot of water. Why wearing a Through the process of convection, air carries heat away from your body. A wool sweater keeps you sweater prevents this from happening by trapping air in many small pockets so warm that it cannot flow and carry the heat away. Similarly, in cold weather birds trap pockets of air by fluffing their feathers. Wind chill If you expose bare skin to cold temperatures, natural convection can quickly become dangerous. As the air surrounding your body warms up, it rises and carries heat away. The effect of air carrying heat away is greatly increased on a windy day when a steady stream of air flows. The faster the wind speed, the more effectively heat is carried away. Antarctic explorers created a commonly used method for “measuring” the chilling effects of the wind, called the Wind Chill Equivalent Temperature or wind chill factor. The wind chill factor was originally based on the temperature at which plastic jugs of water placed on top of a high pole would freeze, given a certain wind speed. Figure 27.9: Convection in water. The hot water at the bottom of the pot rises to the top and replaces the cold water. Why does smoke Convection causes the smoke from the fire in a fireplace to rise up the chimney rise up the instead of entering your home. This is because convection is extremely efficient in chimney? focusing the heat in one direction: up! Smoke particles are carried upward by the rising hot air. If you have ever toasted marshmallows on a campfire, you may have noticed that if you hold the marshmallow on the stick right next to the fire, it gets nice and toasty. However, if you hold the marshmallow directly above the fire, it quickly catches fire and burns to a crisp. The air directly above the fire carries much more heat by convection. At the edge of the fire, the heat is mostly carried by a different kind of heat transfer called radiation. Figure 27.10: Convection is extremely efficient at focusing heat in one direction: up. 464 Chapter 27 Forced convection What is forced Another type of convection is the one in which a mechanical device is used to convection? force the fluid or gas to move, as opposed to the buoyant force. This is called forced convection. Air or liquids can be forced with fans or pumps. Warm fluids can carry heat to cooler regions and cool fluids can take heat away from hot regions. Applications Most heating systems use a combination of forced and natural convection. Let’s of forced return to the example of a radiator for home heating. Water is heated in the convection basement and pumped into the rooms of the house. The process of pumping the hot water through the house is forced convection. In a room, the hot water releases heat to the air surrounding the radiator through conduction. The heat is then carried away from the radiator by natural convection (figure 27.11). The opposite occurs in an air-conditioning system, where cool air is blown through a room with a fan. This forces the cooler air to replace the warmer air in the room. Figure 27.11: Both natural and forced convection help to heat a house. Forced convection In the radiator of a car, there are two examples of forced convection. In the cooling in a car system 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 the water 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. 27.2 Convection 465 Chapter 27 27.3 Radiation Have you ever stood in the sun on a cold day? If it is not too windy, you will feel the sun’s warmth, no matter how cold it is outside. On a warm day, you will feel even hotter if you stand in the sun. How does the warmth of the sun reach the Earth? In this section, you will learn about another type of heat transfer known as radiation that is responsible for the way the sun warms our planet. Radiation is a type of heat transfer that does not require matter to travel through. Electromagnetic radiation Radiation is heat transfer by electromagnetic waves. What is One form of heat transfer due to radiation comes from electromagnetic radiation electromagnetic such as light, ultraviolet rays, X rays, and infrared rays. You know that conduction radiation? and convection require matter to transfer heat. However, as you learned previously, electromagnetic waves can travel through a vacuum. This is fortunate because the Earth receives most of its heat in the form of electromagnetic radiation from the sun. Since space is a vacuum, radiation is the primary way we can receive heat from the sun. Energy-radiation relationships What types of All objects emit radiation due to their thermal properties, or because they have radiation do some internal thermal energy. Some objects emit mostly visible light, some objects emit? ultraviolet, and some infrared. The type of radiation an object emits depends on its temperature. Hotter objects have more energy per molecule than cold objects. Thus hot objects emit light with a higher frequency than cold objects. Ultraviolet photons have more energy than visible light. Visible light has more energy than infrared light. You learned previously how the colors of the rainbow, Red, Orange, Yellow, Green, Blue, and Violet (ROYGBV) are related to the energy of the visible light. 466 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 solar radiation: • • • • • 30% is returned to outer space. 47% is absorbed by the Earth. 23% is used to drive the hydrologic cycle. 0.2% drives the winds. .02% is absorbed by plants to be used in photosynthesis. Chapter 27 What is infrared Infrared radiation has lower energy than visible light. While human eyes cannot radiation? detect infrared radiation, certain species of snakes can. You may have seen popular spy movies where the hero uses an infrared viewer to see people in the dark. In addition, firefighters use infrared equipment to find people in smoke-filled rooms. Color-temperature You may have noticed that when a light bulb on a dimmer is turned on slowly, the relationships bulb will begin to heat up, then glow in the red, then orange, and then yellow areas of the electromagnetic spectrum. This is because different temperatures cause the filament in the light bulb to glow at different colors (figure 27.13). Why do stars Stars also have different colors. The coolest stars are red, such as Antares in the appear in different heart of Scorpio the scorpion. The warmest stars are blue. A bright blue star is colors? Betelgeuse, in the knee of Orion the hunter. Astronomers can tell the temperature of a star by looking at its color. But that does not mean the star only emits light at that color. The star emits light in a range of colors; however, the peak color is what we actually detect with our eyes. The light emitted by a star can be represented using a spectral diagram, like the diagram at the right. Light bulbs If objects have to be incredibly hot in order to glow in the visible spectrum, why doesn’t the filament in a light bulb burn up? Remember that a fire requires two things, fuel and oxygen. One of Thomas Edison’s contributions to engineering was developing the tungsten filament, which could withstand high temperatures. He also had the idea of removing the air from the bulb to prevent the filament from reacting with 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. 27.3 Radiation 467 Chapter 27 Absorption and emission Absorbers and Some objects are good absorbers and some objects are good reflectors. Good reflectors reflectors reflect most of the radiation that hits the object. Shiny metallic objects are great reflectors. Generally, materials that are good conductors of electricity and heat are also good reflectors. White and light colored objects also make good reflectors. Color and Remember that white objects reflect light of all wavelengths. Black and dark absorption objects tend to absorb all light that falls upon them. They take the radiation and convert 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 much light energy as possible. Emitters of Objects that are good absorbers of radiation are also good emitters of radiation. radiation Thus, after sunset, a black road surface emits radiation and cools quickly, whereas a white sandy surface of the beach would not emit radiation efficiently and would cool slowly (figure 27.14). Reflection and planets When we look up in the nighttime sky, we can see stars, planets and our moon. The stars generate their own light, just like our sun. The planets and our moon only reflect light that the sun emits. 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 Mercury or our moon) reflects very little light because rocks tend to absorb light. Clouds tend to have a high reflectivity, thus Venus, Jupiter, and Saturn have very high albedos because they have lots of clouds. Ice and snow also have a very high reflectivity. The icy moons of Saturn reflect over 90 percent of the light that hits their surface. The albedo of Earth varies with the constantly changing cloud cover and the amount of snow covering the planet’s surface. Given the low albedo of the cloudless moon, why do you think it appears so bright in our nighttime sky? 468 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. 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. Set One Set Two 1. heat transfer a. A method of heat transfer by direct contact of particles of matter 1. 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 Concept review 1. What properties make a material a good thermal conductor? Give three examples of good thermal conductors. 6. What is a sea breeze? When and why does it happen? 7. What is a land breeze? When and why does it happen? 2. What properties make a material a good thermal insulator in a solid? Give three examples of solids that are insulators. 8. What is forced convection? 3. Why is air a bad conductor of heat? How can air be used as an insulator? Give two examples. 9. Explain the color-temperature relationship. 10. What properties make a material a good absorber? 4. Why does hot air rise? 5. Why doesn’t convection occur in a solid? 11. What properties make a material a good reflector? 469