chapter 5 Thermal Energy section 2 Conduction, Convection, and What You ll Learn the three ways heat is transferred the difference between insulators and conductors how insulators control the transfer of thermal energy Before You Read There is an old joke that says a sweater is something you put on when your mother is cold. On the lines below, write about a time when you felt warm, but someone else felt cold. Identify Definitions As you read the section, highlight the definition of each word that appears in bold. B Compare and Contrast Make the Foldable shown below to help you understand how conduction, convection, and radiation are similar and different. Conduction Convection Read to Learn Conduction The three ways that thermal energy can be transferred from place to place are conduction, convection, and radiation. In conduction, heat is transferred by direct contact. Conduction occurs when the particles within a substance or between two substances bump into each other and transfer kinetic energy. Conduction occurs because the particles in matter are in constant motion. How does conduction transfer thermal energy? The figure shows how thermal energy is transferred when one end of a metal spoon is heated by a flame. The kinetic energy of the particles near the flame increases. The heated particles bump into surrounding particles. In these collisions, thermal energy is transferred from particles with more kinetic energy to particles with less kinetic energy. The collisions continue as thermal energy is transferred to the other end of the spoon. When heat is transferred by conduction, thermal energy is transferred from place to place without transferring matter. Thermal energy is transferred by the collisions between particles, not by movement of matter. 78 CHAPTER 5 Thermal Energy
What are thermal conductors? Energy can be transferred by conduction in all materials. But energy moves more quickly in some materials than in others. Conduction is faster in solids and liquids than in gases. In gases, particles are farther apart. Collisions with other particles happen less often in gases than in solids or liquids. The best thermal conductors are metals. Remember that an atom has a nucleus surrounded by electrons. In metals, there are electrons that are not bound to individual atoms. They move easily through the metal and bump into other electrons. This helps the electrons transfer thermal energy. Silver, copper, and aluminum are some of the best energy conductors. Convection Thermal energy can also be transferred by a method called convection. Convection is the transfer of thermal energy in a fluid by the movement of warmer and cooler fluids from one place to another. A fluid is a substance that can flow. It can be a liquid or a gas. The movement of fluids from one place to another causes currents. These currents transfer thermal energy. How is heat transferred by convection currents? When a pot of water is heated on a stove, heat is transferred by convection currents. First, the water molecules at the bottom of the pot gain thermal energy from the stove. The water becomes less dense as it is heated. The water at the top of the pot is still cool, so it is more dense than the warm water. Since the warm water is less dense, it rises. As it rises, it is replaced at the bottom of the pot by the sinking, cooler water. The cycle continues until all the water in the pot is at the same temperature. This rising-and-sinking action is a convection current. Also, as the warmer water rises, it heats some of the cooler water that it comes in contact with by conduction. In a convection current, both conduction and convection transfer thermal energy. How do convection and conduction differ? Both convection and conduction transfer heat. However, convection is different from conduction. Convection transfers thermal energy by moving particles from one place to another. Warm particles change places with cooler particles. In conduction, thermal energy is transferred by particles bumping into surrounding particles. But, in conduction, no particles actually move from place to place as they do in convection. C Compare Make the following Foldables to compare conductors and insulators. Conductors: Insulators: 1. Explain why metals are good conductors. 2. Explain A convection oven has a fan to move the air inside it. Is the air in a regular oven also affected by convection? Why or why not? Reading Essentials 79
Picture This 3. Make a Drawing In the first box, draw an icecovered mountain. Then draw arrows to show how radiation is reflected and absorbed by an ice-covered mountain range. In the second box, draw trees to represent a rain forest. Then draw arrows to show how radiation is reflected and absorbed by a dark rain forest. 80 CHAPTER 5 Thermal Energy Reflected by atmosphere Atmosphere Absorbed by atmosphere Outer space How does heat travel through space? There is almost no matter between the Sun and Earth. So heat does not travel by conduction or convection. Instead, heat is transferred from the Sun to Earth by radiation. is the transfer of energy by electromagnetic waves. They carry energy through solids, liquids, and gases. But they can also carry energy through empty space. Energy transferred by radiation is often called radiant energy. When you sit by a fireplace, you feel warm because heat is transferred by radiation from the fire to your skin. How do radiant energy and matter interact? When radiation strikes a material, three things can happen. The material absorbs some of the energy. It also reflects some of the energy. And some of the energy may be transmitted, or passed through, the material. The amount of energy absorbed, reflected, and transmitted depends on the type of material. Light-colored materials reflect more radiant energy, while dark-colored materials absorb more radiant energy. When material absorbs radiant energy, the thermal energy of the material increases. The figure above shows what happens to radiant energy from the Sun as it reaches Earth. How is radiation different in solids, liquids, and gases? Radiant energy can travel through the space between particles in a solid, liquid, or gas. Particles can absorb the radiation and emit some of the energy they absorbed. This energy then travels through the space between particles. It is absorbed and emitted by other particles. The transfer of energy by radiation works best with gases. The particles in gases are much farther apart than in solids or liquids. This extra space allows the radiant energy to travel more easily through gases. Sun Absorbed by Earth Reflected by surface
Controlling Heat Flow You probably do things every day to control the flow of heat. For example, you put on a jacket when you leave your house on a cool morning. And you might wear an oven mitt on your hand when you reach into the oven to pull out a hot pan. In both cases, you use different materials to help control the flow of heat. Your jacket reduces the flow of heat from your body to the surrounding air. The oven mitt reduces the flow of heat from the hot dish to your hand. Animal Adaptations Almost all living things have special features that help them control the flow of heat. The antarctic fur seal has a coat that is about 10 cm thick. This coat helps keep the seal from losing heat. The emperor penguin has a thick layer of blubber and thick, closely spaced feathers. This helps the penguin reduce the loss of body heat. These adaptations help the animals survive in a climate where the temperature is often below freezing. The scaly skin of the desert spiny lizard has the opposite effect. The lizard s skin reflects the Sun s rays and keeps the lizard from becoming too hot. The leathery skin also prevents water loss. An animal s color also can also help it keep warm or cool. The black feathers on a penguin s back allow it to absorb radiant energy. Thermal Insulators When you cook, you want the pan to conduct heat easily from the stove to your food. But you don t want the heat to move easily to the handle of the pan. Most pans have handles that are made from insulators. A thermal insulator is a material in which heat flows slowly. Materials that are insulators include wood, some plastics, fiberglass, and air. An insulator is the opposite of a conductor. Remember, conductors are materials through which heat flows quickly. Metals like silver, copper, and aluminum are excellent conductors. Materials that are good insulators are poor conductors of heat. That is why many pan handles are made of wood or plastic. Air as an Insulator Gases, such as air, are usually better insulators than solids or liquids. Some types of insulators contain many pockets of trapped air. These air pockets conduct heat poorly and keep convection currents from forming. When you wear a fleece jacket, the fibers in the fleece trap air between the jacket and your body. This air slows down the flow of your body heat to the colder air outside the jacket. Your body heat warms the air trapped by the fleece and you are wrapped in a blanket of warm air. 4. Describe How do animals colors help them control their temperature? 5. Draw Conclusions Some of the best cooking pans are made of copper. Why do you think this is? Reading Essentials 81
6. Identify What does building material insulation contain? Picture This 7. Explain How does the shiny surface of a thermos bottle prevent heat transfer? How are buildings insulated? Materials that are insulators help keep warm air from flowing out of buildings in cold weather. They also keep warm air from flowing into buildings in warm weather. These materials are called insulation. Building insulation is usually made of materials, such as fiberglass or foam, which contain pockets of trapped air. Insulation is placed between a building s outer walls and inner walls and between the ceiling and the attic. It reduces the flow of heat between the building and the surrounding air. Insulation also helps furnaces and air conditioners work more effectively. Since the heat doesn t escape or enter the building as easily, the furnaces and air conditioners do not have to work as much. This can save a great deal of energy. In the United States, about 50 percent of the energy used in homes is used for heating and cooling. How does a thermos work? Have you used a thermos bottle like the one in the figure? A thermos bottle lets you keep soup hot or iced tea cold. It does this by reducing the flow of energy into and out of the liquid in the bottle. The temperature of the liquid hardly changes over a number of hours. A thermos bottle has two glass walls. The air between the two walls is Outer case Vacuum Reflective surface removed so there is a vacuum between the glass layers. The vacuum contains little matter. This vacuum prevents heat transfer by conduction or convection between the liquid and the air outside the thermos bottle. The inside and outside glass surface of a thermos bottle is coated with aluminum. The coating makes each surface highly reflective. Electromagnetic waves are reflected at each surface. The inner reflective surface prevents radiation from transferring heat out of the liquid. The outer reflective surface prevents radiation from transferring heat into the liquid. Think about the things you do to stay warm or cool. Sitting in the shade reduces the heat transferred to you by radiation. Opening or closing windows increases or reduces heat transferred by convection. Putting on a jacket reduces the heat transferred from your body by conduction. In what other ways do you control the flow of heat? 82 CHAPTER 5 Thermal Energy
After You Read Mini Glossary conduction: the transfer of thermal energy by collisions between particles in matter convection: when thermal energy is transferred in a fluid by the movement of warmer and cooler fluid from place to place radiation: the transfer of energy by electromagnetic waves thermal insulator: a material in which thermal energy flows slowly 1. Review the terms and their definitions in the Mini Glossary. Write a sentence on the lines below that shows your understanding of the term thermal insulator. 2. Complete the table to organize information about how heat is transferred. How Heat Is Transferred Definition Example Conduction Convection 3. Think about what you have learned. How did identifying definitions help you as you read the section? Chapter 6 End of Section Reading Essentials 83