Thermal Energy. Thermal Energy Transfers

Transcription

1 Thermal Energy Thermal Energy Transfers Key Concepts What is the effect of having a small specific heat? What happens to a material when it is heated? In what ways can thermal energy be transferred? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 3. It takes a large amount of energy to significantly change the temperature of an object with a low specific heat. 4. The thermal energy of an object can never be increased or decreased. Main Ideas and Details Highlight the main idea of each paragraph. Highlight two details that support each main idea with a different color. Use your highlighted copy to review what you studied in this lesson. 1. Define What is radiation? How is thermal energy transferred? Have you ever gotten into a car on a hot summer day? You can guess that the inside of the car is hot even before you touch the door handle. You open the door and hot air seems to pour out of the car. When you touch the metal safety-belt buckle, it is hot. How is thermal energy transferred between objects? Thermal energy is transferred in three ways by radiation, by conduction, and by convection. Radiation The transfer of thermal energy from one material to another by electromagnetic waves is called radiation. All matter, including the Sun, fire, you, and even ice, transfers thermal energy by radiation. Warm objects emit more radiation than cold objects do. You feel the transfer of thermal energy by radiation less when you place your hands near a block of ice than when you place your hands near a fire. Thermal energy from the Sun heats the inside of a car by radiation. Radiation is the only way thermal energy can travel from the Sun to Earth because space is a vacuum. However, radiation also transfers thermal energy through solids, liquids, and gases. 84 Thermal Energy Reading Essentials

2 Conduction Suppose it s a hot summer day and you are outside drinking a glass of cold lemonade. The lemonade has a lower temperature than the surrounding air. Therefore, the particles that make up the lemonade have less kinetic energy than the particles that make up the air. When particles with different kinetic energies collide, the particles with higher kinetic energy transfer energy to particles with lower kinetic energy. In this case, the particles that make up the air collide with and transfer kinetic energy to the particles that make up the lemonade. As a result, the average kinetic energy, or temperature, of the particles that make up the lemonade increases. The hot air transfers thermal energy to, or heats, the cool lemonade. Because kinetic energy is being transferred, thermal energy is being transferred. The transfer of thermal energy between materials by the collisions of particles is called conduction. Conduction continues until the thermal energy of all particles that are in contact is equal. Thermal Conductors and Insulators On a hot day, a metal safety-belt buckle in a car feels hotter than the cloth safety belt. The buckle and safety belt receive the same amount of thermal energy from the Sun. So why does the buckle feel hotter? The reason is that the metal that makes up the buckle is a good thermal conductor. A thermal conductor is a material through which thermal energy flows easily. Atoms in good thermal conductors have electrons that move easily. These electrons transfer kinetic energy when they collide with other electrons and atoms. Metals (like those in safety-belt buckles) are better thermal conductors than nonmetals (like the materials in safety-belt straps). By contrast, the material that makes up a safety belt is a good thermal insulator. A thermal insulator is a material through which thermal energy does not flow easily. The electrons in the atoms of a good thermal insulator do not move easily. These materials do not transfer thermal energy easily because fewer collisions occur between electrons and atoms. Make a three-column chart book to describe the ways thermal energy is transferred. Conduction Radiation Convection 2. Predict Suppose you have a cup of hot tea outside on a cold day. What will happen? (Circle the correct answer.) a. Cold air particles will move into the hot tea. b. Hot tea particles will transfer energy to the cold air particles. c. The average kinetic energy of the tea particles will increase. 3. Contrast What is the difference between a thermal conductor and a thermal insulator? Reading Essentials Thermal Energy 85

3 4. Define What is specific heat? Key Concept Check 5. Summarize What does it mean if a material has a low specific heat? Specific Heat The amount of thermal energy required to increase the temperature of 1 kg of a material by 1ºC is called its specific heat. Every material has a specific heat. The temperature of a material with a low specific heat changes easily. The temperature of a material with a high specific heat does not change easily. Thermal conductors, such as metal safety-belt buckles, have a lower specific heat than thermal insulators, such as cloth safety belts and seat covers. This means it takes less thermal energy to increase a buckle s temperature than it takes to increase the temperature of a cloth safety belt or seat cover by the same amount. Thermal conductors and thermal insulators are shown in the figure below. The specific heat of water is especially high. It takes a large amount of energy to increase or decrease the temperature of water. The high specific heat of water has many beneficial effects. For example, much of your body is water. Water s high specific heat helps prevent your body from overheating. The high specific heat of water is one of the reasons why pools, lakes, and oceans stay cool in summer. Water s high specific heat also makes it ideal for cooling machinery, such as car engines and rock-cutting saws. Specific Heat, Thermal Conductors, and Thermal Insulators Visual Check 6. Analyze Which item in the car requires more thermal energy to change its temperature by 1ºC the metal belt buckle or the cloth seat back? Thermal insulator; high specific heat Thermal conductor; low specific heat Thermal insulator; high specific heat Thermal conductor; low specific heat 86 Thermal Energy Reading Essentials

4 Thermal Expansion and Contraction What happens if you take an inflated balloon outside on a cold day? Thermal energy transfers from the particles that make up the air inside the balloon to the particles that make up the balloon material and then to the cold outside air. As the particles that make up the air in the balloon lose thermal energy, which included kinetic energy, they slow down and move closer together. This cause the volume of the balloon to decrease. Thermal contraction is a decrease in a material s volume when its temperature decreases. How could you reinflate the balloon? You could heat the air inside the balloon with a hair dryer. The particles that make up the hot air coming out of the hair dryer transfer thermal energy, which includes kinetic energy, to the particles that make up the air inside the balloon. As the average kinetic energy of the particles increases, the air temperature increases. Also, as the average kinetic energy of the particles increases, they speed up and spread out. This increases the volume of the air inside the balloon. Thermal expansion is an increase in a material s volume when its temperature increases. Thermal expansion and contraction are most noticeable in gases and less noticeable in liquids. They are least noticeable in solids. Sidewalk Gaps In may locations, the air temperatures are very hot in the summer. The high temperatures cause thermal expansion in structures, such as concrete sidewalks. If the concrete expands too much or expands unevenly, it could crack. Therefore, control joints are cut into sidewalks. If the sidewalk does crack, it should crack smoothly at the control joint. Sidewalks can withstand thermal expansion and contraction because of control joints. Key Concept Check 7. Explain What happens to the volume of a gas when it is heated? 8. Evaluate Thermal expansion is most noticeable in. (Circle the correct answer.) a. water b. rock c. oxygen 9. Consider What process occurs in a sidewalk when the temperature decreases? Reading Essentials Thermal Energy 87

5 10. Identify What process causes a hot-air balloon to rise when the balloonist turns on the burner? 11. Explain Why doesn t ovenproof glass shatter in a hot oven? Key Concept Check 12. Name What are the three processes that transfer thermal energy? Hot-Air Balloons Hot-air balloons float because a burner heats the air in the balloon, causing thermal expansion. The particles that make up the air inside the balloon move faster and faster. The particles collide, and some are forced outside the balloon through the opening at the bottom. Now there are fewer particles in the balloon than in the same volume of air outside the balloon. The balloon is less dense and it begins to rise through denser outside air. To land a hot-air balloon, the balloonist allows the air inside the balloon to gradually cool. The air undergoes thermal contraction. But the balloon does not contract. Instead, denser air from outside the balloon fills the space inside. As the balloon s density increases, it slowly descends. Ovenproof Glass If you put an ordinary drinking glass into a hot oven, the glass might break or shatter. But a hot oven does not damage an ovenproof glass dish. Why? Different parts of ordinary glass expand at different rates when heated. This causes it to crack or shatter. But ovenproof glass is designed to expand less than ordinary glass when heated. This means that it usually does not crack in the oven. Convection When you heat a pan of water on the stove, the burner heats the pan by conduction. This process involves the movement of thermal energy within a fluid. Particles that make up liquids and gases move around easily, transferring thermal energy from one location to another. Convection is the transfer of thermal energy by the movement of particles from one part of a material to another. Convection occurs only in fluids, such as water, air, magma, and maple syrup. Density, Thermal Expansion, and Thermal Contraction If you heat a beaker of water on a burner, the burner transfers thermal energy to the beaker, which transfers thermal energy to the water. Thermal expansion occurs in water nearest the bottom of the beaker. Heating increases the water s volume, making it less dense. At the same time, water molecules at the water s surface transfer thermal energy to the air. This causes cooling and thermal contraction of the surface water. The denser surface water sinks to the bottom, forcing the less-dense water upward. This cycle continues until all the water in the beaker is at the same temperature. 88 Thermal Energy Reading Essentials

6 Convection Currents in Earth s Atmosphere Radiation from the Sun heats Earth unevenly. More thermal energy is transferred to Earth near the equator than anywhere else on Earth. Arid regions are where dry cool air consistently sinks to the surface. This cooler air moves to the equator as a surface wind N 20 N Desert zone Most rain forests are at or near the equator where rising moist air results in precipitation Equator 20 S 30 S Rain forest zone Desert zone 3 1 The higher amount of thermal energy at the equator heats the air. The air becomes less dense and rises. 2 Water vapor in the rising air condenses as the air rises and cools. The water falls back to Earth as rain. Convection Currents in Earth s Atmosphere The movement of fluids in a cycle because of convection is a convection current. Convection currents circulate the water in Earth s oceans and other bodies of water. They also circulate the air in a room and the materials in Earth s interior. Convection currents also move matter and thermal energy from inside the Sun to its surface. On Earth, convection currents move air between the equator and latitudes near 30 N and 30 S. This plays an important role in Earth s climates, as shown in the figure above. The locations of rain forests and deserts are influenced by convection currents. 3 Cooler air sinks back to Earth's surface where it moves to the equator to replace the less dense, rising air. Visual Check 13. Locate At what latitudes are most of Earth s rain forests located? Reading Essentials Thermal Energy 89

7 Mini Glossary conduction: the transfer of thermal energy between materials by the collisions of particles convection: the transfer of thermal energy by the movement of particles from one part of a material to another convection current: the movement of fluids in a cycle because of convection radiation: the transfer of thermal energy from one material to another by electromagnetic waves thermal conductor: a material through which thermal energy flows easily thermal contraction: a decrease in a material s volume when its temperature decreases thermal expansion: an increase in a material s volume when its temperature increases thermal insulator: a material through which thermal energy does not flow easily specific heat: the amount of thermal energy required to increase the temperature of 1 kg of a material by 1 C 1. Review the terms and their definitions in the Mini Glossary. Write a sentence explaining the difference between thermal contraction and thermal expansion. 2. Write yes or no in the appropriate spaces to compare and contrast the processes of conduction and convection. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? Conduction ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. Convection Transfers thermal energy yes yes Transfers thermal energy only in fluids Transfers thermal energy from warmer to cooler materials Transfers thermal energy between objects that touch 3. Use your knowledge of specific heat to explain why you might burn your hand if you touched a copper pan on a hot burner while the water inside the pan was still only warm. END OF LESSON 90 Thermal Energy Reading Essentials