* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Energy - Glencoe
Dark energy wikipedia , lookup
Efficient energy use wikipedia , lookup
Open energy system models wikipedia , lookup
William Flynn Martin wikipedia , lookup
Potential energy wikipedia , lookup
Energy subsidies wikipedia , lookup
Energy storage wikipedia , lookup
100% renewable energy wikipedia , lookup
Low-Income Home Energy Assistance Program wikipedia , lookup
Public schemes for energy efficient refurbishment wikipedia , lookup
Zero-energy building wikipedia , lookup
World energy consumption wikipedia , lookup
Low-carbon economy wikipedia , lookup
Kinetic energy wikipedia , lookup
Gibbs free energy wikipedia , lookup
Alternative energy wikipedia , lookup
Energy Charter Treaty wikipedia , lookup
Regenerative brake wikipedia , lookup
International Energy Agency wikipedia , lookup
Distributed generation wikipedia , lookup
Energy returned on energy invested wikipedia , lookup
Energy policy of the United Kingdom wikipedia , lookup
Internal energy wikipedia , lookup
Life-cycle greenhouse-gas emissions of energy sources wikipedia , lookup
Energy harvesting wikipedia , lookup
Energy policy of Finland wikipedia , lookup
Energy efficiency in transport wikipedia , lookup
Energy in the United Kingdom wikipedia , lookup
Negawatt power wikipedia , lookup
Energy policy of the European Union wikipedia , lookup
Conservation of energy wikipedia , lookup
United States energy law wikipedia , lookup
Energy efficiency in British housing wikipedia , lookup
Energy Independence and Security Act of 2007 wikipedia , lookup
Benchmarks—SC.A.2.3.3 Annually Assessed (pp. 539–541): The student knows that radiation, light, and heat are forms of energy used to cook food, treat diseases, and provide energy; SC.B.1.3.1 Annually Assessed (pp. 535–540, 543): identifies forms of energy and explains that they can be measured and compared; SC.B.1.3.2 (pp. 541, 543); SC.B.1.3.4 (p. 542); SC.B.2.3.1 (p. 542) Also covers: SC.H.1.3.7 Annually Assessed (p. 543) Energy Changes Energy ■ ■ ■ Explain what energy is. Describe the forms energy takes. Describe how energy is used. Energy is a term you probably use every day. You might say that eating a plate of spaghetti gives you energy, or that a gymnast has a lot of energy. Do you realize that a burning fire, a bouncing ball, and a tank of gasoline also have energy? What is energy? The word energy comes from the ancient Energy causes all the changes that take place around you. Review Vocabulary energy transfer: a change in energy from one form to another New Vocabulary energy kinetic energy potential energy thermal energy calorie law of conservation of energy FCAT Vocabulary Figure 1 Lightning causes dramatic change as it lights up the sky. 534 CHAPTER 18 Energy534 Kennan Ward/CORBIS Greek word energos, which means “active.” You probably have used the word energy in the same way. When you say you have a lot of energy, what does this mean? Energy is the ability to cause change. For example, energy can change the temperature of a pot of water, or it can change the direction and speed of a baseball. The energy in a thunderstorm, like the one shown in Figure 1, produces lightning that lights up the sky and thunder that can rattle windows. Energy can change the arrangement of atoms in molecules and cause chemical reactions to occur. You use energy when you change the speed of a bicycle by pedaling faster or when you put on the brakes. What does energy do? Figure 2 Any moving object has energy because it can cause change. Identify a change that the bowling ball is causing. Kinetic Energy One soccer ball is sitting on the ground and another is rolling toward the net. How does the energy of the moving ball compare to the one at rest? A moving ball certainly has the ability to cause change. For example, a moving bowling ball shown in Figure 2 causes the bowling pins to fall. A moving ball has energy due to its motion. The energy an object has due to its motion is called kinetic energy. A football thrown by a quarterback has kinetic energy. A sky diver or a leaf falling toward Earth also has kinetic energy. Mass, Speed, and Kinetic Energy Although moving objects have kinetic energy, not all moving objects have the same amount of kinetic energy. What determines the amount of kinetic energy in a moving object? The amount of kinetic energy an object has depends on the mass and speed of the object, as shown in Figure 3 on the next page. Imagine a small rock and a large boulder rolling down a hillside at the same speed. Which would have more kinetic energy? Think about the damage the rock and the boulder could do if they hit something at the bottom of the hill. The large boulder could cause more damage, so it has more kinetic energy. Even though the rock and the boulder were moving at the same speed, the boulder had more kinetic energy than the rock because it had more mass. Kinetic energy also depends on speed. The faster a bowling ball moves, the more pins it can knock down. When more pins are knocked down, a greater change has occured. So the faster the bowling ball moves, the more kinetic energy it has. Kinetic energy increases as speed increases. Food Contains Chemical Energy You transform energy every time you eat and digest food. The food you eat contains chemical energy. Your body changes this form of energy into other forms of energy that keep your body warm and move your muscles. The amount of chemical energy contained in food is measured in Calories. Check some food labels to see how many Calories your food contains. SECTION 1 Energy Changes 535 Alan Thornton/Getty Images VISUALIZING KINETIC ENERGY Figure 3 T he amount of kinetic energy of a moving object depends on the mass and the speed of the object.Energy is measured in units called joules (J).For example, the fastest measured speed a baseball has been thrown is about 45 m/s.The kinetic energy of a baseball traveling at that speed is about 150 J. ▼ A 600-kg race car, traveling at about 50 m/s, has about 5,000 times the kinetic energy of the baseball. ▼ Earth’s atmosphere is continually bombarded by ▼ There is evidence that a meteorite 10 km in diameter collided with Earth about 65 million years ago and might have caused the extinction of dinosaurs.The meteorite may have been moving 400 times faster than the baseball and would have a tremendous amount of kinetic energy due to its enormous mass and high speed—about a trillion trillion joules. particles called cosmic rays, which are mainly highspeed protons.The mass of a proton is about a 100 trillion trillion times smaller than the mass of the baseball.Yet, some of these particles travel so fast, they have nearly the same kinetic energy as the baseball. ▼ A sprinter with a mass of about 55 kg and running at 9 m/s has kinetic energy about 15 times greater than the baseball. 536 CHAPTER 18 Energy (tl)The Extinction of the Dinosaurs by Eleanor Kish, reproduced by permission of the Canadian Museum of Nature, Ottawa, Canada, (tr)W. Cody/CORBIS, (cr)William Swartz/Index Stock/PictureQuest, (bl)Duomo/CORBIS, (br)CORBIS Potential Energy Suppose the ski lift in Figure 4 takes a skier to the top of a hill. The skier has no kinetic energy when she is standing at the top of the hill. But as she skis down and moves faster, her kinetic energy increases. Where does this kinetic energy come from? Gravity pulls the skier down the hill. If the skier were standing at the bottom of the hill, gravity would not start her moving, as it does when she is at the top of the hill. When the skier’s position is at the top of the hill, she has a form of energy called potential energy. Potential energy is energy that is stored because of an object’s position. By using the ski lift to take her to the top of the hill, the skier increased her potential energy by changing her position. Increasing Potential Energy When you raise an object above its original position, it has the potential to fall. If it does fall, it has kinetic energy. To raise an object, you have to transfer energy to the object. The ski lift uses energy when it takes a skier up a hill and transfers some of that energy to the skier. This energy becomes stored as potential energy in the skier. As the skier goes down the hill, the potential energy she had at the top of the hill is converted to kinetic energy. If the skier were lifted higher, her potential energy would increase. The higher an object is lifted above Earth, the greater its potential energy. Why did the skier’s potential energy increase as she was carried up the hill? Figure 4 Potential and kinetic energy change as the skier moves up and down the slope. Her potential energy is largest at the top of the hill. The skier's potential energy increases as the ski lift carries her up the hill. As she skis down the hill, potential energy transforms into kinetic energy. Here her kinetic energy is greatest and her potential energy is smallest. SECTION 1 Energy Changes 537 SC.B.1.3.2 Comparing Potential Energy Procedure 1. Hold a tennis ball about 1.5 m above a hard surface. 2. Before the ball is dropped, estimate the height the ball will reach on each of its first three bounces. 3. Drop the ball and have a partner use a meterstick to measure the height of the ball’s first three bounces. Analysis 1. When did the ball have the greatest and least amount of potential energy? Explain. 2. Infer why the height of the ball differed in each bounce. 3. How did the mechanical energy of the ball change after each bounce? Converting Potential and Kinetic Energy When a skier skis down a hill, potential energy is transformed to kinetic energy. Kinetic energy also can be transformed into potential energy. Suppose you throw a ball straight up into the air. The muscles in your body cause the ball to move upward when it leaves your hand. Because it is moving, the ball has kinetic energy. Look at Figure 5. As the ball gets higher and higher, its potential energy is increasing. At the same time, the ball is slowing down and its kinetic energy is decreasing. What happens when the ball reaches its highest point? The ball comes to a stop for an instant before it starts to fall downward again. At its highest point the ball has no kinetic energy because it isn’t moving. All the kinetic energy the ball had when it left your hand has been converted to potential energy. As a result, the ball will go no higher. As the ball falls downward, its potential energy is converted back into kinetic energy. Mechanical Energy The sum of the kinetic and potential energy of an object is the mechanical energy of the object. The mechanical energy of the ball wouldn’t change if potential energy is converted only into kinetic energy, and if kinetic energy is converted only into potential energy. However, as the ball moves through the air, the force due to air resistance acts on the ball. This force converts a small amount of the ball’s mechanical energy into another form of energy—thermal energy. As a result, the mechanical energy of the ball when you catch it is slightly less than when you threw it upward. What is the mechanical energy of an object? Figure 5 Energy is transformed as a ball rises and falls. KE PE As the ball leaves the person’s hand, it is moving the fastest and has maximum kinetic energy. 538 CHAPTER 18 Energy KE PE As the ball moves upward, it slows down as its kinetic energy is transformed into potential energy. KE PE As the ball moves downward, it speeds up as its potential energy is transformed into kinetic energy. Forms of Energy Besides kinetic energy and potential energy, there are other forms of energy. Some of these energy forms are due to motion and are a type of kinetic energy. Other energy forms are stored energy and are a type of potential energy. Some of these forms of can be transferred from one object to another. Thermal Energy All matter contains particles, such as atoms or molecules. The particles in matter are always moving. As a result, these particles have energy that is due to their motion. The energy of the particles in matter due to their continual motion is thermal energy. The thermal energy in an object increases when the object’s temperature increases. How does the thermal energy of an object change when its temperature increases? A campfire is used to provide light and warmth. A fire emits radiant energy that you see as light and feel as warmth. Electrical Energy When you plug a hair dryer into an electrical outlet and turn it on, another form of energy is being used— electrical energy. When the hair dryer is turned on, electrically charged particles called electrons flow in various parts of the hair dryer. The energy of these flowing electrons is electrical energy. As electrons flow in a hair dryer, their electrical energy is used to make an electric motor spin and to make parts of the hair dryer hot. Chemical Energy The food that you eat is a source of energy that is used by your body. Food contains chemical compounds. Chemical compounds are made of atoms that are bonded together. The bonds between atoms store chemical energy. Because chemical energy is energy that is stored, it is a type of potential energy. Chemical energy is released when chemical reactions occur. The chemical reactions that occur as your body digests food release chemical energy. This energy is used by your body to grow, to move, and in many other ways. X rays are waves that transfer radiant energy as they move from place to place. The radiant energy transferred by X rays is used to form images of the human body. Radiant Energy When you sit in sunlight, you can feel the warmth of the Sun on your skin. You feel warm because the Sun gives off, or emits, a form of energy called radiant energy. The radiant energy that comes from the Sun sometimes is called solar energy. Radiant energy is energy that is transferred by waves from one place to another. The waves emitted by the Sun transfer radiant energy. These waves travel through space and strike your skin, making it feel warm. Light also is a type of wave that transfers radiant energy. Figure 6 shows examples of other objects that give off radiant energy. A microwave oven uses the radiant energy transferred by microwaves to cook food. Figure 6 Campfires, X-ray images, and microwave ovens all use radiant energy. Identify other objects that emit radiant energy. SECTION 1 Energy Changes 539 (t)Getty Images, (c)Alfred Pasieka/Photo Researchers, (b)Stone/Getty Images LA.B.2.3.4 Topic: Nuclear Reactions in the Sun Visit fl6.msscience.com for Web links to information about the nuclear reactions that occur inside the Sun. Activity Draw a diagram show- AT FO S CU FC ing the structure of the Sun and where in the Sun the nuclear reactions occur. Annually Assessed Benchmark Check SC.B.1.3.1 Compare and contrast chemical energy and nuclear energy. Nuclear Energy The Sun emits an enormous amount of radiant energy every second. This energy is produced by reactions that occur in the Sun. However, these reactions aren’t chemical reactions. Instead, the Sun’s radiant energy is produced by reactions that involve the nuclei of atoms. Nuclear energy is energy stored in the nucleus of an atom. This energy is released when changes occur in an atom’s nucleus. Measuring Energy Recall that in the SI system of units, time is measured in seconds and distance is measured in meters. In the SI system of units, energy is measured using a unit called the joule (J). A student with a mass of 50 kg walking at a speed of 2 m/s has 100 J of kinetic energy. The same student sitting on a ledge 1 m above the ground has about 500 J of potential energy. Sometimes it is more convenient to measure energy using other units. Another unit for energy is the calorie. One calorie is the amount of energy needed to warm one gram of water by 1°C. One calorie (cal) is equal to about 4.19 J. A different energy unit is used to measure the energy content of food. This unit is the Calorie (with a capital C) and is equal to 1,000 cal, or 4,190 J. A candy bar that has 200 Calories supplies your body with almost 838,000 J of energy. Changing Forms of Energy Figure 7 An electric fan converts electrical energy into other forms of energy. Mechanical energy Changing Electrical Energy Many of the devices you use Thermal energy Electrical energy 540 Digital Vision When you throw a ball into the air, the kinetic energy of the ball changes into potential energy as the ball moves upward. As the ball falls, potential energy changes into kinetic energy as the ball speeds up. As the ball rises and falls, one form of energy changes into another form of energy. Every day, all around you and inside you, energy is changing from one form to another. Changes in forms of energy make a car move and make headphones produce sound. Inside your body chemical energy is changed to thermal energy, electrical energy, and mechanical energy. All the events that you see around you involve energy changing form. CHAPTER 18 Energy every day, like the one shown in Figure 7, change electrical energy to other forms of energy. For example, hair dryers, CD players, and refrigerators contain electric motors. In an electric motor, electrical energy makes the shaft of the motor spin, so that electrical energy is changed to mechanical energy. Hair dryers and electric stoves contain heating elements that change electrical energy into thermal energy. A lightbulb produces light by changing electrical energy to radiant energy. Changing Chemical Energy A different energy transfer is used by a stove that burns natural gas. When a fuel like wood, gasoline, or natural gas is burned, chemical energy is changed into thermal energy and light, or radiant energy. Figure 8 shows another example of chemical energy changing form. When you light a candle, wax is burned to produce thermal energy and light. In a car’s engine, gasoline is burned to produce thermal energy. The car converts this thermal energy to mechanical energy that enables the car to move. Your body uses food as fuel. Chemical reactions in your body change the chemical energy stored in food to thermal energy and mechanical energy. Using Energy Where does energy come from? Even though energy changes from one form to another when it is used, energy never is created or destroyed. According to the law of conservation of energy, energy never can be created or destroyed, but only can be changed from one form to another. Sometimes it might seem that energy is being lost. A book sliding on a table slows down and stops. The book’s kinetic energy decreased as it slowed down. Figure 9 shows what happens to this energy. The kinetic energy of the book is not destroyed, but is changed into another form of energy—thermal energy. Friction causes the kinetic energy of the book to be changed into thermal energy. This thermal energy makes the book, the table, and the surrounding air slightly warmer. tacular display of chemical energy changing to thermal energy and radiant energy. AT FO S CU The Law of Conservation of Energy Figure 8 Fireworks are a spec- FC When you use a hair dryer, or toast bread in a toaster, or listen to a CD, you are using energy. In each case, you used electrical energy to do something—dry your hair, cook food, and make sound. When you burn a candle or heat water on the stove you are using chemical energy do something—provide light or heat water. Energy that is used to do something or perform a task is useful energy. Annually Assessed Benchmark Check SC.B.1.3.2 How much thermal energy is produced in an electric motor if 100 J of electrical energy are converted into 92 J of mechanical energy? Figure 9 As the book slides along the table, kinetic energy is changed into thermal energy. When the book stops, all of its kinetic energy has been changed into thermal energy. Thermal energy SECTION 1 Energy Changes 541 Randy Faris/CORBIS Thermal energy Batteries Figure 10 The energy conver- FO S CU FC sions in a flashlight produce some thermal energy that is not useful energy. AT Useful Energy Always Decreases Light bulb Annually Assessed Benchmark Check SC.B.2.3.1 Why does the amount of useful energy decrease when energy changes from one form into another? When energy is used, it usually is changed from one form to another. For example, a battery in a flashlight converts chemical energy into electrical energy. This electrical energy is changed into useful radiant energy by the flashlight bulb. However, some of the electrical energy also is changed to thermal energy, as shown in Figure 10. When energy changes form, some thermal energy is produced. Not all of this thermal energy can be used to do something useful. As a result, when energy changes form, the amount of useful energy is always less than the initial amount of energy. Some energy always is changed into thermal energy that cannot be used to perform useful work. When energy is changed from one form to another, it becomes less concentrated and more spread out. For example, the electrical energy in a flashlight is concentrated in the batteries. When the flashlight is used, the light and thermal energy produced is transferred to the air surrounding the flashlight. Compared to the energy in the batteries, the light and thermal energy transferred to the air is spread out, and is less concentrated. All energy conversions that occur cause energy to become more spread out and less concentrated. Summary Self Check Energy Energy is the ability to cause change. Kinetic energy is the energy an object has because of its motion. Potential energy is stored energy. Forms of Energy Energy comes in different forms. Some of these forms are thermal energy, electrical energy, chemical energy, radiant energy, and nuclear energy. One form of energy can be changed into another form of energy. Using Energy According to the law of conservation of energy, energy cannot be created or destroyed, but only can change form. When energy changes form, some thermal energy is produced and the amount of useful energy decreases. 1. Describe the energy transformations that occur when a lightbulb is turned on. SC.B.1.3.1 2. Explain how the total energy changes when a falling rock hits the ground. SC.B.1.3.2 3. Infer on which part of a roller coaster a roller coaster car has the greatest potential energy. SC.B.1.3.1 4. Determine which has the greater kinetic energy if both are traveling at the same speed—a fully loaded truck or a motorcycle. SC.B.1.3.1 5. Think Critically When a ball is thrown upward, how does the height reached by the ball depend on its initial speed? SC.B.1.3.2 • • • • • • 542 CHAPTER 18 Energy 6. Diagram the energy transformations that occur when you eat breakfast, walk to the bus stop, and ride the bus to school. SC.B.1.3.2 More Section Review fl6.msscience.com