Summary Energy Conversion and Conservation

Transcription

1 Summary 15.1 and Its Forms is the ability to do work. Recall that work is done when a force moves an object. When work is done on an object, energy is transferred, or passed on, to that object. Therefore, work is actually a transfer of energy. Like work, energy is measured in joules (J). There are two general types of energy: kinetic energy and potential energy. Kinetic energy is the energy of motion. All moving objects have kinetic energy. The kinetic energy of a moving object depends on the object s mass and speed. The greater the mass or speed of the object, the more kinetic energy it has. To calculate the kinetic energy of an object, you multiply half the object s mass (m) times its speed (v) squared: Kinetic energy mv 2 Potential energy is energy that is stored in an object as a result of its position or shape. Two types of potential energy are gravitational potential energy and elastic potential energy. An object gains gravitational potential energy when it is raised to a greater height. The gravitational potential energy of an object depends on the object s mass (m), the force of gravity (g), and the object's height (h). The formula for gravitational potential energy is Potential energy mgh An object gains elastic potential energy when it is stretched or is pressed together. For example, stretching a rubber band or pressing a spring gives the object elastic potential energy. can take several different forms. Major forms of energy are mechanical energy, thermal energy, chemical energy, electrical energy, electromagnetic energy, and nuclear energy. 1 2 Mechanical energy is the sum of an object s potential and kinetic energy. A speeding train has mechanical energy. Thermal energy is the sum of the potential and kinetic energy of all the particles in an object. The faster the particles move, the higher the object's thermal energy and the warmer the object becomes. Hot molten metal contains a great deal of thermal energy. Chemical energy is the stored energy that holds together chemical compounds. When the compounds break down, the energy is released. A fuel such as gasoline is a rich store of chemical energy. Electrical energy is the transfer of electric charges. Lightning bolts are produced by electrical energy. Electromagnetic energy is energy that travels through space as waves. Sunlight and X-rays are examples of electromagnetic energy. Nuclear energy is the stored energy that holds together the nucleus of an atom. This energy can be released by breaking apart heavy nuclei. Nuclear fission is a process that splits the nucleus apart to release nuclear energy Conversion and Conservation can be changed from one form to another. The process of changing energy from one form to another form is called energy conversion. A wind-up toy demonstrates energy conversion. When you turn the key of the toy, elastic potential energy is stored in a spring. When the spring is released, the potential energy is changed into kinetic energy, and the toy moves. Physical Science Reading and Study Workbook 171

2 When energy changes from one form to another, the total amount of energy stays the same. This supports the law of conservation of energy. The law states that energy cannot be created or destroyed. This law is one of the most important concepts in science. A very common energy conversion is a change from gravitational potential energy to kinetic energy. This occurs whenever an object falls due to the force of gravity. Throughout the fall, the object s potential energy decreases, while its kinetic energy increases. However, the sum of the object s potential and kinetic energy remains the same because of the conservation of energy. The change from gravitational potential energy to kinetic energy occurs when a pendulum swings downward from its highest point to its lowest point. As the pendulum swings back upward again, the kinetic energy changes back to gravitational potential energy. Another example of this type of energy conversion is a pole vault. A pole-vaulter changes kinetic energy to potential energy to propel herself into the air. Then, gravitational potential energy changes back to kinetic energy as the pole-vaulter falls back to the ground. In the early 1900s, physicist Albert Einstein showed that energy and mass can be changed into each other. In other words, energy is released as matter is destroyed, and matter can be created from energy Resources resources may be renewable or nonrenewable. Nonrenewable energy resources are limited in amount and take millions of years to replace. They include oil, natural gas, coal, and uranium. Several nonrenewable energy resources, including oil and coal, are known as fossil fuels. They are called fossil fuels because they were formed underground from the remains of dead organisms. Fossil fuels are the most commonly used fuels. They are relatively cheap and widely available. However, using fossil fuels creates pollution. Renewable energy resources can be replaced in a relatively short period of time. They include hydroelectric energy energy from flowing water solar energy energy from sunlight geothermal energy energy from the heat beneath Earth's surface biomass energy chemical energy stored in living things These sources of energy can be changed into other, more usable forms of energy, such as electrical or thermal energy. Using renewable energy resources creates less pollution than using fossil fuels. Fossil fuels may become scarce and expensive in the future. We can make these energy resources last longer by using them more slowly. Two ways to make fossil fuels last longer are to reduce our energy needs and to use energy more efficiently. Finding ways to use less energy or to use energy more efficiently is known as energy conservation. Examples of energy conservation include turning off lights when they are not being used carpooling or using mass transportation such as buses and subways using energy-efficient appliances, cars, and light bulbs 172 Physical Science Reading and Study Workbook

3 Section 15.1 and Its Forms (pages ) This section describes how energy and work are related. Kinetic energy and potential energy are defined, and examples are shown for calculating these forms of energy. Examples of various types of energy are discussed. Reading Strategy (page 446) Building Vocabulary As you read, complete the concept map with vocabulary terms and definitions from this section. For more information on this Reading Strategy, see the Reading and Study Skills in the Skills and Reference Handbook at the end of your textbook. kinetic energy which is the energy of motion can be potential energy which can be gravitational potential elastic potential energy and Work (page 447) is the ability to do work. 1. What is energy? 2. When work is done on an object, energy is transferred to that object. 3. Circle the letter of each sentence that is true about work and energy. a. in food is converted into muscle movement. b. is transferred when work is done. c. Both work and energy are usually measured in joules. d. One joule equals one meter per newton. Kinetic (pages ) kinetic energy 4. The energy of motion is called. 5. Is the following sentence true or? You can determine the kinetic energy of an object if you know its mass and its volume. 6. Write the formula used to calculate an object s kinetic energy. Kinetic energy = 1 mv Calculate the kinetic energy of a 0.25-kg toy car traveling at a constant velocity of 2 m/s. 1 KE mv 2 1 (0.25 kg)(2.0 m/s) kg m 2 /s J 2 2 Physical Science Reading and Study Workbook 173

4 Potential (pages ) 8. What is potential energy? It is energy that is stored as a result of position or shape. 9. Is the following sentence true or? The work done by a rock climber going up a cliff decreases her potential energy. 10. An object s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity. 11. Is the following sentence true or? Gravitational potential energy of an object increases as its height increases. true 12. The potential energy of an object that is stretched or compressed is known as elastic potential energy. 13. Complete the table about potential energy. Potential Type Description Example Gravitational Objects raised to heights Diver on a platform high relative to a reference level above the water Elastic Stretched or compressed objects A guitar string stretched to one side Forms of (pages ) For numbers 14 through 19, write the letter of the form of energy that best matches the description. b a e f c d Descriptions 14. stored in gasoline, coal, and wood 15. The sum of an object s potential energy and kinetic energy, excluding atomic-scale movements 16. Produces the sun s heat and light 17. Travels through space in the form of waves 18. Produces lightning bolts 19. Increases as atoms within an object move faster Forms of a. mechanical energy b. chemical energy c. electrical energy d. thermal energy e. nuclear energy f. electromagnetic energy 174 Physical Science Reading and Study Workbook

5 Section 15.2 Conversion and Conservation (pages ) This section describes how energy is converted from one form to another. The law of conservation of energy also is presented. Reading Strategy (page 453) Relating Cause and Effect As you read, complete the flowchart to explain an energy conversion used by some gulls to obtain food. For more information on this Reading Strategy, see the Reading and Study Skills in the Skills and Reference Handbook at the end of your textbook. How Gulls Use Conversions Gull lifts oyster, increasing oyster s gravitational potential energy. The gull drops the oyster, and the oyster s gravitational potential energy is converted into kinetic energy as the oyster falls. (Air resistance can be ignored.) The oyster strikes a rock and breaks, kinetic energy is converted into thermal energy. The kinetic energy and gravitational potential energy of the oyster are now zero. Conversion (page 454) 1. Is the following sentence true or? can be converted from one form to another. true 2. When a wind-up toy is set in motion, elastic potential energy that was stored in a compressed spring is converted into the kinetic energy of the toy s moving parts. 3. Is the following sentence true or? The action of striking a match shows that stored chemical energy in the match can be converted into thermal energy and electromagnetic energy of the flame in a single step. Conservation of (page 455) 4. What does the law of conservation of energy state? cannot be created or destroyed. 5. Is the following sentence true or? When an object slows down because of frictional force acting on it, an amount of energy is destroyed that is equivalent to the decrease in kinetic energy of the object. 6. A moving object slows down because friction causes a continual conversion of kinetic energy into thermal energy. Physical Science Reading and Study Workbook 175

6 Conversions (pages ) 7. As an object falls, the gravitational potential energy of the object is converted into kinetic energy. 8. Circle the letter of each sentence that is true about pendulums. a. A pendulum consists of a weight suspended from a string that swings back and forth. b. The weight at the end of a pendulum reaches maximum kinetic energy at the highest point in the pendulum s swing. c. Potential energy and kinetic energy undergo constant conversion as a pendulum swings. d. Frictional forces enable a pendulum to continue swinging without slowing down. 9. At what point during a pole-vaulter s jump is his gravitational potential energy the greatest? It is greatest at the highest point of the jump. 10. Circle the letter of the type of energy that increases as the pole bends before it propels a pole-vaulter up into the air. a. kinetic energy b. mechanical energy c. frictional force d. elastic potential energy 11. Is the following sentence true or? For a mechanical change in an isolated system, the mechanical energy at the beginning equals the mechanical energy at the end of the process, as long as friction is negligible. true 12. Tell whether the following situations illustrate kinetic energy, potential energy, or both. What Type of Is It? Situation Form of A stationary wind-up toy with a compressed spring Potential energy A descending roller coaster car Both A skier poised to take off at the top of a hill Potential energy A car driving on a flat road Kinetic energy A vibrating guitar string Both and Mass (page 459) 13. What does Einstein s equation imply about mass and energy? energy are equivalent, and mass and energy can be converted into each other. 14. Is the following sentence true or? Einstein s equation, E mc 2, suggests that mass and energy together are conserved. true Mass and 176 Physical Science Reading and Study Workbook

7 Section 15.3 Resources (pages ) This section describes types of energy resources and ways to conserve them. Reading Strategy (page 462) Identifying Main Ideas As you read the section, write the main idea for each heading in the table. For more information on this Reading Strategy, see the Reading and Study Skills in the Skills and Reference Handbook at the end of your textbook. Heading Nonrenewable energy resources Main Idea Nonrenewable energy resources include oil, natural gas, and coal. They exist in limited quantities. Renewable energy resources Renewable energy resources include hydroelectric, solar, geothermal, wind, biomass, and nuclear fusion. Conserving energy resources resources can be conserved by reducing energy needs and by increasing the efficiency of energy use. Nonrenewable Resources (page 462) 1. What are nonrenewable energy resources? 2. List four examples of nonrenewable energy resources. a. b. c. Coal d. They are resources that exist in limited quantities and, once used, cannot be replaced except over the course of millions of years. Oil Natural gas Uranium 3. Circle the letter of each resource that is considered to be a fossil fuel. a. tree b. uranium c. oil d. coal 4. Is the following sentence true or? Although fossil fuels are evenly distributed throughout Earth, they only represent ten percent of total energy consumed. 5. What are some advantages and disadvantages of using fossil fuels as a source of energy? but their use creates pollution. Fossil fuels are relatively inexpensive and are usually readily available, Renewable Resources (pages ) 6. An energy resource that can be replaced in a reasonably short period of time is called a(n) renewable resource. Physical Science Reading and Study Workbook 177

8 7. Circle the letter of each sentence that is true about renewable energy resources. a. Wind and solar energy are both renewable energy resources. b. Renewable energy resources are always more efficient than nonrenewable resources. c. Renewable energy resources can be used to generate electricity and to heat homes. d. Magma generates most renewable energy, either directly or indirectly. 8. Describe one energy conversion that takes place during the generation of hydroelectric power. kinetic energy upon release of the water. Potential energy of water behind a dam is converted to 9. Is the following sentence true or? One disadvantage of hydroelectric power is that it is among the most expensive energy sources. For numbers 10 through 15, match the letter of each renewable energy source to its description. Description Renewable Sources c 10. Water pumped below ground is converted to steam. a. hydroelectric b. solar f 11. The most likely raw material is c. geothermal hydrogen. d. wind b 12. Mirrors concentrate sunlight to produce electricity. e. biomass d a e 13. Kinetic energy of moving air is converted into rotational energy of a turbine. 14. is obtained from flowing water. 15. Chemical energy stored in wood, peat, and agricultural waste can be converted into thermal energy. f. nuclear fusion 16. Is the following sentence true or? Hydrogen fuel cells generate electricity by combining hydrogen with oxygen. true Conserving Resources (page 466) 17. What are two ways that energy resources can be conserved? conserved by reducing energy needs and by increasing the efficiency of energy use. 18. Name two practical ways in which people can conserve energy. resources can be Accept reasonable responses, such as carpooling, walking or biking on short trips, using more efficient appliances, developing and driving more fuel-efficient cars. 178 Physical Science Reading and Study Workbook

9 WordWise Complete the sentences by using one of the vocabulary words below. biomass energy energy conservation fossil fuels solar energy potential energy energy thermal energy gravitational kinetic energy chemical energy nuclear energy When an object is raised to a higher level, its energy increases. potential The motion of microscopic particles in matter partly determines the amount of within it. As a pole-vaulter springs higher into the air, her kinetic energy decreases as her gravitational potential energy increases. This is an example of. Atomic fission and fusion produce. When your muscles move, from the cereal you ate for breakfast is converted into. The of a 100-kg boulder perched high on a cliff is greater than that of a 50-kg boulder at the same height. You can recognize motion and sound. Formed from the remains of once-living organisms, nonrenewable energy resources. Photovoltaic cells convert by the changes it causes, such as into electrical energy. When you sit around a campfire, you are enjoying energy stored in wood a type of. are Physical Science Reading and Study Workbook Level B 179

10 Calculating Potential A 60.0-kg swimmer is standing on the edge of a pier that is 2.5 m above the surface of a lake. The swimmer steps onto a diving board. This raises the swimmer s gravitational potential energy by 10 percent. What is the swimmer s new gravitational potential energy? 1. Read and Understand What information are you given? Mass of person m 60.0 kg Height above lake level h 2.5 m Math Skill: Percents and Decimals You may want to read more about this Math Skill in the Skills and Reference Handbook at the end of your textbook. Acceleration due to gravity g 9.8 m/s 2 2. Plan and Solve What variable are you trying to determine? Gravitational potential energy? What formula contains the given variables? Gravitational potential energy (PE) mgh Initial PE = (60.0 kg)(9.8 m/s 2 )(2.5 m) 1500 J Determine the 10-percent increase of PE. (1500 J)(0.10) 150 J Final PE 1500 J 150 J 1650 J 3. Look Back and Check Is your answer reasonable? This is a reasonable answer because 1650 J is more than 1500 J. When the height of a person or object increases, its gravitational potential energy should increase. Math Practice On a separate sheet of paper, solve the following problems. 1. A.3-kg toy car and a.5-kg toy car are sitting on a shelf that is 2 meters higher than the floor. What is the PE of the two cars? 2. An 80-kg rock climber is standing on a cliff 10 m high, so that his gravitational PE 7840 J. His partner gives him 15 kg of rope and other gear to carry on his way down. What is the climber s gravitational PE now? 180 Physical Science Reading and Study Workbook Level B