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CHAPTER 5LESSON 2Energy and Energy ResourcesEnergy TransformationsKey Concepts What is the law ofconservation of energy? How does friction affectenergy transformations? How are different types ofenergy used?What do you think? Read the two statements below and decidewhether you agree or disagree with them. Place an A in the Before columnif you agree with the statement or a D if you disagree. After you’ve readthis lesson, reread the statements to see if you have changed your mind.BeforeStatementAfter3. Energy can change from one form to another.4. If you toss a baton straight up, total energydecreases as the baton rises.Visual Check1. Identify Which energytransformation pops the cornkernels?1382Changes Between Forms of EnergyHave you ever made popcorn in a microwave oven?Energy changes form when you make popcorn, as shownbelow. A microwave oven changes electric energy intoradiant energy. Radiant energy changes into thermal energyin the popcorn kernels. Thermal energy causes water in thepopcorn kernels to turn to steam. Pressure builds and theyexplode. These changes from one form of energy to anotherare called energy transformations.As you watch TV, energy transformations occur in thetelevision. A television transforms electric energy into soundenergy and radiant energy.Electrical energy transfers from theelectric outlet to the microwave oven.2The microwave oventransforms electricalenergy into radiantenergy.Radiant energy changes into thermal energy asthe popcorn kernels absorb the microwaves.This causes the kernels to become hot and pop.Energy and Energy ResourcesReading EssentialsCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Identify Main IdeasHighlight the sentences inthis lesson that talk abouthow energy changes form.Use the highlightedsentences to review.

Changes Between Kineticand Potential EnergyEnergy transformations also occur when you toss a ballupward, as shown in the figure. The ball slows down as itrises and speeds up as it falls. The ball’s speed and heightchange as the energy changes from one form to another.Copyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Kinetic Energyto Potential EnergyIn the figure, noticethat the ball is movingfastest and has themost kinetic energy(KE) as it leaves thegirl’s hands. As the ballmoves upward, itsspeed and kineticenergy decrease. At thesame time, the ball’spotential energy (PE)increases because theball’s height increases.The ball’s kineticenergy is changing topotential energy as theball moves upward. Atthe ball’s highest point,its gravitationalpotential energy isgreatest and its kineticenergy is the EPEBVisual Check2. Compare When is thegravitational potentialenergy the greatest?TotalenergyKE PEKEPETotalenergyEAPotential Energyto Kinetic EnergyAt the ball’s highest point, it has the most potentialenergy because it is farthest from the ground. As the ballmoves downward, its potential energy decreases because itsheight decreases. At the same time, the ball’s kinetic energyincreases because its speed increases. As the ball drops,potential energy changes to kinetic energy. When the ballreaches the boy’s hands, the ball’s kinetic energy is at thehighest value again.Reading Check3. Explain Why does thepotential energy decrease asthe ball falls?Energy changes between kinetic energy and potentialenergy as the ball moves. However, look at the bars in thefigure. The ball’s total energy does not change.Reading EssentialsEnergy and Energy Resources83

Key Concept Check4. Define What is the lawof conservation of energy?The Law of Conservation of EnergyThe total energy in the universe is the sum of all thedifferent forms of energy everywhere. According to the lawof conservation of energy, energy can be transformed from one forminto another or transferred from one region to another, but energycannot be created or destroyed. The total amount of energy inthe universe does not change.Friction and the Law of Conservation of EnergyVisual Check5. Explain Which type ofenergy is constant in thefigure? Why?Sometimes it seems as if the law of conservation ofenergy is not accurate. Imagine riding a bicycle, as shownbelow. The moving bicycle has mechanical energy. Whathappens to this mechanical energy when you apply thebrakes? According to the law of conservation of energy,the bicycle’s mechanical energy is not destroyed, but itis transformed.When you apply the brakes, the bicycle’s mechanicalenergy is transformed into thermal energy, as shown below.The brake pads rub against the bicycle’s wheels. Theadditional thermal energy warms the brakes, wheels, and airaround the bicycle.Coasting Kinetic Thermal Totalenergy energy energy Kinetic Thermal Totalenergy energy energyStopped Kinetic Thermal Totalenergy energy energyFriction When the bicycle’s brake pads rub against theKey Concept Check6. Specify Friction causeswhat energy transformation?84Energy and Energy Resourcesmoving wheels, friction occurs. Friction is a force that resists themotion of two surfaces that are touching. Friction between thebrake pads and the moving wheels changes the bicycle’smechanical energy into thermal energy. The total amount ofenergy never changes.There is always some friction between any two surfacesthat are rubbing against each other. As a result, somemechanical energy is always transformed into thermalenergy when two surfaces rub against each other.Reading EssentialsCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Applying brakes

Reducing Friction It is easier to pedal a bicycle if there is lessfriction between the bicycle’s parts. With less friction, lessof the bicycle’s mechanical energy gets transformed intothermal energy. One way to reduce friction is to applya lubricant such as oil to surfaces that rub together.Using EnergyYou use different forms of energy to do different things.You use radiant energy from a lamp to light a room. You usechemical energy stored in your body to run a race. Energyusually changes from one form to another when you use it.For example, a lamp changes electric energy to radiantenergy that lights a room. Some of the electric energy alsochanges to thermal energy. Thermal energy causes the lamp’sbulb to become warm to the touch.Make a side-tab book toorganize your notes onenergy alEnergyUsing Thermal EnergyAll forms of energy can be changed into thermal energy.Recall that thermal energy is the energy due to the motionof particles that make up an object. People often use thermalenergy to provide warmth or cook food. An electric spaceheater changes electric energy to thermal energy that warmsa room. In a jet engine, burning fuel releases thermal energythat the engine changes into mechanical energy.Copyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Using Chemical EnergyRecall that chemical energy is energy that is stored infoods and fuels. During photosynthesis, plants change theSun’s radiant energy into chemical energy that they storein chemical compounds. Some of these compounds becomefood for other organisms. Your body transforms chemicalenergy from food into kinetic energy that you use formovement. Your body also transforms some of thechemical energy into thermal energy that keeps you warm.Reading Check7. Name What are twouses of thermal energy?Using Radiant EnergyA cell phone sends and receives radiant energy usingmicrowaves. When you speak into a cell phone, the phonetransforms sound energy into electric energy and then intoradiant energy. Sound waves from your voice carry energyinto the phone. The phone converts the sound energy intoelectric energy and then into radiant energy. Microwavescarry the radiant energy away. When you listen to someoneon a cell phone, the phone is transforming radiant energyinto electric energy and then into sound energy.Reading Essentials8. Sequence What energychanges take place whenyou speak into a cell phone?Energy and Energy Resources85

Using Electric EnergyKey Concept Check9. Restate What happensto energy when it is used?Remember that electric energy is the energy carriedby an electric current. Many of the devices you use everyday, such as a handheld video game, an mp3 player, anda hair dryer, use electric energy. Some devices, such ashair dryers, use electric energy from an electrical powerplant. Other devices, such as handheld video games,transform chemical energy stored in batteries intoelectric energy.Waste EnergyWhen energy changes form, some thermal energy isalways released. For example, a lightbulb converts someelectric energy into radiant energy. However, the lightbulbalso transforms some electric energy into thermal energy.This is what makes the lightbulb hot. Some of this thermalenergy moves into the air and cannot be used.Scientists often refer to thermal energy that cannot beused as waste energy. Whenever energy is used, some energyis transformed into useful energy and some is transformedinto waste energy.Visual Check86Energy and Energy ResourcesReading EssentialsCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.10. Interpret Circle threeareas on the cars where oneform of energy is beingtransformed into wasteenergy.Chemical energy in gasoline makes cars move. However,a car transforms most of the chemical energy in gasolineinto waste energy. The waste energy is thermal energy thatmoves into the air. As shown below, some of this thermalenergy makes the car’s engine hot. Hot gases in the car’sexhaust contain thermal energy produced by burning fuel.Friction between a car’s tires and the road changesmechanical energy to thermal energy.

Mini Glossaryfriction: a force that resists the motion of two surfaces that aretouchinglaw of conservation of energy: energy can be transformedfrom one form into another or transferred from one regionto another, but energy cannot be created or destroyed1. Review the terms and their definitions in the Mini Glossary. Write a sentence thatprovides an example of friction.2. Use what you have learned about energy to complete the table.Energy ChangeKineticto potentialExampleThrow a ball into the air.Electricto thermalCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Potentialto kineticEnergy ChangeChemicalto thermalSoundto radiantAn object falls to the ground.Chemicalto electricExampleA call is received on a cell phone.Electricto radiantChemicalto kineticFood is digested to help a personmove.3. Explain one concept you learned from the sentences that you highlighted.What do you thinkReread the statements at the beginning of thelesson. Fill in the After column with an A if youagree with the statement or a D if you disagree.Did you change your mind?Reading EssentialsConnect EDLog on to ConnectED.mcgraw-hill.comand access your textbook to find thislesson’s resources.END OFLESSONEnergy and Energy Resources87

changes to thermal energy. Thermal energy causes the lamp's bulb to become warm to the touch. Using Thermal Energy All forms of energy can be changed into thermal energy. Recall that thermal energy is the energy due to the motion of particles that make up an object. People often use thermal energy to provide warmth or cook food. An electric space

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