CHAPTER 5 Energy And Energy Transformations - Rcboe

CHAPTER 5LESSON 2Energy and EnergyTransformationsEnergy TransformationsKey Concepts What is the law ofconservation of energy? How does friction affectenergy transformations? How are different typesof energy used?What do you think? Read the three statements below and decidewhether you agree or disagree with them. Place an A in the Beforecolumn if you agree with the statement or a D if you disagree. Afteryou’ve read this lesson, reread the statements to see if you havechanged your mind.BeforeStatementAfter4. Energy can change from one form toanother.5. Energy is destroyed when you apply thebrakes on a moving bicycle or a moving car.6. The Sun releases radiant energy.Identify Main IdeasHighlight the sentences inthis lesson that talk abouthow energy changes form.Use the highlightedsentences to review.Visual Check1. Identify Which energytransformation pops thecorn kernels?Changes Between Forms of EnergyHave you ever made popcorn in a microwave oven to eat whilewatching television? Energy changes form when you makepopcorn, as shown in the figure below. A microwave oven changeselectric energy into radiant energy. Radiant energy changes intothermal energy in the popcorn kernels. These changes from oneform of energy to another are called energy transformations. Asyou watch TV, energy transformations occur in the television. Atelevision transforms electric energy into sound energy andradiant energy.178Energy and Energy TransformationsCopyright McGraw-Hill Education.Energy TransformationEnergy Transformations

Conservation of EnergyChanges Between Kineticand Potential EnergyEnergy transformations also occur when you toss a ball upward.The ball slows down as it rises and speeds up as it falls. The ball’sspeed and height change as the energy changes from one form toanother.Kinetic Energy to Potential EnergyIn the figure to the right, notice that the ball is moving fastestand has the most kinetic energy (KE) as it leaves the girl’s hands.As the ball moves upward, its speed and kinetic energy decrease.However, the ball’s potential energy (PE) increases because theball’s height increases. The ball’s kinetic energy is changing topotential energy. At the ball’s highest point, its gravitationalpotential energy is greatest, and its kinetic energy is lowest.Potential Energy to Kinetic EnergyAs the ball moves downward, its potential energy decreases. Atthe same time, the ball’s kinetic energy increases because its speedincreases. As the ball drops, potential energy changes to kineticenergy. When the ball reaches the boy’s hands, the ball’s kineticenergy is once again at its highest value. Energy changes betweenkinetic energy and potential energy as the ball moves. The bars inthe figure show that the ball’s total energy does not change.The Law of Conservation of EnergyThe total energy in the universe is the sum of all the differentforms of energy everywhere. According to the law ofconservation of energy, energy can be transformed from oneform into another or transferred from one region to another, butenergy cannot be created or destroyed. The total amount ofenergy in the universe does not change.Visual Check2. Compare When is thegravitational potentialenergy the greatest?Copyright McGraw-Hill Education.Friction and the Law of Conservation of EnergySometimes it seems as if the law of conservation of energy isnot accurate. Imagine riding a bicycle. The moving bicycle hasmechanical energy. What happens to this mechanical energy whenyou apply the brakes?When you apply the brakes, the bicycle’s mechanical energy istransformed into thermal energy as the brake pads rub against thebicycle’s wheels. The total amount of energy never changes. Theadditional thermal energy warms the brakes, the wheels, and theair around the bicycle.Energy TransformationsKey ConceptCheck3. Define What is the lawof conservation of energy?Energy and Energy Transformations179

Friction and Thermal EnergyVisual Check4. Explain Which type ofenergy is constant in thefigure? Why?Friction When the bicycle’s brake pads rub against the movingwheels, friction occurs. Friction is a force that resists the slidingof two surfaces that are touching. Friction between the brakepads and the moving wheels transforms the bicycle’s mechanicalenergy into thermal energy as shown in the figure above.There is always some friction between any two surfaces that arerubbing against each other. As a result, some mechanical energy isalways transformed into thermal energy when two surfaces rubagainst each other.Reducing Friction It is easier to pedal a bicycle if there is lessKey ConceptCheck5. Specify How doesfriction affect energytransformations?Math SkillswattskWh hours1,000Appliances typically have apower rating measured inwatts (W).Using EnergyYou use different forms of energy to do different things. Youuse radiant energy from a lamp to light a room. You use chemicalenergy stored in your body to run a race. Energy usually changesfrom one form to another when you use it. For example, a lampchanges electric energy to radiant energy that lights a room. Someof the electric energy also changes to thermal energy. Thermalenergy causes the lamp’s bulb to become warm to the touch.Using Thermal EnergyAll forms of energy can be transformed into thermal energy.People often use thermal energy to cook food or provide warmth.A gas stove transforms the chemical energy stored in natural gasinto the thermal energy that cooks the food. An electric spaceheater transforms the electric energy from a power plant into thethermal energy that warms a room. In a jet engine, burning fuelreleases thermal energy that the engine transforms into mechanicalenergy.6. Solve a One-StepEquation A hair dryer israted at 1,200 W. If you usethe dryer for 0.25 h, howmuch electric energy doyou use?180Energy and Energy TransformationsEnergy TransformationsCopyright McGraw-Hill Education.Electric energy often ismeasured in units calledkilowatt-hours (kWh). Tocalculate the electricenergy used by anappliance in kWh, use thisequation:friction between the bicycle’s parts. With less friction, less of thebicycle’s mechanical energy is transformed into thermal energy. Oneway to reduce friction is to apply a lubricant such as oil to surfacesthat rub against each other.

Using Chemical EnergyDuring photosynthesis, a plant transforms the Sun’s radiantenergy into chemical energy that it stores in chemical compounds.Some of these compounds become food for other organisms. Yourbody transforms the chemical energy from food into the kineticenergy that you use for movement. Your body also transformschemical energy into the thermal energy that keeps you warm.Make a side-tab book toorganize your notes onenergy transformations.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 into radiantenergy. Sound waves from your voice carry energy into the phone.The phone converts the sound energy into electric energy and theninto radiant energy. Microwaves carry the radiant energy away.When you listen to someone on a cell phone, the phone istransforming radiant energy into electric energy and then intosound energy.Using Electric EnergyMany of the devices you use every day, such as a handheldvideo game, an MP3 player, and a hair dryer, use electric energy.Some devices, such as hair dryers, use electric energy from electricpower plants. Other devices, such as handheld video games,transform the chemical energy stored in batteries into electricenergy.Key ConceptCheckWaste EnergyCopyright McGraw-Hill Education.When energy changes form, some thermal energy is alwaysreleased. For example, a lightbulb converts some electric energyinto radiant energy. However, the lightbulb also transforms someelectric energy into thermal energy. This is what makes thelightbulb hot. Some of this thermal energy moves into the air andcannot be used.7. Specify How aredifferent types of energyused?Scientists often refer to thermal energy that cannot be used aswaste energy. Whenever energy is used, some energy istransformed into useful energy and some is transformed into wasteenergy. For example, the chemical energy in gasoline makes carsmove. However, most of that chemical energy ends up as wasteenergy—thermal energy that moves into the air.Reading Check8. Define What is wasteenergy?Energy TransformationsEnergy and Energy Transformations181

Mini Glossaryfriction: a force that resists the sliding of twosurfaces that are touchinglaw of conservation of energy: energy can betransformed from one form into another ortransferred from one region to another, butenergy cannot be created or destroyed1. Review the terms and their definitions in the Mini Glossary. Write a sentence that provides anexample of friction.2. Use what you have learned about energy to complete the table.Energy ChangeKineticto potentialExampleThrow a ball into the air.Electricto thermalPotentialto 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.Copyright McGraw-Hill Education.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?182Energy and Energy TransformationsLog on to ConnectED.mcgraw-hill.comand access your textbook to find thislesson’s resources.END OFLESSONEnergy Transformations

Lesson 2Energy TransformationsScan Lesson 2 in your book. Write three questions you have about energy transformations in yourScience Journal. Try to answer your questions as you read.Changes BetweenForms of EnergyIdentify another term for “energy changing form.”I found this on page.Changes BetweenKinetic and PotentialEnergyDescribe how energy changes as a ball is thrown in the air andreturns to the ground. Indicate the relative amounts of potential energyand kinetic energy at each position.I found this on page.Position CPosition BPosition DPosition APosition EPositionKinetic EnergyPotential EnergyABCopyright McGraw-Hill Education.CDEI found this on page.Complete the equation to summarize the ball’s energy changes.Totalenergy ofthe ball energy energyEnergy and Energy Transformations183

Lesson 2 Energy Transformations (continued)The Law ofConservation of EnergyOrganize information about the law of conservationof energy.I found this on page.Law of Conservation of Energyenergy cannot beenergy can be I found this on page.Define friction.I found this on page.Sequence the events that occur as kinetic energy is changedto thermal energy by friction.A boy in-line skates on a hard-surface pathway. This isan example of.The boy applies his brake pad. The pad rubsagainst the surface of the path.changesenergy intoI found this on page.184energy.Identify one way to reduce friction. Include three examples.Energy and Energy TransformationsCopyright McGraw-Hill Education.The boy starts to slow asbetween the pad and the surface of the path