Thermal energy fromradioactive atoms

Radiant energy from the Sun

Surface ofEarth

Energy Only Changes FormEvery day, energy is used to provide light and to heat and

cool homes, schools, and workplaces. According to the law ofconservation of energy, energy can’t be created or destroyed.Energy only can change form. If a car or refrigerator can’t createthe energy they use, then where does this energy come from?

Energy ResourcesEnergy cannot be made, but must come from the natural

world. As you can see in Figure 15, the surface of Earthreceives energy from two sources—the Sun and radioactiveatoms in Earth’s interior. The amount of energy Earth receivesfrom the Sun is far greater than the amount generated inEarth’s interior. Nearly all the energy you used today can betraced to the Sun, even the gasoline used to power the car orschool bus you came to school in.

Sources of Energy

■ Explain what renewable, non-renewable, and alternativeresources are.

■ Describe the advantages anddisadvantages of using variousenergy sources.

Sources of energy are needed toprovide the energy required for sur-vival and to make life comfortable.

SECTION 3 Sources of Energy 167

Figure 15 All the energy youuse can be traced to one of twosources—the Sun or radioactiveatoms in Earth’s interior.

Benchmarks—SC.A.2.3.3 Annually Assessed (pp. 171–173): The student knows that radiation, light,and heat are forms of energy …; SC.B.2.3.2 (p. 168): knows that most of the energy used today is derivedfrom burning stored energy collected by organisms millions of years ago; SC.D.2.3.2 Annually Assessed (pp. 168–170): knows the positive and negative consequences of human action on the Earth’s systems.

Also covers: SC.G.2.3.1 (pp. 168–175), SC.H.1.3.4 Annually Assessed (pp. 176–177), SC.H.1.3.7 AnnuallyAssessed (pp. 176–177), SC.H.3.3.7 (pp. 176–177)

Review Vocabularyresource: any material that

can be used to satisfy a need

New Vocabularyfossil fuelsnonrenewable resourcerenewable resource

• alternative resource

• inexhaustible resource

FCAT Vocabulary

168 CHAPTER 6 Energy and Energy Resources

Fossil FuelsFossil fuels are coal, oil, and natural gas. Oil and natural gas

were made from the remains of microscopic organisms thatlived in Earth’s oceans millions of years ago. Heat and pressuregradually turned these ancient organisms into oil and naturalgas. Coal was formed by a similar process from the remains ofancient plants that once lived on land, as shown in Figure 16.

Through the process of photosynthesis, ancient plants con-verted the radiant energy in sunlight to chemical energy storedin various types of molecules. Heat and pressure changed thesemolecules into other types of molecules as fossil fuels formed.Chemical energy stored in these molecules is released when fos-sil fuels are burned.

Using Fossil Fuels The energy used when you ride in a car,turn on a light, or use an electric appliance usually comes fromburning fossil fuels. However, it takes millions of years to replaceeach drop of gasoline and each lump of coal that is burned. Thismeans that the supply of oil on Earth will continue to decreaseas oil is used. An energy source that is used up much faster thanit can be replaced is a nonrenewable resource. Fossil fuels arenonrenewable resources.

Burning fossil fuels to produce energy also generates chemi-cal compounds that cause pollution. Each year billions of kilo-grams of air pollutants are produced by burning fossil fuels.These pollutants can cause respiratory illnesses and acid rain.Also, the carbon dioxide gas formed when fossil fuels are burnedmight cause Earth’s climate to warm.

Figure 16 Coal is formed afterthe molecules in ancient plants areheated under pressure for millionsof years. The energy stored by themolecules in coal originally camefrom the Sun.

TimeHeat

Pressure

Radiant energy Radiant energy from the Sun isstored as chemical energy in molecules.

Coal mine

Energy Source OriginsThe kinds of fossil fuelsfound in the grounddepend on the kinds oforganisms (animal or plant)that died and were buriedin that spot. Research coal,oil, and natural gas to findout what types of organ-isms were primarily respon-sible for producing each.

LA.A.2.3.5

1. Nuclear energyof atoms

2. Thermal energyof water

3. Kinetic energyof steam

4. Kinetic energyof turbine

Generator

5. Electrical energy out of generator

SECTION 3 Sources of Energy 169

Nuclear EnergyCan you imagine running an automobile on 1 kg of fuel that

releases as much energy as almost 3 million liters of gasoline?What could supply so much energy? The answer is in the nucleiof some uranium atoms. During the process of nuclear fission,these nuclei release a tremendous amount of nuclear energy. Thisenergy can be used to generate electricity by heating water to pro-duce steam that spins an electric generator, as shown in Figure 17.Because no fossil fuels are burned, using nuclear energy to gener-ate electricity helps make the supply of fossil fuels last longer. Also,unlike fossil fuel power plants, nuclear power plants producealmost no air pollution. In one year, a typical nuclear power plantgenerates enough energy to supply 600,000 homes with power andproduces only 1 m3 of waste.

Nuclear Wastes Like all energy sources, nuclear energy hasadvantages and disadvantages. One disadvantage is that theamount of uranium in Earth’s crust is nonrenewable. Another isthat the waste produced by nuclear power plants is radioactiveand can be dangerous to living things. Some of the materials inthe nuclear waste will remain radioactive for many thousands ofyears. As a result the waste must be stored so no radioactivity isreleased into the environment for a long time. One method is toseal the waste in a ceramic material, place the ceramic in protec-tive containers, and then bury the containers far underground.However, the burial site would have to be chosen carefully sounderground water supplies aren’t contaminated. Also, the sitewould have to be safe from earthquakes and other natural disas-ters that might cause radioactive material to be released.

Figure 17 To obtain electricalenergy from nuclear energy, aseries of energy transformationsmust occur.

170 CHAPTER 6 Energy and Energy Resources

HydroelectricityCurrently, transforming the potential energy of water that is

trapped behind dams supplies the world with almost 20 percentof its electrical energy. Hydroelectricity is the largest renewablesource of energy. A renewable resource is an energy source thatis replenished continually. As long as enough rain and snow fallto keep rivers flowing, hydroelectric power plants can generateelectrical energy, as shown in Figure 18.

Hydroelectricity produces no air pollution. However, onedisadvantage is that dams can disrupt the life cycle of aquaticanimals, especially fish. This is particularly true in theNorthwest where salmon spawn and run. Because salmonreturn to the spot where they were hatched to lay their eggs, thedevelopment of dams has hindered a large fraction of salmonfrom reproducing. This has greatly reduced the salmon popula-tion. Efforts to correct the problem have resulted in plans toremove a number of dams. In an attempt to help fish bypasssome dams, fish ladders are being installed. Like most energysources, hydroelectricity has advantages and disadvantages.

Topic: HydroelectricityVisit fl7.msscience.com for Weblinks to information about the useof hydroelectricity in various partsof the world.

Activity On a map of the world,show where the use of hydroelec-tricity is the greatest.

Is energy consumption outpacing production?

You use energy everyday—to get to school,

to watch TV, and to heator cool your home. Theamount of energy con-sumed by an average per-son has increased overtime. Consequently,more energy must beproduced.

U.S. Energy Overview, 1949–1999

30

60

0

90

120

Year

Ener

gy (q

uadr

illio

n B

tu)

1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999

Produced inthe United States

Consumed inthe United States

Energy imports

Identifying the Problem The graph above shows the energy pro-

duced and consumed in the United Statesfrom 1949 to 1999. How does energy that isconsumed by Americans compare withenergy that is produced in the United States?

Solving the Problem 1. Determine the approximate amount of

energy produced in 1949 and in 1999

and how much it has increased in50 years. Has it doubled or tripled?

2. Do the same for consumption. Has itdoubled or tripled?

3. Using your answers for steps 1 and 2 andthe graph, where does the additionalenergy that is needed come from? Givesome examples.

LA.B.2.3.4

SECTION 3 Sources of Energy 171

Alternative Sources of EnergyElectrical energy can be generated in several ways. However,

each has disadvantages that can affect the environment and thequality of life for humans. Research is being done to develop newsources of energy that are safer and cause less harm to the envi-ronment. These sources often are called alternative resources.These alternative resources include solar energy, wind, and geo-thermal energy.

Solar EnergyThe Sun is the origin of almost all the energy that is used on

Earth. Because the Sun will go on producing an enormousamount of energy for billions of years, the Sun is an inex-haustible source of energy. An inexhaustible resource is anenergy source that can’t be used up by humans.

Each day, on average, the amount of solar energy that strikesthe United States is more than the total amount of energy usedby the entire country in a year. However, less than 0.1 percent ofthe energy used in the United States comes directly from theSun. One reason is that solar energy is more expensive to usethan fossil fuels. However, as the supply of fossil fuels decreases,the cost of finding and mining these fuels might increase. Then,it may be cheaper to use solar energy or other energy sources togenerate electricity and heat buildings than to use fossil fuels.

What is an inexhaustible energy source?

1. Potential energyof water

2. Kinetic energyof water

3. Kinetic energyof turbine

4. Electrical energyout of generator

Long-distancepower lines

Figure 18 The potential energy of water behinda dam supplies the energy to turn the turbine.Explain why hydroelectric power is a renewableenergy source.

Building a SolarCollectorProcedure1. Complete a safety

worksheet.2. Line a 500-mL beaker

with black plastic and fillwith water.

3. Stretch clear plastic wrapover the beaker and tapethe plastic so it is taut.

4. Make a slit in the top andslide a thermometer intothe water.

5. Place your solar collector indirect sunlight and monitorthe temperature changeevery 2 min for 20 min.

6. Repeat your experimentwithout using black plastic.

Analysis1. Graph the temperature

changes in both beakers. 2. Explain why one beaker

became warmer than theother.

SC.A.2.3.3

172 CHAPTER 6 Energy and Energy Resources

Collecting the Sun’s Energy Two types of collectors cap-ture the Sun’s rays. If you look around your neighborhood, youmight see large, rectangular panels attached to the roofs ofbuildings or houses. If, as in Figure 19, pipes come out of thepanel, it is a thermal collector. Using a black surface, a thermalcollector heats water by directly absorbing the Sun’s radiantenergy. Water circulating in this system can be heated to about 70°C. The hot water can be pumped through the house toprovide heat. Also, the hot water can be used for washing andbathing. If the panel has no pipes, it is a photovoltaic (foh toh vol TAY ihk) collector, like the one pictured in Figure 19. A photovoltaic is a device that transforms radiantenergy directly into electrical energy. Photovoltaics are used topower calculators and satellites, including the InternationalSpace Station.

What does a photovoltaic do?

Geothermal EnergyImagine that you could take a journey to the center of

Earth—down to about 6,400 km below the surface. Asyou went deeper and deeper, you would find the temper-ature increasing. In fact, after going only about 3 km, thetemperature could have increased enough to boil water.At a depth of 100 km, the temperature could be over900°C. The heat generated inside Earth is called geother-mal energy. Some of this heat is produced when unstableradioactive atoms inside Earth decay, converting nuclearenergy to thermal energy.

At some places deep within Earth the temperature ishot enough to melt rock. This molten rock, or magma,can rise up close to the surface through cracks in thecrust. During a volcanic eruption, magma reaches thesurface. In other places, magma gets close to the surfaceand heats the rock around it.

Geothermal Reservoirs In some regions wheremagma is close to the surface, rainwater and water frommelted snow can seep down to the hot rock throughcracks and other openings in Earth’s surface. The waterthen becomes hot and sometimes can form steam. Thehot water and steam can be trapped under high pressurein cracks and pockets called geothermal reservoirs. Insome places, the hot water and steam are close enough tothe surface to form hot springs and geysers.

Figure 19 Solar energy can becollected and utilized by individu-als using thermal collectors (topphoto) or photovoltaic collectors(bottom photo).

(t)Dr. Jeremy Burgess/Science Photo Library/Photo Researchers, (b)John Keating/Photo Researchers

Cooling towers

Generator

Electric current

Turbine

Fractures in rock

Hot water

Pump

Cool waterHot water from a geothermal reservoir forces its way through a pipe to the surface where it turns to steam.

The steam turns a turbine that is connected to an electric generator.

The steam is cooled in the cooling towers and condenses into water.

The water is pumped back down into the geothermal reservoir.

SECTION 3 Sources of Energy 173

Geothermal Power Plants In places where the geothermalreservoirs are less than several kilometers deep, wells can bedrilled to reach them. The hot water and steam produced bygeothermal energy then can be used by geothermal powerplants, like the one in Figure 20, to generate electricity.

Most geothermal reservoirs contain hot water under highpressure. Figure 21 shows how these reservoirs can be used togenerate electricity. While geothermal power is an inexhaustiblesource of energy, geothermal power plants can be built only inregions where geothermal reservoirs are close to the surface,such as in the western United States.

Heat Pumps Geothermal heat helps keep the temperature ofthe ground at a depth of several meters at a nearly constant tem-perature of about 10° to 20°C. This constant temperature can beused to cool and heat buildings by using a heat pump.

A heat pump contains a water-filled loop of pipe that isburied to a depth where the temperature is nearly constant. Insummer the air is warmer than this underground temperature.Warm water from the building is pumped through the pipedown into the ground. The water cools and then is pumpedback to the house where it absorbs more heat, and the cycle isrepeated. During the winter, the air is cooler than the groundbelow. Then, cool water absorbs heat from the ground andreleases it into the house.

Figure 20 This geothermalpower plant in Nevada producesenough electricity to power about50,000 homes.

Figure 21 The hot water in ageothermal reservoir is used togenerate electricity in a geother-mal power plant.

Geothermal Education Office

174 CHAPTER 6 Energy and Energy Resources

Energy from the OceansThe ocean is in constant motion. If you’ve been to

the seashore, you’ve seen waves roll in. You may haveseen the level of the ocean rise and fall over a period ofabout a half day. This rise and fall in the ocean level iscalled a tide. The constant movement of the ocean is aninexhaustible source of mechanical energy that can beconverted into electric energy. While methods are stillbeing developed to convert the motion in ocean wavesto electric energy, several electric power plants usingtidal motion have been built.

Using Tidal Energy A high tide and a low tide each occurabout twice a day. In most places the level of the ocean changes byless than a few meters. However, in some places the change is muchgreater. In the Bay of Fundy in Eastern Canada, the ocean levelchanges by 16 m between high tide and low tide. Almost 14 trillionkg of water move into or out of the bay between high and low tide.

Figure 22 shows an electric power plant that has been builtalong the Bay of Fundy. This power plant generates enough elec-tric energy to power about 12,000 homes. The power plant is constructed so that as the tide rises, water flows through a turbine that causes an electric generator to spin, as shown in Figure 23A. The water is then trapped behind a dam. When thetide goes out, the trapped water behind the dam is releasedthrough the turbine to generate more electricity, as shown inFigure 23B. Each day electric power is generated for about tenhours when the tide is rising and falling.

While tidal energy is a nonpolluting, inexhaustible energysource, its use is limited. Only in a few places is the differencebetween high and low tide large enough to enable a large elec-tric power plant to be built.

Figure 23 A tidal power plantcan generate electricity when thetide is coming in and going out.

Ocean

Turbine Turbine

Ocean

Figure 22 This tidal powerplant in Annapolis Royal, NovaScotia, is the only operating tidalpower plant in North America.

As the tide comes in, it turns a turbine con-nected to a generator. When high tide occurs,gates are closed that trap water behind a dam.

As the tide goes out and the ocean level drops,the gates are opened and water from behind the damflows through the turbine, causing it to spin and turna generator.

Carsand-Mosher

Self Check1. Diagram the energy conversions that occur when coal is

formed and then burned to produce thermal energy.

2. Explain why solar energy is considered an inexhaustiblesource of energy.

3. Explain how a heat pump is used to both heat and coola building.

4. Think Critically Identify advantages and disadvantagesof using fossil fuels, hydroelectricity, and solar energyas energy sources.

SummaryNonrenewable Resources

• All energy resources have advantages and disadvantages.

• Nonrenewable energy resources are usedfaster than they are replaced.

• Fossil fuels include oil, coal, and natural gasand are nonrenewable resources. Nuclearenergy is a nonrenewable resource.

Renewable and Alternative Resources

• Renewable energy resources, such as hydro-electricity, are resources that are replenishedcontinually.

• Alternative energy sources include solarenergy, wind energy, and geothermal energy.

5. Use a Ratio Earth’s temperature increases with depth.Suppose the temperature increase inside Earth is 500°Cat a depth of 50 km. What is the temperature increase at a depth of 10 km?

WindWind is another inexhaustible supply of energy. Modern

windmills, like the ones in Figure 24, convert the kinetic energyof the wind to electrical energy. The propeller is connected to agenerator so that electrical energy is generated when wind spinsthe propeller. These windmills produce almost no pollution.Some disadvantages are that windmills produce noise and thatlarge areas of land are needed. Also, studies have shown thatbirds sometimes are killed by windmills.

Conserving EnergyFossil fuels are a valuable resource. Not only are they burned

to provide energy, but oil and coal also are used to make plasticsand other materials. One way to make the supply of fossil fuelslast longer is to use less energy. Reducing the use of energy iscalled conserving energy.

You can conserve energy and also save money by turning offlights and appliances such as televisions when you are not usingthem. Also keep doors and windows closed tightly when it’s coldor hot to keep heat from leaking out of or into your house.Energy could also be conserved if buildings are properly insu-lated, especially around windows. The use of oil could bereduced if cars were used less and made more efficient, so theywent farther on a gallon of gas. Recycling materials such as alu-minum cans and glass also helps conserve energy.

Figure 24 Windmills workon the same basic principles as apower plant. Instead of steam turn-ing a turbine, wind turns the rotors.Describe some of the advantagesand disadvantages of usingwindmills.

SECTION 3 Sources of Energy 175More Section Review fl7.msscience.com

SC.G.2.3.1

SC.B.1.3.5

MA.A.3.3.2

SC.H.3.3.4

Billy Hustace/Stone/Getty Images