California science grade 5 (12)

Sources of Energy Lesson 1 Solar panels harness the Sun’s energy to heat homes and other buildings.. These fossil fuels developed from living creatures that originally used the Sun’s ene

L e s s o n 3

Uses of Resources

6 ES 6 Sources of energy and materials differ

in amounts, distribution, usefulness, and the time

required for their formation.

411

ELA R 6.2.4 Clarify an

understanding of texts

by creating outlines, logical notes, summaries,

or reports • ELA W 6.2.2 Write expository compositions (e.g., description, explanation, comparison and contrast, problem and solution).

Literature

MAGAZINE ARTICLE

412

- Journal Write about it online

@www.macmillanmh.com

Write About It

Response to Literature This article describes a plan for a solar-energy system What problem is the system designed to address? What is the proposed solution? Write a summary describing the problem and the author’s idea for solving it Include the steps involved

in transforming solar energy into a usable form.

The Criswell plan is to transform Luna into a kind of giant orbiting wall plug He advocates building solar power stations on the lunar surface as a way to provide sustainable and affordable electric power back here on Earth

First of all, the airless Moon is already on the receiving end of huge amounts of solar power in the form of sunlight Harnessing just one percent of that sunlight could satisfy Earth’s power needs, Criswell suggests But how?

The concept is called the Lunar Solar Power (LSP)

system Huge “farms” of solar cells would collect sunlight that shines on the Moon Pairs of solar farms would be planted in the lunar highlands, on the east and west limbs

of the Moon, near the equator That sunlight would be converted to microwave energy and beamed across space

to an energy-needy Earth Once here on terra firma, the microwave energy would be transformed into electricity and fed into local energy grids

from ODYSSEY MAGAZINE

413

Sources

of Energy

Lesson 1

Solar panels harness the Sun’s energy to heat

homes and other buildings How else could

solar energy be used?

Solar Two Power Station, Daggit, California

6 ES 6.a Students know the utility of energy sources is determined

by factors that are involved in converting these sources to useful forms and the consequences of the conversion process.

414

ENGAGE

What factors affect the

efficiency of a solar oven?

Make a Prediction

What will happen to water in a solar oven? Write

your answer as a prediction in the form “If water

is placed inside a solar oven, then ”

Test Your Prediction

Measure Draw a 2.5-centimeter border around

the top of a pizza box Be Careful Cut the

three outer sides Fold the flap up along the

back edge.

Glue foil under the flap, shiny side out Glue

another piece of foil inside the box, shiny side

up Tape black construction paper over the foil.

Roll up the newspapers, and place them inside

the edges of the box Tape them in place Tape

plastic wrap under the box lid over the first piece

of foil Add plastic wrap to the top of the lid.

Put the bowl of water in the center of the box

Record the temperature of the water.

Position the oven with the flap facing the Sun

Prop the flap open with a dowel or ruler After

10 minutes measure the temperature of the water.

Draw Conclusions

Analyze How would you explain what happened?

Did your observations support your prediction?

Infer Did the temperature in the box change?

• pizza box

• black construction paper

• small bowl containing

100 milliliters

of water

• thermometer

What other factors might affect the efficiency of the

oven? What would happen if you used white paper

instead of black paper? Form a hypothesis and test it

Materials

Explore More

Step

6 IE 7.b Select and use appropriate tools and technology (including

calculators, computers, balances, spring scales, microscopes, and

binoculars) to perform tests, collect data, and display data.

415 EXPLORE

▶ Main Idea 6 ES 6.a

Converting energy to

a useful form has costs

and consequences, which

differ depending on the

Why is energy important?

Think about all the things you do that require energy You might turn on lights, listen to music, work at a computer, ride in a car or a bus, or use

a home appliance

Where does the energy you use come from?

An energy source is where the light, heat, or electrical energy people use comes from Most forms of energy originate with the Sun For example, power plants produce electricity, but they burn fossil fuels such as coal, oil, or natural gas to make the electricity These fossil fuels developed from living creatures that originally used the Sun’s energy to survive Most of the energy people use can be traced back to fossil fuels

Ac[[O`g

Los Angeles, California

Explore solar energy

with an engineer.

416

EXPLAIN

Types of Energy

Fossil fuels such as coal, oil, and

natural gas are burned to release

energy Coal is a hard, black substance

formed from plants that lived about 300

million years ago Oil , or petroleum,

is a thick, black liquid that forms

underground, over millions of years,

from decaying sea creatures and plants

Oil can be made into gasoline and other

fuels as well as plastics Natural gas

is a mixture of gaseous hydrocarbons

formed from marine organisms

Alternative energy sources include

moving water, wind, and sunlight

Running or falling water can turn

the wheel of a water mill The wheel

spins an axle attached to generators

that produce energy In a hydroelectric

(HIGH•droh•ee•LEK•trik) plant, moving

water spins a turbine (similar to a

waterwheel) that powers an electric

generator Wind can also spin a wheel

Most of the energy on Earth comes from the Sun Solar energy is any form

of energy radiated by the Sun Solar collectors trap sunlight that can be used

to heat water and homes Solar cells convert sunlight into electrical energy Such cells are used in electric cars

Quick Check

Summarize What types of energy

do people use today?

Critical Thinking How do fossil fuels

and other energy sources differ?

liquid absorbs heat

from rooms and

cools the air.

Warmed liquid returns to house.

Inside house warm liquid gives off heat and warms the air in rooms.

417

EXPLAIN

How is energy converted into

usable forms?

Energy cannot be created or destroyed It changes

from one form into another in a process called

energy conversion

Some energy conversions are very simple The

stored chemical energy in a flashlight’s batteries is

converted into light energy and heat energy when the

flashlight is turned on In your body, chemical energy

from food is converted into heat as well as mechanical

energy that you use to move In an electric fan, electric

energy is converted mostly into mechanical energy to

turn the blades

In the solar-powered car shown below, a solar cell

converts sunlight to electricity The electricity is used

to run the car’s motor The motor converts electricity

to mechanical energy, which provides the motion to

propel the car

How a Solar-Powered Car Works

Wind Energy

energy fl ow solar energy

418

EXPLAIN

Other energy conversions consist

of more steps and are more complex

A steam engine converts chemical

energy in fuel into heat energy and

then into mechanical energy A car

engine converts chemical energy

in gasoline into heat energy as it is

burned and then into mechanical

energy to turn the car’s wheels In a

hydroelectric plant, falling water spins

turbines, creating mechanical energy

The turbines power the generators,

changing mechanical energy into

electricity When coal is burned, it

gives off heat that creates steam,

which is then turned into electricity

Scientists continually develop ways

to capture and convert energy into

usable forms All sources of energy have advantages and disadvantages Costs are also involved The monetary cost of the energy conversion is one factor to consider when deciding which energy source is best for a particular purpose Another cost is the possible harm to Earth associated with using a particular energy source

How are the windmills shown in these two photos similar? How are they different?

Clue: Note the designs of the two types of windmills and the ways in which they capture the energy of the wind.

Reading Photos

Quick Check

Summarize What is the purpose

of energy conversion?

Critical Thinking What type

of energy conversion is involved when you use an electric stove?wind farm, Tehachapi Pass, California

419

EXPLAIN

How does cost affect

energy use?

Scientists, government officials, and

economists decide energy policy Among

the most important factors they consider in

deciding which energy source to recommend

are the costs and the benefits of the energy

Cost-effectiveness is determined by

comparing the costs and the consequences

of different ways of doing something For

energy sources, this includes evaluating

the cost of an energy source and the

effects of using it The source that is least

expensive and causes the least damage to the

environment is the most cost-effective

Currently many alternative energy sources

are less cost-effective than fossil fuels The

technology needed to deliver energy from

them is expensive At this time drilling for

oil costs less than building solar panels

Alternative sources also produce less usable

energy than fossil fuels However, as the cost

of using newer technologies decreases and

the amount of usable energy they provide

increases, more people may choose to use

alternative energy sources

▲ Geothermal steam pours from

a power plant in Iceland.

420

EXPLAIN

Efficiency is an important factor

in determining the use of an energy

source or the purchase of a new

appliance Efficiency describes the

amount of usable energy given off

by an energy conversion compared

to the total amount of energy used in

the conversion No energy conversion

is 100% efficient Some energy is

always lost as heat released into the

surrounding atmosphere

When a conversion device puts

out more usable energy, it is more

energy efficient For example,

some automobiles made today can

travel a longer distance on a given

amount of gas than automobiles made

20 years ago Fuel efficiency is a very

important consideration when people

purchase cars, because it determines how much the drivers will have to spend on gas

Energy efficiency also affects the cost of running appliances Items of the same type, such as refrigerators

or air conditioners, may use different amounts of energy The least expensive appliance may not be the most efficient, and it may cost more to run In most cases newer appliances use less energy than older ones

Quick Check

Summarize What is energy

efficiency?

Critical Thinking How could you

decide which of two appliances was more economical to buy?

estimate of the appliance’s annual energy use (The lower the number, the more energy efficient the appliance, and the less

it costs to run.)

range of ratings for similar models

estimate of annual cost

to run this model

421

EXPLAIN

What are the consequences

of energy use?

Using energy has various consequences, all of which are important considerations when people choose energy sources

Consequences include pollution, damage

to land, and changes in the flow of waterways Pollution is a harmful change

in the natural environment It may be the release of substances that harm living things or the release of energy that disrupts

an ecosystem Pollution is usually the result of human activities Some pollution, however, is the result of natural causes, such as volcanic eruptions

is mined comes from the surface, through

strip mining In strip mining, layers of

topsoil are peeled away, exposing the coal underneath This process damages the land.The burning of coal and other fossil fuels

in power plants, factories, and automobiles releases many pollutants For example, some coal deposits contain large amounts

of sulfur Burning these deposits releases harmful substances such as sulfur dioxide Moisture mixes with these substances

to form an acid that can fall to Earth’s

surface as acid rain Acid rain harms soil

and water supplies, and it weathers statues and buildings Pollutants in the atmosphere

can also build up into thick clouds of smog.

▲ Emissions-control laws have

reduced pollution from cars

and trucks This has helped

cut down on smog.

Los Angeles, California

422

EXPLAIN

Hydroelectric Power

Hydroelectric power uses the energy

of falling water to produce electricity

A dam is a barrier constructed to

control a flow of water or to raise

a water level Dams can hold back

millions of tons of water This water

is released through pipes into a power

plant, where it spins the blades of

turbines that power generators to

produce electricity

The efficiency of hydroelectric

power plants can be close to 90%

However, dams can cause harmful

changes to the environment Dam

construction can flood valuable

habitats Dams can also alter the flow

of rivers, interfering with flooding

cycles that replenish wetlands and

add nutrients to the soil

Nuclear Power

A nuclear power plant is a facility

that generates electricity through

nuclear reactions In this controlled

chain reaction, uranium atoms are

split to change the energy locked within

them into heat energy Nuclear power

produces a tremendous amount of energy

compared to other energy sources,

and it does not produce the same kinds

of pollution that fossil fuels do

Even so, disadvantages exist The

reactions create radioactive waste that

can harm people and the environment

Nuclear power plants also release

excess heat, often in the form of hot

water introduced into rivers and

oceans Known as thermal pollution,

this release of heat can destroy habitats

and kill the animals that live in them

Quick Check

Summarize What is pollution,

and what causes it?

Critical Thinking Compare

and contrast the environmental effects of strip mining, burning fossil fuels, hydroelectric plants, and nuclear power plants

The Power of Water

What factors do you think affect how well a waterwheel works?

How can you design the blades on

a waterwheel so that they turn as fast as possible?

Cut eight equally spaced slits from the rim to the base of a plastic cup Fan the sections out to form eight “blades.”

Poke a hole through the center of the bottom of the cup, and insert

a pencil as an axle.

Observe Hold the pencil loosely

at both ends, and place it in

a horizontal position Hold the fanned-out blades under running water What happens?

Will your waterwheel turn faster with more blades? With fewer blades? Experiment to find out.

423

EXPLAIN

Your family probably receives a

monthly energy bill What would you

think if you opened it up one month

and saw that it charged not only

for your energy usage but also for

environmental damage? What

if the charges were for smog, acid

rain, thermal pollution to rivers,

and flooding caused by dams?

This damage represents the

nonmonetary costs , or the

environmental consequences, of

energy use Damage to the

environment, caused by air and water

pollution, affects all living things

Examples of the nonmonetary costs

of fossil fuels are the pollution they

generate and the long-term health

problems they can cause for people

Nonmonetary costs can lead to monetary costs, such as the expense of repairing or cleaning up environmental damage People need to think beyond monetary costs when deciding which energy sources to use Even though fossil fuels are the most cost-effective energy source, they have a high

nonmonetary cost compared to other sources of energy, such as solar power

What are the nonmonetary

costs of energy use?

▼ At this power plant in Florida, manatees are attracted to the warm

water This behavior alters manatees’ migration patterns.

Quick Check

Summarize What is the overall

effect of fossil-fuel use on the environment?

Critical Thinking Does nuclear

power have any nonmonetary costs? Why or why not?

424

EXPLAIN

Summarize the Main Idea

Energy sources include

coal, oil, gas, sunlight,

water, and wind Energy

conversion occurs

when energy changes form (pp 416–419)

Efficiency refers to the

amount of useful energy obtained compared to the total amount in an energy conversion.

Make a

Study Guide

Make a three-tab book (see pp 487–490)

Use the titles shown On the inside of

each tab, summarize each topic.

Think, Talk, and Write Main Idea What are the costs and consequences of using different energy sources?

Vocabulary A hard, black substance formed from the remains of plants that lived about 300 million years

Summarize Explain the difference between cost-effectiveness and energy efficiency.

Critical Thinking How do respiratory problems caused by smog involve both nonmonetary costs and monetary costs?

Test Practice Which type of energy

is obtained from running water?

Energy Use and Pollution

Investigate one type of energy source

used in your community Find out

if there are any concerns about the

pollution it causes and what is being

done about it Write a report, and

present your findings to your class.

Use Energy Guide Labels

Using the Energy Guide label on p 421 for reference, about how much would

it cost to use this washer for 8 years?

- Review Summaries and quizzes online @www.macmillanmh.com 425

EVALUATE

When you communicate you share

information with others You may do

this by speaking, writing, drawing, or

using sign language First, you gather

some information to share with others

Then, you share your information.

Learn It

Scientists look for ways to reduce

energy waste They communicate their

findings to others by writing books and

articles, giving interviews, and making

presentations.

Heating and cooling buildings takes

a lot of energy In fact, heating and

cooling are two of our biggest uses of

energy They are also sources of many

air pollutants To avoid wasting energy,

people can use insulation to retain heat in

winter and keep air cool in summer.

Try It

▶ Some materials are better insulators than others In the

following activity, you will test the insulating qualities of

six different materials You will need down from an old

jacket or glove, a cotton sock, a wool sock, sand, shredded newspaper, plastic air-bubble packing material, coffee cans with snap-on lids, baby food jars with screw-on lids,

a graduated cylinder, warm water, and a thermometer.

▶ Fill each jar carefully with the same amount of warm water Measure the temperature in each jar, and write it on a chart like the one on page 427 Put a lid on each jar, and place each jar

in the center of a can Fill the space between each jar and can with one kind of insulating material Stuff the can to completely surround the jar Label each can to identify the material used Record this on your chart Put the lids on the cans, and wait

1 hour Open each can and then each jar to measure the

temperature of the water Record the temperatures on

your chart.

426

EXTEND

▶ Analyze the data in your chart Which material(s) kept the water

about the same temperature it was when the test started?

▶ Communicate your actions and the results to your classmates

in a written or oral report.

Apply It

▶ Which insulating material do you think would best keep

the air in a building cool when the weather is hot outside?

▶ Repeat the experiment, but this time use very cold water

and one ice cube in each jar.

▶ Analyze your results Communicate your findings to the class

by writing a report, drawing a cartoon strip, or composing

a song.

6 IE 7.d Communicate the steps and results from an investigation in written

EXTEND

Renewable and Nonrenewable Resources

Water provides a renewable source of energy for

making electricity One important use of electricity

is to light people’s homes What type of light bulb

do you think is more efficient?

Lesson 2

Shasta Dam, Sacramento River, California

6 ES 6.b Students know different natural energy and material resources, including air, soil, rocks, minerals, petroleum, fresh water, wildlife, and forests, and know how to classify them as renewable or nonrenewable.

428

ENGAGE

How energy efficient are

different types of light bulbs?

Form a Hypothesis

Are some kinds of light bulbs more efficient than

others? A light bulb gives off light and heat This

heat is wasted energy Do some types of bulbs give

off more heat than others? Develop a hypothesis in

the form “If one type of light bulb is more efficient

than another type, then it will give off ”

Be Careful Let bulbs cool before touching them.

Test Your Hypothesis

Lay the towel on a table Place the lamp at one

end of the towel Put the thermometer at the

other end of the towel where the light from

the lamp will shine on it Measure the distance

between the thermometer and the bulb.

Experiment Unplug the lamp, and screw

in the incandescent light bulb Record the

starting temperature Angle the lamp over the

thermometer, plug in the lamp, and turn it on.

Measure Shine the lamp on the thermometer

for 5 minutes Record the temperature Repeat

steps 2 and 3 for the fluorescent light bulb Allow

the lamp and the table to cool between the trials.

Draw Conclusions

Infer What conclusion can you draw about

which kind of bulb produces less heat?

How do you think the use of incandescent lights

in a home would affect the use of air conditioning

in the summer? Make a prediction and test it.

• 60-watt incandescent light bulb

• 13-watt compact

fl uorescent light bulb

429 EXPLORE

Main Idea What are natural resources?

What would you need to live off the land? All animals need air to breathe, water to drink, and food to eat for energy People need soil to grow food Rocks and minerals are needed for soil formation Forests produce some of the oxygen people breathe as well as raw materials for shelter and fuel for fires Natural resources are materials people take from Earth Almost everything people use comes directly or indirectly from a natural resource

Earth’s natural resources can be classified by the processes and the time needed to produce them In one group are the resources that take

so long to replace that they are considered nonrenewable Nonrenewable resources are available in fixed quantities or are used up more quickly than they can be replaced Fossil fuels are nonrenewable because they take millions

of years to form Once the existing supply of fossil fuels is gone, that fuel cannot be replaced Minerals such as copper and gold, which are limited resources, are also nonrenewable

;OW\7RSO 2SbOWZa

farm near the Russian River, California

▼ This gasoline container holds

a product made from fossil fuels which are nonrenewable resources.

430

EXPLAIN

Other resources can be replaced

Renewable resources are those that

can be replaced by nature, sometimes

at a rate close to their rate of use

If trees are cut down, their seeds

can be planted Water is constantly

replenished through the water cycle

Some resources, such as solar energy,

are almost inexhaustible—there is

virtually no end to the supply

Not all renewable resources can be

replaced at the same rate Resources

such as soil are replaced at a very slow

rate It can take from 500 to 2,000

years to replace just a few centimeters

of topsoil

Even when a resource can be

replaced, only limited quantities of it

may exist Although most of Earth

is covered with water, only a small

amount of water is fresh and available

for use by people If fresh water is

polluted or if people use too much water, then water shortages can occur, despite the fact that water is constantly replaced

Whether a resource is renewable

or nonrenewable, all natural resources must be regarded as Earth’s treasures

To sustain life and protect Earth for future generations, people must make

wise decisions The idea of sustainability

suggests that people should fulfill present needs without endangering the ability of future generations to fulfill their needs

as well

How does this photo show the use of natural resources?

Clue: What plants are shown in the photo?

Reading Photos

Quick Check

Main Idea Why are natural

resources important?

Critical Thinking What is the

difference between renewable and nonrenewable resources?

431

EXPLAIN

What are nonrenewable resources?

oil field in Huntington Beach, California

the elements hydrogen and carbon More than 90% of fossil fuels mined are used for fuel The other 10% of fossil fuels are used to make a variety

of products, such as plastics and

petrochemicals Petrochemicals, which

come from petroleum or natural gas, include ingredients in paints, fertilizers, pesticides, plastics, and medicines

With the growth of industry, the demand for energy also grows Our economy depends on fossil fuels for energy For example, fossil fuels provide the energy to power machines that save time and labor Without fossil fuels the economy would suffer However, the burning of fossil fuels is the greatest contributor to air pollution

Some nonrenewable resources take

hundreds of millions of years to form

Because they take so long to form,

they cannot be replenished in a short

amount of time Much of our current

technology and most of our electronic

devices depend on nonrenewable

resources The two main types of

nonrenewable resources are fossil

fuels and nuclear fuels

Fossil Fuels

Fossil fuels are formed from the

remains of plants and animals that

lived millions of years ago These fuels

include coal, oil, and natural gas Fossil

fuels give off large amounts of energy

when they burn, because they contain

432

EXPLAIN

Nuclear Fuels

Nuclear energy is produced when

atoms are split apart in a controlled

chain reaction Because the forces in

the nucleus of an atom are very strong,

the energy released is much greater

than energy from other sources Splitting

the atoms in 1 kilogram (2.2 pounds)

of uranium yields about as much energy

as burning millions of kilograms of

coal Nuclear fuels are materials, such

as uranium, that can be used in nuclear

reactors as a source of energy Nuclear

fuels can undergo induced fission

(FISH•uhn), or the splitting of atoms

into pieces The nucleus can be split

when struck by a slow-moving neutron

Although uranium is quite common,

uranium 235, the type used in fission,

is relatively rare and is not renewable

Nuclear power plants provide electricity

around the world Without this type of

fuel, other sources would be needed to

meet the demand for energy

Proved Reserves (as of January 1, 2004)

Amount Used (for 2003)

Oil 1,265 billion

barrels

about

80 million barrels per day

Natural gas

6,079 trillion cubic feet

about

96 trillion cubic feet per year Source: U.S Energy Information Administration

Analyze Examine the data in the table.

Communicate Based on the data

in the table, make a line graph that shows how long the world’s supplies of oil and natural gas will last Assume that the rate of use will remain the same over time.

Infer How much time will pass until supplies of oil and natural gas run out?

Quick Check

Main Idea What are fossil fuels

made of?

Critical Thinking What

characteristic is used to classify materials as nonrenewable resources?

nuclear plant, Rancho Seco, California

433

EXPLAIN

What are renewable resources?

▼ Fusion reactors have

the potential to provide

a cheap, clean source

of energy.

Geothermal energy, the energy produced from the heat below Earth’s surface, is almost inexhaustible due to the enormous reserves of heat below Earth’s surface However, there are very few places where geothermal energy can be used

Nuclear fusion offers the potential for an almost unlimited source of energy Fusion is the process of merging nuclei with smaller masses to make

a nucleus with a larger mass This process uses special forms of hydrogen

called deuterium (DOO•tir•ee•uhm)

Renewable resources are those that

can be replenished Resources such

as solar energy, trees, wind, water,

fusion, and biomass are considered

renewable However, some resources

are not completely renewable because

of the time needed to replace them

Truly renewable energy sources include

geothermal energy, fusion, solar

energy, wind, and water Unlike the

use of trees or biomass, these types

of energy are fully renewable because

there is no time gap between using

them and replacing them

434

EXPLAIN

and tritium (TRI•tee•uhm) Nuclear

fusion is similar to the reaction that

produces the energy given off by

the Sun and other stars Scientists

around the world are working on the

development of fusion reactors that they

hope will become practical and

cost-efficient

Solar energy, wind, and water are

natural resources that are harvested

for energy use Solar panels on

buildings collect the Sun’s energy

and use it to provide heat and

electricity Wind is collected through

a turbine and converted into electricity

Hydroelectric power , which harnesses

the force of falling or running water,

is also converted into electricity

Some resources, such as trees and

biomass—organic matter such as plants

and animal wastes—are not always

fully renewable because of the amount

of time needed to replace them Trees

that are cut down for construction or

fuel can be replaced with new trees

However, they can be replaced only

if the rate at which they are used

does not exceed the time needed for

regrowth Too often, when forests are

cleared, the trees cannot be replaced

quickly enough to cope with demand

Biomass conversion is a method

for changing both plant and animal

remains into high-quality fuels, such as

ethanol Land and soil are needed to

grow the plants used in the conversion

However, too much cultivation can

damage the land and soil and lead

to habitat destruction, reducing the

supply of biomass

Quick Check

Main Idea How is biomass

conversion a form of solar energy?

Critical Thinking Why are some

renewable resources not always fully renewable?

▲ Trees are a renewable resource

However, replacing a forest can take decades.

435

EXPLAIN

How is hydroelectric power

dependent on solar energy?

Hydroelectric power is a renewable-energy resource

generated by the force of falling or running water

Hydroelectric power depends on the water cycle As the

Sun warms water on Earth’s surface, the heat causes water

to evaporate, or turn into water vapor As the water vapor

rises into the atmosphere, it cools and condenses In time

the water falls back to Earth in the form of precipitation,

adding to Earth’s rivers, lakes, and oceans

Water flowing from streams or rivers feeds hydroelectric

plants Water in reservoirs, pipelines, or canals can also be

used for hydroelectric power Dams are often built to hold

back water, creating large reservoirs The dam can be opened

and closed to control the flow of water All of these water

sources depend on precipitation to maintain their levels

A Hydroelectric Plant

436

EXPLAIN