Download Generators supply electrical energy.

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KEY CONCEPT
Generators supply
electrical energy.
BEFORE, you learned
NOW, you will learn
• Magnetism is a force exerted
by magnets
• A moving magnetic field can
generate an electric current in
a conductor
• Generators use magnetism to
produce current
• How power plants generate
electrical energy
• How electric power is measured
• How energy usage is calculated
VOCABULARY
THINK ABOUT
electric power p. 102
watt p. 104
kilowatt p. 104
kilowatt-hour p. 105
How can falling water
generate electrical energy?
This photograph shows the Hoover Dam
on the Nevada/Arizona border, which
holds back a large lake, almost 600 feet
deep, on the Colorado River. It took
thousands of workers nearly five years to
build the dam, and it cost millions of
dollars. One of the main purposes of the
Hoover Dam is the generation of current.
Think about what you have read about
generators. How could the energy of
falling water be used to generate current?
Generators provide most of the world’s
electrical energy.
The tremendous energy produced by falling water provides the turning
motion for large generators at a power plant. The power plant at the
Hoover Dam supplies energy to more than a million people.
VOCABULARY
Use a description wheel to
take notes about electrical
power.
Other sources of energy at power plants include steam from
burning fossil fuels, nuclear reactions, wind, solar heating, and ocean
tides. Each source provides the energy of motion to the generators,
producing electrical energy. Electric power is the rate at which electrical
energy is generated from another source of energy.
Check Your Reading
E
102 Unit: Electricity and Magnetism
What do power plants that use water, steam, and wind all have
in common?
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Generating Electrical Energy
How does the power plant convert the energy of motion into electrical
energy? Very large generators in the plant hold powerful electromagnets
surrounded by massive coils of copper wire. The illustration below
shows how the energy from water falling from the reservoir to the
river far below a dam is converted to electrical energy.
1
As the water falls from the reservoir, its kinetic energy increases
and it flows very fast. The falling stream of water turns a fan-like
device, called a turbine, which is connected to the generator’s shaft.
2
The rotation of the shaft turns powerful electromagnets that are
surrounded by the coil of copper wires. The coil is connected to a
step-up transformer that sends high-voltage current to power lines.
3
Far from the plant, step-down transformers reduce the voltage so
that current can be sent through smaller lines to neighborhoods.
Another transformer reduces the voltage to the level needed to
operate lights and appliances.
RESOURCE CENTER
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Find out more about
dams that generate
current.
How Electrical Power Is Generated
Power plants use generators to convert kinetic energy into electrical energy.
step-up
transformers
step-down
transformers
3
shaft
2
turbine
1
2
1
Falling water provides energy to turn
the turbine of the
generator.
3
The shaft turns a
powerful electromagnet within a coil of
wire, generating electrical current.
Current is sent along
power lines at a high
voltage. The voltage
level is adjusted by
transformers.
How is kinetic energy turned into electrical energy
visuals question
in aReading
power plant?
Chapter 3: Magnetism 103
E
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Electric power can be measured.
You have read that electric power is the rate at which electrical energy is
generated from another source of energy. Power also refers to the rate at
which an appliance converts electrical energy back into another form of
energy, such as light, heat, or sound.
In order to provide electrical energy to homes and factories, power
companies need to know the rate at which energy is needed. Power
can be measured so that companies can determine how much energy
is used and where it is used. This information is used to figure out
how much to charge customers, and it is used to determine whether
more electrical energy needs to be generated. To provide energy to
an average home, a power plant needs to burn about four tons of
coal each year.
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Learn more about
energy use and
conservation.
Watts and Kilowatts
The unit of measurement for power is the watt (W). Watts measure
the rate at which energy is used by an electrical appliance. For instance,
a light bulb converts energy to light and heat. The power rating of the
bulb, or of any device that consumes electrical energy, depends on both
the voltage and the current. The formula for finding power, in watts,
from voltage and current, is shown below. The letter I stands
for current.
Electrical Power 5 Voltage p Current
P5VpI
Typical Power Ratings
Appliance
E
Watts
DVD player
20
Radio
20
Video game system
25
Electric blanket
60
Light bulb
75
Stereo system
100
Window fan
100
Television
110
Computer
120
Computer monitor
150
Refrigerator
700
Air conditioner
1000
Microwave oven
1000
Hair dryer
1200
Clothes dryer
3000
104 Unit: Electricity and Magnetism
You have probably seen the label on a light bulb that gives
its power rating in watts—usually in the range of 40 W to
100 W. A brighter bulb converts energy at a higher rate than
one with a lower power rating.
The chart at the left shows typical power ratings, in watts,
for some appliances that you might have in your home. The
exact power rating depends on how each brand of appliance
uses energy. You can find the actual power rating for an
appliance on its label.
The combined power rating in a building is likely to be a
fairly large number. A kilowatt (kW) is a unit of power equal
to one thousand watts. All of the appliances in a room may
have a combined power rating of several kilowatts, but all
appliances are not in use all of the time. That is why energy is
usually calculated based on how long the appliances are in use.
check your reading
Explain what kilowatts are used to measure.
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Power
How would you use your electrical energy?
SKILL FOCUS
Making models
PROCEDURE
1
On a sheet of graph paper, outline a box that is 10 squares long by 18
squares wide. The box represents a room that is wired to power a total
of 1800 W. Each square represents 10 W of power.
2 From the chart on page 104, choose appliances that you want in your
MATERIALS
• graph paper
• colored pencils
TIME
30 minutes
room. Using colored pencils, fill in the appropriate number of boxes for
each appliance.
3 All of the items that you choose must fit within the total power available,
represented by the 180 squares.
WHAT DO YOU THINK?
• How did you decide to use your electrical energy?
• Could you provide enough energy to operate everything you wanted at
one time?
CHALLENGE During the summer, power companies sometimes cannot
produce enough energy for the demand. Why do you think that happens?
Calculating Energy Use
The electric bill for your energy usage is calculated based on the rate
at which energy is used, or the power, and the amount of time it is
used at that rate. Total energy used by an appliance is determined by
multiplying its power consumption by the amount of time that
it is used.
Energy used 5 Power p time
E 5Ppt
The kilowatt-hour is the unit of measurement for
energy usage. A kilowatt-hour (kWh) is equal to one
kilowatt of power for a one-hour period. Buildings
usually have meters that measure how many kilowatthours of energy have been used. The meters display
four or five small dials in a row, as shown in the photograph on the right. Each dial represents a different place
value—ones, tens, hundreds, or thousands. For example,
the meter in the photograph shows that the customer has
used close to 9,000 kWh of energy—8,933 kWh, to be
exact. To find how much energy was used in one month, the
last month’s reading is subtracted from this total.
Chapter 3: Magnetism 105
E
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To determine the number of kilowatt-hours of energy used by an
appliance, find its wattage on the chart on the page 104 or from the
label. Then, substitute it into the formula along with the number of
hours it was in use. Solve the sample problems below.
Finding Energy Used
Sample Problem
How much energy is used to dry clothes in a 3 kW dryer for 30 minutes?
What do you know? Power 5 3.0 kW, time 5 0.5 hr
What do you want to find out? Energy used
Write the formula: E 5 P p t
Substitute into the formula: E 5 3.0 kW p 0.5 hr
Calculate and simplify: E 5 1.5 kWh
Check that your units agree: Unit is kWh. Unit for energy used is kWh.
Units agree.
Answer: 1.5 kWh
Practice the Math
1. All of the appliances in a computer lab are in use for 6 hours every day
and together use 3.3 kW. How much energy has been used in 1 day?
2. How much energy is used when a 1.2 kW hair dyer is in use for 0.2 hr?
Energy prices vary, but you can estimate the cost of using an
electrical appliance by using a value of about 8 cents/kWh. You can
calculate how much energy you can save by turning off the lights or
television when you are not using them. Although the number may
seem small, try multiplying your savings over the course of a month
or year.
KEY CONCEPTS
CRITICAL THINKING
1. How do power plants generate
electrical energy from kinetic
energy?
4. Apply Think about reducing
energy usage in your home.
What changes would make
the largest difference in the
amount of energy used?
2. Explain what watts measure.
3. How is energy use determined?
E
106 Unit: Electricity and Magnetism
5. Calculate How much energy
is used if a 3000 W clothes
dryer is used for 4 hours?
CHALLENGE
6. Calculate An electric bill for
an apartment shows 396 kWh
of energy used over one
month. The appliances in the
apartment have a total power
rating of 2.2 kW. How many
hours were the appliances
in use?