Download 3 Electric Currents from Magnetism

Name
CHAPTER 18
Class
Date
Magnetism
SECTION
3 Electric Currents from Magnetism
KEY IDEAS
As you read this section, keep these questions in mind:
• What is electromagnetic induction?
• How are electricity and magnetism related?
• What are the basic parts of a transformer?
What Is Electromagnetic Induction?
Remember that electric motors use magnetism to
convert electric energy into mechanical energy. In
contrast, electrical power plants use magnetism to convert
mechanical energy into electricity. They use technology
discovered by British physicist Michael Faraday in 1831.
Faraday produced a current by pushing a magnet
through a coil of wire. The movement of the magnet into
and out of the coil causes electrons in the wire to move.
The moving electrons produce a current. This process is
called electromagnetic induction.
Look at the loop of wire moving between two magnets
in the figure below. As the wire moves into and out of the
magnetic field, the magnetic field induces a current in the
loop. Rotating the loop or changing the strength of the
magnetic field also induces a current in the loop. In each
case, a changing magnetic field passes through the loop.
In general, a current is induced in a circuit if the
magnetic field passing through the circuit changes. This
is known as Faraday’s law.
READING TOOLBOX
Compare After you read this
section, make a table showing the features of step-up
and step-down transformers.
READING CHECK
1. Describe What is
electromagnetic induction?
Magnetic field
S
N
Current
When the loop moves
in the magnetic field,
a current is induced in
the wire.
Direction
of loop's
motion
EHHDBG@<EHL>K
2. Infer If you held the wire
still and moved the
magnets forward and
backward, would there
be a current in the wire?
You can use magnetic field lines to figure out whether
the magnetic field through a circuit is changing. If the
number of magnetic field lines passing through the circuit
changes, the magnetic field through the circuit changes.
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Electric Currents from Magnetism continued
What Happens to Energy During Induction?
READING CHECK
3. Explain Why doesn’t
electromagnetic induction
break the law of conservation
of energy?
READING CHECK
Electromagnetic induction does not break the law of
conservation of energy. It does not create energy from
nothing. Actually, induction requires energy. Pushing a
loop through a magnetic field requires work. The stronger
the magnetic field, the more force you need to push the
loop. The energy needed for that task comes from an
outside source, such as your muscles.
What Do Generators Do?
Remember that electric motors convert electrical
energy into mechanical energy. Generators have the
opposite effect. They convert mechanical energy into
electrical energy. For example, look at the simple
generator shown in the figure below. Notice that it is
similar in structure to an electric motor.
4. Define What is a
generator?
Slip rings
N
S
EHHDBG@<EHL>K
5. Compare How is a
generator similar to an
electric motor?
A generator converts
the mechanical energy
used to turn the wire
into electrical energy.
Brush
Brush
READING CHECK
6. Identify What is an
alternating current?
You must use energy to turn the wire in the magnetic
field. As the coil turns in the magnetic field, a current is
produced in the wire. This current can be used to light a
light bulb, for example,
Each time the loop rotates halfway around, the
current in the wire changes direction. A current that
changes direction at regular intervals is called an
alternating current (AC).
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GENERATORS AND AC CURRENT
The diagram below shows the amount of current an
AC generator produces as the loop rotates. Before the
loop begins to rotate, it is perpendicular to the magnetic
field. There is no current in the loop. When the loop has
rotated 90°, it is parallel to the magnetic field. Then, the
current is at its maximum.
When the loop rotates another 90°, to 180°, it is
again perpendicular to the magnetic field. Therefore,
the current is zero. At 270°, the loop is parallel to
the magnetic field. The current is at its maximum.
However, because the sides of the loop are in opposite
locations, the current in the loop is in the opposite
direction. As the loop continues to rotate, the current
continues to change direction.
READING CHECK
Direction of wire
motion
Graph of current versus
angle of rotation
zero current
Current
Magnetic field
Direction
of wire
motion
Amount of
current
maximum
current
Current
Position of loop
0º
0º
90º
90º
180º
180º
270º
270º
360º
360º
7. Explain Why does the
direction of the current
change as the loop rotates?
Rotation
angle
Rotation
angle
Magnetic
field
Direction
of wire
motion
Direction of wire
motion
Magnetic
field
Magnetic field
Direction
of wire
motion
maximum
current
(opposite
direction)
Current
Current
zero current
0º
90º
180º
270º
360º
Rotation
angle
Rotation
angle
0º
90º
180º
270º
360º
EHHDBG@<EHL>K
8. Identify At which three
angles of rotation is the
current zero?
zero current
Current
Magnetic field
Rotation
angle
0º
90º
180º
270º
360º
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Electric Currents from Magnetism continued
AC CURRENT IN THE HOME
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9. Explain Why is the
current from a power plant
an alternating current?
(Hint: What type of current
do generators produce?)
Remember that batteries produce direct current. That
is, the current from a battery does not change with time.
In contrast, most electrical outlets supply alternating current. This is why many battery-powered devices, such
as radios, require special adaptors to use power from an
outlet.
The electrical energy supplied through electrical outlets comes from generators at power plants. The mechanical energy needed to turn these generators can come
from many sources.
Some sources, such as wind, turn the generators
directly. Other sources, such as coal and oil, are burned
to release heat energy. The heat energy is used to boil
water and produce steam. The steam turns special fans
called turbines. The turbines are connected to the generator. As the steam turns the turbines, the generator produces electricity.
What Is the Electromagnetic Force?
READING CHECK
10. Identify What are three
forms of energy that
are produced by the
electromagnetic force?
Recall that moving electric charges produce magnetic
fields and that changing magnetic fields cause electric
charges to move. Those two facts show that electricity
and magnetism are two aspects of a single force, called
the electromagnetic force.
The electromagnetic force produces electromagnetic
energy. Visible light is a form of electromagnetic energy.
So are other forms of radiation, such as radio waves and
X rays. All are electromagnetic waves, or EM waves.
The figure below shows the nature of EM waves. As you
can see, they consist of vibrating electric and magnetic
fields that are perpendicular to each other. All EM waves
have the two fields in common, whatever their frequency.
Vibrating magnetic field
EHHDBG@<EHL>K
11. Describe How do the
vibration directions of the
electric and magnetic fields
in an electromagnetic wave
compare?
Vibrating electric field
Direction of the
electromagnetic wave
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GENERATING AND REGENERATING
An EM wave’s electric and magnetic fields are both
perpendicular to the direction in which the wave travels.
Therefore, EM waves are transverse waves.
As an EM wave travels, the changing electric field
generates the magnetic field. At the same time, the
changing magnetic field generates the electric field.
Because each field generates the other, EM waves have
a special property: they can travel through a vacuum.
Without that property, light from the sun would not be
able to reach Earth.
What Are Transformers?
You have probably seen metal cylinders on power
line poles. These cylinders contain devices called
transformers that increase or decrease the voltage of
alternating current. The simplest type of transformer
consists of two coils of wire wrapped around opposite
sides of a closed iron loop.
In the transformer shown below, one wire is attached
to a source of alternating current, such as a power outlet. This wire is called the primary coil. The other wire
is attached to an appliance, such as a lamp. This is the
secondary coil.
Current in the primary coil creates a changing
magnetic field. This, in turn, magnetizes the transformer’s
iron core. The changing magnetic field of the iron core
then induces a current in the secondary coil. That
current’s direction changes every time the direction of
the current in the primary coil changes.
Primary coil
READING CHECK
12. Explain Why can
EM waves travel through
a vacuum?
READING CHECK
13. Define What is a
transformer?
Secondary coil
EHHDBG@<EHL>K
14. Identify Label the core
of the transformer in the
figure.
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STEP-UP AND STEP-DOWN TRANSFORMERS
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15. Apply Concepts A
transformer has 10 turns
in its primary coil and 15
turns in its secondary coil.
Is it a step-up or step-down
transformer?
The voltage induced in a transformer’s secondary coil
depends on the number of loops, or turns, in the coil. If
the secondary coil has more turns than the primary coil,
the voltage in the secondary coil is higher. Then, the
transformer is called a step-up transformer, shown at the
top of the figure below.
If the secondary coil has fewer turns than the primary
coil, the voltage in the secondary coil is lower. Then, the
transformer is a step-down transformer, shown at the
bottom of the figure below.
Primary coil
Secondary coil
In a step-up transformer, the voltage in the secondary coil is higher than the
voltage in the primary coil.
Primary coil
Secondary coil
EHHDBG@<EHL>K
16. Identify Circle the side
of each transformer that has
the higher voltage.
READING CHECK
17. Describe How does
the current in the secondary
coil of a step-up transformer
compare to the current in the
primary coil?
In a step-down transformer, the voltage in the secondary coil is lower than
the voltage in the primary coil.
TRANSFORMERS AND ENERGY CONSERVATION
You might think that step-up transformers provide
something for nothing. They do not, because voltage is
only one aspect of electrical power. The secondary coil’s
power output cannot exceed the primary coil’s power
input. Therefore, if the voltage in a coil increases, the
current in the coil must decrease.
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TRANSFORMERS ON POWER LINES
Real transformers are not perfectly efficient. Because
of resistance in the coils, they lose some of the energy
put into them as heat. The power loss increases as the
current increases.
Power companies want to decrease loss to maximize
the amount of energy they transmit. Therefore, they use
high voltages and low currents in their long-distance
power lines. However, it is safer for appliances in the
home to use lower voltage. Therefore, there are
transformers on power lines.
Step-up and step-down transformers help to transmit
electrical energy from power plants to homes and
businesses. Step-up transformers near power plants
increase the voltage to about 120,000 V. Then, near
homes, step-down transformers reduce the voltage to the
120 V that is safe to use in the home.
READING CHECK
18. Identify What causes
energy loss in transformers?
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19. Compare How does the
current in power lines compare to the current in wires
in the home? Explain your
answer.
Step-down transformers like the ones shown here are used to reduce the
voltage across power lines when they enter people’s homes.
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Section 3 Review
SECTION VOCABULARY
alternating current an electric current that
changes direction at regular intervals
(abbreviation, AC)
electromagnetic induction the process of
creating a current in a circuit by changing a
magnetic field
generator a machine that converts mechanical
energy into electrical energy
transformer a device that increases or decreases
the voltage of alternating current
1. Compare Describe how the current produced by a battery is different from the
current produced by a generator.
2. Identify What is Faraday’s law?
3. Describe How can you use magnetic field lines to determine whether a magnetic
field will induce a current in a circuit?
4. Explain How can the chemical energy in oil be used to produce electrical energy?
5. Describe The primary coil of a transformer has 1,000 turns of wire. The secondary
coil has 500 turns of wire. Identify what type of transformer this is, and describe
how the voltage in the two coils will be different.
6. Compare How is a generator different from an electric motor?
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