Chapter 5 Basic Concepts of Electricity and Magnetism

Transcription

TR AINER IMPLEMENTAT I O N Man ual
Chapter 5
Basic Concepts of Electricity
and Magnetism
© Cengage Learning. All rights reserved. No distribution allowed without express authorization.
Career Profile
Ray Shorkey “enjoys electrical work in and of itself.” He believes that “being an
electrician is a career that provides you with a variety of work, and is challenging both
mentally and physically. There is a sense of satisfaction in building something [and] then
see it completed and operating properly.”
seeing
Shorkey is currently a journeyman electrician and lead-man at Merit Electric in Fort Collins, Colorado.
Describing his responsibilities on a typical day, he says, “We start early, usually six or seven in the morning,
and, depending on the job, work between eight and twelve hours. As a lead-man you must be aware of the
job schedule, plan and prioritize tasks for your crew, and ensure that the needed materials are available to
complete those tasks. In addition to your own tasks, you need to be available to assist your crew members
with questions and troubleshooting.”
Additionally, he notes, “Some jobs or projects don’t always work out as planned. Sometimes you run into a
situation that wasn’t already taken into account, or even known about. You have to solve problems quickly,
using the resources that are available. Time management is also a challenge. It requires planning to balance
everything and to be able to prioritize. It takes practice to think clearly in stressful situations.”
Because the challenges of managing projects and crews are so many and so diverse, “proper training
and education are important,” Shorkey states. “The more knowledge you have, the more value you
hold in your field.” Shorkey endorses IEC programs (like the one he completed at the Rocky Mountain
chapter) because it “teaches theory and provides you with a deeper knowledge of the electrical trade
than you learn in the field.” He adds, “The education you receive at IEC is also instrumental in preparing
for your journeyman license exam.”
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Chapter Outline
A. The Atom (Figure 5-1, Figure 5-2, Figure 5-3, Figure 5-4, Figure 5-5)
B. Electric Charge (Figure 5-6, Figure 5-7)
C. Electricity—Electrons in Motion
1. Valence Electrons
2. Free Electrons and Electron Flow
3. Ions
4. Static Electricity
a. Practical Applications of
Static Electricity (Figure 5-8,
Figure 5-9)
b. Nuisance Charges of Static
Electricity
D. Electric Sources
1. Direct Current
2. Alternating Current
E. Magnetism (Figure 5-10)
1. Magnets
2. Fields, Flux, and Poles (Figure 5-11)
3. Electromagnetism (Figure 5-12,
Figure 5-13)
4. Magnetic Devices
Explain that when we apply voltage to a conductor, we are merely “exciting,” or moving, the electrons that are already present in the conductor. Some students have the
misconception that the application of voltage forces electrons into the conductor.
Remind students that any exposure to dc should be treated with the same respect afforded to ac voltage. Explain that where ac can cause a severe shock, the same can
occur with dc, along with the added damage of severe burns.
Dc is not commonly used because it can’t be transmitted long distances.
Ac can be transmitted for long distances with the use of a transformer to increase the
voltage and thereby decrease the current level.
Help the students understand that magnetism is a basic concept of electricity.
Explain that everything that is run with electricity, whether ac or dc, will involve the use
of magnets or magnetism.
Explain that all magnets produce “lines of flux”; it is important to understand that lines
of flux orient themselves North and South and that the magnetism flows in the flux lines
from North to South. The lines of flux, however, are stationary and do not move.
Chapter Outline
Key Terms
THE ATOM
Air-core magnet
Ampere-turns
ELECTRIC CHARGE
Atom
Atomic number
ELECTRICITY—ELECTRONS IN MOTION
ELECTRIC SOURCES
Compound
Conductor
Electrical current
Electromagnet
Electron
MAGNETISM
Element
Free electrons
Ions
Iron-core magnet
Law of charges
Chapter Objectives
After completing this chapter, you will be able to: [Review Questions
1. Identify the three principle parts of an atom. 1, 2, 3, 4, 5]
[Review Questions 6, 7] 2. State the law of charges, and describe its importance to current flow.
[Review Questions 8, 9, 10, 11, 12, 13] 3. Explain electron current flow, and contrast direct and alternating current.
[Review Questions 14, 15] 4. Describe current flow through a conductor, and discuss why heat is produced.
[Review Questions 16, 17, 18, 19, 20] 5. Define magnetic terms, and discuss magnetic principles and concepts as
they relate to electricity.
Life Skills Covered
Managing Stress
Critical Thinking
Take Action
Magnetic field
Magnetism
Molecule
Neutron
Nucleus
Permanent magnet
Permeability
Polarity
Poles
Proton
Reluctance
Residual magnetism
Saturation
Static electricity
Temporary magnet
Transformer
Valence shell
Life Skill Goals
The life skill goals covered in this chapter are Managing Stress, Critical
Thinking, and Take Action. You will practice the reading strategies introduced
in Chapter 2. You will also be asked to think about how you are fitting reading
into your schedule. Finally, as in every chapter, you have opportunities to check
your understanding throughout the chapter.
124 TRAINER IMPLEMENTATION MANUAL Chapter 5 Basic Concepts of Electricity and Magnetism
Where You Are Headed
Identifying the parts of an atom and understanding the laws of charges will help
you realize that electricity is not magic! There are real and practical reasons
why and how current flows through a conductor. Atomic theory, laws of charges,
electron movement (current flow), and the basic principles of magnetism are all
building block materials. And like the foundations of the houses and buildings you
will someday be wiring, this information will serve as the foundation for more
advanced electrical topics.
Introduction
It has been over 250 years since Ben Franklin discovered two kinds of electrical charges.
It has been almost 200 years since Hans Oersted and Michael Faraday developed ideas
of electric fields, currents, and magnetism. At about the same time, British electrician/
engineer William Sturgeon invented the electromagnet. Because of these men and
other pioneers in the fields of magnetism and electricity, we enjoy watching television,
listening to our stereos, surfing the Internet on our computers, and heating our food in
microwave ovens. This chapter covers the basic concepts of electricity and magnetism;
learn about the forces that improve the quality of our lives.
We learn about atomic theory to help us understand electricity, but few of us
realize that much of an atom’s mass is in its dense nucleus, which stores enormous
amounts of energy. This energy can be released by breaking up the nucleus. Why
is this important? Our understanding of this basic atomic structure has been used
as the basis of nuclear power plants and many modern medical applications, such
as radiation therapy for cancer and medical imaging used for noninvasive medical
testing. This knowledge has also enabled scientists to make weapons. The advent
of atomic weapons influenced the way World War II ended, its aftermath, and the
power plays between nations at work today.
How long do you think it will take you to read this chapter? When do you think you can
fit that much time into your schedule?
Practice SQR3.
• Survey through Section 5-1, The Atom.
• Skim through the section. Take a look at the pictures. Read the Self-Check
questions at the end of the section.
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TRAINER IMPLEMENTATION MANUAL Chapter 5 Basic Concepts of Electricity and Magnetism 125
•
•
•
•
Create the questions you are going to answer by reading this section.
Read this section. Take some notes. Find the answers to your questions.
Recite these answers.
Review. Go back and highlight the main points in the section. Add to your notes.
Atom the smallest
particle into which
an element can
be divided without
losing its identity.
Element a group
of identical atoms.
Molecule consists
of two or more
atoms.
The Atom
An atom is the smallest particle into which an element can be divided without losing
its identity. A group of identical atoms is called an element. All matter is composed
of atoms. Whether we are discussing metal, wood, or glass, they are all made up of
atoms. Before we go any further, let’s take a step back and look at some useful terms.
The smallest particle of a substance that still has all of its characteristics is called a
molecule. A molecule consists of two or more atoms. If a molecule of metal is divided
into smaller parts, it is no longer metal.
There are over a hundred different types of atoms and as many elements as
there are atoms. A few of the more common elements are iron, nitrogen, and oxygen;
important elements in terms of electrical conductivity are copper, silver, and gold.
Figure 5-1 lists the table of elements.
FIGURE 5-1
Table of elements.
ATOMIC
VALENCE
NUMBER NAME ELECTRONS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
16891_ch05_108-129.indd 111
Hydrogen
Helium
Lithium
Beryllum
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Argon
Potassium
Calcium
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Krypton
1
2
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
2
2
2
1
2
2
2
2
1
2
3
4
5
6
7
8
SYMBOL
H
He
Li
Be
B
C
N
O
F
Ne
Na
Ma
Al
Si
P
S
Cl
A
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
ATOMIC
VALENCE
NUMBER NAME ELECTRONS
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Technetium
Ruthenium
Rhodium
Palladium
Silver
Cadmium
Indium
Tin
Antimony
Tellurium
Iodine
Xenon
Cesium
Barium
Lanthanum
Cerium
Praseodymium
Neodymium
Promethium
Samarium
Europium
Gadolinium
Terbium
Dysprosium
Holmium
Erbium
Thulium
Ytterbium
Lutetium
Hafnium
1
2
2
2
1
1
2
1
1
–
1
2
3
4
5
6
7
8
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
ATOMIC
VALENCE
SYMBOL NUMBER NAME ELECTRONS SYMBOL
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
Tantalum
Tungsten
Rhenium
Osmium
Iridium
Platinum
Gold
Mercury
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
Francium
Radium
Actinium
Thorium
Protactinium
Uranium
93
94
95
96
97
98
99
100
101
102
103
Neptunium
Plutonium
Americium
Curium
Berkelium
Californium
Einsteinium
Fermium
Mendelevium
Nobelium
Lawrencium
2
2
2
2
2
1
1
2
3
4
5
6
7
8
1
2
2
2
2
2
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bl
Po
At
Rd
Fr
Ra
Ac
Th
Pa
U
Artificial Elements
2
2
2
2
2
2
2
2
2
2
2
Np
Pu
Am
Cm
Bk
Cf
E
Fm
Mv
No
Lw
From HERMAN. Delmar’s Standard Textbook of Electricity, 5E. © 2011 Cengage Learning
5-1
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112 CHAPTER 5 BASIC CONCEPTS OF ELECTRICITY AND MAGNETISM
Electron an
elementary particle
carrying one unit of
negative electrical
charge.
Proton found in
the nucleus and
has a positive
electrical charge.
Neutron a small
particle that
possesses no
electrical charge
and is typically
found within an
atom’s nucleus.
Nucleus the
very dense
region consisting
of protons and
neutrons at the
center of an atom.
FIGURE 5-2
Compound exists
when two kinds
of atoms combine
chemically.
Because there are so many different materials in the world, most materials are
made up of more than one element. When different kinds of atoms combine chemically,
they form materials referred to as compounds. Two very familiar compounds are
table salt and water. Table salt is made up of the elements sodium and chloride; water is
made up of the elements hydrogen and oxygen.
Because we need to develop an understanding of what electricity is, let’s take a
closer look at the atom. There are three principle parts of the atom. They are the electron,
proton, and neutron. An electron is an elementary particle carrying one unit of negative
electrical charge. The proton is found in the nucleus and has a positive electrical charge
equal to the negative charge of an electron. Neutrons and protons have about the same
weight, but the neutron has no electrical charge. Figure 5-2 illustrates a helium atom. The
nucleus is the very dense region consisting of protons and neutrons at the center of an
atom. The electrons revolve around the nucleus in an elliptical, or oval-shaped, path.
The orbiting electrons have a negative charge, the protons have a positive charge,
and the neutrons display no charge. Because the neutron has no charge, the nucleus
has a net positive charge. The element type is determined by how many protons are
in the nucleus. Helium, as already noted, has 2 protons in its nucleus; copper has 29.
Helium atom illustrating electrons, protons, and neutrons.
Electron
Neutron
-
16891_ch05_108-129.indd 112
+ +
Proton
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CHAPTER 5 BASIC CONCEPTS OF ELECTRICITY AND MAGNETISM
FIGURE 5-3
113
Positive charge of a proton with lines of force extending outward.
FIGURE 5-4
Electron lines of force.
Electron
Atomic number
equal to the number
of protons in the
nucleus.
16891_ch05_108-129.indd 113
From HERMAN. Delmar’s Standard Textbook of
Electricity, 5E. © 2011 Cengage Learning
+
From HERMAN. Delmar’s Standard Textbook of
Electricity, 5E. © 2011 Cengage Learning
Proton
The atomic number of an element is equal to the number of protons in the nucleus.
Figure 5-3 shows that the positive charge of the proton produces lines of force that
extend outward in all directions.
We saw in Figure 5-2 that the electrons orbit outside the nucleus. Although an
electron is about three times larger than a proton, the proton weighs over 1800 times
more. Because electrons are negatively charged, the lines of force produced by an
electron come in from all directions. See Figure 5-4.
Electrons revolve around the nucleus of the atom similar to the way the Earth
rotates around the sun. In atoms that contain more than one electron (all atoms except
hydrogen), each electron has its own orbit. It is possible for two or more atoms to share
common space; in many materials, closely spaced atoms share both electrons and space.
Figure 5-5 shows the electron orbits of an atom.
The two electrons closest to the nucleus are said to occupy the first shell, or orbit,
of the atom. This first shell can only have two electrons. Atoms that have more than two
electrons, like iron, with 26 electrons, must have a second, third, and fourth shell, or
orbit.
Have you ever heard the expression “Opposites attract and likes repel?” Well,
much like the magnetic poles of a magnet repel if like poles are brought together and
attract if unlike poles are brought together, the law of charges states that opposite
charges attract and like charges repel. This basic law will help you understand the
importance of attraction and repulsion of electrons and protons in the atom.
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FIGURE 5-5
Electron orbits.
Nucleus
Electron
There are many key terms in this chapter. It would be a good idea to create some
flashcards. Put the term on one side and the meaning on the other side. Find someone
in class or at home to quiz you on your words.
Self-Check 1
1. List three common elements.
2. What are the three principal parts of the atom?
Polarity the
type (positive or
negative) of charge.
Law of charges
opposite charges
attract and like
charges repel.
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5-2
Electric Charge
Electrons and protons exhibit electric charges of opposite polarity. Polarity refers to
the type (positive or negative) of charge. The proton possesses a positive charge, and the
electron is negatively charged. As illustrated in Figure 5-3, these charges produce lines
of force. A basic law of physics, known as the law of charges, states that opposite
charges attract and like charges repel. Figures 5-6 and 5-7 illustrate this law.
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Self-Check 1
1. Answer: Iron, nitrogen, and oxygen; important elements in terms of electrical conductivity are copper,
silver, and gold.
2. Answer: Electron, proton, and neutron.
FIGURE 5-7
Proton
Electron
+
-
Unlike charges attract each other.
Like charges repel each other.
Proton
Proton
+
+
FIGURE 5-6
115
When considering the electric charge of an atom, the neutron can be ignored
because it has no electric charge. An atom such as helium, illustrated in Figure 5-2 in
its natural state, is electrically neutral. Helium has two protons in the nucleus and two
electrons orbiting the nucleus. Even though the electrons and protons are electrically
charged, the net electric charge is zero.
Electricity is not magic. Current is simply electrons in motion. Next, we learn
how those electrons are put into motion.
What was the most important thing that you learned about electric charge in this section?
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Self-Check 2
1. What does the law of charges state?
2. Define polarity.
5-3
Electricity—Electrons in Motion
•
•
•
•
•
•
Practice SQR3.
Survey through Section 5-3, Electricity—Electrons in Motion.
Skim through the headings. Take a look at the pictures. Read the Self-Check questions at the end of the section.
As an electrician, it becomes very important to understand the chemical
reactions and electric currents that occur in the outer shell of an atom. This section will
define this outer shell, where materials such as conductors come together, free electrons
can escape, and positive or negative ions are created. Static electricity will also be
discussed; this transfer of electrons from one object to another has both practical and
problematic applications.
Valence Electrons
Valence shell the
outermost shell, or
ring, of an atom.
Conductor a
material that
readily conducts
electricity.
The outermost shell of an atom is referred to as the valence shell, or ring. Electrons
located in this valence shell are called valance electrons. These electrons are atomic
particles involved in chemical reactions and electric currents. The valence shell cannot
hold more than eight electrons. A conductor, for example, is comprised of a material
that contains between one and three valence electrons. A conductor is a material that
readily conducts electricity.
One of the forces that helps to keep electrons in orbit is the force of attraction
between unlike charges. The closer together two particles of opposite electric charges
are, the greater the electrical attraction between them. So, the attraction between the
proton in the nucleus and the orbiting electron decreases as the electron gets farther
from the nucleus. Because of this, valence electrons are more loosely held than the
electrons in the inner shells. Here are a few characteristics of valence electrons:
•
•
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They can be more easily removed from the atom than electrons in the inner shell.
Valence electrons possess more energy than electrons in the inner shells because
the farther an electron is from the nucleus, the more energy it possesses.
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Self-Check 2
1. Answer: The law of charges states that opposite charges attract and like charges repel.
2. Answer: Polarity refers to the type (positive or negative) of charge.
117
Self-Check 3
1. What is a conductor?
2. When does the attraction between the proton in the nucleus and the orbiting
electron decrease?
Free Electrons and Electron Flow
Free electrons
valence electrons
that have been
temporarily
separated from an
atom.
Electrical
current the flow of
electrons.
Valence electrons that have been temporarily separated from an atom are called free
electrons. They are not attached to any particular atom. Only the valence electrons in the
outermost orbit can become free electrons. Recall that the nucleus and the attraction of the
protons keep electrons in the inner shells held tightly. A valence electron can be freed from
an atom when energy is added to the atom. This energy permits the loosely held valence
electron to escape the force of attraction between the electron and the nucleus. A free
electron possesses more energy than it did as a valence electron.
Electrical current is the flow of electrons. Heating an atom or subjecting an
atom to an electric field provides the necessary energy to free the valence electron.
After traveling a short distance, the free electron enters the valence orbit of a different
atom. When it returns to orbit, some or all of the gained energy is released in the form
of heat. This is why conductors become warm when current flows through them. If
current through a conductor becomes excessive, a fire may result.
Self-Check 4
1. What are free electrons?
2. Define electrical current.
Ions
Ions atoms that
have more or less
than their normal
complement of
electrons.
The departure of a valence electron leaves a previously neutral atom with an excess
of positive charge, or more protons than electrons. Atoms that have more or less than
their normal complement of electrons are referred to as ions. When an atom has an
excess of electrons, it becomes a negative ion; when an atom loses electrons, it becomes
a positive ion. The amount of energy necessary to cause a valence electron to become
free is different with each element.
The number of an atom’s valence electrons determines how much energy is
required to create a free electron. Typically, if there are fewer electrons in the valence
orbit, less energy is required to free that valence electron.
Self-Check 5
1. How does an ion become positive?
2. How does an ion become negative?
Static electricity
the accumulation of
an electric charge
on an insulated
body.
Static Electricity
Static electricity is a common phenomenon that all of us have observed at one
time or another. Static electricity is the accumulation of an electric charge on an
insulated body. An example of static electricity is the shock we feel when walking
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Self-Check 3
1. Answer: A conductor is comprised of a material that contains between one and three valence
electrons. A conductor is a material that readily conducts electricity.
2. Answer: The attraction between the proton in the nucleus and the orbiting electron decreases as the
electron gets farther from the nucleus.
Self-Check 4
1. Answer: Free electrons are valence electrons that have been temporarily separated from an atom.
2. Answer: Electrical current is the flow of electrons.
Self-Check 5
1. Answer: When an atom loses electrons, it becomes a positive ion.
2. Answer: When an atom has an excess of electrons, it becomes a negative ion.
across a high pile carpet and reaching for a metal doorknob. Another example is
hair clinging to a comb. The best example of natural static electricity is lightning,
but it is potentially harmful. Lightning occurs when a static charge builds up in
clouds that contain large amounts of moisture. Most lightning takes place within
a cloud, but discharges from cloud to cloud and cloud to ground are common.
All of these discharges have one thing in common; they all involve the transfer of
electrons from one object to another. A lightning discharge, or bolt, has an average
voltage of 15,000,000 volts.
Practical Applications of Static Electricity
FIGURE 5-8
Most of the practical applications of static electricity do not operate by taking
advantage of static discharges. Useful applications of static electricity generally make
use of the forces caused by the repulsion of like charges and the attraction of unlike
charges. These forces are used to move charged particles to desired locations. An
example of this application would be removing dust particles from the air. Figure 5-8
illustrates how air is forced between negatively charged rods and positively charged
plates. The device in Figure 5-8 is referred to as an electrostatic dust precipitator. Dust
particles are given a negative charge; they are attracted to positively charged plates
removing the dust.
Spray painting is another application that works on the principle of electric
charges attracting and repelling. A high-voltage grid is placed in front of a spray gun.
An electronic air cleaner.
Charged plates ( – )
High-voltage DC
power supply
–
Airflow
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+
Charged wires (+)
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Spray painting using static charges.
Object to be painted receives
a negative charge.
Wire grid used to charge
particles of paint is positive.
–
+
Spray gun
FIGURE 5-9
119
A positive charge is applied to this grid. The object to be painted has a negative charge.
Figure 5-9 shows how static charges are used in spray painting.
As the paint droplets pass through the grid, the positive charge causes electrons
to be removed from the paint droplets. The positively charged droplets are attracted
to the negatively charged object. Using the static charge paint operation, less paint is
wasted, and the object has a more uniform finish.
Nuisance Charges of Static Electricity
Static electricity is the static charge that accumulates on a person’s body as he or she
walks across a carpet. Although static charges have some useful applications, they are
often a nuisance. Here are some examples of nuisance static charges:
•
•
When reaching for a metal doorknob or other metal object, the discharge in the
form of an electric shock can be painful. Static buildup while walking across
a carpeted floor happens because carpets are made of nylon or other types of
insulators, and heating systems generally remove moisture from the air. The
installation of a humidifier usually prevents the buildup of static charges.
A static charge causes clothes moving through the dry air in a clothes dryer to
cling together. Synthetic fabrics, which are the best insulators, retain electrons
more than natural fabrics do. So, synthetic fabrics build up the greatest static
charge.
You may already know that batteries, fuel cells, and solar cells produce direct
current. This current always flows in the same direction. You may have also heard
the term alternating current. This type of current reverses, or alternates, 60 times per
second (in the United States). The next section examines both types of electric sources
and their defining characteristics.
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Self-Check 6
1. Answer: • Static electricity is
the static charge that
accumulates on a
person’s body as he
or she walks across a
carpet. When reaching
for a metal doorknob or
other metal object, the
discharge in the form
of an electric shock
can be painful. Static
buildup while walking
across a carpeted
floor happens because
carpets are made of
nylon or other types of
insulators, and heating
systems generally
remove moisture from
the air. The installation
of a humidifier typically
prevents the buildup of
static charges.
• A static charge
causes clothes moving
through the dry air in a
clothes dryer to cling
together. Synthetic
fabrics, which are the
best insulators, retain
electrons more than
natural fabrics do. So,
synthetic fabrics build
up the greatest static
charge.
2. Answer: Static
electricity is the
accumulation of an
electric charge on an
insulated body.
Self-Check 6
1. List two examples of nuisance charges of static electricity.
2. Define static electricity.
5-4
Electric Sources
Little is commonly known about the early days of current and the so-called “War of
Currents.” In the 1890s, a bitter rivalry took place between Thomas Edison and Nikola
Tesla. Edison adamantly supported his direct current (dc) delivery of electricity,
whereas Tesla and George Westinghouse were developing alternating current (ac)
delivery systems. The only real casualties in this “war” were the animals Edison publicly
electrocuted using Tesla’s high-voltage ac system. Edison did this to prove the dangers
of alternating current systems.
There are several methods of providing the power required to set a valence
electron free, including chemical, magnetic, and photovoltaic, to list a few. The two
major categories of electric sources are direct current (dc) and alternating current (ac).
Direct Current
A direct current (dc) power source provides a voltage whose polarity and output
voltage never change direction. Dc power sources provide two terminal devices:
one terminal negative and the other positive, with respect to each other. Direct
current sources of energy include batteries, dc generators, and electronic power
supplies.
Self-Check 7
1. What never changes in direct current?
Alternating Current
Transformer an
electrical device by
which alternating
current of one
voltage is changed
to another voltage.
Alternating current (ac) power sources produce a voltage that periodically reverses
polarity and magnitude. An example of an ac power source is an ac generator.
Alternating current has one major advantage over direct current; the voltage can be
increased or decreased by using a transformer. A transformer is an electrical device
by which alternating current of one voltage is changed to another voltage.
When electric current runs through a conductor, a magnetic field is produced.
Magnetism makes possible the operation of motors, transformers, and even your
doorbell. The next section discusses magnetism, electromagnets, and some of the
devices that work because of this force.
Self-Check 8
1. What is one major advantage alternating current has over direct current?
16891_ch05_108-129.indd 120
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Self-Check 7
1. Answer: Polarity and output voltage.
Self-Check 8
1. Answer: The voltage can be increased or decreased by using a transformer.
121
Practice SQR3.
•
•
•
•
Survey through Section 5-5, Magnetism.
Skim through the headings. Take a look at the pictures. Read the Self-Check questions at the end of the section.
5-5
Magnetism a
force field that acts
on some materials,
but not on other
materials.
Magnetism
Magnetism is a force field that acts on some materials, but not others. Physical
devices that possess this force are referred to as magnets. Lodestone is a natural
magnet. However, magnets used today are all manufactured. They are made from
alloys containing elements such as nickel, copper, aluminum, and iron. Many of today’s
instruments operate because of magnetism. A compass is an example of such a device.
A compass works on the principle that a small, lightweight magnet balanced on
a nearly frictionless pivot point turns toward the north. Why? It is because the Earth
itself contains magnetic poles. Figure 5-10 illustrates the positions of the true North
FIGURE 5-10
The Earth’s poles.
True North Pole
Magnetic flux lines
Magnetic south pole
N
Polar
axis
S
Magnetic north pole
16891_ch05_108-129.indd 121
True South Pole
•
•
13/02/12 6:12 PM
Temporary
magnet acts as
a magnet only as
long as it is in the
magnetic field
produced by a
permanent magnet
or an electric
current.
Magnets
The two general types of magnets are known as permanent and temporary.
Permanent magnets are magnets that do not require any power or force to
maintain their field. A temporary magnet acts as a magnet only as long as it is in
the magnetic field produced by a permanent magnet or an electric current. Magnetic
materials from which permanent magnets are made are called hard magnetic
materials; materials from which temporary magnets are made are known as soft
magnetic materials.
Permanent
magnet a magnet
that does not
require power or
force to maintain
its field.
and South Poles, referred to as the axis of the earth. This figure also shows the positions
of the magnetic poles.
Note that magnetic north is not located at the true North Pole of the earth. This
is the reason why navigators must distinguish between true north and magnetic north.
The angular difference between the two is known as the angle of declination.
Self-Check 9
1. What is the difference between permanent and temporary magnets?
Fields, Flux, and Poles
Magnetic field
the result of the
force of magnetism.
To better understand magnetism and devices that operate because of magnetic
fields, such as transformers, motors, and loudspeakers, we must develop a basic
understanding of magnetism.
A magnetic field is the result of the force of magnetism. The magnetic field
extends out from the magnet in all directions. The invisible lines of force that make up
this magnetic field are known as magnetic flux. Figure 5-11 illustrates how lines of
flux surround the entire magnet.
Magnetic lines of flux.
FIGURE 5-11
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13/02/12 6:12 PM
Self-Check 9
1. Answer: Permanent magnets are magnets that do not require any power or force to maintain their
field. A temporary magnet acts as a magnet only as long as it is in the magnetic field produced by a
permanent magnet or an electric current.
Poles the two
areas (ends) of a
magnet where the
magnetic force is
the greatest.
123
A magnetic field is stronger where lines of flux are dense. Where lines of
flux are sparse, the field is weak. The lines of flux are most dense at the ends of a
magnet. So, the magnetic field is strongest at the ends of the magnet. The ends of
a magnet are referred to as the magnetic poles and are designated north (N) and
south (S). Magnetic poles are the two areas (ends) of a magnet where the magnetic
force is the greatest. Lines of flux are always assumed to leave the north pole and
enter the south pole of a magnet.
Bringing the north (or south) poles of two magnets together will create a
repelling force. If you bring a north pole of a magnet close to the south pole of another
magnet, a force of attraction is created. So, unlike poles attract each other.
1. A magnetic field is the result of what?
2. Where is magnetic force the greatest on a magnet?
Electromagnetism
Electromagnet
an object that acts
like a magnet,
but its magnetic
force is created
and controlled by
electricity.
Electricity and magnetism cannot be separated. Whenever an electric current runs
through a wire, a magnetic field is created. An electromagnet is simply an object
that acts like a magnet, but its magnetic force is created and controlled by electricity.
Electromagnets operate because current flowing through a conductor produces a
magnetic field around the conductor, as illustrated in Figure 5-12.
FIGURE 5-12
Magnetic field around a current-carrying conductor.
Current flowing out
Current flowing through
two conductors in the
opposite direction
16891_ch05_108-129.indd 123
Current flowing in
Current flowing through
two conductors in the
same direction
Self-Check 10
13/02/12 6:12 PM
Self-Check 10
1. Answer: The force of magnetism.
2. Answer: A magnetic field is stronger where lines of flux are dense. Where lines of flux are sparse, the
field is weak. The lines of flux are most dense at the ends of a magnet.
Air-core magnet
an electromagnet
comprised of a
coil wound around
a nonmagnetic
material.
Permeability
a term used to
describe the
measure of a
material’s ability
to become
magnetized.
Reluctance
resistance to
magnetism.
Saturation the
state reached
when an increase
in applied external
magnetizing field
cannot increase the
magnetization of
the material further.
FIGURE 5-13
The left-hand rule.
N
16891_ch05_108-129.indd 124
Iron-core magnet
an electromagnet
made by wrapping a
coil of wire around
an iron core.
If a conductor is wound into a coil, the magnetic field is concentrated and
stronger because of the added effect of each turn of the wire.
Electromagnetic field strength is also affected by the amount of current
flowing through the wire. If current is increased, the magnetic field strength is
stronger. The strength of an electromagnet is proportional to its ampere-turns.
Ampere-turns are determined by multiplying the number of turns of wire by the
current flow.
Wrapping a wire around an iron core greatly increases the magnetic field.
This kind of coil is referred to as an iron-core magnet. Coils can also be wound
around a nonmagnetic material such as wood or plastic. These coils are called
air-core magnets. Air-core magnets are electromagnets that have a coil wound
around a nonmagnetic material. Permeability is a term used to describe the measure
of a material’s ability to become magnetized. A magnetic core material, such as iron,
provides an easy path for the flow of magnetic lines in the same way a conductor
provides an easy path for the flow of electrons.
The polarity of an electromagnet can be determined by using what has become
known as the left-hand rule. When fingers on your left hand are placed around the
windings in the direction of current flow, your thumb points to the north magnetic
pole, as can be seen in Figure 5-13. If the direction of current flow is reversed, the
polarity of the magnetic field also reverses.
Another measurement used when working with magnetic devices is
reluctance, which is simply resistance to magnetism. Materials like soft iron
or nickel have high permeability and low reluctance because they can be easily
magnetized. However, graphite, for example, has low permeability and high
reluctance.
When the current is increased in an electromagnet and the magnetic strength
increases only slightly, a condition known as saturation occurs. Saturation is the state
reached when an increase in an applied external magnetizing field cannot increase the
magnetization of the material further. A stronger magnetic field can be produced, but
a larger core material is necessary.
From HERMAN. Delmar’s Standard Textbook of Electricity, 5E.
© 2011 Cengage Learning
Ampere-turns
determined by
multiplying the
number of turns of
wire by the current
flow.
13/02/12 6:12 PM
Self-Check 11
1. Answer: An
electromagnet
is an object that
acts like a magnet,
but its magnetic
force is created
and controlled
by electricity.
Electromagnets
operate because
current flowing
through a conductor
produces a magnetic
field around the
conductor.
2. Answer: The
polarity of an
electromagnet can
be determined by
using what has
become known as
the left-hand rule.
When fingers on your
left hand are placed
around the windings
in the direction of
current flow, your
thumb points to
the north magnetic
pole. If the direction
of current flow is
reversed, the polarity
of the magnetic field
also reverses.
Residual
magnetism
the amount of
magnetism
left in a
material
after the
magnetizing
force has been
removed.
125
When the current flow through the coil of an electromagnet is interrupted,
some magnetism may remain in the core material. The amount of magnetism left
in a material after the magnetizing force has been removed is called residual
magnetism.
Self-Check 11
1. What is an electromagnet?
2. How can you determine the polarity of an electromagnet?
Magnetic Devices
Magnetism is responsible for the operation of many devices we use every day. Some of
the more common devices include measuring instruments, transformers, motors, and
electromagnets. One useful characteristic of an electromagnet is that magnetic force
can be varied by changing the amount and direction of current going through the coils
around its core. Tape recorders, speakers, and earphones operate because of varying
current that flows through the winding of an electromagnet. Some other applications of
electromagnetic devices are listed here:
•
•
•
•
•
Scrap-yard crane with a huge electromagnet to pick up and move junked autos
Electromagnetic door lock
Doorbell ringer
Electric motor
Maglev trains
Self-Check 12
1. What devices operate because of varying current?
2. List three applications of electromagnetic devices.
How long did it take you to read this chapter? In what part of the day did you read it?
Was it a time when you were able to concentrate, or should you find another time of
day to do your reading?
Summary
•
•
16891_ch05_108-129.indd 125
All matter is comprised of atoms, and an atom is the smallest particle into which
an element can be divided without losing its identity.
Some common elements include iron, nitrogen, oxygen, copper, silver,
and gold.
Self-Check 12
1. Answer: Tape recorders, speakers, and earphones operate because of varying current that flows
through the winding of an electromagnet.
2. Answer: Any THREE of the following:
• Scrap-yard crane with a huge electromagnet to pick up and move junked autos
• Electromagnetic door lock
• Doorbell ringer
• Electric motor
• Maglev trains
13/02/12 6:12 PM
•
•
•
•
•
•
•
•
•
•
•
•
•
•
The three principal parts of an atom are the electron, proton, and neutron.
The center of an atom, called the nucleus, contains positively charged protons as
well as neutrons, which have no electrical charge.
Negatively charged electrons orbit the nucleus.
The law of charges states that opposite electrical charges attract and like electrical
charges repel.
The outermost shell of an atom is called the valence shell, or ring.
A conductor is a material that contains between one and three valence electrons
and readily conducts electricity.
Valence electrons temporarily separated from an atom are free electrons.
Electrical current is the movement, or flow, of free electrons.
Atoms that have more or less than their normal complement of electrons
are ions.
•
•
Static electricity is the accumulation of an electric charge on an insulated
body.
Applications of static electricity include dust precipitators and spray
painting.
The two major categories of electric sources are direct current and alternating
current.
Magnetism is a force field that acts on some materials, but not on other
materials.
An electromagnet is an object that acts like a magnet, but its magnetic field is
created and controlled by electricity.
Devices that operate because of magnetism include measuring instruments,
transformers, motors, and electromagnets.
Applications of electromagnets include door lock, doorbell ringer, electric motor,
and maglev train.
Review Questions
True/False
1. A molecule is the smallest particle into which an element can be divided without
losing its identity. (True, False) See 5-1.
2. An electron is an elementary particle carrying one unit of negative electrical
charge. (True, False) See 5-1.
3. Protons and neutrons are found in the nucleus of an atom. (True, False) See 5-1.
4. The neutron found in the nucleus has a net positive charge. (True, False) See 5-1.
5. Protons revolve around the nucleus of the atom. (True, False) See 5-1.
16891_ch05_108-129.indd 126
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127
Multiple Choice
6. What is the law of charges?
A.
B.
C.
D.
Opposite charges repel.
Like charges repel.
Like charges are always negative.
Like charges are always positive. See 5-2.
7. Neutrons
A.
B.
C.
D.
.
Have a positive charge
Have a negative charge
Revolve around the nucleus
Have no electrical charge See 5-2.
8. The outermost shell of an atom is referred to as the
A.
B.
C.
D.
9. A material that readily conducts electricity is a(n)
A.
B.
C.
D.
.
Variance shell
Ion shell
Valence shell
Free electron shell See 5-3.
.
Element
Molecule
Compound
Conductor See 5-3.
10. What is the term for valence electrons that have been temporarily separated from
an atom?
A. Free electrons
B. Ions
C. Neutrons
D. Free protons See 5-3.
Fill in the Blanks
11. Electrical
current
is the flow of free electrons. See 5-3.
12. Atoms that have more or less than their normal complement of electrons are
called
13.
ions
Static
. See 5-3.
electricity is an accumulation of an electric charge on an
insulated body. See 5-3.
direct
current power source provides a voltage whose
polarity and output voltage never change direction. See 5-4.
14. A(n)
transformer is an electrical device by which alternating current of
one voltage is changed to another voltage. See 5-4.
15. A(n)
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16.
17.
Magnetism
is a force field that acts on some materials, but not on other
materials. See 5-5.
Permanent magnets are magnets that do not require any power or
force to maintain their field. See 5-5.
18. The ends of a magnet are called magnetic
poles
. See 5-5.
19. A(n) electromagnet is an object that acts like a magnet, but its magnetic
force is created and controlled by electricity. See 5-5.
Reluctance
is resistance to magnetism. See 5-5.
20.
16891_ch05_108-129.indd 128
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ACTIVITY 5-1
Bar magnets.
S
Give the following directions to your
students: Place two magnets on a flat
surface (with like poles facing each
other) far enough apart that no reaction
is demonstrated. Move the magnets
together to demonstrate that like charges
repel, once you have asked the following
questions.
Place two magnets on a flat surface (with
unlike poles facing each other), far enough
apart that no reaction is demonstrated.
Move the magnets together to demonstrate
that unlike charges attract, once you have
asked the following question.
Electrical Theory—Law of Charges
N
N
S
From HERMAN. Delmar’s
Standard Textbook of
Electricity, 5E. © 2011
Cengage Learning
FIGURE 5-14
129
Materials
2 bar magnets with north and south poles identified (See Figure 5-14.)
Whiteboard or chalkboard
Part A
Your instructor will give you directions on how to proceed.
Questions
1. Explain the law of charges.
2. Explain what action will take place once the magnets are slowly brought together.
Part B
Your instructor will give you directions on how to proceed.
Question
1. Explain what action will take place once the magnets are slowly brought
together.
Follow-up Questions
1. Identify some objects that use magnets to function.
2. What happens to the magnetic field when a conductor is wound into a coil?
16891_ch05_108-129.indd 129
Activity 5-1 Answer Key
Part A
1. Answer: Like charges repel and opposite charges attract.
2. Answer: Following the law of charges, the magnets will repel each other.
13/02/12 6:12 PM
Part B
1. Answer: Following the law of charges, the magnets will attract each other.
Follow-up
1. Suggested Answers: compass, electric can openers, electric motors and generators, doorbell ringers,
scrap-yard crane
2. Answer: The magnetic field is concentrated and stronger because of the cumulative effect of each
turn of the wire.
Lab 5-1Electrical Theory—Law of Charges
Task Performance Objective Sheet
S
N
N
From HERMAN. Delmar’s
Standard Textbook of
Electricity, 5E.
© 2011 Cengage Learning
Name ___________________________________________________ Date ____________________________
S
Figure 5-15
Given:
2 magnets with north and south poles identified (See Figure 5-15.)
Task Performance Objective:
Demonstrate the theory of charges. Place two magnets on a flat surface, far enough apart that no
reaction to the lab is demonstrated. Have students identify what action will take place once the
magnets are slowly brought together.
Scoring:
Criteria
Completes
Completes
with Assistance
Does Not
Complete
Date
Initials
Student explains the
theory of charges.
Student relates
and demonstrates
the reaction of the
two magnets when
brought together
with both north poles
facing each other.
Student relates
and demonstrates
the reaction of the
two magnets when
brought together with
opposite poles facing
each other.
©Cengage Learning 2013
Student Signature: Instructor Signature: 145
NOTES
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146
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Chapter 5 Basic Concepts of Electricity and Magnetism

Transcription

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