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Chapter 5
Basic Concepts of Electricity and Magnetism
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 then seeing it completed
and operating properly.”
Shorkey is currently a journeyman electrician and lead-man at Merit Electric in Fort Collins,
CO. 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.” “The education you
receive at IEC is also instrumental in preparing for your journeyman license exam,” he adds.
Life Skills Covered:
Managing Stress
Critical Thinking
Take Action
Look through the chapter. Take a quick look at the outline, key terms, and headings.
Look through the pictures in the chaper. What will you be learning about as you read this
chapter?
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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?
Chapter Outline
Introduction
The Atom
Electric Charge
Electricity – Electrons in Motion
Valence Electrons
Free Electrons and Electron Flow
Ions
Static Electricity
Electric Sources
Direct Current
Alternating Current
Magnetism
Magnets
Fields, Flux and Poles
Electromagnetism
Magnetic Devices
Key terms
Air-core magnet
Ampere-turns
Atom
Atomic number
Compound
Conductor
Electrical current
Electromagnet
Electron
Element
Free electrons
Ions
Iron-core magnet
Law of charges
Magnetic field
Magnetism
Molecule
Neutron
Nucleus
Permanent magnet
Permeability
Polarity
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Poles
Proton
Reluctance
Residual magnetism
Saturation
Static electricity
Temporary magnet
Transformer
Valence shell
Chapter Objectives
After completing this chapter, you should be able to:
1.
2.
3.
4.
5.
Identify the three principle parts of an atom.
List the laws of charges and describe their importance to current flow.
Explain electron current flow and contrast direct and alternating current.
Describe current flow through a conductor and discuss why heat is produced.
Define magnetic terms and discuss magnetic principles and concepts as they
relate to electricity.
Life Skills Goals
You have three life skills goals for this chapter: critical thinking, managing stress, and
self-check. 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.
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’s 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.
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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.
/H2/ SELF CHECK
•
•
•
•
•
Practice SQR3.
Survey through section The Atom.
Skim through the headings. Take a look at the pictures. Read the self-check
questions at the end of the section.
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.
/H1/
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. There are 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.
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Figure 5-1 Table of elements.
[“Standard Textbook of Electricity” Herman – Figure 1-4 on page 32]
Since 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.
Since 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.
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Figure 5-2 Helium atom illustrating electrons, protons, and neutrons.
[“Electronic Devices” Floyd– Figure 2-1 on page 44]
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. 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.
Figure 5-3 Positive charge of a proton with lines of force extending outward.
[“Standard Textbook of Electricity” Herman – Figure 1-7 on page 34]
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. Since
electrons are negatively charged, the lines of force produced by an electron come in from
all directions. See Figure 5-4.
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Figure 5-4 Electron lines of force.
[“Standard Textbook of Electricity” Herman – Figure 1-9 on page 34]
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.
Figure 5-5 Electron orbits.
[“Standard Textbook of Electricity” Herman – Figure 1-14 on page 38]
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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 that will 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.
/H2/ SELF CHECK
1. List three common elements.
2. What are the three principle parts of the atom?
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.
/H1/
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.
Figure 5-6 Unlike charges attract each other.
[“Standard Textbook of Electricity” Herman – Figure 1-11 on page 35]
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Figure 5-7 Like charges repel each other.
[“Standard Textbook of Electricity” Herman – Figure 1-12 on page 36]
When considering the electric charge of an atom, the neutron can be ignored since it has
no electric charge. An atom such as helium, illustrated in Figure 5-2 in its natural state, is
electrically neutral. Helium has 2 protons in the nucleus and 2 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 will learn how
those electrons are put into motion.
/H2/ SELF CHECK
1. What does the law of charges state?
2. Define polarity.
Practice SQR3.
• Survey through section 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.
• 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.
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/H1/
Electricity – Electrons in Motion
As an electrician, the chemical reactions and electric currents that occur in the outer shell
of an atom become very important to understand. 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.
/H2/ Valence Electrons
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 8 electrons. A conductor, for example, is comprised of a material that
contains between 1 and 3 valence electrons. A conductor is a material that readily
conducts electricity.
One of the forces that help 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:
•
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.
/H2/ SELF CHECK
1. What is a conductor?
2. When does the attraction between the proton in the nucleus and the orbiting
electron decrease?
/H2/ Free Electrons and Electron Flow
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
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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.
/H2/ SELF CHECK
1. What are free electrons?
2. Define electrical current.
/H2/ Ions
The departure of a valence electron leaves a previously neutral atom with an excess of
positive charge, or more protons than electrons. Atoms which 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.
/H2/ SELF CHECK
1. How does an ion become positive?
2. How does an ion become negative?
/H2/ 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 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, but potentially harmful, is lightning. This is
when a static charge builds up in clouds that contain large amounts of moisture. Most
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lightning take 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.
/H3/ Practical Applications of Static Electricity
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 to remove 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.
Figure 5-8 A dust precipitator.
[“Electricity Principles and Applications” Fowler Figure 1-7 on page 12]
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. 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.
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Figure 5-9 Spray painting using static charges.
[“Standard Textbook of Electricity” Herman – Figure 3-12 on page 91]
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.
/H3/ Nuisance Charges of Static Electricity
Although static charges have some useful applications, they can often be a nuisance. Here
are some examples of nuisance static charges:
•
•
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.
The static charge that causes clothes in a dryer to cling together. Clothes that
move through the dry air cause this type of static charge. Man-made fabrics,
which are the best insulators, retain electrons easier than natural fabrics. So, manmade 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 U.S.). The next section examines both types of electric sources and
their defining characteristics.
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/H2/ SELF CHECK
1. List two examples of nuisance charges of static electricity.
2. Define static electricity.
/H1/
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 was adamantly supporting his direct current (DC) delivery of electricity,
while 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 voltage system. Edison did this to prove the
dangers of alternating current systems. The early victims were dogs and cats, but Edison
eventually electrocuted an elephant named Topsy.
There are several methods of providing the power required to set a valence electron free.
They include chemical, magnetic, and photovoltaic, to list a few. The two major
categories of electric sources are direct current (dc) and alternating current (ac).
/H2/ 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.
/H2/ SELF CHECK
1. What never changes in direct current?
/H2/ Alternating Current
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 motors, transformers, and even your doorbell possible. The next
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section will discuss magnetism, electromagnets, and some of the devices that work
because of this force.
/H2/ SELF CHECK
1. What is one major advantage alternating current has over direct current?
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.
• 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.
/H1/
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, points toward the north. Why? It is because the earth itself
contains magnetic poles. Figure 5-10 illustrates the positions of the true North and South
Poles, referred to as the axis of the earth. This figure also shows the positions of the
magnetic poles.
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Figure 5-10 The earth’s Poles.
[“Standard Textbook of Electricity” Herman – Figure 4-2 on page 98]
Note that the magnetic north is not located at the true North Pole of the earth. This is the
reason that navigators must distinguish between true north and magnetic north. The
angular difference between the two is known as the angle of declination.
/H2/ 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.
/H2/ SELF CHECK
1. What is the difference between permanent and temporary magnets?
/H2/ Fields, Flux and Poles
In order 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.
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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.
Figure 5-11 Magnetic lines of flux.
[“Standard Textbook of Electricity” Herman – Figure 4-9 on page 102]
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.
/H2/ SELF CHECK
1. A magnetic field is the result of what?
2. Where is magnetic force the greatest on a magnet?
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/H2/ Electromagnetism
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. Figure 5-12 shows how current flowing through a
conductor produces a magnetic field around that conductor.
Figure 5-12 Magnetic field around a current-carrying conductor.
[“Standard Textbook of Electricity” Herman – Figure 4-13 on page 104]
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.
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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 as 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. If the direction of
current flow is reversed, the polarity of the magnetic field also reverses. Figure 5-13
illustrates the left-hand rule.
Figure 5-13 The left-hand rule.
[“Standard Textbook of Electricity” Herman – Figure 4-17 on page 109]
Another measurement used when working with magnetic devices is reluctance.
Reluctance 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 a low permeability and high reluctance.
If when the current is increased in an electromagnet and the magnetic strength increases
only slightly, a condition known as saturation has occurred. 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 would be necessary.
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.
/H2/ SELF CHECK
1. What is an electromagnet?
2. How can you determine the polarity of an electromagnet?
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/H2/ 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 the fact 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 include:
•
•
•
•
•
Scrap yard crane with a huge electromagnet to pick up and move junked autos
Electromagnetic door lock
Doorbell ringer
Electric motor
Maglev trains
/H2/ SELF CHECK
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? What part of the day did you read it?
Was it a time when you could concentrate, or should you find another time of day to do
your reading?
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
/H2/ Summary
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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.
The three principle parts of an atom are the electron, proton, and neutron.
The center of an atom, called the nucleus, contains positively charged protons and
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 1 and 3 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 which 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.
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
/H2/ Review Questions
True/False
1. A molecule is the smallest particle into which an element can be divided without
losing its identity. (True, False)
2. An electron is an elementary particle carrying one unit of negative electrical
charge. (True, False)
3. Protons and neutrons are found in the nucleus of an atom. (True, False)
4. The neutron found in the nucleus has a net positive charge. (True, False)
5. Protons revolve around the nucleus of the atom. (True, False)
Multiple Choice
6.
A.
B.
C.
D.
The law of charges states
Opposite charges repel
Like charges repel
Like charges are always negative
Like charges are always positive
7.
A.
B.
C.
D.
Neutrons
Have a positive charge
Have a negative charge
Revolve around the nucleus
Have no electrical charge
8.
A.
B.
C.
D.
The outermost shell of an atom is referred to as the
Variance shell
Ion shell
Valence shell
Free electron shell
9.
A.
B.
C.
D.
A material that readily conducts electricity is a(n)
Element
Molecule
Compound
Conductor
10. Valence electrons that have been temporarily separated from an atom are called
A. Free electrons
B. Ions
C. Neutrons
D. Free protons
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
Fill in the Blanks
11. Electrical ____________________ is the flow of free electrons.
12. Atoms which have more or less than their normal complement of electrons are
called ____________________.
13. ____________________ electricity is an accumulation of an electric charge on an
insulated body.
14. A (n) ____________________ current power source provides a voltage whose
polarity and output voltage never change direction.
15. A (n) ____________________ is an electrical device by which alternating current
of one voltage is changed to another voltage.
16. ____________________ is a force field that acts on some materials but not on
other materials.
17. ____________________ magnets are magnets that do not require any power or
force to maintain their field.
18. The ends of a magnet are called magnetic ____________________.
19. A (n) ____________________ is an object that acts like a magnet, but its
magnetic force is created and controlled by electricity.
20. ____________________ is resistance to magnetism.
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
/H2/ Review Questions - Answers
True/False
1. A molecule is the smallest particle into which an element can be divided without
losing its identity. (True, False)
2. An electron is an elementary particle carrying one unit of negative electrical
charge. (True, False)
3. Protons and neutrons are found in the nucleus of an atom. (True, False)
4. The neutron found in the nucleus has a net positive charge. (True, False)
5. Protons revolve around the nucleus of the atom. (True, False)
Multiple Choice
6.
A.
B.
C.
D.
The law of charges states
Opposite charges repel
Like charges repel
Like charges are always negative
Like charges are always positive
7.
A.
B.
C.
D.
Neutrons
Have a positive charge
Have a negative charge
Revolve around the nucleus
Have no electrical charge
8.
A.
B.
C.
D.
The outermost shell of an atom is referred to as the
Variance shell
Ion shell
Valence shell
Free electron shell
9.
A.
B.
C.
D.
A material that readily conducts electricity is a(n)
Element
Molecule
Compound
Conductor
10. Valence electrons that have been temporarily separated from an atom are called
A. Free electrons
B. Ions
C. Neutrons
D. Free protons
Fill in the Blanks
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
11. Electrical current is the flow of free electrons.
12. Atoms which have more or less than their normal complement of electrons are
called ions.
13. Static electricity is an accumulation of an electric charge on an insulated body.
14. A (n) direct current power source provides a voltage whose polarity and output
voltage never change direction.
15. A (n) transformer is an electrical device by which alternating current of one
voltage is changed to another voltage.
16. Magnetism is a force field that acts on some materials but not on other materials.
17. Permanent magnets are magnets that do not require any power or force to
maintain their field.
18. The ends of a magnet are called magnetic poles.
19. A (n) electromagnet is an object that acts like a magnet, but its magnetic force is
created and controlled by electricity.
20. Reluctance is resistance to magnetism.
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
/H2/ Glossary of Terms
Air-core magnet an electromagnet comprised of a coil wound around a nonmagnetic
material
Ampere-turns determined by multiplying the number of turns of wire by the current
flow
Atom the smallest particle into which an element can be divided without losing its
identity
Atomic number equal to the number of protons in the nucleus
Compound exists when two kinds of atoms combine chemically
Conductor a material that readily conducts electricity
Electrical current the flow of electrons
Electromagnet an object that acts like a magnet, but its magnetic force is created and
controlled by electricity.
Electron an elementary particle carrying one unit of negative electrical charge
Element a group of identical atoms
Free electrons valence electrons that have been temporarily separated from an atom.
Ions atoms which have more or less than their normal complement of electrons
Iron-core magnet an electromagnet made by wrapping a coil of wire around an iron core
Law of charges opposite charges attract and like charges repel
Magnetic field the result of the force of magnetism
Magnetism a force field that acts on some materials but not on other materials
Molecule consists of two or more atoms
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
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Electrical Pre-Apprenticeship & Workforce Development Manual, Chapter 5
DRAFT 05/14/11 (Figures are not final but for placement only.)
Permanent magnet a magnet that does not require power or force to maintain its field
Permeability a term used to describe the measure of a material’s ability to become
magnetized
Polarity the type (positive or negative) of charge
Poles the two areas (ends) of a magnet where the magnetic force is the greatest
Proton found in the nucleus and has a positive electrical charge
Reluctance resistance to magnetism
Residual magnetism the amount of magnetism left in a material after the magnetizing
force has been removed
Saturation the state reached when an increase in applied external magnetizing field
cannot increase the magnetization of the material further
Static electricity the accumulation of an electric charge on an insulated body
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
Transformer an electrical device by which alternating current of one voltage is changed
to another voltage
Valence shell the outermost shell or ring of an atom
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