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52C11A03-TATS
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LESSON TITLE: MAGNETISM
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Magnetism
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1. Introduction:
a. You will now continue your study of the logistic phase with Distance Learning Lesson
52C11A03, Magnetism. The basic two fundamentals of electricity are electronic
force and magnetic force, which will be discussed in this lesson. These are the forces
that allow the operation of electrical motors, generators, meters, relays, and countless
other electrical devices. You have just studied the electric force as explained by the
electron theory. In this lesson you will learn the following:
 The relationship between magnetism and electricity.

The characteristics of magnets.
 The characteristics of a magnetic field around a current
carrying conductor.
 The factors that effect the strength of electromagnets.
 The three requirements needed to develop an electromotive
force (EMF).
b. As you can see, this is the third distance learning lesson in annex A:
52C11A01 Course Introduction
52C11A02 Elements of Electricity
52C11A03 Magnetism
52C11A04 Ohm’s Law
52C11A05 Series Circuits
52C11A06 Parallel-Circuits
52C11A07 Series Parallel Circuits
52C11A08 Direct Current Generators
52C11A09 Direct Current Motors
52C11A10 Direct Current Diagrams
52C11A11 Alternating Current Circuits
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52C11A12 Rectifier Circuits
52C11A13 Alternating Current Generators
52C11A14 Alternating Current Motors
52C11A15 Three-Phase Circuits
52C11A16 Alternating Current Diagrams
52C11A17 Heat and Heat Transfer
52C11A18 Pressure and Pressure Measurement
52C11A19 Basic Refrigeration Cycle
52C11A20 Refrigerant and Refrigerant Usage
52C11A21 Small Appliances High Pressure Systems
52C11A22 Written Test (IDT)
2. Instructions:
a. Read through this lesson, 52C11A03, Magnetism.
b. Answer the questions or solve the problems in the Self Check Test at the end of this
lesson.
c. Check your answers with the solutions provided on the Self Check Solution Sheet.
a. You will be required to pass a written test in lesson A22, covering material presented
in the distance learning portion, annex A. Save this lesson booklet. You will be
allowed to use it during the tests.
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1.0. General.
In order to properly understand the principles of electricity, it is necessary to study magnetism
and the effects of magnetism on electrical equipment. Magnetism and electricity are so closely
related that the study of either subject would be incomplete without at least mentioning the
basics of the other. Much of today’s modern electrical and electronic equipment could not
function without magnetism. Modern computers, tape recorders, and video reproduction
equipment use magnetized tape. High-fidelity speakers use magnets to convert amplifier outputs
into audible sound. Electrical motors use magnets to convert electrical energy into mechanical
motion; generators use magnets to convert mechanical motion into electrical energy. It is
magnetic force that attracts small bits of iron and steel to the end of a common horseshoe
magnet. It is magnetic force that swings a compass needle toward the north. Magnets may be
conveniently divided into two groups, natural magnets and artificial magnets. A natural magnet
is an iron ore called magnetite that possesses magnetic qualities. An artificial magnet is
produced from magnetic materials. This lesson will help you to develop an understanding of the
artificial magnets used in electric equipment.
2.0. Nature of Magnetism.
To understand the artificial magnets used in electric equipment, it is necessary to know the nature
of magnetism. Magnetism is the power to attract as possessed by a magnet.
2.1. Magnetic Substance.
Those materials that are attracted to a magnet are called magnetic substances. Two common
magnetic substances are iron and steel. There are other magnetic substances that are attracted by
a magnet but not as strong as iron and steel. Some of these magnetic substances are cobalt,
nickel, and manganese. It is interesting to note that some of the best permanent magnets are
made from alloys. For example, alnico is made of aluminum, nickel, and cobalt. Alnico is
derived from the first two letters of the three elements of which it is composed.
2.2. Nonmagnetic Substance.
Most other substances are not attracted by a magnet and are nonmagnetic. Examples of
nonmagnetic substances are air, wood, paper, glass, copper, aluminum, lead, tin, and silver.
Magnetic force acts through any nonmagnetic substance, which can be demonstrated by moving a
magnet beneath a piece of paper (nonmagnetic substance) on which there are iron filings. As the
magnet is moved, movement of the iron filings can be observed.
2.3. Theory of Magnetism.
One common theory of magnetism is that a piece of iron or steel consists of millions of tiny
magnets. This is based on the assumption that each of the molecules of a magnetic substance is a
tiny magnet.
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2.3.1. Unmagnetized.
The molecular magnets that compose an unmagnetized bar of iron or steel are arranged at
random as shown in Figure 1(A). With this random arrangement, the magnetism of each of the
molecules is neutralized by that of adjacent molecules, and no external magnetic force is
produced.
2.3.2. Magnetized.
The molecular magnets that compose a magnetized bar of iron or steel are aligned so that the
north poles point one direction and the south poles point the other direction as shown in Figure
1(B). With this aligned arrangement, the magnetism of the molecules is, added and a north pole
is set up at one end of the bar and a south pole at the other end.
Figure 1. Theory of Magnetism
2.3.3. Broken Magnet.
If a bar magnet is broken into several parts as shown in Figure 2, each part constitutes a magnet.
The north and south poles of these small magnets are in the same respective directions as those
of the original magnet. If each of these parts are again broken, the resulting parts are likewise
magnets. If this breaking process continued, smaller pieces would retain their magnetism until
each part was reduced to a molecule. It is therefore logical to assume that each of these
molecules is a magnet.
Figure 2. Magnetic Poles of a Broken Magnet
N
S
N
S
N
S
2
N
S
N
S
N
S
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2.4. Magnetic Poles.
The magnetic poles of the magnetized bar are the areas where the magnetic force is the greatest.
If the magnetized bar were to be suspended from a string, one end would point to the north and
the other end would point to the south. Long ago, when this fact was first discovered, it was
decided to call the north-seeking pole of the magnet the north pole. Likewise, the south-seeking
pole was called the south pole. These designations for poles of a magnet are still used.
2.5. Magnetic Field.
A magnetic field is the space surrounding a magnet in which the magnet force acts. A visual
representation of the magnetic field around a magnet can be obtained by placing a plate of glass
over a magnet and sprinkling iron filings onto the glass. The filings arrange themselves in
definite paths between the poles. This arrangement of the filings shows the pattern of the
magnetic field around the magnet as shown in Figure 3. Actually, this pattern of the magnetic
field is only one of number of possible patterns that can be produced by using one or more
magnets.
Figure 3. Magnetic Field Pattern
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N
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2.5.1. Unlike Poles.
If the area surrounding the north pole of one bar magnet and the south pole of a second bar
magnet is sprinkled with iron filings, the pattern shown in Figure 4 will be obtained. A study of
this pattern shows one of the fundamental laws of magnetism —unlike magnetic poles attract
each other.
Figure 4. Unlike Poles (Attraction)
2.5.2. Like Poles.
If the area surrounding the north pole of one bar magnet and the north pole of a second bar
magnet is sprinkled with iron filings, the pattern shown in figure 5 will be obtained. (Two south
poles will give the same effect). A study of this pattern shows another of the fundamental laws
of magnetism –like magnetic poles repel each other.
Figure 5. Like Poles (Repulsion)
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2.6. Lines of Force.
To represent the direction and intensity of the magnet field about a magnet a drawing is used.
This is done by representing the forces in a magnetic field with a few lines called lines of force as
shown in figure 6. Note that arrowheads have been placed on each of the lines of force. The
arrowheads indicate that lines of force leave the magnet at the north pole and enter the magnet at
the south pole.
Figure 6. Lines of Force
NORTH
END
N
FIELD OF FORCE
ABOUT NORTH POLE
LINES LEAVE POLE
IN ALL DIRECTIONS
SIDE VIEW
N
SOUTH
END
S
FIELD OF FORCE ABOUT A BAR MAGNET
LINES OF FORCE SHOWN IN ONE
PLANE ONLY, ACTUALLY, LINES
EXTEND IN ALL DIRECTIONS AS
SHOWN IN END VIEWS AND, THEORETICALLY, THROUGHOUT ALL SPACE
5
S
FIELD OF FORCE
ABOUT SOUTH POLE
LINES ENTER POLE
FROM ALL DIRECTIONS
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2.7. Magnetic Shielding.
There is not a known insulator for a magnetic field. If a nonmagnetic material is placed in a
magnetic field, the magnetic field penetrates the nonmagnetic material. However, if a magnetic
material such as soft iron is placed in a magnetic field, the magnetic field may be redirected.
This property is used to protect the sensitive mechanism of electric instruments and meters that
can be influenced by stray magnetic fields and will cause errors in their readings. This is
accomplished by placing a soft iron case called a magnetic shield about the instrument as shown
in Figure 7.
Figure 7. Magnetic Shield
11 12
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10
2
9
3
8
4
5
7
SOFT IRON
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3.0. Artificial Magnets.
Inserting the bar into a coil of insulated wire and passing a direct current through the coil as
shown in Figure 8(A) can magnetize an iron, steel, or alloy bar. The same bar may also be
magnetized if it is stroked with a bar magnet as shown in Figure 8(B). The best method of
making artificial magnets is by electrical means and is described in detail.
Figure 8. Methods of Producing Artificial Magnets
COIL METHOD
(B)
STROKING METHOD
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3.1. Electromagnetism.
Electrons moving in a conductor cause a magnetic field to build up around the conductor.
Placing a compass needle near a current carrying wire can prove this. The compass will point to
the wire’s magnetic field instead of to the earth’s magnetic north as shown in Figure 9. When an
electric current flowing through a conductor causes magnetism, it is called electromagnetism.
Figure 9. Electromagnetism
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COMPASS
NEEDLE
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B
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3.1.1. Magnetic Field about a Conductor.
The magnetic field produced by electron flow always lies at right angles to the current that
produces it. The truth of this statement can be proven by the experiment shown in Figure 10.
Lines of force as shown in Figure 11 normally represent the magnetic field.
Figure 10. Magnetic Field
CARDBOARD
-
COMPASS
IRON FILINGS
A
LINES
OF
FORCE
DIRECTION OF
CURRENT
FLOW
Figure 11. Lines of Force about a Conductor
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B
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3.1.2. Left Hand Rule for a Conductor.
A simple rule has been set up for finding the direction of the magnetic field when the direction of
current is known. Encircle the conductor with the left hand and point the thumb in the direction
of the current flow. The fingers will point in the direction of the magnetic field as shown in
Figure 12.
Figure 12. Left Hand Rule for a Conductor
3.1.3. Magnetic Field about A Loop.
If a current-carrying conductor is bent in the form of a loop, the lines of force surround the wire
as when it was straight as shown in Figure 13(A). When the conductor is bent into a loop, all the
lines of force will enter on one side of the loop and leave on the other side of the loop as shown
in Figure 13(B). Thus, a North Pole is created on one side of the loop and a South Pole on the
other side of the loop.
Figure 13. Lines of Force about a Loop
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3.1.4. Magnetic Field about a Coil.
The magnetic field resulting under the conditions shown in Figure 14 is relatively weak. But if
several loops of wire are wound to form a coil, the strength of the magnetic field within the coil
is increased. This is caused by the concentration of magnetic lines of force in a smaller area. We
can also say that the density of the field is increased. The combined magnetic field results in a
North Pole on one end of the coil and a South Pole on the other end of the coil as shown in
Figure 14.
Figure 14. Lines of Force about a Coil
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3.1.5. Left Hand Rule for a Coil.
When the direction of current flow through a coil is known, then the north and south poles can be
determined by the use of the left-hand rule for coils. Grasp the coil in the left hand, with the
fingers wrapped around in the direction of current flow. The thumb will point toward the North
Pole as shown in Figure 15.
Figure 15. Left Hand Rule for a Coil
3.2. Effects of Cores.
An electromagnet is normally designed to make use of a soft core. Iron has the ability to gather
and convey lines of force. This characteristic is called permeability.
3.2.1. Permeability.
Permeability is the ease with which a material conducts magnetic lines of force. A steel core has
a higher permeability than air. An iron core has a higher permeability than steel. Examples of
the effect of permeability on a coil are shown in Figure 16.
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Figure 16. Permeability
AIR CORE
STEEL CORE
IRON CORE
AIR CORE
STEEL CORE
IRON CORE
Figure 17. Lines of Force about a Loop
3.2.2. Retentivity.
Retentivity is the ability of a material to retain an amount of residual magnetism after current
flow has stopped. What happens to an electromagnet after the current flow has been stopped?
An air core, of course, retains no magnetism. A soft iron core loses most of its magnetism after
current flow is stopped. A hard steel core retains most of its magnetism after current flow is
stopped and is said to be a permanent magnet. Examples of retentivity of air, steel, and iron
cores, are shown in figure 17.
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3.2.3. Residual Magnetism.
Residual magnetism is the amount of magnetism that remains in a temporary magnet. The type
of material used for the temporary magnet will determine the amount of residual magnetism that
will be left over after the magnetic field is removed.
4.0. Relationship between Magnetism and Electricity.
 Electricity current flow (movement of free electrons) through a conductor will always
produce some form of magnetic field around that conductor.
 Magnetism has the reverse affect on current flow. Magnetism induced in a conductor will
produce current flow.
5.0. Characteristics of Magnets.
 All magnets (permanent or electro) have a field of force.
 All magnets (permanent or electro) have two poles (north and south). Lines of force are
directed from the North Pole to the South Pole.
 Like poles repel and unlike poles attract.
 Magnetism can be induced in all magnetic materials.
6.0. Strength of Electromagnets.
The strength of an electromagnet depends on three things. The number of turns in the coil, the
permeability of the core, and the amount of current flow through the conductor.
 Number of turns in the coil — increasing the number of turns in the coil will increase the
strength of the electromagnet by more magnetic lines of force.
 The permeability of the core — increasing the permeability of the core will increase the
strength of the electromagnet by increasing the ease in which the material conducts magnet
lines of force. Air has zero permeability.
Steel has good retentivity (the ability of a substance to retain magnetism after the current flow
has stopped), but low permeability.
Soft iron has good permeability (the ease with which a material conducts magnetic lines of
force), but low retentivity.
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 The current flows through the conductor by increasing the strength of the electromagnet and
increasing the strength of the magnetic lines of force.
7.0. Three Things needed for an Induced Voltage.
 Conductor — a coil of wire
 Magnetic Field — without the magnetic field, we will have zero lines of force.
 Relative Motion — the conductor must be moving inside the magnetic field cutting the lines
of force, or the conductor can be stationary and the magnetic field can rotate around the
conductor.
8.0. Inductance and Transformers.
 Inductance —The property of a circuit which tends to oppose a change in the existing current.
 Transformer — An electrical device consisting of two coils very close together, but
electrically insulated yet are not touching. Refer to figure 18.
Figure 18. Transformer Construction
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9.0. Summary.
In this lesson you have learned that an electric current (flow of electrons) always produces a
magnetic field. In a wire, the current flow causes the magnetic lines of force to circle the wire. It
is thought that these lines of force result from the movement of the electrons along the wire. As
they move, the electrons “send out” the lines of force. When many electrons move, there are
many lines of force (the magnetic field is strong). Few electrons in motion mean a weak
magnetic field or few magnetic lines of force.
The magnetic field produced by current flowing in a loop of wire must follow the curve of the
wire. If two loops are made in the conductor, the magnetic lines of force will circle the two loops
almost as though they were a single loop. However, the magnetic field will be twice as strong
since the lines of force of the two loops combine. Increasing the number of loops further
increases the magnetic field accordingly. Many loops of wire are normally referred to as a coil.
The magnetic strength of a coil can be greatly increased by wrapping the loops of wire around an
iron core. The iron core passes the lines of force with much greater ease than air. With this great
increase in the number of lines of force, the magnetic strength of the electromagnet is greatly
increased, even though no more current flows through it. Practically, all electromagnets use an
iron core of some kind.
Electromagnets can be in many shapes and have many uses. The coils in electric gauges and
meters, the field coils in motors and generators, solenoids, relays, and magnetic circuit breakers
are just a few of the uses of an electromagnet.
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SELF CHECK TEST
Instructions: Given lesson 52C11A03, Magnetism, answer each question or solve each problem
to the best of your ability, then check your answers with the solution sheet.
1. What do you call materials that are attracted to a magnet?
2. What are some of the magnetic substances?
3. What would each part constitute if a bar magnet were broken into several parts?
4. What would happen if a magnetic material such as soft iron were placed in a magnetic field?
5. What can be determined when the direction of current flowing through a coil is known?
6. Where will all the lines of force enter when the conductor is bent into a loop?
7. What do you call the ability to retain magnetism after the current flow has stopped?
8. What has a reverse affect on current flow?
9. What three things affect the strength of an electromagnet?
10. What three things are needed to cause an induced voltage?
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SELF CHECK SOLUTION SHEET
1. Magnetic Substances. (page. 1, para 2.1)
2. Cobalt, Nickel, and Manganese. (page. 1, para 2.1)
3. Magnet. (page. 2, para 2.3.3)
4. The magnetic field may be redirected. (page. 6, para 2.7)
5. That the north and south poles can be determined by use of the left-hand rule for coils. (page.
12, para 3.1.5)
6. Lines of force will enter on one side of the loop and leave on the other side of the loop. (page.
10, para 3.1.3)
7. Retentivity. (page. 13, para 3.2.2)
8. Magnetism. (page. 14, para 4.0.)
9. The number of turns in the coil, the permeability of the core, and the amount of current flow
through the conductor. (page. 14, para 6.0)
10. Conductor, magnetic field, and relative motion. (page. 15, para 7.0)
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