Basic Electrical Engineering
CHAPTER 18: Magnetism
18-1 QUESTIONS
1. What are the three types of magnets?
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Natural magnet
Artificial magnet
Electromagnet
2. What are the basic shapes of magnets?
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Horseshoe magnet
Bar or Rectangle magnet
Ring magnet
3. How are the ends of a magnet identified?
The magnet aligns in a north-south direction. Ends can be identified by color code. Red
is for the North Pole, while blue is for the South Pole.
4. What are the two laws of magnetism?
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Unlike magnetic poles attract each other.
Like magnetic poles repel each other.
5. What are flux lines?
Flux lines are the invisible lines of force that surround a magnet.
18-2 QUESTIONS
1. How can a magnetic field be shown to exist when a current flows through a wire?
When current flows through a wire, it generates a magnetic field around the wire. This
can be shown by placing a compass next to a wire that has no current flowing through it. The
compass aligns itself with the earth’s magnetic field. When current passes through the wire,
however, the compass needle is deflected and aligns itself with the magnetic field generated by
the current.
2. How can the direction of flux lines around a wire be determined?
The north pole of the compass indicates the direction of the flux lines. The direction of
the flux lines can also be determined if the direction of the current flow is known, using the lefthand rule for conductors. If the wire is grasped with the left hand, with the thumb pointing in
the direction of current flow, the fingers point in the direction of the flux lines. When the polarity
of the voltage source is reversed, the flux lines are also reversed.
3. What happens when two current-carrying wires are placed next to each other with the current
flowing in: a) the same direction? b) Opposite directions?
If two wires are placed next to each other with current flowing in opposite directions, they
create opposing magnetic fields that repel each other. On the other hand, if the two wires carry
current flowing in the same direction, the fields combine.
4. What are three ways to increase the strength of an electromagnetic field?
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Increase the number of wire/coil turns.
Increase the current; the greater the current, the greater the number of flux line.
The third method of increasing the strength of the magnetic field is to insert a
ferromagnetic core into the center of the coil.
5. How can the polarity of an electromagnet be determined?
The polarity of an electromagnet can be determined using the left-hand rule for coils.
Grasp the coil with the left hand, with the fingers pointing in the direction of the current flow. The
thumb then points in the direction of the North Pole.
18-3 QUESTIONS
1. How can the length of a magnet be increased without physically altering the magnet?
The length of a magnet can be increased without physically altering the magnet by
placing an iron bar in the magnetic field.
2. What is residual magnetism?
Residual magnetism is the weak magnetic field remaining in a material when the
magnetizing force is removed.
3. How does a magnetic shield work?
Magnetic shield works by diverting the magnetic flux and drawing the magnetic field lines
into the shielding material rather than them passing into the protected space.
4. How is electromagnetic induction used to generate electricity?
Electromagnetic induction generates electrical energy by moving a metal wire through a
magnetic field. As the wire moves through the field, it causes the magnetic field to change,
which creates an electromotive force. This force can generate a voltage that can be used to
power electronic devices.
5. What does Faraday’s law state?
The induced voltage in a conductor is directly proportional to the rate at which the
conductor cuts the magnetic lines of force.
18-4 QUESTIONS
1. What are the differences between an AC and a DC generator?
Alternating Current (AC) and Direct Current (DC) generators both use electromagnetic
induction to generate electricity. However, the process they use is different. An AC generator
creates an alternating current that periodically reverses direction. But in a DC generator, a direct
current flows in one direction.
2. Why are relays important?
A relay is an electromechanical switch that opens and closes with an electromagnetic
coil. It controls one electrical circuit by opening and closing contacts in another circuit. Relays
are important because they are used to protect the electrical system and minimize damage to
the equipment connected to the system due to overcurrent or voltages.
3. How does a loudspeaker produce sound?
Most speakers today are constructed of a moving coil around a permanent magnet. The
magnet produces a stationary magnetic field. As current is passed through the coil, it produces
a magnetic field that varies at the rate of the audio signal. The varying magnetic field of the coil
is attracted and repelled by the magnetic field of the permanent magnet. The coil is attached to
a cone that moves back and forth in response to the audio signal. The cone moving back and
forth moves the air, reproducing the audio sound.
4. What is the principle behind DC motors and meter movements?
The operation of a DC motor and meter movements depend on the principle that a
current-carrying conductor, placed in and at right angles to a magnetic field, tends to move at
right angles to the direction of the field.
5. How does an electromagnetic field produce an image on a screen?
Electron beams can be deflected by an electromagnetic field to produce images on
Television, radar and oscilloscope screens.
CHAPTER 18- SELF- TEST
1. How can the domain theory of magnetism be verified?
The domain theory of magnetism can be verified by jarring the domains into a random
arrangement by heating or hitting with a hammer. The magnet will eventually lose its
magnetism.
2. What three methods can be used to increase the strength of an electromagnet?
The strength of an electromagnet can be increased by increasing the number of turns of
wire, by increasing the current flow, or by inserting a ferromagnetic core in the center of the coil.
3. Explain the left-hand rule for conductors.
The left-hand rule for conductors: Grasp the wire in the left hand with the thumb pointing
in the direction of the current flow; the fingers will point in the direction of the flux lines.
4. Explain how a DC generator operates through one cycle.
In Figure 18–15 (page 189), when the loop is rotated from position A to position B, a
voltage is induced when the motion is at right angles to the magnetic field. As the loop is rotated
to position C, the induced voltage decreases to 0 volts. As the loop continues to position D, a
voltage is again induced, but the commutator reverses the output polarity so it is the same as
was first output by the DC generator. The output pulsates in one direction, varying twice during
each revolution between 0 and maximum
5. Show in a drawing how an electromagnet works.
6. Explain how to determine the polarity of an electromagnet.
Polarity of an electromagnet can be determined by grasping the coil with the left hand,
with the fingers pointing in the direction of current flow; the thumb will point in the direction of the
North Pole.
7. Explain the left-hand rule for generators.
Left-hand rule for generator: Holding the left hand with the thumb, index, and middle
fingers extended at right angles to each other (Figure 18-13, page 188). With the thumb pointing
in the direction of movement, the index finger in the direction of flux lines, the middle finger
points in the direction of current flow
8. Draw and identify the parts of a DC generator.
9. Describe how a DC motor operates.
DC motor operation is dependent on the principle that a current-carrying conductor
placed at right angles to a magnetic field will move at right angles to the direction of the field. A
commutator reverses the direction of the current flow at the top or zero torque position, resulting
in the DC motor armature rotating.
10. What other devices can use a magnetic field to operate besides DC motors and generators?
The basic meter movement relies on the same principles of the DC motor. A pointer is
attached to a rotating coil and moves according to current flow. The pointer moves across a
graduated scale and indicates the amount of current flow.