Line D Flashcards
1
Q
What is the term for a wire wound into a helical coil for the express purpose of creating a stronger magnetic field.
1) Relay
2) Rheostat
3) Armature
4) Solenoid
A
Solenoid
2
Q
The core of lifting magnets is made from a material with ________ retentivity.
A
Low
3
Q
A ______________ is a type of electromagnetic switch.
A
Relay
4
Q
The magnetic flux in a circuit is directly proportional to the ________ and inversely proportional to the ___________ of the magnetic circuit?
A
magnetomotive force and reluctance
5
Q
What is the purpose of the spring device in a relay?
A
The spring pulls the armature away from the coil when current stops flowing.
6
Q
What happens to bell contacts as the armature strikes the bell?
A
The contacts open.
7
Q
The opposition to the formation of flux lines in a magnetic circuit is called ?
1) reluctance
2) coercive force
3) magnetomotive force
4) theoretical saturation
A
reluctance
8
Q
How is the voltage of a generator normally controlled?
A
The output voltage is usually controlled by varying a field rheostat that controls the current flowing through the field coils . This varies the flux density of the field.
9
Q
The point at which all of the domains in the magnetic material have aligned themselves with the magnetic field is referred to as the_______
1) reluctance
2) coercive force
3) magnetomotive force
4) theoretical saturation
A
Theoretical saturation
10
Q
Briefly explain the operating principle of the D’Arsonval meter movement.
A
Current flow in a coil sets up magnetic flux around that coil. The coil is free to turn in the field of a strong permanent magnet.
11
Q
What device must be used with the D’Arsonval meter movement to enable the meter to read AC?
A
Rectifier
12
Q
Residual magnetism is a measure of the amount of ____________ remaining in the magnetic material after the magnetizing force has been removed
1) reluctance
2) flux density
3) coercive force
4) magnetomotive force
A
Flux Density
13
Q
What is meant by “damping” a meter movement?
A
Preventing pointer oscillation in a meter
14
Q
The amount of reverse magnetizing force required to remove the residual magnetism is called the .
1) reluctance
2) coercive force
3) magnetomotive force
4) theoretical saturation
A
Coercive force
15
Q
The mirror strip on the face of some meters is there for what purpose?
A
To assist the user in taking an accurate reading
16
Q
What is the purpose of the zero-adjust screw in the D’Arsonval meter movement?
A
To set the pointer to zero when there is no coil current
17
Q
The type of core material used for lifting electromagnets would have a _______ retentivity
A
low
18
Q
What term refers to the spreading out of the flux lines in an air gap?
1) Laminating
2) Hysteresis
3) Fringing
4) Leakage
A
Fringing
19
Q
Magnetic cores that are constructed of alternating layers of magnetic material and electrical insulators are said to be_______
A
Laminated
20
Q
As the reluctance of a magnetic circuit is increased, the flux density of the circuit is__________?
A
Decreased
21
Q
If the magnetomotive force applied to a magnetic circuit is increased, the flux density of the circuit is increased.
A
True
22
Q
The ease with which flux is set up in a unit length and cross-section of a material is referred to as the _______?
1) hysteresis
2) reluctance
3) permeability
4) capacitance
A
Permeability
23
Q
The term for the lag of flux density behind magnetizing intensity is __________?
1) hysteresis
2) reluctance
3) permeability
4) capacitance
A
Hysteresis
24
Q
Magnetic cores are usually constructed from materials with a high ________ and a low ___________
A
permeability and a low reluctance
25
The amount of energy lost due to the constant reversal of the magnetic field in AC circuits is referred to as ____________ Loss
1) hysteresis
2) reluctance
3) permeability
4) capacitance
Hysteresis
26
in Hysteresis loss is the energy lost normally by heat?
true
27
The arrows on the magnetic lines of force always point from north to south outside of the magnet, and south to north on the inside of the magnet.
1) True
2) False
True
28
The temperature at which a magnetic material gains its ferromagnetic properties is known as its Curie temperature?
1) True
2) False
False
29
Unlike poles of magnets repel each other.
1) True
2) False
False
30
Magnetic screens block magnetic flux lines.
1) True
2) False
False
31
The north pole of the Earth is actually a south magnetic pole.
1) True
2) False
True
32
It is not possible to have a north pole without having an accompanying south pole.
1) True
2) False
True
33
Why are there no magnetic poles in the field surrounding a current-carrying conductor?
There is no point of convergence or divergence of the flux lines. There is no point of flux concentration.
34
As the direction of current in a conductor is reversed, what happens to compasses surrounding the conductor?
The compasses also reverse direction.
35
As the direction of current in a conductor is reversed, the compasses surrounding the conductors also reverse direction. What does this action indicate?
reversal of the magnetic field direction
36
As the current in a conductor is increased, what happens to the magnetic lines of force surrounding the conductor?
1) Decreased
2) Increased
3) Remain the same
4) Lower flux density
Increased
37
Is the field around the conductor more dense or less dense closer to the conductor?
more dense
38
When the current remains steady, does the field disappear?
No, it becomes stationary.
39
Why is the field around a conductor more dense closer to the conductor?
The outer lines of force exert an inward force on the inner lines of force.
40
As the current in a conductor is decreased, what happens to the magnetic lines of force surrounding the conductor?
The lines of force collapse inward.
41
In moving four times as far away from a current-carrying conductor, the density of the flux lines will have _____ by _____ times.
decreased, four
42
Parallel conductors carrying currents in the same direction _____ each other and parallel conductors carrying currents in opposite directions _____ each other.
attract, repel
43
Using the left-hand rule for coils, the thumb points in the direction of
flux lines or the north pole of the coil
44
List four things that affect the strength of an electromagnetic coil.
spacing between coils in an air-core electromagnet
number of turns in the coil
magnitude of current flow through the coil
type of core material
45
Materials that act in a similar way to iron in a magnetic field are referred to as _____ materials.
ferromagnetic
46
Give two examples of non-magnetic materials.
air, wood, copper, aluminum, ect.
47
What is the most practical way of varying the strength of an electromagnet?
Vary the current through the coil.
48
What device connected in series with the electromagnet is normally used to varying the strength of an electromagnet?
1) Relay
2) Switch
3) Rheostat
4) Transformer
Rheostat
49
As you move two times the distance away from a conductor, the flux density of the field will decrease by _____ times.
two
50
Why must electrical busbars be securely braced?
Strong magnetic forces occur between the bus bars if there is a short circuit.
51
In the dot-and-cross convention used to indicate the direction of current flow, a _____ represents a current coming out of the page toward you.
dot
52
What device connected in series with the electromagnet is normally used to varying the strength of an electromagnet?
1) Relay
2) Switch
3) Rheostat
4) Transformer
3) Rheostat
53
As the direction of current in a conductor is reversed, what happens to compasses surrounding the conductor?
1) They spin continuously
2) They also reverse direction
3) They remain pointing in the same direction
2) They also reverse direction
54
Is the field around the conductor more dense or less dense closer to the conductor?
1) More dense
2) Less Dense
1) More dense
55
What happens to the flux density of a field as you move two times the distance away from a conductor?
1) Decrease by two times
2) Increase by two times
3) Decrease by four times
4) Increase by four times
1) Decrease by two times
56
Any time a charge moves, there must be a magnetic field set up around that charge?
1) True
2) False
1) True
57
What is another name for magnetic lines of force?
Magnetic flux
58
Why don’t magnetic lines of force collapse into the center of a magnet?
1) Magnetic flux
2) Mutual repulsion
3) Alternating current
4) Magnetic induction
Mutual repulsion
59
Magnetic lines of force never cross
a. true
b. false
a. True
60
Magnetic lines of force start at the north pole and end at the south pole
a. true
b. false
b. false
61
It is not possible to have a north pole without having an accompanied south pole.
a. true
b. false
a. true
62
The north pole of the earth is actually a south magnetic pole
a. true
b. false
a. true
63
The arrows on the magnetic lines of force always point from north to south outside of the magnet, and south to north on the inside of the magnet.
a. true
b. false
a. true
64
unlike poles of magnets repel each other.
a. true
b. false
b. false
65
Magnetic screens are made of material with a low reluctance.
a. true
b. false
a. true
66
magnetic screens block magnetic flux lines.
a. true
b. false
b. false
67
list three ways of creating magnets.
using an electric current
stroking with a magnet
lining up with another magnetic field (such as Earth's) and tapping gently
68
list three ways of demagnetizing a material
Heating up to the Curie temperature
using an alternating current and reducing the field strength slowly
tapping or vibrating the magnet
69
which method of demagnetizing is commonly used by watchmakers?
the alternating current method
70
The temperature at which a magnetic material gains its ferromagnetic properties is known as its Curie temperature?
false
71
when a piece of ferromagnetic material is brought near a magnet, the ferromagnetic material becomes magnetized. this process is referred to as ___________
magnetic induction
72
What three quantities affect the magnitude of an induced emf?
field flux density
active length of the conductor
velocity of the conductor relative to the magnetic field
73
The active length of the conductor is that portion of the conductor that actually cuts through the flux lines as the conductor moves.
True
74
what is the purpose of the field rheostat placed in series with a generators field windings?
the rheostat varies the generated voltage by varying the current though the field coils, which in turn, varies the flux density of the field
75
in Fleming's left-hand rule, in which direction does the first finger point?
in the direction of magnetic flux
76
What is meant by the electromagnetic term Prime mover?
1) The electrical generator is a prime mover
2) The device supplying electrical energy to the generator
3) The device supplying the mechanical energy to the generator
4) The magnetic field created by the generator
The device supplying the mechanical energy to the generator
77
A conductor has an active length of 2 meters . to induce a voltage of 240V, at which velocity must it be pushed through a field of flux density 1.1T?
109 m/s
78
if a 2 meter conductor is pushed through 1.1t field at 126 m/s what will the voltage be
277 V
79
A conductor is to be pushed through a field with a flux density of 1.5 T at a velocity of 100 m/s to produce a voltage of 347 V. what must the active length of the conductor be?
2.31 m
80
In Flemings left hand rule , in what direction does the thumb point ?
the direction of motion of the conductor relative to the magnetic field
81
To increase the field flux density in a generator, would the resistance of the field rheostat have to be increased or decreased?
decreased
82
When a piece of ferromagnetic material is brought near a magnet, the ferromagnetic material becomes magnetized. What is this process referred to as?
1) Magnetic flux
2) Mutual repulsion
3) Alternating current
4) Magnetic induction
Magnetic induction
83
What are three factors affect the magnitude of an induced voltage?
Flux density
active length of the conductor
velocity of the conductor through the field
84
Which of these is used most commonly to control generated emfs?
a) Flux density
b) active length of an conductor
c) Velocity of the conductor through the field
Flux density
85
What is the device that supplies mechanical energy to the alternator called?
1) Slip ring
2) Armature
3) Brush arm
4) Prime mover
Prime mover
86
What is the name given to a current that Periodically changes in both magnitude and directions?
alternating current
87
What is the term that represents the distance from one point on a waveform to where it repeats itself?
1) RPM
2) Cycle
3) Frequency
4) Hertz
Cycle
88
As the speed of the prime mover increases , what happens to the frequency of the alternating current?
1) Increases
2) Decreases
3) Remains constant
Increases
89
How many electrical degrees are there in each complete cycle?
360
90
In flemings left hand rule , what does the thumb represent
Thrust of the conductor
91
In Fleming's left hand rule , what does the first finger represent ?
direction of the flux lines
92
Alternators can be manufactured with higher voltage and power ratings than DC generators.
True
93
list five applications where direct current is preferred or required instead of alternating current
direct current is used to produce rotating magnetic fields for alternators and synchronous motors
electrochemical processes require DC voltages
high-intensity light sources such as carbon arcs
precise speed control, although variable frequency drives are changing this
DC traction motors for transportation equipment
94
What does Epp stand for?
1) Average value
2) Effective value
3) Peak-to-peak value
4) Instantaneous value
Peak-to-Peak value
95
What is another name for the root mean square value?
1) Average value
2) Effective value
3) Peak-to-peak value
4) Instantaneous value
Effective value
96
at what angles in a sine wave does the peak value occur?
90 and 270 degrees
97
with respect to the peak value, what is the average value of a complete AC sine wave?
zero
.637 x peak value
1.414 x 0.707 = 1
360
zero
98
with respect to the peak value, what is the average value of one alternation of an ACsine wave
.637 multiplied by the peak
99
which value of an AC voltage or current do we normally refer to?
effective or RMS
100
Select three variables that affect the magnitude of an induced EMF?
Pole alignment
flux density
active length of the conductor
direction of the EMF
velocity of the conductor relative to the magnetic field
resistance of conductors
flux density
active length of the conductor
velocity of the conductor relative to the magnetic field
101
Which value of an AC voltage or current do we normally refer to?
Average value
Effective value
Peak-to-peak value
Instantaneous value
Effective value
102
As the frequency of the AC increases, the effective resistance
Increases
Decreases
Remains the same
Increases
103
What is the easiest and most common method of controlling a generators voltage?
1) A field rheostat in series with the field coils
2) Adjusting the speed of the prime mover
3) Changing the active length of the conductor
4) Alternating prime movers
1) A field rheostat in series with the field coils
104
In Fleming’s left-hand rule, in which direction does the first finger point?
1) The first finger points in the direction of the magnetic flux, from south to north
2) The first finger points in the direction of the magnetic flux, from north to south
3) The first finger points in the direction of the magnetic flux, from west to east
4) The first finger points in the direction of the magnetic flux, from east to west
2) The first finger points in the direction of the magnetic flux, from north to south
105
What is meant by the electromagnetic term “prime mover”?
1) The electrical generator is a prime mover
2) The device supplying electrical energy to the generator
3) The device supplying the mechanical energy to the generator
4) The magnetic field created by the generator
3) The device supplying the mechanical energy to the generator
106
If a 1.5 metre conductor is pushed through a field of flux density 1.2T at a velocity of 126 m/s, what voltage will be generated?
227 V
107
What affects the magnitude of an induced voltage?
1) flux density
2) active length of the conductor
3) the velocity of the conductor through the field
4) all choices are correct
4) all choices are correct
108
Alternators can be manufactured with higher voltage and power ratings than DC generators.
True
109
DC motors are generally smaller, simpler and more reliable than AC motors with similar ratings.
False
110
Alternating voltages can be easily stepped up or down. This can be used to reduce line drop and line loss.
True
111
Effective resistance is the property of an electric circuit that dissipates electric energy.
True
112
What is the term used to refer to the circulating currents in a magnetic core?
1) Eddy currents
2) Henry currents
3) Sinusoidal currents
4) Synchronous currents
1) Eddy currents
113
What two meters must be used to determine the effective resistance of an AC circuit?
1) voltmeter and ammeter
2) wattmeter and ammeter
3) ohmmeter and ammeter
4) wattmeter and voltmeter
2) wattmeter and ammeter
114
Define effective resistance
Effective resistance is the property of an electric circuit (AC or DC) that dissipates electric energy.
115
Effective resistance is made up of what six things?
```
pure ohmic resistance
skin effect losses
hysteresis losses
eddy current losses
dielectric losses
radiation losses
```
116
What two meters must be used to determine the effective resistance of an AC circuit?
wattmeter and ammeter
117
As the frequency of the AC increases, the effective resistance_________
Increases
118
There are no magnetic poles in the field surrounding a current-carrying conductor
1) True
2) False
True
119
As the current in a conductor is increased, what happens to the magnetic lines of force surrounding the conductor?
1) Decreased
2) Increased
3) Remain the same
4) Lower flux density
Increased
120
Of the following materials, the most permeable is:
iron.
copper.
aluminum.
brass.
iron.
121
Refer to Diagram 17-06. Which of the following points starting at “A” and travelling up towards “D” on the BH curve represents the point of "Practical saturation”?
Point A.
Point B.
Point C.
Point D.
Point C.
122
Cast iron, silicon iron, and cast steel are used for electromagnetic cores because they:
are permanent magnets.
are easy to magnetize and to demagnetize.
are difficult to magnetize and demagnetize.
are easy to magnetize.
are easy to magnetize and to demagnetize.
123
A B-H magnetization curve is commonly used to determine an electromagnet's:
magneto motive force.
turns.
permeability.
polarity.
permeability.
124
Magnetic resistance is called:
reluctance.
coercive force.
permeability.
retentivity
reluctance.
125
The active length of the conductor is that portion of the conductor that actually cuts through the flux lines as the conductor moves.
True
126
Flux density is the number of lines of magnetic force divided by the core's:
length.
cross-section area.
mmf.
permeability.
cross-section area.
127
A conductor is to be pushed through a field with a flux density of 1.5 T at a velocity of
100 m/s to produce a voltage of 347 V. What must the active length of the conductor be?
1) 2.31 m
2) 3.47 m
3) 231 m
4) 52050 m
2.31m
128
Materials having a low permeability number are:
only good for temporary magnets.
easy to magnetize.
only good for permanent magnets.
difficult to magnetize.
difficult to magnetize.
129
In what direction are the flux lines in coil a?
Clockwise dotted line
130
The Magnetomotive force of an electromagnet is dependent on:
current and number of turns
131
Which factor is most commonly manipulated to control the generated emf?
flux density
active length of the conductor
the velocity of the conductor through the field
all choices are correct
flux density
132
What affects the magnitude of an induced voltage?
flux density
active length of the conductor
the velocity of the conductor through the field
all choices are correct
all choices are correct
133
ohm's law for magnetic circuits formula is
Fancy R = mmf / 0 with a slash thru it
134
What is another name for for the root mean square value?
average value
effective value
peak-to-peak value
instantaneous value
effective value
135
Refer to Diagram 17-02. If free to move, the magnet shown in the centre of Figure 2 will:
|S N| |S N| |N S|
turn counterclockwise.
be attracted to the magnet on the right.
turn clockwise.
be attracted to the magnet on the left.
be attracted to the magnet on the left.
136
Magnetic flux lines outside the magnet extend from:
south to north.
north to south.
north to south.
137
When two parallel conductors carrying large currents in the same direction are placed side by side, their combined fields:
cancel each other's field.
push each other apart.
form a north pole.
draw the conductors together.
draw the conductors together.
138
The magnetic flux in a circuit is directly proportional to _______ of the magnetic circuit.
reluctance
flux density
coercive force
magnetomotive force
magnetomotive force
139
If the iron core of an electromagnet is replaced by copper core, the:
flux density is reduced.
mmf is reduced.
flux density is increased.
mmf is increased.
flux density is reduced.
140
A magnetic alloy with a low coercive force factor is:
difficult to demagnetize.
usually a permanent magnet.
difficult to magnetize.
easy to demagnetize.
easy to demagnetize.
141
Increasing the current in a coil will result in:
higher core permeability.
lower core permeability.
lower mmf.
higher mmf.
higher mmf.
142
Cast iron, silicon iron, and cast steel are used for electromagnetic cores because they:
are permanent magnets.
are easy to magnetize and to demagnetize.
are difficult to magnetize and demagnetize.
are easy to magnetize.
are easy to magnetize and to demagnetize.
143
An electromagnet is saturated when:
an increase in current causes a large increase in flux.
an increase in current causes a small increase in flux.
a reverse current is required to remove residual magnetism.
Eddy currents start to flow in the core.
an increase in current causes a small increase in flux.
144
As the reluctance of a magnetic circuit is increased, the flux density of the circuit is increased
False
145
What part of a relay pulls the armature away from the coil when the current stops flowing?
1) Spring
2) Solenoid
3) Contacts
4) Coercive force
Spring
146
Select three variables that affect the magnitude of an induced EMF?
1) Pole alignment
2) flux density
3) active length of the conductor
4) direction of the EMF
5) velocity of the conductor relative to the magnetic field
6) resistance of conductors
flux density
active length of the conductor
velocity of the conductor relative to the magnetic field
147
What is the term for a wire wound into a helical coil for the express purpose of creating a stronger magnetic field.
Relay
Rheostat
Armature
Solenoid
Solenoid
