NEETS MODULE 2; APENDIX D; CHAPTER 1,3,4,5 Flashcards
1
Q
Define direct current?
A
An electrical current which flows in one direction only.
2
Q
Define alternating current?
A
An electrical current which is constantly varying in amplitude, and which changes
direction at regular intervals.
3
Q
what type of current is caused by a coil rotating in a magnetic field which regularly changes direction
A
alternating current
4
Q
what is determined by the number of electrons flowing past a point in a circuit in one second
A
current
5
Q
alternating current can be stepped up or down in amplitude by what type of device
A
transformer
6
Q
what is a disadvantage of a direct current system with respect to supply voltage
A
the voltage must be generated at the level required by the load
7
Q
what type of current is used in most modern power distribution systems
A
ac
8
Q
a pattern which results in changes in amplitude with a respect to time plotted on paper
A
waveform
9
Q
the study of how magnetism is affected by an electric current and also how electricity is affected by magnetism
A
electromagnetism
10
Q
if you grasp a conductor in your left hand with the thumb extended in the direction of current flow, your fingers will point in the direction of the magnetic lines of force
A
left hand rule for a conductor
11
Q
what happens when two adjacent parallel conductors are carrying current in the same direction
A
the magnetic lines of force combine and increase the strength of the field around the conductors
12
Q
what happens when two adjacent parallel conductors are carrying current in opposite directions
A
the conductors repel eachother
13
Q
what are characteristics of a coil’s intensity
A
number of turns, amount of current, ratio of coil length to width, type of core material
14
Q
in order to reverse the poles in a coil by changing current direction the reversal of all the atoms requires that power be expended, and this power is lost
A
hysteresis loss
15
Q
two alterations represents ______ complete _______ of rotation
A
one, cycle
16
Q
one cycle per second?
A
Hz
17
Q
the number of complete cycles of ac current completed per second
A
frequency
18
Q
an individual cycle of any sine wave represents a definite amount of
A
f= 1/t
19
Q
the time required to complete one cycle of a waveform
A
period
20
Q
the variation which occurs during the time the voltage is positive?
A
positive alternation
21
Q
the variation which occurs during the time the voltage is negative
A
negative altenration
22
Q
the distance from zero to the maximum value of each alernation?
A
amplitude
23
Q
the distance traveled by the sine wave during a period
A
wavelength
24
Q
the time is takes for a sine wave to complete one cycle
A
period
25
the point at which the dc and ac voltages are equal
maximum or peak value
26
the difference between positive peak and negative peak values
peak to peak
27
the value of voltage or current in an ac cycle at one particular instant
instantaneous value
28
how is average value calculated
E or I avg = .636 * peak (current or voltage)
29
this value of the ac voltage or current will have the same effect on a resistance as a comparable value of direct voltage
effective value
30
how is effective value calculated
E or I eff = .707 * peak (current or voltage)
31
what is another name for effective value
RMS
32
when two sine waves are precisely in step with one another they are said to be
in phase
33
when two voltages do not go through their maximum and minimum points at the same time what exists between the waves
phase difference
34
what type of current must be generated at the voltage level required by the load
DC
35
this type of current can be generated at a high level of voltage and stepped down at the consumer end
AC
36
this type of current must be transmitted at low voltage and high current
DC
37
this type of current can be transmitted at high voltage low current
AC
38
when placed in the vicinity of a current carrying conductor a compasses needle will point in what direction
at right angles to the conductor
39
when wire carrying current is wound around a core?
Magnetic field
40
what effect occurs when a conductor is in a magnetic field and either the field or the conductor moves causing a voltage to be induced in the conductor
electromagnetic induction
41
the number of cycles of ac per second
frequency
42
the time required to complete on cycle of a waveform
period
43
Define the terms "capacitor" and "capacitance".
(a) A capacitor is a device that stores electrical energy in an electrostatic field.
(b) Capacitance is the property of a circuit which opposes changes in voltage.
44
State four characteristics of electrostatic lines of force
(a) They are polarized from positive to negative.
(b) They radiate from a charge particle in straight lines and do not form closed loop.
(c) They have the ability to pass through any known material.
(d) They have the ability to distort the orbits of electrons circling the nucleus.
45
An electron moves into the electro static field between a positive charge and a negative charge. Towards which charge will the electron move?
Toward the positive charge.
46
What are the basic parts of a capacitor?
Two pieces of conducting material separated by an insulator.
47
Define the term "farad".
A farad is the unit of capacitance.
48
What is the mathematical relationship between a farad, a microfarad, and a picofarad?
(a) One microfarad equals 10^-6 farad.
| (b) One picofarad equals 10 ^-12 farad.
49
State three factors that affect the capacitance of a capacitor.
(a) The area of the plates.
(b) The distance between the plates.
(c) The dielectric constant of the material between the plates.
50
Name two types of power losses associated with a capacitor.
(a) Hysteresis
| (b) Dielectric
51
a)Define the term "working voltage" of a capacitor.
(a) It is the maximum voltage the capacitor can work without risk of damage.
52
State what happens to the electrons in a capacitor circuit when (a) the capacitor is charging and (b) the capacitor is discharging.
(a) When the capacitor is charging, electrons accumulate on the negative plate and leave the positive plate until the charge on the capacitor is equal to the battery voltage.
(b) When the capacitor is discharging, electrons flow from the negatively charge plate to the positively charged plate until the charge on each plate is neutral.
53
At what instant does the greatest voltage appear across the resistor in a series RC circuit when the capacitor is charging?
At the instant of the initiation of the action.
54
What is the voltage drop across the resistor in an RC charging circuit when the charge on the capacitor is equal to the battery voltage?
Zero.
55
What is the RC time constant of a series RC circuit that contains a 12-megohm resistor and a 12-microfarad capacitor?
144 seconds
t = R (megohms) x C (microfarads)
t = 12 x 12
t = 144 seconds
56
(a) An oxide-film dielectric is used in what type of capacitor?
(a) Electrolytic capacitor
57
What effect does an inductor have on a change in current?
An inductor opposes a change in current.
58
What is the phase relationship between current and voltage in an inductor?
Current lags voltage by 90º (ELI).
59
What is the term for the opposition an inductor presents to ac?
Inductive reactance.
60
What is the formula used to compute the value of this opposition?
XL = 2πfL
61
What happens to the value of XL as frequency increases?
XL increases.
62
What happens to the value of XL as inductance decreases?
XL decreases.
63
What effect does the capacitor have on a changing voltage?
The capacitor opposes any change in voltage.
64
What is the phase relationship between current and voltage in a capacitor?
Current leads voltage by 90º (ICE).
65
What is the term for the opposition that a capacitor presents to ac?
Capacitive reactance.
66
What happens to the value of XC as frequency decreases?
XC increases.
67
What happens to the value of XC as capacitance increases?
XC decreases
68
What is the formula for determining total reactance in a series circuit where the values of XC and XL are known?
X = XL - XC or X = XC - XL
69
What term is given to total opposition to ac in a circuit?
Impedance.
70
What is the unit of measurement of true power?
Watt.
71
What is the true power in an ac circuit?
True power is the power dissipated in the resistance of the circuit or the power actually used in the circuit.
72
What is the formula for calculating true power?
True Power = I2 x R
73
What is the reactive power in an ac circuit?
Is the power returned to the source by the reactive components of the circuit.
74
What is the unit of measurement for reactive power?
var. volt amps reactive
75
What is apparent power?
The power that appears to the source because of circuit impedance, or the
combination of true power and reactive power.
76
What is the unit of measurement for apparent power?
VA (volt-amperes).
77
What is the power factor of a circuit?
PF is a number representing the portion of apparent power actually dissipated in a circuit.
78
Which symbol is used to represent impedance?
Z
79
What is meant by "transformer action?"
The transfer of energy from one circuit to another circuit by electromagnetic induction.
80
What are, the three basic parts of a transformer?
Primary winding; secondary winding; core.
81
What are three materials commonly used as the core of a transformer?
Air; soft iron; steel.
82
What are the two main types of cores used in transformers?
Hollow-core type; shell-core type.
83
Which transformer windings are connected to an ac source voltage and to a load, respectively?
Primary to source; secondary to load.
84
A transformer designed for high-voltage applications differs in construction in what way from a
transformer designed for low-voltage applications?
Additional insulation is provided between the layers of windings in the high-voltage transformer.
85
What is meant by a "no-load condition" in a transformer circuit?
A voltage is applied to the primary, but no load is connected to the secondary.
86
What is meant by "exciting current" in a transformer?
Exciting current is the current that flows in the primary of a transformer with the secondary
open (no load attached).
87
What causes a voltage to be developed across the secondary winding of a transformer?
The magnetic lines generated by the current in the primary cut the secondary windings and
induce a voltage into them.
88
What is "leakage flux?"
Lines of flux generated by one winding which do not link the other winding.
89
Name the three power losses in a transformer.
Copper loss, eddy-current loss, and hysteresis loss.
90
List five different types of transformers according to their applications.
a. Power transformer
b. Autotransformer
c. Impedance-matching transformer
d. Audio-frequency transformer
e. Radio-frequency transformer
91
The leads to the primary and to the high-voltage secondary windings of a power transformer
usually are of what color?
Primary leads-black; secondary leads-red.
92
What is the cause of most accidents?
Carelessness.
93
Before working on electrical equipment containing capacitors, what should you do to the
capacitors?
Discharge them by shorting them to ground.
94
When working on electrical equipment, why should you use only one hand?
To minimize the possibility of providing a path for current through your body.
95
The amplitude of the voltage induced in the secondary is dependent upon the efficiency of the transformer and the turns ratio. The efficiency of a transformer is
related to the power losses in the windings and core of the transformer. Efficiency (in percent) equals
Pout/Pin × 100. A perfect transformer would have an efficiency of 1.0 or 100%.
TRANSFORMER EFFICIENCY
96
A transformer with two or more windings wound on a laminated iron core. The transformer is used to supply stepped up and stepped down values of voltage to the various circuits in electrical equipment.
POWER TRANSFORMER
97
A transformer with a single winding in which the entire winding can be used as the primary and part of the winding as the secondary, or part of the winding can be used as the primary and the entire winding can be used as the secondary.
AUTOTRANSFORMER
98
A transformer used in audio-frequency circuits to
| transfer af signals from one circuit to another.
AUDIO-FREQUENCY TRANSFORMER
99
A transformer used in a radio-frequency circuit to
| transfer rf signals from one circuit to another
RADIO-FREQUENCY TRANSFORMER
100
A transformer used to match the impedance of
the source and the impedance of the load. The mathematical relationship of the turns and impedance of the transformer is expressed by the equation:
IMPEDANCE-MATCHING TRANSFORMER
101
The current which appears to flow through a capacitor?
DISPLACEMENT CURRENT
102
Induced circulating currents in a conducting material that are caused by a varying magnetic field.
EDDY CURRENT
103
Losses caused by random current flowing in the core of a transformer. Power is lost in the form of heat.
EDDY CURRENT LOSS
104
The generation of a magnetic field around a current carrying conductor
ELECTROMAGNETISM
105
The basic unit of capacitance. A capacitor has a capacitance of 1 farad when a voltage change
of 1 volt per second across it produces a current of 1 ampere.
FARAD
106
The basic unit of frequency equal to one cycle per second.
HERTZ (Hz)
107
Any material that affords a path for magnetic flux lines in a coil.
CORE
108
An inductive device created by looping turns of wire around a core.
COIL
109
An insulator; a term applied to the insulating material between the plates of a capacitor.
DIELECTRIC
110
The property of a circuit which tends to oppose a change in the existing current flow. The symbol for inductance is L.
INDUCTANCE
111
The current induced in a circuit due to its motion in a magnetic field or to a change in its magnetic flux in such a direction as to exert a mechanical force opposing the motion or to oppose the change in flux
LENZ'S LAW
112
(b) A screw adjustment is used to vary the distance between the plates of what type of capacitor
Trimmer capacitor
