Cheat sheet Flashcards
Pass the poxy thing (96 cards)
1
Q
CAME UP
A material may become demagnitised if it..?
Q27
A
Recieves a heavy shock/
By applying an alternating magnetic field, reducing it to zero
2
Q
What is the basic law of magnetism?
A
Unlike poles attract.
3
Q
Magnetic flux is measured in what unit?
A
Tesla
4
Q
The transfer of electric energy from one conductor to another is known as…?
A
Induction
5
Q
A circuit with a, 10-omh resistor, 20-ohm induction and a 30 ohm capacitor is..?
A
Capacitive.
6
Q
CAME UP
How can you improve the efficiency of a transformer?
A
Insulate the laminations with varnish to prevent losses.
(Inversely proportional)
7
Q
CAME UP DIFF FIG
An AC generator is running at 12,00RPM generating 400Hz.
How many pairs of poles are in the generator?
A
F= NP/120
F=Freq
N=RPM
P=Poles
Ans= 2 pairs
8
Q
What effect will adding a capacitor to an AC generator, in relation to power factor?
A
It will increase.
9
Q
A material has a positive temperature coefficient. As the temperature increases, resistance will?
A
Increase.
10
Q
A material has a negative temperature coefficient. As temperature increases its resistance will?
A
Decrease.
11
Q
What is the opposite of conductance?
A
Resistance.
12
Q
Peak power is always…?
(in relation to RMS)
A
Greater than the RMS/ effective value. (They are averages)
13
Q
In a conductor, 360 coulombs pass one point in 5 minutes. What is the current in the wire?
A
1.2 amp
C=IT
14
Q
came up exactly
A motor consumes 4 amps for 16 seconds. How many coulombs pass through the conductor?
A
64 coulombs
C=IxT
15
Q
In a DC current of 2 amps, how many coulombs pass per second?
A
2 coulombs per second
One Ampere is equivalent to one Coulomb per second
16
Q
A generator with field windings in parallel is a…?
A
Shunt-Wound DC generator
17
Q
What are the main types of DC motors?
A
Series, shunt and compound wound.
18
Q
What are the field windings of a DC series motor made of?
A
Few turns of heavy wire
Pg 527
19
Q
Power in a series circuit
A
??high resisotrs give high power??
20
Q
A 600W landing light is supplied with 30V, what value is the current?
Q20
A
20Amps
I=P/V
21
Q
“Effeactive power” is the same as?
A
RMS power
Real power
22
Q
What is the time period of 1k hz?
A
Period of time=1/F
0.001s=1 msec
23
Q
RMS current is the equivelent to…?
A
DC current
24
Q
Regarding an AC current, the effective voltage value is
A
Less than peak voltage.
25
In a 3 phase motor, increasing the number of poles will?
| Q48
Reduce the speed at which the motor runs.
26
An advantage of a shaded pole motor is...?
They have a weak starting torque
27
# came up exactly
A circuit has an applied voltage of 30v with 10+20 ohm resistors in series.
What is the voltage drop across the 10 ohms resistor?
| Q9
10v
Vⁿ= IⁿRⁿ
| I = V/R
28
A 48v source is required to furnish 192 watts to a parallel circuit consisitng of three resistors of equal value.
What is the value of each resistor?
| Q10
36 ohms.
I = P / V = 192 / 48 = 4 amps,
V=IxR, R=12
1/RT=1/Rx formula=36ohms
29
Which element has a negative temperature coefficient?
Carbon
30
# asked about tolerance of silver
If a resistor has no fourth band, what is its tolerance?
20%
31
When an electrical supply becomes 'open-circuit'...
the loss of continuity will prevent its component from functioning
32
# came up but about capacitors
If a number of resistors are connected in parallel, the total resistance is...
Less than the smallest resistance
33
A 24v source is required to furnish 48 watts to a parallel circuit consisting of four equal resistors. What is the voltage drop across each resistor?
24 volts.
Resistors in parallel, voltage drop across them is the same and equal to the source voltage.
34
# came up but opposite
What will cause the resistance of a conductor to decrease?
(Dimensions )
Decrease the length or increase the cross-sectional area
35
# came up twice
Which type of filter only allows a narrow band of frequencies to pass?
A band pass filter.
36
A simple circuit with a capacitor in parallel and a resistor in series is a...?
An intergrator circuit
37
The resistance of a battery terminal...
| Q8
Reduces the output voltage
38
Battery charging current depends on
state of charge of the battery
39
Mutual inductance is found in
transformers
40
# came up exactly
**What is true in relation to electromagnetic induction?**
1. a voltage is induced in a conductor when it is moving parallel to the magnetic field
2. a voltage is induced whenever the magnetic field linking that circuit changes
3. the direction of an induced voltage is always such as to assist the change that is producing it.
Q30
A voltage is induced whenever the magnetic field linking that circuit changes
| Google
41
Real load/power is measured in...?
Watts
42
Electrostatic charge is created when…?
A charged body is near uncharged body
43
# came up
The electrostatic charge distribution on charged metallic plates would be greatest at the...?
Greatest at edge/corners
Extremities
44
What effect does increasing the distance between two charged bodies have regarding force?
The force decreases
45
If your RMS value is 5v, what is your peak voltage?
7.07v
(x1.414)
46
Two, 1uF capacitors are in parallel. What is the total value of capacitance?
2 Micro-farads
47
A circuit contains 3 capacitors of 60uF, 40uF, 120uF in series. What is the total value of capacitance?
20 uF
(Capacitors in series formula)
48
Two capacitors of 12mF and 4mF are in series, what is the total value of capacitance?
3mFarad
49
AC voltage 500 peak to peak, what is the voltage at 45 degrees of the sin wave?
177v
49
In an AC circuit, the effective voltage is
less than the maximum instantaneous voltage (peak)
50
A PARALLEL circuit at resonance has its impedance at a
Maximum
51
The impedance of a SERIES tuned circuit at resonance is
minimum
52
# came up exaclty
RMS 200v, what is the peak voltage?
282.2v
(x1.414)
53
When two charges are placed close to one another, doubling one of the charges values will have which effect on the force between them?
q1
Doubling the force
54
A semiconductor normally has which type of temperature coefficient of resistance?
Negative coefficient
55
# came up but used two negativley charged materials
Two glass rods rubbed with a silk material will...
they repel each other and will be positively charged
56
What describes the direction of current flow in a European electrical circuit diagram?
Conventional current flow
57
180 coulombs of charge pass through a point in 3 minutes. Calculate the current.
1 ampere
| 1 coulomb/second = 1 ampere
C/T=I
58
# came upish
Which of the following has the smallest value of resistance?
10 meters long, 1mm² wire
100 meters long, 1mm² wire
10 meters long, 1.5mm² wire
| Q6
10 meters long, 1.5mm² wire
## Footnote
The resistance of a conductor is inversely proportional to its cross sectional area
59
If three resistors of 3 ohms, 5 ohms and 22 ohmns are connected in series in a 28v circuit, how much current will flow through the 3 ohms resistor?
| Q11
0.93 amperes
| V = IR
60
Three parallel connected resistors each of value 90 ohms, the total resistance is;
| Q12
30 ohms
61
The total amount of resistance in this circuit is
(Q13 DIA)
10 ohms
1/Rt= 1/Rn+1/Rn
62
# came up
The tolerance of a resistor is normally shown through which method?
| Q15
Using the last colour band on the resistor.
63
A resistor with a positive temperature coefficent means that, as the temperature increases, its resistance...?
## Footnote
Q16
Increases
64
Data plate of electric motor says, U 230v DC and 10 Ohms, what power is needed?
| Q18
5.29kw
| P=VI
65
A resistor of 120kiloOhm is connected in series with a 50 microfarad capacitor. A voltage of 10v is applied. What will the voltage be after 6 seconds?
| Q21
6.3V
| T=RC, 63% charge
66
The ability for electric fields to flow thorugh a particular substance is determined by its?
| Q22
Permittivity
67
# came up
When a number of capacitors are connected in series, the total capacitance of the network is always...?
| Q23
Of less capacitance than the value of the smallest capacitor
68
Through which material will magnetic lines of force pass most easily?
1. Copper
2. Iron
3. Aluminium
| Q24
Iron
| pg 386
69
Magnetomotive force is obtained by a product of...?
| Q26
the current and number of turns on a coil
| pg 415
70
The RMS value of an AC voltage is...
| Q28
0.707 times the peak voltage
71
# came up exactly
A relay with a moving core is usually know as a?
| Q29
Solenoid
72
How can the rotation of a DC shunt motor be reversed?
| Q31
By reversing the feild winding connections
73
The direction of rotation of a DC motor can be reversed by..?
| Q32
Reversing the current flow through either the exciter windings OR armature
74
Armature reaction is
| Q33
Distortion of the main magnetic field
75
A 3 phase AC circuit is star connected, the phase to phase voltage is 1700V, what is the voltage between phase and neutral?
| Q34
1700x1/√3
76
# came up
What is the phase relationship between voltage and current in an inductor? (coil=inductor)
| Q35
Voltage is 90 degrees before the current
CIVIL
77
# came up
What is the RMS value of a sine voltage of max value 100v?
| Q36
70.7v
| x0.707
78
# came up
In an inductor, the voltage and the current...
(relationship) (coil=inductor)
| Q37
The current lags the voltage by 90 degrees
CIVIL
79
If frequency increases, XC
(capacitive resistance)
| Q38
Decreases
80
Inductive reactance is a function of...?
| Q39
Frequency
| pg 625
81
A tranformer has 12v at the secondary winding. Given that the primary winding has 2000 turns and the secondary 100 turns, what is the primary voltage?
| Q40
240v
82
What happens to current in a voltage step-up transformer with a ratio of 1 to 4?
| Q42
Current decreases by a ratio of 1;4
83
The RC circuit that blocks all frequencies below the cut off frequency is called a
| Q43
High-pass filter
84
The basic function of a filter in a circuit is;
To pass some frequencies and block others
85
Brushless AC generators use diodes mounted in the
Rotor
(NOT IN NOTES)
86
A three phase AC generator feeds a delta connected balanced load with 200v between phases, the current on this line is 5amps and the power factor is 0.75. The power consumption of the load is...?
| q46
1.299kW
When using delta 3 phase, Remember to multiply by root 3
87
# came up
An induction motor
1. Has AC applied to its rotor
2. Has AC applied to rotor and stator
3. Does not have its rotor connected to any power source
Does not have its rotor connected to any power source
(AC power is provided to the stator only)
88
# EXAM QUESTION
Why is the core of a magnet made of a soft iron core as apposed to harder substance?
Soft iron is easier to magnetise
89
# EXAM QUESTION
Why will a battery not freeze until a very low temperature?
Because of the electrolyte composition
91
Faradays law states
92
In a coil, what is the relation between current and voltage?
Voltage leads the current
CIVIL, coil is an inductor
93
Two resistors of value 100ohms and 200ohms are connected in series across a 30 volts supply.
What is the power consumed across the 200 ohms resistor?
R total=100Ω+200Ω=300Ω
I= V/R total = 30V/300Ω=0.1A
P=I^2×R
P=(0.1)2×200=0.01×200=2W
94
If two negatively charged bodies are brought into close proximity, the forces between the two bodies will..?
Increase
95
The wire in a wire-wound resistor is wound around a...?
Cermatic/Porcelain
95
A wheatstone bridge circuit is constructed of...?
3 fixed resistors, 1 variable
