DC CIRCUIS EXCEL SOURCE Flashcards

PROBLEMS AND OBJECTIVE TYPE QUESTIONS

1
Q

If a complex circuit is reduced to an equivalent circuit consisting of a single voltage source in series with a single resistor, this is an example of:

A

Thevenin`s theorem

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2
Q

Two equal resistors when connected in series across a supply draws a total power of 20 watts. What is the total power drawn when the two resistors are connected in parallel across the same supply?

A

20 W

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3
Q

Assuming a power source to have a fixed value of internal impedance, maximum power will be transferred to the load when:

A

The load impedance equals the internal impedance of the source.

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4
Q

Two resistors x and y are connected in parallel. Resistor x having a value of 6 ohms while resistor y being adjustable over a wide range. The parallel combination is then connected through a 2 – ohm line resistor to a 120 V DC source. For what resistance y will its power be the greatest?

A

1.5 Ω

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5
Q

A load is connected to a network. At terminals to which the load is connected, Rth = 10 Ω and Vth = 40 V. The maximum possible power supplied to the load is:

A

40 W

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6
Q

An ampere is how many abampere?

A

0.1

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7
Q

If a complex circuit is reduced to an equivalent circuit consisting of a single current source in parallel with a single resistor, this is an example of:

A

Norton`s theorem

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8
Q

In any closed loop in a network, the algebraic sum of the voltage drops (i.e. products of current and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop. This is known as:

A

Voltage law

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9
Q

In any network made up of linear resistances and containing more than one source of e.m.f., the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately. What is this theorem?

A

Superposition theorem

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10
Q

The current in any branch of a network is that which would result if an e.m.f. equal to the p.d. across a break made in the branch, were introduced into the branch, all other e.m.f.’s being removed and represented by the internal resistances of the sources.

A

Thevenin`s theorem

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11
Q

The power transferred from a supply source to a load is at its maximum when the resistance of the load is _______ the internal resistance of the source.

A

equal to

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12
Q

The terminal voltage of a battery is 11.8 V when delivering 20W of power to an external load resistor R. If the battery has an emf of 15V, find the internal resistance of the battery.

A

1.89 Ω

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13
Q

Three series resistors A, B and C are connected across a 120 – V supply. If the resistor A = 50 ohms and the voltage across resistor B = 40V. when the current is ½ A. what is the resistance in resistor C in ohms?

A

110 Ω

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14
Q

Under the conditions of maximum power transfer, a voltage source is delivering a power of 15 W to the load. What is the power generated by the source?

A

30 W

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15
Q

What is the efficiency for maximum power transfer?

A

50%

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16
Q

What is the first step in Norton’s theorem?

A

Short the load under test

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17
Q

What is the first step in Thevenin’s theorem?

A

Open the load under test

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18
Q

A conductor whose diameter is 0.175 inch has a resistance of 0.5 ohm. The wire is drawn through a series of dies until its diameter is reduced to 0.08 inch. Assuming the specific resistance of the material remains constant, what is the resistance of the lengthened conductor?

A

11.45 Ω

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19
Q

In a voltage divider network, the total resistance

A

Should be as small as the power supply will allow

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20
Q

Two (2) 115-V incandescent lamps A and B are connected in series across a 230-V source. If lamp A is rated at 75 watts and lamp B is rated 50 watts, determine the current by the series connection.

A

0.52 A

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21
Q

A process equipment contains 100 gallons of water at 25°C. It is required to bring it to boiling point in 10 minutes. The heat loss is estimated to be 5%. What is the kW rating of the heater?

A

208.47

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22
Q

The resistance of a transformer winding is 0.25 ohm at 25°C. When operating at full load, the temperature of the winding is 75°C. The temperature coefficient of resistance for copper is 0.00427 per °C. What is the winding resistance at full load?

A

0.298 Ω

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23
Q

Resistors are identified as to wattage by:

A

size

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24
Q

Derived units are obtained from various combinations of ______.

A

fundamental units

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25
Good engineering practice usually requires that a series-parallel resistive network be made:
From resistors that are all the same.
26
How much heat in kcal is produced by a current of 20 Amperes flowing for half of an hour in a circuit having a resistance of 6 ohms?
1032
27
The resistance of a copper wire at 30°C is 50 ohms. If the temperature coefficient of copper at 0°C is 0.00427°C^-1, what is the resistance at 100°C?
63.25
28
A copper bar is 1/2 inch by 4 inches by 20 feet. If the resistivity of the copper bar is 10.37 ohms-cmf, then what is the resistance of the copper bar?
8.1445 x 10ˉ⁵ Ω
29
If the reference point is changed to the earth point, then any voltage taken w.r.t. the earth is known as ____.
absolute potential
30
If a 220 V heater us used on 110 V supply, heat produced by it will be ____ as much
one-fourth
31
A wire with a smaller cross-sectional area will produce:
more heat
32
For a given line voltage, four heating coils will produce maximum heat when connected:
All in parallel
33
You have an unlimited supply of 1-W, 100-Ω resistors. You need to get a 100-Ω, 10-W resistor. This can be done most cheaply by means of a series-parallel matrix of:
4 x 4 resistors
34
Scientific notation is a method _____.
all of the choices
35
What is the size of 250MCM in square millimeter?
126.677
36
Four wires of same material, same cross – sectional area and same length when connected in parallel gives a resistance of 0.25 ohm. If the same four wires are connected in series, its effective resistance will be equal to ____.
4 ohms
37
A carbon resistor dissipates 60 watts of power from a 120-volt source at 20°C. How much power will be dissipated in the resistor at 120°C if connected across the same source? Assume that the temperature coefficient of carbon at 20°C is – 0.0005 per °C.
63.16 W
38
The power taken by a 500-ft resistive coil made of copper wire is 250 watts at 100 volts. The resistivity of copper is 10.4 ohm-cm per ft. Calculate the cross-sectional area of the coil in circular mil.
107
39
A 500 MCM ACSR cable has 37 strands. Determine the diameter in mils of each strand.
116.25
40
The resistance of two wires is 25 ohms when connected in series and 6 ohms when joined in parallel. The resistance of each wire is ____.
15 ohms and 10 ohms
41
Suppose you double the voltage in a simple dc circuit and cut the resistance in half. The current will become:
Four times as great
42
In a series-connected string of holiday ornament bulbs, if one bulb gets shorted out, which of these is most likely?
The current in the string will go up.
43
The presence of electric current is only made known by the effect if produces. Three important effects are:
C. generation, chemical & electric shock
44
It is a cell designed to produce electric current and can be recharged.
A. Secondary cell
45
Which is the best conductor of electricity? A. Aluminum B. Silver C. Copper D. Carbon
B. Silver
46
What is the other term for American Wire Gauge? A. JIS B. Brown & Sharpe C. DIN D. VDE
B. Brown & Sharpe
47
The Ampere – Hour capacity of a battery depends on ___ of the plates. A. Area B. Strength C. Thickness D. Distance
D. Distance
48
A 120-volt dc motor has 100A rated current. The allowable voltage drop is 3% of the input voltage. What is the maximum distance from the supply if the wire has a diameter of 316 mils and resistivity of wire is 10.4 ohm-cmf? A. 508 ft B. 316 ft C. 178 ft D. 203 ft
C. 178 ft
49
A one-meter rod of 2 cm diameter is drawn until its resistance is 100 times the initial resistance, its length afterward is ___. A. 8 m B. 100 m C. 12.5 m D. 10 m
D. 10 m
50
What is the area in circular mils (CM) of a conductor whose diameter is 0.25 inch? A. 65,000 B. 62,500 C. 70,000 D. 67,500
B. 62,500
51
A wire has a resistance of 1Ω. If the length is increased four times and the area is doubled the original, find the new resistance of the wire. A. 2 Ω B. 4 Ω C. 3 Ω D. 5 Ω
A. 2 Ω
52
An aluminum wire has a diameter of 2 mm. What must be the diameter of copper wire of the same resistance and length as aluminum? A. 1.33 mm B. 1.56 mm C. 1.45 mm D. 1.75 mm
B. 1.56 mm
53
A copper bar has a length of 20 ft, width of 4 inches and thickness of 0.5 inch. If the resistivity of copper is 10.37 ohm – cmf, what is the resistance of the bar? A. 81.4 µΩ B. 814 µΩ C. 8.14 µΩ D. 0.814 µΩ
A. 81.4 µΩ
54
A copper wire is drawn down such that its length is doubled but the volume remains the same, what is the new resistance of the wire? A. R/4 B. 2R C. R/2 D. 4R
D. 4R
55
The resistance of a copper wire at 30°C is 50 ohms. If the temperature coefficient of copper at 0°C is 0.00427 per °C, what is the resistance at 100°C? A. 63.24 ohms B. 58.2 ohms C. 24.5 ohms D. 72.26 ohms
A. 63.24 ohms
56
A copper wire whose resistivity is 0.0043/℃ at 0℃ has its resistance increased by 25% of its initial value at 20℃. What is the operating temperature? A. 56.8℃ B. 52.5℃ C. 83.1℃
C. 83.1℃
57
A 120V DC motor draws a current of 100A and is located 1000ft from the supply. If the diameter of the copper transmission line is 0.45 in, what must be the voltage supply? A. 128.32V B. 137.05V C. 129.82V D. 130.24V
D. 130.24V
58
An electric water heater has a rating of 1kW, 230V. The coil used as the heating element is 10 m long and has a resistivity of 1.724x10^-6 ohm – cm. Determine the required diameter of the wire in mils. A. 2.52 mils B. 2.43 mils C. 3.21 mils D. 1.36 mils
A. 2.52 mils
59
A hot resistance of an incandescent lamp is 10 ohms and the rated voltage is 50V. Find the series resistance required to operate the lamp from an 80V supply. A. 8 ohms B. 4 ohms C. 6 ohms D. 10 ohms
C. 6 ohms
60
What is the total resistance of eleven 33 kΩ resistors connected in parallel? A. 33 kΩ B. 3 kΩ C. 333 kΩ D. 11 kΩ
B. 3kΩ
61
Two Resistors R1 and R2 are connected in parallel. R1 is twice of R2. If the current in R1, is I, what is the current in R2?
A. I/2
62
Find the total resistance if 3Ω is connected in series with the parallel combination of 4Ω,5Ω and 20Ω
B. 5Ω
63
According to superposition theorem, in a network of linear resistances containing more than one generator (or source of emf), (a) the current flow at any point is the sum of all the currents which would flow at that point if each generator were considered separately, and (b) all the other generators replaced for the time being by resistances equal to their voltage drops. A. Only statement a is true. B. Only statement b is true. C. Both statements a and b are true. D. Both statements a and b are false.
A. Only statement a is true
64
It is a property of matter that results from the presence or movement of electric charge. A. Conductivity B. Electricity C. Admittance D. Electromagnetism
B. Electricity
65
These are materials that allow the essentially free passage of current when connected to a battery or other source of electrical energy. A. conductors B. Insulators C. semiconductors D. resistors
A. conductors
66
These are materials that possess low conductivity or offer a relatively high resistance to the flow of electric current. A. Conductors B. Insulators C. Semiconductors D. Resistors
B. Insulators
67
These are materials that have conductivity about midway between good conductors and good insulators. A. conductors B. insulators C. semiconductors D. resistors
C. semiconductors
68
It is the driving force behind current flow. A. voltage B. current C. resistance D. mmf
A. voltage
69
It is the motion or transfer of charges from one region of a conductor to another. A. voltage B. emf C. current D. mmf
C. current
70
It is the difference in electric potentials of two charged bodies. A. voltage B. current C. potential difference D. magnetomotive force
C. potential difference
71
Delta to wye or wye to delta transformation technique is applied to a ______ network. A. one-terminal B. two-terminal C. three-terminal D. complex
C. three-terminal
72
A load of 10 Ω was connected to a 12-V battery. The current drawn was 1.18 A. What is the internal resistance of the battery? A. 0.35 Ω C. 0.20 Ω B. 0.25 Ω D. 0.30 Ω
C. 0.20 Ω
73
Which of the following safety rules should be observed while working on live electric circuit? A. Keep yourself well insulated from earth ground. B. When making measurements in a live circuit place one hand behind your back or in your pocket. C. Make resistance measurements only in a live circuit. D. Both A and B.
D. Both A and B
74
“The sum of the emf’s and potential differences around a closed loop equals zero” is a consequence of: A. Newton’s third law B. Newton’s second law C. Conservation of energy D. Conservation of charge
C. Conservation of energy
75
A Barangay power station supplies 60 kW to a load over 2,500 ft, 100 mm2 , two-conductors copper feeder, the resistance of which is 0.078 ohm per 1000 ft. The bus bar voltage is maintained constant at 600 V. Determine the load current. A. 105 A C. 108 A B. 110 A D. 102 A
C. 108 A
76
A circuit of three resistors rated: 10 ohms, 15 ohms and 20 ohms are connected in delta. What would be the resistances of the equivalent wye connected load? A. 0.30, 0.23 and 0.15 ohm B. 3.0, 4.0 and 5.0 ohms C. 3.33, 4.44 and 6.66 ohms D. 5.77, 8.66 and 11.55 ohms
C. 3.33, 4.44 and 6.66 ohms
77
The hot resistance of an incandescent lamp is about _____ times its cold resistance. A. 10 C. 5 B. 50 D. 100
A. 10
78
A certain factory takes 250 A at 220 V. It is supplied through a pair of feeders 73.2 meters long and 1.29 cm2 in cross section. What voltage must be maintained at the supply end of the feeders? (Use  = 1.78 -cm) A. 111.5 V C. 333.5 V B. 222.5 V D. 444.5 V
B. 222.5 V
79
Four cubic meters of water is to be heated by means of four 1.5 kW, 230-V immersion heating elements. Assuming the efficiency of the heater as 90%, determine the time required boiling the water if the initial temperature is 20oC and if all four elements are connected in parallel. A. 71 hrs C. 63 hrs B. 69 hrs D. 66 hrs
B. 69 hrs
80
The equivalent wye element of 3 equal resistors each equal to R and connected in delta is A. R C. 3/2 R B. R/3 D. 3R
B. R/3
81
A coil of aluminum wire has a resistance of 50  when its temperature is 0°C. Determine its resistance at 100°C if the temperature coefficient of resistance of aluminum at 0°C is 0.0038/°C. A. 69 Ω C. 96 Ω B. 65 Ω D. 90 Ω
A. 69 Ω
82
What is the significance of connecting loads in parallel? A. It makes power consumption less B. It provides greater efficiency C. It increases the safety factor D. It allows independent operations of loads 14. A wire with a smaller cross-sectional area will produce
D. It allows independent operations of loads
83
5. Halving the cross-sectional area of a conductor will: A. Not affect the resistance. B. Quarter the resistance. C. Half the resistance. D. Double the resistance.
D. Double the resistance.
84
An aluminum wire has a resistance of 50  at 20°C. The wire is heated to a temperature of 100°C. Determine the resistance of the wire at 100°C, assuming that the temperature coefficient of resistance at 0°C is 0.004/°C A. 64.81 Ω C. 68.14 Ω B. 61.84 Ω D. 63.16 Ω
A. 64.81 Ω
85
.If the diameter of a wire is doubled, its current carrying capacity becomes A. one-fourth B. half C. twice D. four times
D. four times
86
Suppose you double the voltage in a simple dc circuit, and cut the resistance in half. The current will become: A. Four times as great B. Twice as great C. The same as it was before D. Half as great
A. Four times as great
87
In a series circuit, the current, I, is A. different in each resistor. B. the same everywhere. C. the highest near the positive and negative terminals of the voltage source. D. different at all, the largest resistance has
. B. the same everywhere
88
When a material becomes a superconductor, its resistivity becomes A. very small B. zero C. about 10% of normal value D. about 20% of normal value
B. zero
89
Two (2) 115-V incandescent lamps A and B are connected in series across a 230-V source. If lamp A is rated 75 watts and lamp B is rated 50 watts, determine the current drawn by the series connection. A. 0.52 A C. 0.64 A B. 0.48 A D. 0.57 A
A. 0.52 A
90
. A network contains linear resistors and ideal voltage sources. If values of all the resistors are doubled, then the voltage across each resistor is A. increase by 4 times B. half C doubled D. no change
D. no change
91
Which of the following is neither a basic physical law nor derivable from one? A. Ohm’s law B. Coulomb’s law C. Kirchhoff’s first law D. Kirchhoff’s second law
. A. Ohm’s law
92
A process equipment contains 100 gallons of water at 25oC. It is required to bring it to boiling in 10 minutes. The heat loss is estimated to be 5%. What is the kW rating of the heater? A. 125 kW C. 252 kW B. 50.5 k D. 207 kW
D. 207 kW
93
If a resistor in a series circuit becomes open, how much is the voltage across each of the remaining resistors that are still good? A. Each good resistor has the full value of applied voltage. B. The applied voltage is split evenly among the good resistors. C. 0 V. D. It cannot be determined.
C. 0 V.
94
A series circuit in which desired portions of the source voltage may be tapped off for use in equipment. A. Voltage selector B. Dividing network C. Voltage trap D. Voltage divider
D. Voltage divider
95
If a complex circuit is reduced to an equivalent circuit consisting of a single voltage source in series with a single resistor, this is an example of: A. Norton's theorem B. Thevenin's theorem C. Ohm's law D. Kirchhoff's law
B. Thevenin's theorem
96
Two resistances of 10 and 15 ohms, each respectively are connected in parallel. The two are then connected in series with a 5-ohm resistance. It is then connected across a 12-V battery, what are the current and power? A. 1.2 A, 17.28 W B. 0.96 A, 11.52 W C. 1.09 A, 13.1 W D. 1.5 A, 20.25 W
C. 1.09 A, 13.1 W
97
Twelve 1 Ω resistance are used as edges to form a cube. What is the resistance between two diagonally opposite corners of the cube? A. 5/6 Ω C. 1 Ω B. 6/5 Ω D. 3/2 Ω
A. 5/6 Ω
98
The polarity of a resistor’s voltage drop is determined by A. the direction of current through the resistor. B. how large the resistance is. C. how close the resistor is to the voltage source. D. how far away the resistor is from the voltage source
A. the direction of current through the resistor.
99
The electric energy required to raise the temperature of a given amount of water is 1000 kWh. If heat losses are 25%, the total heating energy required is _____ kWh. A. 1500 C. 1250 B. 1333 D. 1000
B. 1333
100
. The resistance of a 5 m length of wire is 600 . Determine the resistance of an 8 m length of the same wire. A. 960 Ω C. 690 Ω B. 906 Ω D. 609 Ω
C. Provides more powe
101
What is expected when two 20 k, 1 watt resistor in parallel are used instead of one 10 k, 1 watt? A. Provides higher current B. Provides less power C. Provides more power D. Provides wider tolerance
C. Provides more power
102
Twelve identical wires of resistance 6 ohms each are arranged to form the edge of a cube. A current of 40 mA is led into the cube at one corner and out at the other diagonally opposite corner. Calculate the potential difference developed between these corners. A. 0.20 V C. 0.28 V B. 0.22 V D. 0.24 V
A. 0.20 V
103
The superposition theorem is used when the circuit contains A. reactive elements B. active elements C. number of voltage sources D. single voltage source
C. number of voltage sources
104
.If a resistance element is part of two loops, how many voltage drops must be calculated for that component? A. Two C. One B. None D. Three
A. Two
105
In a normally operating parallel circuit, the individual branch currents are A. independent of each other. B. not affected by the value of the applied voltage. C. larger than the total current, IT. D. None of the above.
A. independent of each other
106
. What is a closed path made of several branches of the network called? A. Junction C. Node B. Terminal D. Loop
. D. Loop
107
What is expected when two 20 k, 1 watt resistor in parallel are used instead of one 10k, 1 watt? A. Provides higher current B. Provides less power C. Provides more power D. Provides wider tolerance
C. Provides more power
108
A 5-ohm resistance is connected in parallel with a 10-ohm resistance. Another set, a 6-ohm and 8-ohm resistances are also connected in parallel. The two sets are connected in series. What is the equivalent resistance? A. 6.76 ohms C. 9.25 ohms B. 14.4 ohms D. 21.2 ohms
A. 6.76 ohms
109
.In a parallel circuit, the individual branch currents are A. not related to the branch resistance values B. directly proportional to the branch resistance values C. inversely proportional to the branch resistance values. D. None of the above
C. inversely proportional to the branch resistance values.
110
In any network made up of linear resistances and containing more than one source of e.m.f., the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately. What is this theorem? A. Thevenin's theorem B. Norton's theorem C. Superposition theorem D. Millman's theorem
. C. Superposition theorem
111
Two resistors of 4 ohms and 6 ohms are connected in parallel. If the total current is 30 amperes, find the current through each. A. 17 A and 13 A B. 18 A and 12A C. 20 A and 30 A D. 16 A and 14 A
B. 18 A and 12A
112
If an emf in circuit A produces a current in circuit B, then the same emf in circuit B produces the same current in circuit A. This theorem is known as A. Maximum power transfer theorem B. Millman’s theorem C. Reciprocity theorem D. Norton’s theorem
C. Reciprocity theorem
113
. A 100- resistor, R1, and a 300- resistor, R2, are in parallel across a dc voltage source. Which resistor dissipates more power? A. The 300- resistor. B. Both resistors dissipate the same amount of power. C. The 100- resistor. D. It cannot be determined.
C. Reciprocity theorem
114
The power transferred from a supply source to a load is at its maximum when the resistance of the load is _______ the internal resistance of the source. A. lower than B. greater than C. equal to D. one-half of the
C. equal to
115
What If you have a 12-strand conductor and each strand has a radius (one-half the diameter) of 2/10 inch, what is the square mil area of the conductor? A. 1507965 sq. mil B. 1705965 sq. mil C. 1607955 sq. mil D. 1907565 sq. mil
A. 1507965 sq. mil
116
A 5- and 10- resistor are connected in series across a dc voltage source. Which resistor will dissipate more power? A. the 5- resistor. B. the 10- resistor. C. It depends on how much the current is. D. They will both dissipate the same amount or power.
B. the 10- resistor.
117
The first step in Thevenin's theorem is to: A. determine the short-circuit current B. open the load under test C. short the load under test D. determine the open-circuit current
B. open the load under test