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AC CIRCUITS EXCEL SOURCE Flashcards

PROBLEMS AND OBJECTIVE TYPE QUESTIONS (145 cards)

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1
Q
  1. An incandescent lamp load generally
    considered to be made up of resistors
    take 4.8 kW from a 120 V AC
    source. What is the instantaneous
    maximum value of power?
    A. 4800 W C. 2400 W
    B. 480 W D. 9600W
A
  1. D. 9600 W
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2
Q

The p.f. of an RC circuit is ___.
A. often zero
B. between zero and 1
C. always unity
D. between zero and -1

A
  1. B. between zero and 1
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3
Q
  1. A 25-ohm resistor connected in series
    with a coil of 50 ohms resistance and
    150 mH inductance. What is the
    power factor of the circuit?
    A. 85% C. 80%
    B. 90% D. 75%
A
  1. C. 80%
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4
Q
  1. The length of time between a point in
    one cycle and the same point in the
    next cycle of an ac wave is the:
    A. Frequency C. Magnitude
    B. Period D. Polarity
A
  1. B. Period
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5
Q
  1. Which of the following can vary with
    ac, but not with dc?
    A. Power C. Voltage
    B. Frequency D. Magnitude
A
  1. B. Frequency
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6
Q
  1. Three loads, units A, B, and C are
    connected in parallel and take
    currents that are respectively 12, 10,
    and 15 A respectively. Assuming Ia
    to be the reference phasor. Ib leads by
    30o
    and Ic lags behind Ia by 65 degrees
    ,
    calculate the total (resultant) current.
    A. 28.33 A C. 30.21 A
    B. 26.46 A D. 32.10 A
A
  1. A. 28.33 A
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7
Q
  1. If R increases in an RC circuit, but
    the capacitance and the frequency are
    nonzero and constant, then the vector
    in the RC plane will:
    A. Get longer and rotate clockwise
    B. Get longer and rotate
    counterclockwise
    C. Get shorter and rotate clockwise
    D. Get shorter and rotate
    counterclockwise
A
  1. B. Get longer and rotate
    counterclockwise
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8
Q
  1. A 50 and 100-microfarad capacitors
    are connected in series and across a
    100 sin (ωt + 30o
    ) voltage. Write the
    equation of the current.
    A. 1.26 sin (ωt + 120o)
    B. 1.26 sin (ωt + 90o)
    C. 5.65 sin (ωt + 120o)
    D. 5.56 sin (ωt + 90o)
A
  1. A. 1.26 sin (ωt + 120o
    )
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9
Q
  1. On a spectrum analyzer, a pure ac
    signal, having just one frequency
    component, would look like:
    A. A single pip
    B. A perfect sine wave
    C. A square wave
    D. A sawtooth wave
A
  1. A. A single pip
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10
Q

10.If admittance Y = 0.06 – j0.08 mho,
then conductance G equals,
A. – 0.06 C. 0.06
B. 0.08 D. – 0.08

A
  1. C. 0.06
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11
Q
  1. Three-phase ac:
    A. Has waves that add up to three
    times the originals
    B. Has three waves, all of the same
    magnitude
    C. Is what you get at a common wall
    outlet
    D. Is of interest only to physicists
A
  1. B. Has three waves, all of the same
    magnitude
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12
Q
  1. A parallel circuit consists of a
    resistor having a conductance of 4
    mhos, an inductive reactor having a
    susceptance of 8 mhos and a
    capacitive reactor having a
    susceptance of 5 mhos. What is the
    impedance of the circuit?
    A. 0.11 + j0.13
    B. 0.13 + j0.11
    C. 0.12 + j0.16
    D. 0.16 + j0.12
A
  1. D. 0.16 + j0.12
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13
Q
  1. The period of an ac wave is:
    A. The same as the frequency
    B. Not related to the frequency
    C. Equal to 1 divided by the
    frequency
    D. Equal to the amplitude divided by
    the frequency
A
  1. C. Equal to 1 divided by the
    frequency
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14
Q
  1. An industrial coil has a resistance of
    32 ohms and a reactance of 24 ohms
    and rated 440 volts at 60 Hz. A
    factory will connect the coil to a 440
    V, 50 Hz supply. Solve for the value
    of a series resistor needed to avoid
    overcurrent condition.
    A. 2.07 Ω C. 2.64 Ω
    B. 2.44 Ω D. 2.25 Ω
A
  1. C. 2.64 Ω
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15
Q
  1. A triangular wave:
    A. Has a fast rise time and a slow
    decay time
    B. Has a slow rise time and a fast
    decay time
    C. Has equal rise and decay rates
    D. Rises and falls abruptly
A
  1. C. Has equal rise and decay rates
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16
Q
  1. A non-inductive resistor R is
    connected in parallel with an
    inductive reactance of 10 ohms. The
    combination is then connected in
    series with a capacitive reactance of
    5 ohms. The whole combination is
    connected across a 100-V, 60 Hz AC
    source. If R is equal to 5 ohms, solve
    for the voltage across the parallel
    combination.
    A. 87.53 V C. 88.34 V
    B. 89.44 V D. 91.87 V
A
  1. B. 89.44 V
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17
Q

17.If two waves have the same
frequency and the same amplitude,
but opposite phase, the composite
wave is:
A. Twice the amplitude of either
wave alone
B. Half the amplitude of either wave
alone
C. A complex waveform, but with
the same frequency as the
originals
D. Zero

A
  1. D. Zero
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18
Q

18.If R increases in an RC circuit, but
the capacitance and the frequency are nonzero and constant, then the vector
in the RC plane will:
A. Get longer and rotate clockwise
B. Get longer and rotate
counterclockwise
C. Get shorter and rotate clockwise
D. Get shorter and rotate
counterclockwise

A
  1. B. Get longer and rotate
    counterclockwise
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19
Q
  1. An advantage of ac over dc in utility
    applications is:
    A. Ac is easier to transform from one
    voltage to another
    B. Ac is transmitted with lower loss
    in wires
    C. Ac can be easily gotten from dc
    generators
    D. Ac can be generated with less
    dangerous by-products
A
  1. A. Ac is easier to transform from
    one voltage to another
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20
Q
  1. A 240 V, 25 Hz sinusoidal generator
    is connected to a 20 ohms resistor.
    Determine the instantaneous current
    when elapsed time is 0.01 second.
    A. 15.43 A C. 16.30 A
    B. 16.97 A D. 12.00 A
A
  1. B. 16.97 A
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21
Q
  1. As the frequency of a wave gets
    lower, all other things being equal,
    the value of XC for a capacitor:
    A. Increases negatively
    B. Decreases negatively
    C. Does not change
    D. Depends on the current
A
  1. A. Increases negatively
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22
Q
  1. Each impedance R + jXL:
    A. Corresponds to a unique point in
    the RL plane
    B. Corresponds to a unique
    inductance reactance
    C. Corresponds to a unique
    resistance
    D. All of the above
A
  1. D. All of the above
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23
Q
  1. An impedance draws a current i = 10
    cos (ωt – 30o) from a voltage v = sin
    ωt. What is the maximum power?
    A. 2200 W C. 1100 W
    B. 190.5 W D. 1320 W
A
  1. C. 1100 W
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24
Q
  1. A resistor of 6 ohms and an unknown
    impedance coil in series draws 12 A
    from a 120-V, 60-Hz line. If the real
    power taken from the line is 1152
    watts, what is the coil inductance?
    A. 15.9 mH C. 10.0 mH
    B. 20.0 mH D. 1.59 mH
A
  1. A. 15.9 mH
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25
25. Two waves are in phase coincidence. One has a peak value of 3V and the other a peak value of 5V. The resultant will be: A. 8 V peak, in phase with the composites B. 2 V peak, in phase with the composites C. 8 V peak, in phase opposition with respect to the composites D. 2 V peak, in phase opposition with respect to the composites
25. A. 8 V peak, in phase with the composites
26
26.In vector diagrams, the progression of time is sometimes depicted as: A. Movement to the right B. Movement to the left C. Rotation counterclockwise D. Rotation clockwise
26. C. Rotation counterclockwise
27
27. A capacitance is connected to a 115 V, 25 Hz mains and takes 5 A. What current will it take when the capacitance and the frequency are both doubled? A. 2.5 A C. 5 A B. 20 A D. 10 A
27. B. 20 A
28
28. A resistance of 5 ohms is connected in series with a capacitor of 442.1 μF. The combination is then connected in parallel with an inductance of 21.22 mH. Solve for the resultant current if the circuit is connected across a 120 V, 60 Hz AC source. A. 9.44 A C. 10.68 A B. 11.29 A D. 10.34 A
28. D. 10.34 A
29
29. Given three impedances: Z1 = 10 + j0, Z2 = 3 + j4 and Z3 = 8 – j6. Impedance Z2 and Z3 are connected in parallel and that combination is connected in series with impedance Z1 across 120 V single-phase 60 Hz source. Find the total power drawn by the impedances. A. 1008 W C. 1204 W B. 1038 W D. 1103 W
29. A. 1008 W
30
30.In a purely reactive circuit, the phase angle is ___. A. increasing C. decreasing B. 0 degrees D. 90
30. D. 90
31
31.If e = 100 sin (wt + 30o) – 50 cos 3wt+ 25 sin (5wt + 150o) and i = 20 sin (wt + 40o) + 10 sin (3wt + 30o) – 5 sin (5wt – 50o). Calculate the power in watts. A. 1177 C. 918.5 B. 1043 D. 922.4
31. C. 918.5
32
32.In a purely resistive circuit, the phase angle is ____. A. increasing C. decreasing B. 0 degrees D. – 90
32. B. 0 degrees
33
33.In vector diagrams, the progression of time is sometimes depicted as: A. Movement to the right B. Movement to the left C. Rotation counterclockwise D. Rotation clockwise
33. C. Rotation counterclockwise
34
34. A series circuit composed of a 0.2 henry inductor and a 74-microfarad capacitor is connected to a 60 V variable frequency source. At what frequency is the current be 4 amperes with a lagging power factor? A. 50 Hz C. 51 Hz B. 48 Hz D. 49 Hz
34. B. 48 Hz
35
35.In vector diagrams, the progression of time is sometimes depicted as: A. Movement to the right B. Movement to the left C. Rotation counterclockwise D. Rotation clockwise
35. C. Rotation counterclockwise
36
36. What is the wavelength of the carrier frequency of 100 MHz? A. 3.0 m C. 7.5 m B. 1.5 m D. 6.0 m
36. A. 3.0 m
37
37. The impedance vector 0 – j22 represents: A. A pure resistance B. A pure inductance C. A pure capacitance D. An inductance combined with a resistance
37. C. A pure capacitance
38
38. Across a 230 V, 60 Hz powers supply is a 15-ohm non-inductive resistor. What is the equation of the resulting current? A. 21.68 sin 377t B. 26.55 sin 377t C. 15.33 sin 377t D. 28.16 sin 377t
38. A. 21.68 sin 377t
39
39. Each point in the RL plane: A. Corresponds to a unique resistance B. Corresponds to a unique inductance C. Corresponds to a unique combination of resistance and inductive reactance D. Corresponds to a unique combination of resistance and inductance
39. C. Corresponds to a unique combination of resistance and inductive reactance
40
40. Three impedances Za = 3+ j4 ohms, Zc = 4 – j4 ohms and Zr = 0 + j3 ohms are connected in parallel. Solve for the PF of the combination. A. 0.653 leading B. 0.554 lagging C. 0.503 leading D. 0. 620 lagging
40. B. 0.554 lagging
41
41.In vector diagrams, length of the vector represents: A. Average amplitude B. Frequency C. Phase difference D. Peak amplitude
41. D. Peak amplitude
42
42. A capacitor is rated 100 kVAR, 380 volts, 50 Hz. What will its rating be at 60 Hz, 220 volts? A. 50 kVAR B. 40 kVAR C. 90.9 kVAR D. 57.7 kVAR
42. B. 40 kVAR
43
43. A sine wave: A. Always has the same general appearance B. Has inductance C. Is in the same phase as a cosine wave D. Rises very fast, but decays slowly
43. A. Always has the same general appearance
44
44.If the resistance R and the inductance reactance XL both vary from zero to unlimited values, but are always in the ratio 3:1, the points in the RL plane for all the resulting impedances will fall along: A. A vector pointing straight up B. A vector pointing “east” C. A circle D. A ray of unlimited length
44. D. A ray of unlimited length
45
45.If two waves have the same frequency and the same phase, the composite wave: A. Has a magnitude equal to the difference between the two originals B. Has a magnitude equal to the sum of the two originals C. Is complex, with the same frequency as the originals D. Is zero
45. B. of the two originals
46
46. Which of the following is not a general characteristic of an ac wave? A. The wave shape is identical for each cycle B. The polarity reverses periodically C. The electrons always flow in the same direction D. There is a definite frequency
46. C. The electrons always flow in the same direction
47
47. A resistor and a capacitor are connected in series across a supply of 250 V. When the supply frequency is 50 Hz the current in the circuit is 5 A. When the supply frequency is 60 Hz, the current is 5.8 A. Find the value of the capacitance. A. 58.3 μF C. 60.2 μF B. 69.1 μF D. 70.2 μF
47. B. 69.1 μF
48
48.In a perfect sine wave, the pk-pk value is: A. Half of the peak value B. The same as the peak value C. 1.414 times the peak value D. Twice the peak value
48. D. Twice the peak value
49
49. A circuit consists of Xa = j5 ohms, Xc = - 5j ohms and R = 5 ohms all are connected in parallel. Find the equivalent impedance. A. 5.5 Ω C. 5.0 Ω B. 4.8 Ω D. 5.2 Ω
48. D. Twice the peak value
50
50. A 1-hp, 220 V, 60 Hz capacitor-start motor has main and auxiliary winding impedance at starting of 3.5 + j2.5 ohms and 8.6 + j2.5 ohms, respectively. Determine the value of the starting capacitance that will place the main and auxiliary winding currents 90o apart at starting. A. 186.75 μF B. 174.35 μF C. 182.43 μF D. 170.67 μF
50. C. 182.43 μF
51
1. The rms value of sine wave is equal to _____. A. 0.637 max value B. 0.606 max value C. 0.707 max value D. 1.414 max value
1. C.0.707 max value
52
2. The instantaneous equation of a voltage is given as 325.3 sin(377t) volts. What is the rms value of the voltage? A. 325.3 volts B. 460 volts C. 212 volts D. 230 volts
2. D. 230 volts
53
3. The instantaneous value of a voltage is given as 100sin377t volts. What is its average value? A. 63.7 volts B. 70.7 volts C. 50 volts D. 122 volts
3. A. 63.7 volts
54
4. The average value of a sinusoidal current is 100 amperes. What is its rms value? A. 64 A B. 141.4 A C. 88.8 A D. 111 A
4. D. 111 A
55
5. What is the complex expression for a given alternating current, i = 250sin(ωt+25°)? A. 227+j106 A B. 160 – j75 A C. 227 – j106A D. 160+j75 A
5. D. 160+j75 A
56
6. The voltage in a circuit follows the law v = 200sinωt. If the frequency is 50Hz, how long will it take for the voltage to rise to 120 volts? A. 0.002 s B. 0.02 s C. 0.2 s D. 2 s
6. A. 0.002 s
57
7. The rms value of a sinusoidal AC current is equal to its value at an angle of _____ degrees. A. 30 B. 45 C. 60 D. 90
7. B. 45
58
8. The rms value and mean value is the same in the case of _____. A. Triangular wave B. Cosine wave C. Square wave D. Half wave rectified sine wave
8. C. Square wave
59
9. An ac voltage of 60Hz has a maximum value of 100V. Its value after 1/300 second after the instant the current is zero will be _____. A. 25 V B. 50 V C. 95.1 V D. 77.9 V
9. C. 95.1 V
60
10. An alternating voltage is given by v = 30 sin314t. What is the time taken by the voltage to reach 30 volts for the first time? A. 0.005 s B. 0.02 s C. 0.03 s D. 0.1 s
10. A. 0.005 s
61
11. What is the form factor of half wave rectified sinusoidal wave? A. 1.57 B. 1.11 C. 1.0 D. 1.15
11. A. 1.57
62
12. A circuit carries a direct current of 10 amperes on which is super imposed in a sinusoidal component of 5 ampere maximum value. What is the value of the rms effective current? A. 8.2 A B. 10.6 A C. 12.74 A D. 14.14 A
12. B. 10.6 A
63
13. A purely resistive circuit has an impedance of ____ A. Z=R+jXL B. Z= R-jXC C. Z=R D. Z=R+j(XL-XC)
13. C. Z=R
64
14. In a purely inductive circuit _____. A. the current leads the voltage by 90 degrees. B. the is in phase with the voltage C. the current can either lag or lead the voltage by 90 degrees. D. the current lags the voltage by 90 degrees.
14. D. the current lags the voltage by 90 degrees.
65
15. Which of the following circuit component opposes the change in the circuit voltage? A. Resistance B. Inductance C. Capacitance D. Impedance
15. C. Capacitance
66
16. Actual power is zero in _____. A. a purely inductive circuit B. a purely resistive circuit C. an impedance that has a resistance of 10 ohms and capacitance of 10 microfarads D. an impedance that has a resistance of 10 ohms and inductance of 10 millihenry.
16. A. a purely inductive circuit
67
17. A 240-volt, 60Hz, source, is supplying an impedance of 3.5+j2 ohms. What will be the value of the current when the resistance is reduced to by 50%? A. 55 A B. 90 A C. 72.4 A D. 66.7 A
17. B. 90 A
68
18. What is the reactance of a 50mH inductance connected to a 100-volt source, 50Hz? A. 12.8 ohms B. 18.85 ohms C. 37.7 ohms D. 15.7 ohms
18. D. 15.7 ohms
69
19. A 60mH inductance is connected in series with a 10-ohm resistor. If the voltage is 220 volts, 60Hz, what is the current drawn? A. 6 A B. 8 A C. 10 A D. 12 A
19. B. 8 A
70
20. Determine the power angle in the series circuit which consist of R=25 ohms and L=0.2H across a power supply of 220V, 60Hz. A. 71.7 degrees B. 46.6 degrees C. 32.3 degrees D. 56.2 degrees
20. A. 71.7 degrees
71
21. What is the reactance of an ac circuit having 20 ohms inductive reactance and 10 ohms capacitive reactance? A. 5 ohms B. 10 ohms C. 20 ohms D. 30 ohms
21. B. 10 ohms
72
22.Find the value of impedance which requires -1540 VAR at 230V and 10A. A. 17.4 – j15 ohms B. 16.8 – j15.7 ohms C. 17.52 – j 14.5 ohms D. 17.09 – j 15.4 ohms
22. D. 17.09 – j 15.4 ohms
73
23. A single-phase circuit is supplied from a 220V, 60Hz, source. It draws 2000 watts of power and 10 amperes of current. What is the power factor of the circuit? A. 90.9% B. 72.5% C. 82.8% D. 41.14%
23. A. 90.9%
74
24. The input of an AC circuit having power factor of 0.8 lagging is 40kVA. The power drawn by the circuit is _____. A. 12 kW B. 20 kW C. 32 kW D. 36 kW
24. C. 32 kW
75
25. The apparent power drawn by an AC circuit is 10kVA and active power is 8kW. The reactive power in the circuit is _____. A. 4 kVAR B. 8 kVAR C. 6 kVAR D. 12 kVAR
25. C. 6 kVAR
76
26. A coil is connected across a 200V,50Hz supply and takes a current of 10A. The loss in the coil is 1000W. The impedance and resistance of the coil are _____. A. 10 & 8 B. 15 & 8 C. 10 & 10 D. 20 & 10
26. D. 20 & 10
77
27. An electric iron designed for 110V AC supply was rated at 500 Watts. It was then connected across 220V supply. Assuming an ideal scenario, what will be the new wattage? A. 2000 Watts B. 2500 Watts C. 500 Watts D. 250 Watts
27. A. 2000 Watts
78
28. A 40-micro-farad capacitor, 100- millihenry inductance and a 20-ohm resistance are connected in series. At 220-volt,60Hz, what is the current of the circuit? A. 6.3 Amperes B. 4.47 Amperes C. 5.78 Amperes D. 7.2 Amperes
28. A. 6.3 Amperes
79
29. A coil connected in series with a resistance are supplied from a 240- volt, 60Hz source. The voltage across the resistance and the coil are both equal to 160 volts. If the circuit draws 1 Ampere from the source, what will be the value of coil resistance? A. 20 B. 60 C. 30 D. 45
29. A. 20
80
30.For a series RLC circuit with elements R=50Ω, L=8mH, C=2.22μF. What is the equivalent impedance of this circuit at 796Hz? A. 50 + j50 ohms B. 50+j120 ohms C. 50 – j50 ohms D. 50 – j120 ohms
30. C. 50 – j50 ohms
81
31. In a series resonant circuit, the impedance of the circuit is _____. A. maximum B. minimum C. zero D. approaching infinity
31. B. minimum
82
32. In a series circuit on resonance, the following will occur except _____. A. V=VR B. XL=XC C. VL=VR D. Z=R
32. C. VL=VR
83
33. At resonance, a series RLC circuit has A. ω 2LC=1 B. ωLC=1 C. ωL2C=1 D. ω 2L 2C=1
33. A. ω2LC=1
84
34. An 8Ω resistance, 16Ω inductive reactance and an unknown capacitor are connected in series across a 100V, 50Hz supply. The current drawn by the circuit is 12.5 A. The value of capacitance of the capacitor at resonance is A. 50 μF B. 150 μF C. 200 μF D. 250 μF
34. C. 200 μF
85
35. Impedance ZC is connected in series to the two impedances ZA=10+j10Ω and ZB=20 – j30Ω that are connected in parallel. What should be the value of impedance ZC to produce resonance? A. – j13.1 B. j13.1 C. j5.4 D. – j5.4
35. D. – j5.4
86
36. A 100-pF capacitor is connected in series with a coil having inductance of 100 μH and a resistance of 100Ω. At what frequency will this network have a unity pf? A. 1.3 MHz B. 1.5 MHz C. 1.23 MHz D. 1.6 MHz
36. D. 1.6 MHz
87
37. The resistance of 50 ohms, inductance of 50 mH and capacitance of 50 micro-farads are connected in series across a 240-volt source. What is the resonant frequency? A. 100 Hz B. 50 Hz C. 1000 Hz D. 37.7 Hz
37. A. 100 Hz
88
38. Two impedances Z1=2+j4 and Z2=R+j0 are connected in parallel. Determine R so that the pf of the circuit is 0.9 lagging. A. 2.4 ohms B. 3.2 ohms C. 4.2 ohms D. 6.5 ohms
38. B. 3.2 ohms
89
39. A source of 200 volts rms is supplying a resistance of 50 ohms in parallel with a 150 mH inductor. To what angle is the current lag or lead? A. 41.48 leading B. 46.7 lagging C. 46.7 leading D. 41.48 lagging
39. D. 41.48 lagging
90
40. Three loads are connected in parallel across 1400V rms, 60Hz, single phase supply. Load 1: Inductive load, 125kVA at 0.28 pf Load 2: Capacitive load, 10kW and 40kVAR Load 3: Resistive load, 15kW What is the total power factor? A. 60% lagging B. 60% leading C. 48% lagging D. 75% leading
40. A. 60% lagging
91
41. A single-phase induction motor is rated 5Hp, 75% power factor and 220 volts. What approximate size of capacitor is necessary to raise the power factor to about 95%? A. 2 kVAR B. 2.5 kVAR C. 3 kVAR D. 3.5 kVAR
41. A. 2 kVAR
92
42. A load of 10,000 kVA, 85% power factor lagging is connected to a 13,200-volt, line. How much reactive power is needed to correct the power factor to 97% lagging? A. 5,160 kVAR B. 2, 600 kVAR C. 3,140 kVAR D. 4,750 kVAR
42. C. 3,140 kVAR
93
43. A 5Hp, 220V, 60Hz, single-phase induction motor, operates at an efficiency and power factor or 0.88 and 0.8 respectively. What capacitance should be connected across the motor in order for the feeder supplying this motor to operate at unity power factor? A. 321.55 μF B. 163.62 μF C. 174.22 μF D. 112.86 μF
43. C. 174.22 μF
94
44. A plant has a load of 290 kW with an average power factor of 70%. The owner requests you to correct the power factor to reduce its power consumption. How much reactive power in kVAR is required to increase the power factor to 90%? A. 155.39 B. 162.75 C. 150.34 D. 154.58
44. A. 155.39
95
45.Poor power factor results to the following except _____. A. Reduces load handling capacity of electrical system. B. Overloads alternators, transformers and distribution lines. C. Results to additional power losses in the line. D. Results into lesser voltage drop across the lines.
45. D. Results into lesser voltage drop across the lines.
96
The length of time between a point in one cycle and the same point in the next cycle of an ac wave is the: A. Frequency B. Magnitude C. Period D. Phase
Period
97
What is the term used to identify an AC voltage that would cause the same heating in a resistor as a corresponding value of DC voltage? A. Root mean sqaure B. Average Value C. Maximum Value D. Size
Root mean square (RMS)
98
Two relays each with 20 ohms resistance and 0.16 H inductance are connected in series. What is the equivalent impedance?
40 + j120.63
99
The ohmic resistance of a large magnetic contactor is measured to be 20 Ω. A 230 V is impressed on the contractor and the current is taken as 3.2 A. Neglecting core loss, determine the inductance of the contactor. AC 1 - EXAMS
183 mH
100
5 What is the wavelength of a carrier wave with a frequency of 100 MHz?
3 m
101
6 Across a 230 V, 60 Hz power supply is a 15 Ω non-inductive resistor. What is the equation of the voltage and the resulting current?
e = 325.5 sin 377t, i = 21.6 sin 377t
102
7 A sinusoidal current wave has a maximum value of 12.7 A. What is the average value of one-half cycle?
8.085
103
8 A voltage wave is given by v(t) = 100 sin 314t. How long does it take this waveform to complete one-fourth cycle?
5 ms
104
9 A resistor and a coil are connected in series with a voltage source. If the voltage across the coil is 10 sin (866t + 70°) V and the current flowing thru the resistor is 2 cos (866t - 80°) A, what is the resistance of the coil?
5 Ω
105
10 What part of an alternating current is defined as a current which will produce the same heating effect as an equivalent direct current?
Effective value
106
11 The period of an ac wave is:
Equal to 1 divided by the frequency
107
12 If two waves have the same frequency and the same phase, the composite wave:
Has a magnitude equal to the sum of the two originals
108
13 A rotating vector is known as a/an ______.
phasor
109
14 What is the complex expression for a given alternating current, i = 15 sin (ωt – 15°)?
14.49 – j3.88
110
15 The average value of a symmetrical alternating quantity, (such as a sine wave), is the average value measured over a ____ cycle.
half
111
16 Calculate the effective value of v(t) = 100 sin 400t + 50 sin 800t + 10 cos 1200t V.
79.4V
112
17 Which of the following can vary with ac, but not with dc?
Frequency
113
18 If two waves have the same frequency and the same amplitude, but opposite phase, the composite wave is:
Zero
114
19 A telephone circuit makes power available at a pair of terminals. The open circuit voltage across the terminals in 1 V and the impedance looking into the terminals is 500 – j500 Ω. What is the maximum power than can be drawn from the circuit?
0.001 W
115
20 What figure of merit describes the ratio of the maximum value and the rms value?
Peak factoR
116
21 A 240-V, 25 Hz sinusoidal generator is connected to a 20 Ω resistor. Determine the instantaneous current when elapsed time is 0.01 s.
16.97 A
117
22 On a spectrum analyzer, a pure ac signal, having just one frequency component, would look like:
A single pip
118
24 An advantage of ac over dc in utility applications is:
AC is easier to transform from one voltage to another
119
25 Two current sources deliver current to a common load. The first source delivers a current whose equation is 25 sin 100πt A while the second delivers a current whose equation is 15 cos 100πt A. What is the rms value of the current in the load?
20.6 A
120
1 The ohmic resistance of a coil is measured to be 5 Ω. A 115 V is impressed on the coil and the current is taken as 7.6 A. Determine the inductance of the circuit.
37.9 mH
121
2 As the number of turns in a coil increases, the current in the coil will eventually _____.
decrease to near zero
122
Each impedance R – jXc:
Represents a unique combination of resistance and reactance
123
What is the complex expression for a given voltage equation, v = 311 cos (ωt – 15°)?
56.917 + j212.417
124
5 What is the average value of a half-wave rectified sinusoidal curve with maximum value of 20 V?
6.366 V
125
6 If R increases in an RC circuit, but Xc is always zero, then the vector in the RC plane will:
Always point straight towards the right
126
7 A wire carries a current, i = 3 cos 314t A. What is the average current over 6 s?
0
127
8 Find the average voltage in a resistance R = 10 ohms if the current in series form is i = 10 sin ωt + 5 sin 3ωt + 2 sin 5ωt A.
80.311 V
128
9 As the number of turns in a coil increases, the reactance ______.
increases
129
10 A resistor and an inductor are connected in series with a voltage source. If the voltage across the coil is 10 sin 377t V and the current flowing thru the resistor is 2 cos (377t - 80°) A, what is the resistance?
4.7 Ω
130
11 Each point in the RC plane:
Corresponds to a unique combination of resistance and reactance
131
12 A series RL circuit has L = 2 mH and an impedance of 15 Ω. When a sinusoidal voltage is applied, the current lags the voltage by 45°. What is the value of the angular frequency?
5300 rad/s
132
13 If the dielectric material between the plates of a capacitor is changed, all other things being equal:
You can`t say what happens to Xc without more data
133
14 As the size of the plates in a capacitor increase, all other things being equal:
The value of Xc decreases negatively
134
15 Determine the effective value of v(t) = 150 + 100 sin ωt.
165.83 V
135
16 The effective voltage across a circuit element is 100 + j50 V and the effective current through the element is 4 + j3 A. Calculate the reactive power taken by the element.
100 VAR leading
136
17 As the frequency of a wave gets lower, all other things being equal, the value of Xc for a capacitor ______.
increases negatively
137
18 In an RC circuit, as the ratio of capacitive reactance to resistance, - Xc/R, gets closer to zero, the phase angle ________.
gets closer to 0 degrees
138
19 As the frequency of an ac wave gets lower, the value of X_L for a particular coil _____.
decreases
139
20 Each point in the RL plane:
Corresponds to a unique combination of resistance and inductive reactance
140
21 A circuit makes power available at a pair of terminals. The open circuit voltage across the terminals in 5 V and the impedance looking into the terminals is 500 – j500 Ω. What is the apparent power than can be drawn from the circuit?
0.035 VA
141
22 Which of the following effects are generally less for an alternating current than for a direct current?
Chemical effects
142
23 If R increases in an RC circuit, but the capacitance and the frequency are nonzero and constant, then the vector in the RC plane will:
Get longer and rotate counterclockwise
143
24 A series RL circuit has L = 0.02 H and an impedance of 17.85 Ω. When a sinusoidal voltage is applied, the current lags the voltage by 63.5°. What is the value of the angular frequency?
800 rad/s
144
25 An impedance draws a current i = 10 cos (ωt – 45°) A from a v = 220 sin ωt V. What is the apparent power?
1100 VA
145
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