1. | With regards to measuring current and voltage in an AC circuit, modern AC instruments are calibrated to read: |
2. | The current waveform for a purely resistive circuit: A. | leads the voltage by 90 ° |
B. | is in phase with the voltage |
C. | lags the voltage by 90° |
D. | alternately leads and lags the voltage |
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3. | Look at the following figure: Figure shows the voltage, current and power waveshapes for a purely resistive circuit supplied with a sinusoidal AC voltage. The waveshapes show that for a purely resistive circuit, the power curve: A. | has an average value equal to the area under the voltage wave |
B. | has a negative value when both the voltage and current are negative |
C. | completes two cycles for each complete cycle of current or voltage |
D. | will only have a negative value if the voltage and current are positive |
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4. | The power consumed by a purely resistive AC circuit can be determined using the following formula: P = Vrms x Irms In the formula, the symbol ‘P’ stands for the: A. | peak value of the power |
B. | RMS value of the power |
C. | maximum value of the power |
D. | average value of the power |
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5. | Figure shows a non-inductive resistor: The non-inductive effect is produced by winding: A. | half in a clockwise direction, and then the other half anticlockwise |
B. | coils for magnetic fields inside the inner core from a DC voltage |
C. | all the coils in the same direction to produce a self-induced voltage |
D. | the resistor with many turns of very fine a high resistance wire |
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6. | A pure resistance of 15 ohms has been connected across an AC power supply that generates a pure sinewave of 84.84 volts peak voltage. The average power consumed by this resistor will be approximately: |
7. | In an inductive AC circuit, the current is continually changing in value and direction, generating an induced EMF that will continually: A. | assist the change of current flow |
B. | assist a change in supply frequency |
C. | oppose the change of current flow |
D. | oppose the resistance of the circuit conductors |
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8. | Figure 3 shows the waveforms of the voltage and current in a purely inductive AC circuit: Using the voltage phasor as the reference, the current phasor: B. | is in phase with the voltage |
C. | leads and then lags by 90°E |
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9. | On AC, the change in current flow gives rise to an induced EMF that opposes the current flow. The effect of this current opposition is called: |
10. | The inductive reactance in an AC circuit can be calculated from the formula: XL = 2 π f L In the formula, the symbol ‘L’ stands for the: A. | length of the circuit in metres |
B. | inductance of the circuit in Henrys |
C. | low voltage value of the conductors |
D. | frequency of the supply in Hertz |
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11. | A coil has an inductance of 0.04 H. The inductive reactance of the coil at a frequency of 50 Hz will be: |
12. | A 230 V 50 Hz supply has been applied to a coil with an inductance of 0.15 H. The current in the circuit will be approximately: |
14. | Two inductors, one with an inductive reactance of 15 Ω, and the second with an inductive reactance of 10 Ω have been connected in series across a 230 V 50 Hz supply. The total inductive reactance will be: |
15. | Two inductors, one with an inductive reactance of 12 Ω, and the second with an inductive reactance of 8 Ω have been connected in parallel across a 230 V 50 Hz supply. The total inductive reactance will be: |
16. | Figure shows the voltage, current and power waveshapes for purely inductive circuit: The power waveshape shows that power is: A. | fed into the inductor continuously |
B. | continuously fed into the AC supply |
C. | only consumed by the inductor when both V & I are positive |
D. | alternately fed into and returned from the inductor |
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17. | A pure inductor with an inductive reactance of 150 W has been connected to a 230 V AC circuit. The average power consumed by this inductor is: |
18. | In a purely capacitive circuit the current: C. | is in phase with the voltage |
D. | leads and then lags the voltage |
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19. | Figure shows the waveshapes of voltage, current and power for a purely capacitive circuit: In this circuit, the current: A. | lags the applied voltage by 90°E |
B. | leads the applied voltage by 180°E |
C. | leads the applied voltage by 90°E |
D. | lags the applied voltage by 180°E |
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20. | The capacitive reactance of a capacitor can be determined using the formula: In the formula the symbol ‘f’ stands for the: B. | speed of the capacitor in m/s |
C. | current in the circuit in amperes |
D. | frequency of the supply in Hertz |
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21. | A 16 uF capacitor has been connected to a 230 V 50 Hz supply. The capacitive reactance of this capacitor in this circuit will be approximately: |
22. | When two capacitors are connected in series, the total capacitance: A. | is double the capacitance of any one |
D. | remains the same as the largest one |
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23. | When two capacitors are connected in parallel, the total capacitance: D. | equals the difference between the two |
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24. | An 22 uF capacitor has been connected in series with a 47 uF capacitor. The total capacitance of the combination is approximately: |
25. | An 16 uF capacitor has been connected in parallel with a 22 uF capacitor. The total capacitance of the combination is approximately: |
26. | Two 10 uF capacitors have been connected in parallel across a 230 V AC supply. The current drawn from the supply will be approximately: |
27. | The average power consumed by a purely capacitive circuit: A. | is maximum when the current is leading |
C. | equals the value of the supply voltage |
D. | will be minimum when the current is lagging |
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28. | Figure shows a resistor and an inductor connected in series across an AC supply: In this circuit, the current will: A. | be in phase with the voltage |
D. | always be a large value |
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29. | Figure shows the phasor diagram for an AC circuit with resistor and capacitor in series: The diagram shows that in this type of circuit, the current phasor: A. | lags the voltage across the capacitor |
B. | leads the voltage across the resistor |
C. | lags the voltage across the resistor |
D. | leads the supply voltage phasor |
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30. | In an AC circuit containing resistance, inductance and capacitance in series, the voltage drop across the inductor will: A. | be 180°E out of phase with the voltage drop across the capacitance |
B. | lead the voltage drop across the capacitance by 90°E |
C. | lag the voltage drop across the capacitance by 90°E |
D. | be in phase with the voltage drop across the capacitance |
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31. | The following phasor diagram is for a resistor, inductor and capacitor connected in series across an AC supply: The value of the phase angle F is: |
32. | The following formula can be used to determine the impedance of an AC circuit with resistance, inductance and capacitance in series: Z = √(R2 + (XL – XC)2) In the formula, the term XL stands for the value of the: A. | capacitive reactance in Ohms |
C. | inductive reactance in Ohms |
D. | reactance of the capacitor in microfarads |
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33. | A resistance of 50 Ω has been connected in series with an inductive reactance of 160 Ω and a capacitive reactance of 40 Ω. The impedance of the circuit will be: |
34. | In a parallel AC circuit, the voltage is: A. | common to all the components |
B. | equal to the sum of the branch voltages |
C. | larger than any branch voltage |
D. | equal the phasor sum of the branch currents |
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35. | Figure shows a resistor and inductor connected in parallel across an AC supply: In this circuit, the current through the inductor will: A. | lead the supply voltage |
B. | lag the supply voltage |
C. | be in phase with the supply voltage |
D. | equal the current through the resistance |
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36. | Figure shows a resistor connected in parallel, with an inductor and resistor in series, across an AC supply. In this circuit, the current through the inductor will: A. | lag the supply voltage by 90°E |
B. | lead the supply voltage by 90°E |
C. | lead the supply voltage by less than 90°E |
D. | lag the supply voltage by less than 90°E |
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37. | Figure 10 shows the phasor diagram for a resistor and capacitor connected in parallel across an AC supply: In this circuit, the current through the capacitor: A. | leads the supply voltage by 90°E |
B. | is in phase with the supply voltage |
C. | leads the supply voltage by approximately 30°E |
D. | lags the supply voltage by 90°E |
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38. | When drawing the phasor diagram for R, L and C in parallel across an AC supply, the reference phasor is normally the: A. | current through inductor |
C. | current through the capacitor |
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39. | In an, AC circuit with R, L and C in parallel, the total supply current is: A. | the arithmetic sum of the branch currents |
C. | the phasor sum of the branch currents |
D. | equal to the resistive current minus the inductive current |
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40. | A resistance of 57.5 Ω has been connected to a 230 V 50 Hz supply, in parallel with an inductive reactance of 57.5 Ω and a capacitive reactance of 230 Ω. The total supply current will be: |
41. | An AC circuit with R, L and C in parallel, has the following branch currents. Resistive branch – 12 A Inductive branch – 11 A Capacitive branch – 6 A The phase angle between the supply voltage and the supply current will be approximately: |
42. | When resistance and inductance are combined in one circuit, there will be a value of power consumed that is dependent on the: A. | capacitive load, in the circuit |
B. | resistive load, in the circuit |
C. | inductive load, in the circuit |
D. | size of the inductive reactance |
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43. | The true power consumed by a single-phase AC circuit can be determined using the formula: |
44. | In an AC the product of the measured line voltage and line current is called the: |
45. | The reactive power in an AC circuit is sometimes called: |
46. | Figure shows the power triangle for an AC circuit: In the diagram, the side marked ‘S’ represents the: |
47. | In an AC circuit, the power factor is the factor by which the apparent power is multiplied to obtain the: |
48. | For all electrical power work with sinusoidal waveforms, the power factor is equal to the: A. | phase angle between the voltage and the current |
B. | sine of the phase angle between the voltage and the current |
C. | tangent of the phase angle between the voltage and the current |
D. | cosine of the phase angle between the voltage and the current |
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49. | Generally, the lower the value of the power factor in an AC circuit, the: A. | greater will be the current required to supply the same true power |
B. | less will be the current required to supply the same true power |
C. | less will be the phase angle between the voltage and current |
D. | greater will be time taken for the current to pass through |
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50. | One of the major causes of a low power factor is: A. | transformers running near full load |
B. | lightly loaded electric motors |
D. | diesel driven alternators |
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51. | The power factor of an AC circuit can be found using the formula, Power factor = : B. | volt-amperes x current |
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52. | A single phase motor draws 2.175 A from a 230 V supply. A wattmeter in the circuit shows 400 W. The power factor of this circuit is approximately: |
53. | An inductor draws 11.5 A on 230 V DC and 5.75 A when connected to 230 V AC. The angle of lag between the voltage and current when on AC will be: |
54. | Look at the following diagram: When the switch S1 is closed in figure, the reading on the ammeter will: D. | be the capacitor current only |
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55. | Look at the following diagram: When the switch S1 is closed in figure, the power factor of the circuit will: D. | move toward 0.1 lagging |
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56. | Figure shows the phasor diagram of an electric motor connected to 230 V AC supply: If a capacitor is connected in parallel with the motor, the phasor to represent the capacitor current would be drawn: A. | vertically down from position 1 |
B. | vertically down from position 3 |
C. | vertically up from position 2 |
D. | vertically up from position 1 |
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57. | A single-phase 230 V 50 Hz induction motor draws 7.5 A at 0.6 power factor. If a 47 μF capacitor is connected across the line, then the combined line current will be approximately: |
58. | The purpose of using capacitors to improve the power factor is to provide a: A. | lagging current to counteract the lagging current drawn by the load |
B. | leading current to counteract the leading current drawn by the load |
C. | leading current to counteract the lagging current drawn by the load |
D. | lagging current to counteract the leading current drawn by the load |
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59. | When using a power triangle to solve AC circuits the reactive power can be found using the formula: Q = V I √1 – PF2) In the formula the symbol PF stands for the: C. | power factor of the circuit |
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60. | When an electrical circuit has its power factor corrected to unity, the current is: C. | same value as the voltage |
D. | brought into phase with the voltage |
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61. | In an AC circuit, when the capacitive reactance and inductive reactance are exactly equal, the circuit is said to be: |
62. | The major characteristics of the series resonant circuit are a power factor of unity and a: D. | maximum capacitive reactance |
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63. | When an inductor and capacitor are connected in parallel and their respective reactances are equal, the reactive currents are: A. | equal but 90° out of phase with each other |
B. | not equal but 180° out of phase with each other |
C. | equal but 180° out of phase with each other |
D. | not equal but 90° out of phase with each other |
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64. | When the supply frequency to a parallel resonant circuit is varied, the resistance in the circuit is unchanged but the impedance will be: A. | minimum only at the resonant frequency |
B. | maximum at frequencies other than the resonant frequency |
C. | maximum only if the resonant frequency is 50 Hz |
D. | maximum only at the resonant frequency |
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65. | In an AC circuit at resonance, energy is being transferred back and forth from the: A. | electromagnetic field of the inductor to the electrostatic field of the capacitor |
B. | electrostatic field of the inductor to the electrostatic field of the capacitor |
C. | electrostatic field of the inductor to the electromagnetic field of the capacitor |
D. | electromagnetic field of the inductor to the electromagnetic field of the capacitor |
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66. | A 10 Ω resistor, 0.25 H inductor and a 40.52 μF capacitor have been connected in series across a variable frequency AC supply. The resonant frequency of the circuit will be: |