13. AC Theory

31. A sine wave of RMS value 7.07 volts has a peak to peak value of. Option A. 20.0 volts. Option B. 10.0 volts. Option C. 0.707 volts.

Correct Answer is. 20.0 volts. Explanation. Peak = RMS * 1.414 7.07 * 1.414 = 10V Peak-to-peak = 2 * peak = 20V.

13. If a 1 ohm circuit produces a 50 W output, the phase angle is. Option A. 45°. Option B. 0°. Option C. 90°.

Correct Answer is. 0°. Explanation. Going by the power being quoted in Watts (rather than VA) the circuit is purely resistive and therefore the phase angle is 0 degrees.

2. What is the phase difference in a circuit with 100V, drawing 0.5 amps, consuming 50 Watts?. Option A. 0°. Option B. 45°. Option C. 90°.

Correct Answer is. 0°. Explanation. When power is in Watts, it is all true power - there is no apparent power.

30. Form Factor for a sine wave AC output is. Option A. 1.1. Option B. 1.414. Option C. 0.707.

Correct Answer is. 1.1. Explanation. Form Factor = RMS / Average = 0.707 / 0.63 (for a sine wave).

49. Peak factor in a standard AC circuit is. Option A. 1.414 times max. value of applied voltage. Option B. 1.11 times max. value of applied voltage. Option C. 0.707 time max. value of applied voltage.

Correct Answer is. 1.414 times max. value of applied voltage. Explanation. Peak Factor (or Crest Factor) = Peak/RMS = 1.414 for a sinusoidal wave.

27. The peak factor for a sine wave is. Option A. 1.11. Option B. 0.707. Option C. 1.414.

Correct Answer is. 1.414. Explanation. Peak factor (sometimes called 'crest factor') is calculated by peak value/RMS value. For a sine wave it is 1.414.

12. A 10 ohm resistor has a 14.14 V peak drop across it. What power is dissipated?. Option A. 10 W. Option B. 19.99 W. Option C. 1.414 W.

Correct Answer is. 10 W. Explanation. Since it mentions peak, it is AC. Always use RMS voltage when calculating power. RMS voltage = peak voltage / 1.414 (more than coincidence that 14.14 / 1.414 = 10). Power = V2/ R.

34. The RMS value of 200 volts peak to peak is. Option A. 127.4 volts. Option B. 70.7 volts. Option C. 141.4 volts.

Correct Answer is. 141.4 volts. Explanation. RMS = 0.707 * peak. Peak is 1/2 of peak-to-peak.

46. A circuit has 115 V RMS, 2.5A at 60 degrees, what is the power dissipated?. Option A. 79 W. Option B. 300 VA. Option C. 143 W.

Correct Answer is. 143 W. Explanation. Apparent Power = 115 * 2.5 = 287.5. PF = cos 60 = 0.5. True Power = Apparent Power * PF.

21. How many amperes will a 28-volt generator be required to supply to a circuit containing five lamps in parallel, three of which have a resistance of 6 ohms each and two of which have a resistance of 5 ohms each?. Option A. 1 ampere. Option B. 25.23 amperes. Option C. 1.11 amperes.

Correct Answer is. 25.23 amperes. Explanation. resistance, then use ohms law (I=V/R) to find the total current. It is a tricky calculation to do without a calculator, so round the total resistance up (to 1 ohms) then choose the nearest answer for I.

42. A sine wave has a periodic time of 0.5 milliseconds, what is its frequency?. Option A. 200 Hz. Option B. 2Khz. Option C. 20Khz.

Correct Answer is. 2Khz. Explanation. Frequency is 1/time period. Watch that prefix 'milli'. f = 1/T = 1/0.0005 = 10,000/5 = 2000 Hz = 2 kHz.

33. The average value of 100 volts peak AC is. Option A. 70.7 volts. Option B. 141.4 volts. Option C. 63.7 volts.

Correct Answer is. 63.7 volts. Explanation. Average value of a sine wave is 0.637 * peak.

40. If an AC sine wave has an RMS value of 5V its peak value is. Option A. 6.37 V. Option B. 7.07 V. Option C. 14.14 V.

Correct Answer is. 7.07 V. Explanation. Peak value = 1.414 * RMS = 1.414 * 5 = 7.07 V.

7. A sine wave has 5 amps RMS value. What is the peak value?. Option A. 7.07 amps. Option B. 6.37 amps. Option C. 10 amps.

Correct Answer is. 7.07 amps. Explanation. Peak = 1.414 * RMS (also remember that 5 is half of 10).

29. The mark-to-space ratio of the waveform shown is. Option A. A to D / D to E. Option B. D to E / D to C. Option C. A to B / B to C.

Correct Answer is. A to D / D to E. Explanation. Mark-to-space is the ratio of the duration of the square wave's positive amplitude (represented by a mark) to its negative amplitude (represented by a space).

20. One of the chief advantages of alternating current is that it can be transmitted at a high voltage with a low power loss; the voltage can then be changed to any desired value of. Option A. DC by means of transformers. Option B. DC by means of inverters. Option C. AC by means of transformers.

Correct Answer is. AC by means of transformers. Explanation. NIL.

16. One advantage of using AC electrical power in aircraft is. Option A. the greater ease in stepping the voltage up or down. Option B. AC electrical motors can be reversed while DC motors cannot. Option C. the effective voltage is 1.41 times the maximum instantaneous voltage; therefore, less power input is required.

Correct Answer is. AC electrical motors can be reversed while DC motors cannot. OR the greater ease in stepping the voltage up or down. Explanation. NIL.

19. In an AC circuit how is the value of true power calculated?. Option A. By voltmeter readings multiplied by ammeter readings. Option B. By volts multiplied by amps multiplied by power factor. Option C. By watt meter readings multiplied by power factor.

Correct Answer is. By volts multiplied by amps multiplied by power factor. Explanation. NIL.

14. What does a rectifier do?. Option A. Changes alternating current into direct current. Option B. Reduces voltage. Option C. Changes direct current into alternating current.

Correct Answer is. Changes alternating current into direct current. Explanation. NIL.

54. What is the power factor in a purely resistive circuit?. Option A. Equal to 1. Option B. Less than 1 but greater than zero. Option C. Greater than 1.

Correct Answer is. Equal to 1. Explanation. NIL.

45. What happens to the current in a series resistive resonant circuit?. Option A. It is maximum. Option B. It is minimum. Option C. It is zero.

Correct Answer is. It is maximum. Explanation. At resonance the impedance is minimum (zero apart from the resistance in the circuit) and so current is highest.

32. The true power in an AC circuit is given by. Option A. volts * amps. Option B. PF * volts * amps. Option C. voltmeter reading * PF.

Correct Answer is. PF * volts * amps. Explanation. True power = PF * Apparent power. Apparent power = volts * amps.

4. If you apply this waveform to this circuit, what is the output across the capacitor?. Option A. Pulsed negative. Option B. Pulsed positive. Option C. Zero.

Correct Answer is. Pulsed positive. Explanation. With the output taken across the capacitor, this is a basic integrator - so the output is.

36. The Form Factor of an AC waveform can be found by. Option A. RMS value divided by the average value. Option B. average value divided by the RMS value. Option C. average value times the RMS value.

Correct Answer is. RMS value divided by the average value. Explanation. Form Factor = RMS / Average.

26. The value of an AC sine wave that will give an equivalent heating effect in a DC resistor is the. Option A. peak value. Option B. RMS value. Option C. average value.

Correct Answer is. RMS value. Explanation. NIL.

6. What shape is the waveform when the input pulse and the time base are unequal?. Option A. Saw tooth. Option B. Square. Option C. Rectangular.

Correct Answer is. Rectangular. Explanation. NIL.

5. What value is the same as the equivalent D.C. heating effect?. Option A. Peak. Option B. Average. Option C. Root Mean Square.

Correct Answer is. Root Mean Square. Explanation. NIL.

39. What sort of wave would be used in a CR oscilloscope to control sweep?. Option A. Square. Option B. Sine wave. Option C. Sawtooth.

Correct Answer is. Sawtooth. Explanation. A sawtooth wave is used in an oscilloscope to control sweep.

25. Which wave form is stepped positive and negative with unequal length sides?. Option A. Rectangular wave. Option B. Trapezoidal wave. Option C. Saw tooth wave.

Correct Answer is. Trapezoidal wave. Explanation. A trapezoidal waveform ALWAYS has unequal (vertical) sides. However, a sawtooth wave could also be considered to have unequal length sides (but does not necessarily have).

8. What is the wave shape of the x-axis input of a traversing oscilloscope image?. Option A. Rectangular. Option B. Triangular. Option C. Square.

Correct Answer is. Triangular. Explanation. NIL.

41. Apparent power in an AC circuit is found by. Option A. V * I * cosØ. Option B. V * I * sinØ. Option C. V * I.

Correct Answer is. V * I. Explanation. Apparent power = amps * volts.

18. Which aircraft circuit would be most likely to use frequency wild 200V AC?. Option A. Hydraulic pump. Option B. Windscreen heating. Option C. Standby compass.

Correct Answer is. Windscreen heating. Explanation. Frequency wild can only be used on resistive circuits (such as heating and lighting).

28. An integrated step input at unequal time base produces. Option A. a sine wave. Option B. a square wave. Option C. a saw tooth wave.

Correct Answer is. a saw tooth wave. Explanation. Integrating a step input produces a ramp' or sawtooth waveform.

53. A differentiator has a time constant that will give. Option A. a long pulse. Option B. a short pulse. Option C. a continuous output.

Correct Answer is. a short pulse. Explanation. A differentiator turns a ramp signal into a steady state (of level corresponding to the slope of the ramp).

10. When comparing the average values of an AC generator output, to the peak values. Option A. average voltage = 0.707 * peak voltage. Option B. average voltage = 0.63 * peak voltage. Option C. average voltage = same as the peak voltage.

Correct Answer is. average voltage = 0.63 * peak voltage. Explanation. Don't get confused with RMS value.

48. Power factor is. Option A. sin theta. Option B. tan theta. Option C. cos theta.

Correct Answer is. cos theta. Explanation. Power factor is cos of the phase angle (theta).

35. The power factor of an AC circuit is the. Option A. cosine of the phase angle. Option B. tangent of the phase angle. Option C. sine of the phase angle.

Correct Answer is. cosine of the phase angle. Explanation. NIL.

17. Unless otherwise specified, any values given for current or voltage in an AC circuit are assumed to be. Option A. effective values. Option B. instantaneous values. Option C. maximum values.

Correct Answer is. effective values. Explanation. Effective values' is another term for RMS because it is the same heating 'effect' as DC.

9. In a purely resistive AC circuit, the current vector is. Option A. +90° out of phase with the voltage vector. Option B. in phase with the voltage vector. Option C. -90° out of phase with the voltage vector.

Correct Answer is. in phase with the voltage vector. Explanation. Purely resistive - current and voltage must be in phase.

24. In an AC circuit, the effective voltage is. Option A. less than the maximum instantaneous voltage. Option B. equal to the maximum instantaneous voltage. Option C. greater than the maximum instantaneous voltage.

Correct Answer is. less than the maximum instantaneous voltage. Explanation. Effective voltage is another term for RMS voltage, due to it being equal to the DC heating effect.

3. The power factor of a circuit containing an imbalance of inductive and capacitive reactance is. Option A. greater than unity. Option B. unity. Option C. less than unity.

Correct Answer is. less than unity. Explanation. NIL.

11. A parallel circuit at resonance has. Option A. maximum impedance. Option B. zero impedance. Option C. minimum impedance.

Correct Answer is. maximum impedance. Explanation. A 'series' circuit at resonance has zero impedance. A 'parallel' circuit at resonance has maximum impedance.

47. In a resonant parallel circuit, current is. Option A. zero. Option B. minimum. Option C. maximum.

Correct Answer is. minimum. Explanation. At resonance in a PARALLEL circuit, current is minimum (would be zero if it were not for the resistance in the circuit).

38. If the frequency is increased in an AC circuit of pure resistance, the effect is. Option A. nothing. Option B. decreased resistance. Option C. increased resistance.

Correct Answer is. nothing. Explanation. NIL.

44. The impedance of an AC circuit is measured in. Option A. ohms. Option B. kilovolt-amperes. Option C. amperes.

Correct Answer is. ohms. Explanation. Impedance is 'resistance to electron flow' in an AC circuit and is measured in Ohms.

52. The Sine wave signals of a circular time base are. Option A. in phase. Option B. phase separated by 90°. Option C. phase separated by 180°.

Correct Answer is. phase separated by 90°. Explanation. The two sine wave time bases (X and Y) for a CRT are displaced by 90° to produce a circle on the screen.

15. The ratio between apparent power and true power is the. Option A. power rating. Option B. power factor. Option C. efficiency.

Correct Answer is. power factor. Explanation. Power factor = true power / apparent power.

1. Convention requires that in a symmetrical 3 phase system, the. Option A. red voltage is taken as the reference phase. Option B. yellow voltage phase leads the red phase by 120 degrees. Option C. red voltage phase leads the blue phase by 120 degrees.

Correct Answer is. red voltage is taken as the reference phase. Explanation. NIL.

50. The controlling wave in a CRT is. Option A. square. Option B. sawtooth. Option C. sinusoidal.

Correct Answer is. sawtooth. Explanation. A sawtooth wave is required to control the electron beam vertical and horizontal scan.

22. Frequency (Hz) is the number of cycles per. Option A. minute. Option B. second. Option C. revolution.

Correct Answer is. second. Explanation. NIL.

37. A differentiator has a time constant which is. Option A. long. Option B. equal to the input pulse. Option C. short.

Correct Answer is. short. Explanation. A differentiator provides a voltage proportional to the rate of change of the input. So for a square wave input, it needs to give a pulse on the step up, nothing on the straight bit of the square wave, then a negative pulse as the input drops back to zero. Only a very short time constant can do this.

51. A wave form having equal amplitude and time base is a. Option A. rectangular. Option B. sawtooth. Option C. square waveform.

Correct Answer is. square waveform. Explanation. NIL.

43. Phase angle is. Option A. the cosine of the peak value of an ac voltage. Option B. the difference in degrees of rotation between 2 alternating voltages or currents or a voltage and a current. Option C. the difference in degrees between the positive and negative parts of an AC sine wave.

Correct Answer is. the difference in degrees of rotation between 2 alternating voltages or currents or a voltage and a current. Explanation. Phase angle is the difference in degrees of rotation between 2 alternating voltages or currents or a voltage and a current.

23. The frequency of a power wave in an AC resistance circuit is. Option A. half the frequency for voltage and current. Option B. the same as the frequency for voltage and current. Option C. twice the frequency for voltage and current.

Correct Answer is. twice the frequency for voltage and current. Explanation. Look at a diagram of a power wave.