ELEC50B FINAL - Practice Test

This practice test is about a third longer than the actual test will be.

1.  Series resonance is a condition where the circuit’s:

  1. Resistance equals reactance
  2. Capacitive reactance exceeds its inductive reactance
  3. A purely resistive circuit occurs
  4. Resistance and reactance are equal

2.  At series resonance the total impedance of a circuit is equal to:

  1. R
  2. XL + XC
  3. 0 ohms
  4. X

3.  The resonant frequency in a series circuit is a point of:

  1. High current and high impedance
  2. High current and low impedance
  3. Low current and high impedance
  4. Low current and low impedance

4.  The quality factor Q is:

  1. Measure of an inductors ratio of reactance to series resistance (including coil resistance)
  2. Determines how much reactance there is in the circuit
  3. Used to calculate the phase angle
  4. Measure of magnification of voltage across an inductor vs VR

5.  An RCL series resonant circuit freely passes current at the resonant frequency and:

  1. Limits frequencies at fo
  2. Attenuates current at frequencies above and below resonance
  3. Magnifies current at frequencies above and below the resonant frequency
  4. Blocks all current at frequencies above resonance

6.  The power factor of a circuit in which resistance equals reactance is:

  1. .707
  2. 1
  3. 0

7.  In order to make a capacitive circuit look purely resistive without removing the capacitor, you can:

  1. Make their power factor equal to 0
  2. Remove the inductors from their loads
  3. Remove capacitance
  4. Add Inductance
  5. Make the apparent power equal the reactive power

8.  In a series RCL circuit operating at fo , the voltage across the inductor may be larger than the source voltage. What will cause KVL to still be true?

  1. Current will be maximized thereby reducing resistance
  2. Because the reactance is high
  3. Because KVL does not apply at resonant frequency
  4. The voltage in the capacitor will have an equal magnitude but opposite direction
  5. Because RS of the circuit is usually lower

9.  If True power is 5W and Reactive Power was 12VA, then Apparent Power is:

  1. Equal to the true power
  2. Equal to the reactive power
  3. 17VA
  4. 13VA
  5. Less than reactive power

10.  If the resistance in a parallel RCL is smaller than either the capacitive or inductive reactances, then:

  1. The total impedance is larger than the value of R
  2. The circuit has more total reactance than total resistance.
  3. Has a phase θ less than 45˚
  4. The circuit will be operating at resonance

11.  Which of the following are measured in Watts?

  1. Apparent Power
  2. True Power
  3. Reactive Power
  4. Inductive Power

12.  What is the resonant frequency of a series RCL circuit in which R = 30Ω, L = 15mH, C=.022uF?

  1. 8.76 kHz
  2. 3.2 kHz
  3. 44 uHz
  4. 225 MHz

13.  In the question above, if R was change to 5 ohms, then the resonant frequency would:

  1. Go up
  2. Go down
  3. Stay the same
  4. I like macaroni

14.  In question 12 above, if the capacitor value went up, then the circuit phase angle would:

  1. Become more negative
  2. Stay the same
  3. Become more positive
  4. Flying saucers

15.  If the voltage angle across a capacitor is 29˚, what is the voltage angle across an inductor that is in series with the capacitor?

  1. 119˚
  2. -61˚
  3. 209˚
  4. -107˚

16.  T or F In an AC series RC circuit, the voltage across a resistor is in phase with VS.

  1. True
  2. False

17.  T or F In an AC series RL circuit, the voltage in the resistor is leading the voltage in the inductor by 90 degrees.

18.  If XL = R in a series RL circuit the impedance angle θ is:

  1. -90˚
  2. -45˚
  3. 0˚
  4. 45˚

19.  In a resonant AC series RLC circuit, VCL (the voltage across the capacitor and inductor combined) is:

  1. Equal to VR
  2. Equal to the VS
  3. Equal to 0V
  4. Equal to VC or VL (same thing)

20.  In an AC parallel RLC circuit, at resonant frequency, VC:

  1. Has an angle of 0degrees
  2. Is 0V
  3. Is smaller than VR
  4. Is in phase with IC

21.  In a series RLC circuit where XL = 7Ω, XC = 7Ω, R = 7Ω, which of the following is true?

  1. There is less current in the circuit than if XL = 5Ω
  2. The impedance angle looks purely resistive.
  3. Resonant frequency is 10Hz
  4. The voltage across VC or VL is 0V

22.  T or F In a parallel RLC circuit the voltage angle across the inductor is in phase with the voltage across the capacitor. A. True B. False

23.  T or F In a parallel RLC circuit the current angle through the inductor is in phase with the current through the resistor. A. True B. False

24.  In a series circuit, if VC = 15Ð-70˚ and VL = 20Ð110˚, which of the following is true?

  1. XL must be smaller than XC
  2. There must be a resistor in the circuit too
  3. The circuit is mostly capacitive
  4. VC leads VS
  5. VS has an angle of 110˚

25.  T or F The following formula would work to find the decibel level for power: dB = 20log(VOUT/VIN)

  1. True b. False

26.  In a filter circuit, if PIN = 10W and POUT = 4000W, what is the gain in dB?

  1. 52dB
  2. 26dB
  3. 2.6dB
  4. 5.2dB
  5. 400dB
  6. 3990dB

27.  What is true about the cutoff frequency fC?

  1. It signal is 3dB less than maximum power
  2. Its power is 70.7% of maximum
  3. It is at a frequency 10 times larger than fo
  4. It’s voltage level is attenuated to 50% of max

28.  In which case is the gain given in dB positive?

  1. At resonant frequency
  2. When VOUT > VIN
  3. If the circuit loses power from input to output
  4. If power is 50% of max

29.  Which of the following is true about a HPF:

  1. It attenuates DC signals
  2. It magnifies the high frequencies
  3. It blocks the cutoff frequency
  4. It’s composed of a capacitor and inductor in which XL = XC
  5. It limits low frequencies

30.  What happens to the phase angle between IT and IC in a series RC circuit when frequency increases?

  1. It begins to look mostly resistive
  2. It gets larger
  3. It stays the same
  4. It gets smaller

31.  What happens to the circuit phase angle of an series RLC circuit if frequency goes down?

  1. q goes up
  2. q goes down
  3. q stays the same
  4. Taco bell has great cheesy gordita crunches (Although this is true, it is not the answer)

32.  What happens to the phase angle between the supply voltage and the inductor voltage in a series RL circuit when frequency increases?

  1. It stays the same
  2. It becomes purely inductive
  3. It gets smaller
  4. It gets larger

33.  T or F In a series RC circuit, current increases as frequency increases.

34.  T or F Phase and magnitude of voltage is the same throughout a parallel RCL circuit across any component

35.  T or F The voltage angle across a resistor in a series RL circuit lags the voltage across an inductor by 90 degrees

36.  T or F VS in a series RL circuit leads VL by 90 degrees

37.  T or F VS in a series RC circuit lags VC.

38.  Varying the inductance of a circuit will vary the circuit's:

  1. resonant frequency
  2. capacitive reactance
  3. resistance
  4. source voltage

39.  In a practical parallel resonant circuit at resonance, the line current is:

  1. Maximum
  2. 70.7% of maximum
  3. 50% of max power
  4. Minimum

40.  In an ideal parallel resonant circuit, the impedance of the tank portion if the circuit is:

  1. 0 ohms
  2. Infinite ohms
  3. Equal to XC + XL

41.  If Q increases, what happens to BW of a series resonant circuit?

  1. stays the same
  2. increases
  3. decreases
  4. Flaming Hot Cheetos are addictive

42.  With the voltmeter across the resistor in a RCL series resonant circuit, how can you tell if you are at resonance?

  1. Both d and e
  2. VL will be minimum
  3. VL will equal VS
  4. VR will be in phase with VS
  5. VR will be maximized

PUT ALL ANSWERS IN POLAR COORDINATES! SHOW WORK FOR PARTIAL CREDIT

43.  Find the current in each branch.

44.  Find the total impedance

45.  Find the total current

46.  What is the circuit phase angle?

47.  Is the total circuit more inductive or capacitive

48.  fo = ______Hz

49.  BW = ______Hz

50.  fCutoff1 = ______Hz

51.  fCutoff2 = ______Hz

52.  VC at fo = ______V

Short Answer

53.  Draw 2 different types of HPF’s and 2 different types of LPF’s.

54.  Draw and explain how a BPF would work

55.  In an AC circuit with a frequency of 500Hz, what size inductor should be used if the capacitor C = 4uF and you wanted the phasor diagram to look neither capacitive nor inductive?

L = ______

56.  What is the difference between a step up and a step down transformer?

57.  What is the most fundamental difference between AC and DC signals?

58.  If you were given a 100uF capacitor, could you tell me the capacitive reactance? Why or why not?

59.  Inductors act like a/an ______and capacitors act like a/an ______when the switch on a series DC circuit is first closed. Choose from “short” or “open”.

60.  Explain how a capacitor can act like a resistor under AC conditions.

61.  What information do rectangular coordinates give that polar coordinates do not?

62.  In a transformer, when the voltage is stepped up, the power in the primary and the secondary are still the same value. How can this be so?

63.  We named probably 20 opposites between capacitors and inductors. Name 3.

64.  Describe what t is used for in our 50B class.

65.  In class we talked about how ELI means the voltage in an inductor leads current in and inductor by 90 degrees and the opposite for a capacitor.

  1. For which type of circuit would you use the acronym ELICE?
  1. Describe what ELICE means for this circuit? (besides a computer virus J )
  1. What other parameter could represent the phase angle of the I?
  1. What do you think the acronym would be for a parallel RCL circuit?

66.  Name one thing that is similar and one thing that is dissimilar about resistance and reactance.

62.  In the screen shot of the oscilloscope above there are 3 voltage signals. They are measuring VC, VR, and VS in the RC circuit above. Please draw an arrow to each and label which one is which. (You do not need to do any calculations to figure this out.) [1pt]

63.  Calculate the impedance phase angle of the circuit above [1pt]:

64.  Keeping in mind ZT=VSIT, explain how to determine the impedance phase angle you calculated in the last question using the voltages in the screen shot of the oscilloscope instead. [1pt] (This question is NOT asking about how to count tick marks and convert it into a degrees.)

65.  Using the screen shot of the oscilloscope above, explain how to tell how many degrees the source voltage is out of phase with the source current. [1pt] (This question is NOT asking about how to count tick marks and convert it into a degrees.)

66.  How many degrees is VS and IS out of phase by? Which is leading which?

67.  The screen shot of the oscilloscope above was not taken using the oscilloscopes used in class. List 2 reasons why would you never be able to see these waveforms simultaneously using the oscilloscope from class.

68.  Given the RC circuit above, identify and label on both the sine wave graphic and on the phasor diagram, each of the following voltages: VR, VC, and VS. In the space to the right of the diagrams, explain your reasoning behind each choice.

69.  Indicate on both diagrams the impedance phase angle, θ.

70.  Explain why θ, as indicated in these diagrams, is associated with impedance, Z.

Lab practical: 25% of the test

Know how to use the oscilloscope to measure:

VPP and convert to VRMS

Phase angle

Frequency,

Period

Determine whether a signal is lagging or leading.

Cutoff frequency for a filter and compare to a calculated cutoff frequency for a filter

Study Guide for 50B final

Do not assume this list is the only things you need to know for the final, but rather use it as a spring board for where to spend your time studying. (For example, notice there are no questions on magnetism.)

The final will focus on the last 3 chapters, 24-26, however a good foundation from the previous chapters is needed in order to fully understand chpts 24-26 (clearly.) Note: There will be a few questions aimed at the general understanding of concepts in earlier chapters.

1.  Describe and define series resonance.

2.  Describe what happens to a circuit at resonant frequency.

3.  Define the quality factor Q in different ways and/or different formulas.

4.  Define and describe what power factor is.

5.  Define and describe apparent power, true power and reactive power, as well as how they are related to each other.

6.  Describe what phase angle is and what types of phase angles are associated with capacitive and inductive loads.

7.  Know the units for different types of power.

8.  Calculate circuit parameters at resonant frequency.

9.  Calculate capacitor and/or inductor size to yield a resonant frequency

10.  Understand what ELI and ICE stand for, what they mean, and how to apply them.

11.  Understand phasor diagrams

12.  Describe why instantaneous power in a purely resistive AC circuit is always positive.

13.  Calculate gain given input/output powers or voltages.

14.  Describe, define and calculate cutoff frequency, bandwidth, Q factor and how they are related mathematically.

15.  Describe and identify what a Low Pass Filter, High Pass Filter, Band Pass Filter, and Band Stop Filter circuits are.

16.  Know how to construct the LPF and HPF.

17.  Understand the term “attenuates.”

18.  Be able to solve voltage, current and impedance parameters for a series/parallel combo AC circuit.

19.  Know how to convert an inductive circuit into a purely resistive one.

20.  Know the basics on transformers.