ISCET AC Circuits and Analysis 4 — Questions and Answers
Question 1: When analyzing AC circuits using phasors, inductive impedance is represented as:
- -jXL
- +jXL (Correct answer)
- XL∠-90°
- 1/(jωL)
Correct answer: +jXL
Inductive impedance is ZL = +jXL = jωL, placing it on the positive imaginary axis in the complex plane.
Question 2: If an AC circuit has a power factor of 0.6 lagging and apparent power of 100 VA, what is the true power?
- 166.7 W
- 60 W (Correct answer)
- 80 W
- 40 W
Correct answer: 60 W
True power P = S × PF = 100 VA × 0.6 = 60 W.
Question 3: Which of the following best describes 'leading' power factor?
- Current lags voltage, caused by inductive loads
- Current leads voltage, caused by capacitive loads (Correct answer)
- Voltage and current are in phase
- Current leads voltage, caused by inductive loads
Correct answer: Current leads voltage, caused by capacitive loads
A leading power factor means current leads voltage, which occurs in circuits where capacitive reactance dominates.
Question 4: The bandwidth of a series resonant circuit with fr = 500 kHz and Q = 50 is:
- 10 kHz (Correct answer)
- 25 kHz
- 100 kHz
- 1 kHz
Correct answer: 10 kHz
BW = fr/Q = 500 kHz / 50 = 10 kHz.
Question 5: In a transformer, the secondary current compared to the primary current is determined by:
- The inverse of the turns ratio (Correct answer)
- The square of the turns ratio
- The same turns ratio as voltage
- The core material only
Correct answer: The inverse of the turns ratio
Current transforms inversely to voltage: Is/Ip = Np/Ns, the inverse of the voltage turns ratio.
Question 6: Which component causes a 180° phase shift between input and output when used as a simple inverting element in an AC circuit?
- Capacitor
- Inductor
- Resistor in a voltage divider fed to a common-emitter amplifier stage
- Transformer with reversed secondary connections (Correct answer)
Correct answer: Transformer with reversed secondary connections
Reversing the secondary connections of a transformer inverts the output signal, producing a 180° phase shift.
Question 7: What does a Bode plot primarily display for an AC circuit?
- Time-domain waveforms
- Gain and phase response versus frequency (Correct answer)
- Phasor diagrams at a single frequency
- Power dissipation versus load resistance
Correct answer: Gain and phase response versus frequency
A Bode plot shows how the gain (magnitude) and phase of a circuit's transfer function vary with frequency, typically using a logarithmic frequency axis.
ISCET AC Circuits and Analysis 4
When analyzing AC circuits using phasors, inductive impedance is represented as: