ISCET AC Circuits and Analysis 3 — Questions and Answers
Question 1: In a parallel RLC circuit at resonance, the impedance is:
- Zero
- Equal to XL
- Maximum (theoretically infinite for ideal components) (Correct answer)
- Equal to R only
Correct answer: Maximum (theoretically infinite for ideal components)
In an ideal parallel RLC circuit at resonance, the LC tank presents maximum (infinite) impedance because the circulating tank current is maximum while source current is minimum.
Question 2: The Q factor (quality factor) of a series resonant circuit is best described as:
- The ratio of reactive power to true power
- The ratio of resonant frequency to bandwidth (Correct answer)
- The ratio of bandwidth to resonant frequency
- The ratio of resistance to reactance
Correct answer: The ratio of resonant frequency to bandwidth
Q = fr/BW, representing how selective (narrow-bandwidth) the resonant circuit is.
Question 3: What does the term 'skin effect' refer to in AC circuits?
- Insulation breakdown at high voltages
- Tendency of AC current to flow near the conductor surface at high frequencies (Correct answer)
- Capacitive coupling between conductors
- Impedance mismatch at circuit boundaries
Correct answer: Tendency of AC current to flow near the conductor surface at high frequencies
The skin effect causes AC current to concentrate near the outer surface of a conductor as frequency increases, effectively reducing the conductive cross-section.
Question 4: A low-pass RC filter has R = 1 kΩ and C = 0.1 µF. What is its cutoff frequency?
- approximately 1592 Hz (Correct answer)
- approximately 159 Hz
- approximately 15,920 Hz
- approximately 6283 Hz
Correct answer: approximately 1592 Hz
fc = 1/(2πRC) = 1/(2π × 1000 × 0.0000001) ≈ 1592 Hz.
Question 5: In AC circuit analysis, Kirchhoff's Voltage Law (KVL) applies to:
- Only DC circuits
- Only resistive AC circuits
- Phasor (complex) voltages around a loop (Correct answer)
- Only series circuits, not parallel
Correct answer: Phasor (complex) voltages around a loop
KVL applies to AC circuits using phasor (complex number) representation, so phasor voltages around any closed loop sum to zero.
Question 6: A capacitor is rated 47 µF, 50V AC. Operating it at 60V would most likely cause:
- Increased capacitance
- Dielectric breakdown and possible failure (Correct answer)
- Reduced capacitive reactance
- Improved power factor
Correct answer: Dielectric breakdown and possible failure
Exceeding the voltage rating stresses the dielectric, risking breakdown and permanent damage to the capacitor.
Question 7: What is the relationship between true power (P), apparent power (S), and reactive power (Q)?
- S = P + Q
- S² = P² + Q² (Correct answer)
- P² = S² + Q²
- Q = P × S
Correct answer: S² = P² + Q²
The power triangle gives S² = P² + Q², forming a right triangle relationship among the three power components.
ISCET AC Circuits and Analysis 3
In a parallel RLC circuit at resonance, the impedance is: