ISCET AC Circuits and Analysis 2 — Questions and Answers
Question 1: In a series RLC circuit at resonance, what happens to the impedance?
- Impedance equals infinity
- Impedance equals zero
- Impedance equals the resistance R (Correct answer)
- Impedance equals XL + XC
Correct answer: Impedance equals the resistance R
At resonance, XL and XC cancel, leaving only the resistive component R as the total impedance.
Question 2: What is the phase angle between voltage and current in a purely capacitive AC circuit?
- Current leads voltage by 90° (Correct answer)
- Voltage leads current by 90°
- Current and voltage are in phase
- Current lags voltage by 45°
Correct answer: Current leads voltage by 90°
In a purely capacitive circuit, current leads the voltage by 90 degrees.
Question 3: If the frequency of an AC signal doubles, what happens to the capacitive reactance (XC)?
- XC doubles
- XC quadruples
- XC halves (Correct answer)
- XC remains the same
Correct answer: XC halves
Since XC = 1/(2πfC), doubling frequency halves the capacitive reactance.
Question 4: The power factor of an AC circuit is defined as:
- Apparent power divided by true power
- True power divided by apparent power (Correct answer)
- Reactive power divided by apparent power
- True power divided by reactive power
Correct answer: True power divided by apparent power
Power factor equals true (real) power divided by apparent power, or equivalently cos(θ).
Question 5: A transformer has a primary winding of 200 turns and a secondary winding of 50 turns. If 120V AC is applied to the primary, what is the secondary voltage?
- 480V
- 30V (Correct answer)
- 60V
- 240V
Correct answer: 30V
Using the turns ratio: Vs = Vp × (Ns/Np) = 120 × (50/200) = 30V.
Question 6: In an AC circuit, reactive power is measured in:
- Watts (W)
- Volt-amperes (VA)
- Volt-amperes reactive (VAR) (Correct answer)
- Decibels (dB)
Correct answer: Volt-amperes reactive (VAR)
Reactive power, which represents energy stored and returned by reactive components, is measured in volt-amperes reactive (VAR).
Question 7: What is the resonant frequency of a series LC circuit with L = 10 mH and C = 10 µF?
- approximately 503 Hz (Correct answer)
- approximately 1592 Hz
- approximately 159 Hz
- approximately 50 Hz
Correct answer: approximately 503 Hz
fr = 1/(2π√LC) = 1/(2π√(0.01 × 0.00001)) ≈ 503 Hz.
ISCET AC Circuits and Analysis 2
In a series RLC circuit at resonance, what happens to the impedance?