CET AC Circuit Analysis & Power 2 — Questions and Answers
Question 1: In a series RLC circuit at resonance, which statement is TRUE?
- Impedance is at its maximum value
- Current is at its minimum value
- Inductive and capacitive reactances are equal and cancel (Correct answer)
- Power factor equals zero
Correct answer: Inductive and capacitive reactances are equal and cancel
At resonance, XL = XC, so they cancel, leaving only resistance in the circuit.
Question 2: What is the phase relationship between voltage and current in a purely capacitive AC circuit?
- Voltage leads current by 90°
- Current leads voltage by 90° (Correct answer)
- Voltage and current are in phase
- Current lags voltage by 45°
Correct answer: Current leads voltage by 90°
In a purely capacitive circuit, current leads voltage by 90° (ELI the ICE man mnemonic — I before E in C).
Question 3: A circuit has a true power of 300 W and an apparent power of 500 VA. What is the reactive power?
- 200 VAR
- 400 VAR (Correct answer)
- 800 VAR
- 150 VAR
Correct answer: 400 VAR
Reactive power Q = √(S² − P²) = √(500² − 300²) = √(250000 − 90000) = √160000 = 400 VAR.
Question 4: Which instrument is used to measure the phase angle between voltage and current in an AC circuit?
- Wattmeter
- Power factor meter (Correct answer)
- Ammeter
- Frequency counter
Correct answer: Power factor meter
A power factor meter directly measures the cosine of the phase angle (power factor) between voltage and current.
Question 5: What happens to the capacitive reactance (XC) of a capacitor if the frequency of the applied AC signal is doubled?
- XC doubles
- XC quadruples
- XC is halved (Correct answer)
- XC remains unchanged
Correct answer: XC is halved
Since XC = 1/(2πfC), doubling frequency halves the capacitive reactance.
Question 6: In an AC circuit, what does the Q factor (quality factor) of a resonant circuit indicate?
- The ratio of reactive power to true power
- The bandwidth relative to the resonant frequency (Correct answer)
- The power factor of the circuit
- The ratio of resistance to reactance
Correct answer: The bandwidth relative to the resonant frequency
The Q factor equals the resonant frequency divided by the bandwidth, indicating how selective (sharp) the resonance is.
Question 7: What is the impedance of a series circuit containing a 40 Ω resistor, XL = 80 Ω, and XC = 50 Ω?
- 170 Ω
- 50 Ω (Correct answer)
- 90 Ω
- 60 Ω
Correct answer: 50 Ω
Z = √(R² + (XL − XC)²) = √(40² + 30²) = √(1600 + 900) = √2500 = 50 Ω.
CET AC Circuit Analysis & Power 2
In a series RLC circuit at resonance, which statement is TRUE?