Junior Engineer (Electrical/Electronics) MCQs – Series 1 (Q21-30)
Q21. In a series RLC circuit at resonance, the impedance is:
- A) Minimum and equal to R
- B) Zero
- C) Infinite
- D) Maximum
Answer: A
At resonance, XL = XC, so they cancel out and the total impedance equals only the resistance R, which is minimum.
Q22. The Thevenin equivalent resistance is found by:
- A) Opening all sources
- B) Shorting all sources
- C) Shorting all voltage sources and opening all current sources
- D) Opening all voltage sources and shorting all current sources
Answer: C
To find Rth, independent voltage sources are replaced by short circuits and independent current sources by open circuits.
Q23. In a purely capacitive AC circuit, the current:
- A) Lags voltage by 90°
- B) Lags voltage by 180°
- C) Leads voltage by 90°
- D) Is in phase with voltage
Answer: C
In a pure capacitor, current leads the applied voltage by exactly 90°.
Q24. The Q-factor of a series resonant circuit is given by:
- A) R/(ωL)
- B) ωL/R
- C) 1/(ωRC)
- D) ωC/R
Answer: B
At resonance, Q-factor = XL/R = ωL/R, indicating the sharpness of resonance.
Q25. Maximum power transfer theorem states that maximum power is delivered to the load when load resistance equals:
- A) Zero
- B) Twice the source resistance
- C) Source (Thevenin) resistance
- D) Half the source resistance
Answer: C
Maximum power transfer occurs when RL = Rth (source’s internal resistance).
Q26. In a balanced three-phase delta-connected system, the line current is:
- A) 3 times the phase current
- B) √3 times the phase current
- C) Phase current divided by √3
- D) Equal to the phase current
Answer: B
In a delta connection, IL = √3 × Iph, while voltage remains the same across line and phase.
Q27. The superposition theorem is applicable only to:
- A) Linear bilateral circuits
- B) Nonlinear circuits
- C) Circuits with only resistors
- D) DC circuits only
Answer: A
Superposition theorem holds only for circuits that are linear and bilateral; it fails for nonlinear elements.
Q28. Norton’s theorem replaces a complex network with:
- A) A voltage source in series with a resistance
- B) A pure resistance only
- C) Two voltage sources
- D) A current source in parallel with a resistance
Answer: D
Norton’s equivalent circuit consists of a current source IN in parallel with resistance RN.
Q29. The time constant of an RC charging circuit is defined as the time required for capacitor voltage to reach:
- A) 37% of its final value
- B) 100% of its final value
- C) 50% of its final value
- D) 63.2% of its final value
Answer: D
One time constant (τ = RC) corresponds to the capacitor charging to about 63.2% of the final steady-state voltage.
Q30. In a purely inductive AC circuit, the average power consumed is:
- A) Maximum
- B) Zero
- C) Equal to I²R
- D) Equal to VI
Answer: B
Since current lags voltage by 90° in a pure inductor, the average power over a cycle is zero.
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