Alternating Current Important Questions.

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1. Q1Medium

   In an LCR circuit at resonance, what happens to the phase difference between the source voltage and the current?

   • A.90°
   • B.180°
   • C.0°
   • D.45°
   Answer: 0°

   Explanation: At resonance, the inductive reactance (X_L) and capacitive reactance (X_C) cancel each other out, resulting in purely resistive impedance. This means the current and voltage are in phase, eliminating any phase difference.

2. Q2Medium

   Which of the following components in an AC circuit does NOT dissipate average power?

   • A.Resistor
   • B.Inductor
   • C.Capacitor
   • D.Transformer
   Answer: Capacitor

   Explanation: Purely reactive components like capacitors and inductors do not consume average power over a complete cycle. Only resistive elements dissipate power due to Joule heating.

3. Q3Medium

   What is the primary function of a transformer in an AC circuit?

   • A.To dissipate power
   • B.To cancel out reactance
   • C.To step up or step down AC voltage efficiently
   • D.To increase resistance in the circuit
   Answer: To step up or step down AC voltage efficiently

   Explanation: Transformers utilize mutual induction to change the voltage level between primary and secondary windings, enabling efficient transmission and distribution of electrical power over long distances.

4. Q4Medium

   Which of the following statements is true about the phase relationship between current and voltage in a capacitor?

   • A.Current lags voltage by 90°
   • B.Current and voltage are in phase
   • C.Current leads voltage by 90°
   • D.Current and voltage are out of phase by 45°
   Answer: Current leads voltage by 90°

   Explanation: In a capacitor, the current leads the voltage by 90° due to the capacitive reactance, which opposes changes in voltage rather than current.

5. Q5Medium

   What is the RMS value of an AC voltage if its peak value is 283 V?

   • A.141 V
   • B.283 V
   • C.200 V
   • D.400 V
   Answer: 200 V

   Explanation: The RMS value of an AC voltage is calculated as the peak value divided by the square root of 2. For a peak voltage of 283 V, RMS voltage = 283 / √2 ≈ 200 V.

6. Q6Medium

   What is the impedance of an AC circuit if the resistance is 3 Ω, inductive reactance is 4 Ω, and capacitive reactance is 4 Ω?

   • A.4 Ω
   • B.5 Ω
   • C.3 Ω
   • D.7 Ω
   Answer: 3 Ω

   Explanation: At resonance, the inductive reactance and capacitive reactance cancel each other out, leaving only the resistance as the impedance of the circuit.

7. Q7Medium

   In a series LCR circuit, what happens to the power factor when the circuit is at resonance?

   • A.It becomes 0
   • B.It remains unchanged
   • C.It becomes 1 (unity)
   • D.It becomes 0.5
   Answer: It becomes 1 (unity)

   Explanation: At resonance, the impedance is purely resistive, leading to a phase angle of 0°, which results in a power factor of 1 (cosφ = cos(0°) = 1).

8. Q8Medium

   Which of the following is the correct phasor representation for the voltage across an inductor?

   • A.Voltage lags current by 90°
   • B.Voltage and current are in phase
   • C.Voltage leads current by 90°
   • D.Voltage and current are out of phase by 45°
   Answer: Voltage leads current by 90°

   Explanation: In an inductor, the voltage phasor leads the current phasor by 90° due to the inductive reactance, which stores energy in the magnetic field.

9. Q9Medium

   What does the power factor indicate in an AC circuit?

   • A.Frequency of the circuit
   • B.Impedance of the circuit
   • C.Efficiency of power utilization
   • D.Voltage amplitude
   Answer: Efficiency of power utilization

   Explanation: The power factor is the ratio of real power to apparent power, indicating how effectively the circuit utilizes the supplied electrical power.

10. Q10Medium

    Which law is applied to analyze steady-state solutions in AC circuits?

    • A.Ohm’s Law
    • B.Faraday’s Law
    • C.Kirchhoff’s Laws
    • D.Newton’s Laws
    Answer: Kirchhoff’s Laws

    Explanation: Kirchhoff’s laws, when applied using phasor analysis, are used to determine voltage and current relationships in AC circuits under steady-state conditions.

11. Q11Medium

    What is the natural frequency of an LC circuit with an inductance of 30 mH and capacitance of 30 μF?

    • A.90 rad/s
    • B.182.04 rad/s
    • C.364 rad/s
    • D.45 rad/s
    Answer: 182.04 rad/s

    Explanation: The natural frequency ω₀ of an LC circuit is given by ω₀ = 1/√(LC). Substituting L = 30 mH and C = 30 μF, ω₀ ≈ √(1/(30×10⁻³ × 30×10⁻⁶)) ≈ 182.04 rad/s.

12. Q12Medium

    In a series LCR circuit, what is the rms potential drop across the resistor if the source voltage is 230 V and the circuit is at resonance?

    • A.Less than 230 V
    • B.Equal to the inductive reactance
    • C.Equal to the capacitive reactance
    • D.230 V
    Answer: 230 V

    Explanation: At resonance, the total impedance is purely resistive, so the entire source voltage appears across the resistor. Thus, the rms potential drop across the resistor equals the source voltage.

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