IEC Electrical Theory and Principles 5 — Questions and Answers
Question 1: The time constant (τ) of an RL circuit is defined as:
- τ = R × C
- τ = L / R (Correct answer)
- τ = R / L
- τ = L × C
Correct answer: τ = L / R
The time constant of an RL circuit is τ = L/R, representing the time for current to reach approximately 63.2% of its final value.
Question 2: What is the phase angle in a circuit with R = 30 Ω and XL = 40 Ω?
- 36.9°
- 53.1° (Correct answer)
- 45°
- 60°
Correct answer: 53.1°
θ = arctan(XL/R) = arctan(40/30) = arctan(1.333) ≈ 53.1°.
Question 3: Reactive power in an AC circuit is measured in:
- Watts (W)
- Volt-amperes (VA)
- Volt-amperes reactive (VAR) (Correct answer)
- Kilowatts (kW)
Correct answer: Volt-amperes reactive (VAR)
Reactive power, which represents energy stored and released by inductors and capacitors, is measured in volt-amperes reactive (VAR).
Question 4: A 240V circuit draws 20A with a power factor of 0.8. What is the true power consumed?
- 4800 W
- 3840 W (Correct answer)
- 6000 W
- 2400 W
Correct answer: 3840 W
True power P = V × I × PF = 240 × 20 × 0.8 = 3840 W.
Question 5: Which of the following materials is classified as a semiconductor?
- Copper
- Silver
- Silicon (Correct answer)
- Aluminum
Correct answer: Silicon
Silicon is a semiconductor, having electrical conductivity between that of conductors and insulators.
Question 6: The skin effect in AC conductors causes current to:
- Flow uniformly throughout the conductor cross-section
- Concentrate toward the outer surface of the conductor (Correct answer)
- Concentrate at the center of the conductor
- Flow only at very low frequencies
Correct answer: Concentrate toward the outer surface of the conductor
At higher AC frequencies, current tends to flow near the outer surface (skin) of the conductor, increasing effective resistance.
Question 7: What is the total impedance of a series circuit with R = 6 Ω, XL = 10 Ω, and XC = 2 Ω?
- 18 Ω
- 10 Ω (Correct answer)
- 6 Ω
- 14 Ω
Correct answer: 10 Ω
Z = √(R² + (XL - XC)²) = √(36 + (10-2)²) = √(36 + 64) = √100 = 10 Ω.
IEC Electrical Theory and Principles 5
The time constant (τ) of an RL circuit is defined as: