IEC Electrical Theory and Principles 3 — Questions and Answers
Question 1: A 10 µF capacitor is connected to a 120V, 60 Hz supply. What is the capacitive reactance?
- 265 Ω (Correct answer)
- 132 Ω
- 530 Ω
- 66 Ω
Correct answer: 265 Ω
Xc = 1/(2πfC) = 1/(2π × 60 × 10×10⁻⁶) ≈ 265 Ω.
Question 2: What is the power factor of a purely resistive circuit?
- 0
- 0.5
- 1.0 (Correct answer)
- 0.707
Correct answer: 1.0
A purely resistive circuit has voltage and current in phase, giving a power factor of 1.0 (unity).
Question 3: Kirchhoff's Current Law (KCL) states that:
- The sum of voltages around any closed loop equals zero
- The sum of currents entering a node equals the sum leaving it (Correct answer)
- Current is proportional to resistance
- Voltage drops are additive in parallel circuits
Correct answer: The sum of currents entering a node equals the sum leaving it
KCL states that the algebraic sum of all currents at any node in a circuit equals zero (current in = current out).
Question 4: Two 100 Ω resistors are connected in parallel. What is the equivalent resistance?
- 200 Ω
- 100 Ω
- 50 Ω (Correct answer)
- 25 Ω
Correct answer: 50 Ω
For two equal resistors in parallel, Req = R/2 = 100/2 = 50 Ω.
Question 5: Which formula correctly expresses electrical power in terms of voltage and resistance?
- P = V × R
- P = V² / R (Correct answer)
- P = R / V²
- P = V / R²
Correct answer: P = V² / R
Substituting I = V/R into P = VI gives P = V²/R.
Question 6: What is the term for the opposition that a capacitor offers to AC current flow?
- Resistance
- Inductance
- Capacitive reactance (Correct answer)
- Impedance
Correct answer: Capacitive reactance
Capacitive reactance (Xc) is the opposition a capacitor presents to AC current, measured in ohms.
Question 7: In a 3-phase wye (Y) connected system, the line voltage is related to the phase voltage by:
- VL = Vφ
- VL = Vφ / √3
- VL = Vφ × √3 (Correct answer)
- VL = Vφ × 3
Correct answer: VL = Vφ × √3
In a wye system, line voltage equals phase voltage multiplied by √3 (approximately 1.732).
IEC Electrical Theory and Principles 3
A 10 µF capacitor is connected to a 120V, 60 Hz supply.
What is the capacitive reactance?