IEC Electrical Theory and Principles 4 — Questions and Answers
Question 1: A conductor carrying current is placed in a magnetic field. The resulting force on the conductor is described by:
- Faraday's Law
- The Left-Hand Rule (motor action) (Correct answer)
- Lenz's Law
- The Right-Hand Rule for generators
Correct answer: The Left-Hand Rule (motor action)
The Left-Hand Rule (Fleming's) determines the direction of force on a current-carrying conductor in a magnetic field (motor action).
Question 2: What is the effect of increasing wire gauge number (AWG) on conductor resistance?
- Resistance decreases
- Resistance increases (Correct answer)
- Resistance is unaffected
- Resistance doubles
Correct answer: Resistance increases
Higher AWG numbers indicate smaller diameter conductors, which have higher resistance per unit length.
Question 3: Which of the following is true about apparent power in an AC circuit?
- It is measured in watts
- It equals true power times power factor
- It is the product of RMS voltage and RMS current (Correct answer)
- It can never exceed true power
Correct answer: It is the product of RMS voltage and RMS current
Apparent power (S) in volt-amperes (VA) equals the product of RMS voltage and RMS current: S = Vrms × Irms.
Question 4: Lenz's Law is best described as:
- The induced EMF drives a current that opposes the change causing it (Correct answer)
- The magnetic field is proportional to current
- Voltage equals current times resistance
- Power equals voltage squared divided by resistance
Correct answer: The induced EMF drives a current that opposes the change causing it
Lenz's Law states that the induced current flows in a direction to oppose the change in magnetic flux that caused it.
Question 5: What is the resonant frequency of a series LC circuit with L = 50 mH and C = 20 µF?
- 159 Hz (Correct answer)
- 1000 Hz
- 503 Hz
- 79.6 Hz
Correct answer: 159 Hz
fr = 1/(2π√LC) = 1/(2π√(0.05 × 20×10⁻⁶)) ≈ 159 Hz.
Question 6: In a delta (Δ) connected 3-phase system, the relationship between line current and phase current is:
- IL = Iφ
- IL = Iφ × √3 (Correct answer)
- IL = Iφ / √3
- IL = Iφ × 3
Correct answer: IL = Iφ × √3
In a delta connection, the line current equals the phase current multiplied by √3.
Question 7: Which property of a material determines how easily it can be magnetized?
- Reluctance
- Permeability (Correct answer)
- Resistivity
- Conductance
Correct answer: Permeability
Permeability (µ) measures how easily a material supports the formation of a magnetic field within it.
IEC Electrical Theory and Principles 4
A conductor carrying current is placed in a magnetic field.
The resulting force on the conductor is described by: