IEC Electrical Theory and Principles 2 — Questions and Answers
Question 1: What is the phase relationship between voltage and current in a purely inductive circuit?
- Current leads voltage by 90°
- Current lags voltage by 90° (Correct answer)
- Current and voltage are in phase
- Current leads voltage by 45°
Correct answer: Current lags voltage by 90°
In a purely inductive circuit, current lags voltage by 90° because the inductor opposes changes in current.
Question 2: A transformer has 500 turns on the primary and 100 turns on the secondary. If the primary voltage is 240V, what is the secondary voltage?
- 1200V
- 48V (Correct answer)
- 240V
- 120V
Correct answer: 48V
Using the turns ratio formula: Vs = Vp × (Ns/Np) = 240 × (100/500) = 48V.
Question 3: Which of the following best describes impedance in an AC circuit?
- The DC resistance only
- The total opposition to AC current flow including resistance and reactance (Correct answer)
- The inductive reactance only
- The capacitive reactance only
Correct answer: The total opposition to AC current flow including resistance and reactance
Impedance (Z) is the total opposition to AC current and is the vector sum of resistance and reactance.
Question 4: What happens to capacitive reactance as frequency increases?
- It increases proportionally
- It remains constant
- It decreases (Correct answer)
- It doubles
Correct answer: It decreases
Capacitive reactance (Xc = 1/2πfC) is inversely proportional to frequency, so it decreases as frequency increases.
Question 5: In a series RLC circuit at resonance, which statement is true?
- Inductive reactance exceeds capacitive reactance
- Capacitive reactance exceeds inductive reactance
- Inductive and capacitive reactances are equal and cancel (Correct answer)
- Impedance is at its maximum value
Correct answer: Inductive and capacitive reactances are equal and cancel
At resonance, XL = XC, they cancel each other, and the circuit impedance equals only the resistance.
Question 6: What is the unit of magnetic flux?
- Tesla
- Weber (Correct answer)
- Henry
- Gauss
Correct answer: Weber
The Weber (Wb) is the SI unit of magnetic flux, where 1 Wb = 1 V·s.
Question 7: According to Faraday's Law, EMF is induced in a conductor when:
- Current flows through it
- It is placed in a static magnetic field
- The magnetic flux through it changes (Correct answer)
- It is heated
Correct answer: The magnetic flux through it changes
Faraday's Law states that induced EMF is proportional to the rate of change of magnetic flux through the conductor.
IEC Electrical Theory and Principles 2
What is the phase relationship between voltage and current in a purely inductive circuit?