NECA Electrical Theory & Principles 3 — Questions and Answers
Question 1: What is the phase relationship between voltage and current in a purely capacitive AC circuit?
- Current leads voltage by 90° (Correct answer)
- Voltage leads current by 90°
- They are in phase
- Current lags voltage by 45°
Correct answer: Current leads voltage by 90°
In a purely capacitive circuit, current leads voltage by 90° because the capacitor must charge before a voltage develops across it.
Question 2: Which of the following best describes electric potential difference (voltage)?
- The rate of electron flow past a point
- The work done per unit charge to move charge between two points (Correct answer)
- The opposition to current flow
- The total energy stored in a circuit
Correct answer: The work done per unit charge to move charge between two points
Voltage (potential difference) is defined as the work done per unit charge (W/Q) required to move a charge between two points in an electric field.
Question 3: A 120V circuit has a load drawing 15A. What is the power consumed?
- 8 W
- 1,800 W (Correct answer)
- 135 W
- 0.125 W
Correct answer: 1,800 W
Power P = V × I = 120 V × 15 A = 1,800 W (1.8 kW).
Question 4: In a series RLC circuit at resonance, the impedance is:
- Maximum and purely reactive
- Equal to the inductive reactance
- Minimum and equal to resistance only (Correct answer)
- Zero
Correct answer: Minimum and equal to resistance only
At resonance, inductive reactance equals capacitive reactance and they cancel, leaving only resistance — making impedance minimum and purely resistive.
Question 5: The skin effect in conductors causes high-frequency AC current to:
- Flow uniformly through the cross-section
- Concentrate near the outer surface (Correct answer)
- Increase conductor resistance permanently
- Reduce conductor temperature
Correct answer: Concentrate near the outer surface
The skin effect causes AC current at higher frequencies to flow primarily near the conductor's surface, effectively reducing the useful cross-sectional area and increasing resistance.
Question 6: What does the term 'impedance' refer to in an AC circuit?
- Only the resistance of the circuit
- Total opposition to AC current flow, combining resistance and reactance (Correct answer)
- The ratio of inductance to capacitance
- The peak voltage divided by peak current
Correct answer: Total opposition to AC current flow, combining resistance and reactance
Impedance (Z) is the total opposition to AC current, expressed in ohms, combining both resistance (R) and reactance (X) as Z = √(R² + X²).
Question 7: Lenz's Law states that the direction of an induced current is such that it:
- Aids the change in flux that caused it
- Opposes the change in flux that caused it (Correct answer)
- Is always clockwise
- Increases the source EMF
Correct answer: Opposes the change in flux that caused it
Lenz's Law states that induced current flows in a direction that opposes the change in magnetic flux that caused it, consistent with conservation of energy.
NECA Electrical Theory & Principles 3
What is the phase relationship between voltage and current in a purely capacitive AC circuit?