NETA AC/DC Theory and Circuits 5 — Questions and Answers
Question 1: In a three-phase delta-connected system with a line voltage of 208 V, what is the phase voltage?
- 120 V
- 208 V (Correct answer)
- 360 V
- 416 V
Correct answer: 208 V
In a delta connection, the phase voltage equals the line voltage, so Vphase = 208 V.
Question 2: What happens to the total capacitance when capacitors are connected in series?
- Total capacitance equals the sum of all capacitances
- Total capacitance is less than the smallest individual capacitance (Correct answer)
- Total capacitance equals the largest individual capacitance
- Total capacitance doubles with each added capacitor
Correct answer: Total capacitance is less than the smallest individual capacitance
Capacitors in series combine like resistors in parallel; the total is always less than the smallest individual value.
Question 3: Skin effect in AC conductors causes current to:
- Concentrate at the center of the conductor
- Distribute evenly throughout the conductor cross-section
- Concentrate near the outer surface of the conductor (Correct answer)
- Flow only in one direction
Correct answer: Concentrate near the outer surface of the conductor
At higher AC frequencies, skin effect causes current density to concentrate near the conductor surface, increasing effective resistance.
Question 4: A 60 Hz AC circuit has an inductance of 0.1 H. What is the inductive reactance?
- 6 Ω
- 37.7 Ω (Correct answer)
- 60 Ω
- 376.99 Ω
Correct answer: 37.7 Ω
XL = 2πfL = 2π × 60 × 0.1 ≈ 37.7 Ω.
Question 5: Which law states that the algebraic sum of all currents entering and leaving a node in a circuit equals zero?
- Ohm's Law
- Kirchhoff's Voltage Law
- Kirchhoff's Current Law (Correct answer)
- Faraday's Law
Correct answer: Kirchhoff's Current Law
Kirchhoff's Current Law (KCL) states that the sum of currents entering a node equals the sum leaving it.
Question 6: An AC waveform has a peak value of 170 V. What is the approximate RMS value?
- 85 V
- 120 V (Correct answer)
- 170 V
- 240 V
Correct answer: 120 V
Vrms = Vpeak / √2 = 170 / 1.414 ≈ 120 V.
Question 7: In a DC circuit, if the supply voltage remains constant and the circuit resistance is doubled, the power dissipated will:
- Double
- Quadruple
- Remain the same
- Be reduced by half (Correct answer)
Correct answer: Be reduced by half
P = V²/R; with constant voltage, doubling R halves the power dissipated.
NETA AC/DC Theory and Circuits 5
In a three-phase delta-connected system with a line voltage of 208 V, what is the phase voltage?