PC General Theory and Calculations 2 — Questions and Answers
Question 1: A 240V single-phase circuit draws 15A. What is the apparent power in VA?
- 1,800 VA
- 3,600 VA (Correct answer)
- 240 VA
- 960 VA
Correct answer: 3,600 VA
Apparent power S = V × I = 240V × 15A = 3,600 VA.
Question 2: Which formula correctly expresses Ohm's Law for current?
- I = R / V
- I = V × R
- I = V / R (Correct answer)
- I = P / R
Correct answer: I = V / R
Ohm's Law states I = V / R, where I is current, V is voltage, and R is resistance.
Question 3: Three resistors of 6Ω, 12Ω, and 4Ω are connected in parallel. What is the total equivalent resistance?
- 22 Ω
- 2 Ω (Correct answer)
- 7.33 Ω
- 1.5 Ω
Correct answer: 2 Ω
1/R_T = 1/6 + 1/12 + 1/4 = 2/12 + 1/12 + 3/12 = 6/12, so R_T = 2Ω.
Question 4: What is the power factor of a purely resistive AC circuit?
- 0
- 0.5
- 0.707
- 1.0 (Correct answer)
Correct answer: 1.0
A purely resistive circuit has zero phase angle between V and I, so PF = cos(0°) = 1.0.
Question 5: A conductor has a resistance of 0.5Ω and carries 20A. What is the power dissipated as heat?
- 10 W
- 200 W (Correct answer)
- 0.025 W
- 40 W
Correct answer: 200 W
P = I² × R = (20)² × 0.5 = 400 × 0.5 = 200 W.
Question 6: 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)
- Total impedance equals the sum of XL and XC
Correct answer: Inductive and capacitive reactances are equal and cancel
At resonance, XL = XC, they cancel, and the circuit impedance equals resistance only.
Question 7: What does a lagging power factor indicate about a load?
- The load is purely resistive
- The current leads the voltage
- The load is predominantly inductive (Correct answer)
- The load is predominantly capacitive
Correct answer: The load is predominantly inductive
A lagging power factor means current lags behind voltage, which is characteristic of inductive loads.
PC General Theory and Calculations 2
A 240V single-phase circuit draws 15A.
What is the apparent power in VA?