309A Circuit Theory and Calculations 4 — Questions and Answers
Question 1: What is Kirchhoff's Voltage Law (KVL)?
- The sum of currents entering a node equals the sum leaving
- The sum of all voltages around a closed loop equals zero (Correct answer)
- Voltage is directly proportional to current
- Power equals voltage times current
Correct answer: The sum of all voltages around a closed loop equals zero
Kirchhoff's Voltage Law states that the algebraic sum of all voltage drops and rises around any closed loop in a circuit must equal zero, reflecting conservation of energy.
Question 2: A 10 kW, 240 V single-phase load operates for 8 hours. How many kWh of energy are consumed?
- 40 kWh
- 80 kWh (Correct answer)
- 120 kWh
- 1920 kWh
Correct answer: 80 kWh
Energy = Power × Time = 10 kW × 8 hours = 80 kWh. Energy consumption is the product of power and the duration of use.
Question 3: What is the phase angle between voltage and current in a purely capacitive circuit?
- 0°
- 45°
- 90° (current leads voltage) (Correct answer)
- 90° (voltage leads current)
Correct answer: 90° (current leads voltage)
In a purely capacitive circuit, current leads voltage by 90°. This is remembered by the mnemonic 'ICE' — In a Capacitor, current (I) leads voltage (E).
Question 4: What is the total resistance of four 100 Ω resistors connected in parallel?
- 25 Ω (Correct answer)
- 50 Ω
- 100 Ω
- 400 Ω
Correct answer: 25 Ω
For identical resistors in parallel: Rt = R/n = 100/4 = 25 Ω. The total resistance is always less than the smallest individual resistor.
Question 5: A transformer has a turns ratio of 10:1. If the primary voltage is 4800 V, what is the secondary voltage?
- 48 V
- 240 V
- 480 V (Correct answer)
- 48,000 V
Correct answer: 480 V
Secondary voltage = Primary voltage / Turns ratio = 4800 / 10 = 480 V. A step-down transformer reduces voltage by the turns ratio.
Question 6: What is the reactive power in a circuit drawing 20 A at 240 V with a power factor of 0.8 lagging?
- 1920 VAR
- 2880 VAR (Correct answer)
- 3840 VAR
- 4800 VAR
Correct answer: 2880 VAR
Apparent power S = 240 × 20 = 4800 VA. Real power P = 4800 × 0.8 = 3840 W. Reactive power Q = √(S² - P²) = √(4800² - 3840²) = √(23,040,000 - 14,745,600) = √8,294,400 = 2880 VAR.
309A Circuit Theory and Calculations 4
What is Kirchhoff's Voltage Law (KVL)?