ISCET AC Circuits and Analysis Questions and Answers — Questions and Answers
Question 1: In a series RLC circuit, what is the condition known as resonance?
- The total impedance is equal to the resistance. (Correct answer)
- The inductive reactance is greater than the capacitive reactance.
- The capacitive reactance is greater than the inductive reactance.
- The phase angle between voltage and current is 90 degrees.
Correct answer: The total impedance is equal to the resistance.
Resonance in a series RLC circuit occurs at the specific frequency where the inductive reactance (XL) equals the capacitive reactance (XC). When XL = XC, they cancel each other out in the impedance formula Z = √(R² + (XL - XC)²). This simplifies the impedance to Z = √R², meaning the total impedance (Z) is equal to the resistance (R), and the circuit behaves as purely resistive. At this point, the current in the circuit reaches its maximum value.
Question 2: A technician is analyzing an AC circuit and notes that the current waveform leads the voltage waveform. Which of the following best describes the load?
- Purely resistive
- Primarily inductive
- Primarily capacitive (Correct answer)
- The load is at resonance.
Correct answer: Primarily capacitive
In an AC circuit, the phase relationship between current and voltage depends on the type of load. For a capacitive load, the current leads the voltage. Conversely, for an inductive load, the current lags the voltage. In a purely resistive circuit, the current and voltage are in phase.
Question 3: What is the primary purpose of improving the power factor in an AC circuit, ideally towards unity (1.0)?
- To increase the circuit's resonant frequency.
- To decrease the apparent power consumed by the load.
- To maximize the phase angle between voltage and current.
- To reduce the total current drawn from the source for the same amount of real power. (Correct answer)
Correct answer: To reduce the total current drawn from the source for the same amount of real power.
Power factor is the ratio of real power (used to do work) to apparent power (total power supplied). A low power factor indicates that a significant portion of the current is not performing useful work. By improving the power factor towards unity (1.0), the apparent power becomes nearly equal to the real power. This means less total current is required from the source to deliver the same amount of real power, which reduces energy losses in the distribution system and allows for more efficient operation.
Question 4: A series AC circuit consists of a 30 Ω resistor and a capacitor with a capacitive reactance (XC) of 40 Ω. What is the total impedance (Z) of this circuit?
- 70 Ω
- 10 Ω
- 50 Ω (Correct answer)
- 1200 Ω
Correct answer: 50 Ω
For a series RC circuit, the total impedance (Z) is calculated using the formula Z = √(R² + XC²). In this case, R = 30 Ω and XC = 40 Ω. So, Z = √(30² + 40²) = √(900 + 1600) = √2500 = 50 Ω. Impedance represents the total opposition to current flow in an AC circuit.
Question 5: Which of the following will cause the resonant frequency of a series LC circuit to decrease?
- Decreasing the inductance.
- Increasing the capacitance. (Correct answer)
- Increasing the source voltage.
- Adding a resistor in series.
Correct answer: Increasing the capacitance.
The formula for resonant frequency (f₀) in an LC circuit is f₀ = 1 / (2π√(LC)). This formula shows that the resonant frequency is inversely proportional to the square root of both the inductance (L) and the capacitance (C). Therefore, increasing either the capacitance or the inductance will cause the resonant frequency to decrease.
Question 6: A technician measures an AC motor load and finds the real power is 1200 W and the apparent power is 1500 VA. What is the power factor of this load?
- 1.25 leading
- 0.8 lagging (Correct answer)
- 1.0 (unity)
- 0.9 lagging
Correct answer: 0.8 lagging
The power factor (PF) is defined as the ratio of real power (P) to apparent power (S). The formula is PF = P / S. Given P = 1200 W and S = 1500 VA, the power factor is 1200 / 1500 = 0.8. Since motors are inductive loads, they cause the current to lag the voltage, resulting in a lagging power factor.
ISCET AC Circuits and Analysis Questions and Answers
In a series RLC circuit, what is the condition known as resonance?