Ques.21. It is advisable to have _____ power factor.

Low

Large

Zero

No

Answer.2. Large

Explanation:-

The power factor of a circuit is a measure of its effectiveness in utilizing the apparent power. Thus 0.5 pf. of a circuit means that it will utilize only 50% of apparent power whereas OR p.f. would mean 80% utilization of apparent power. For this reason, we wish that the power factor of the circuit should be as large as possible (maximum value is 1).

22. At zero frequency the capacitive reactance is ______

Zero

Maximum

Infinity

Minimum

Answer.3. Infinity

Explanation:-

The capacitive reactance of RLC series circuit is

X_{C} = 1/2πfC

Now f = 0

X_{C} =1/2π0C

X_{C} = 1/0 = ∞

At zero frequency the capacitive reactance is infinity and the capacitor behaves like an open at 0 Hz frequency.

Ques.23. At zero frequency the Inductive reactance is ________

Zero

Maximum

Infinity

Minimum

Answer.1. Zero

Explanation:-

The Inductive reactance of RLC series circuit is

X_{L} = 2πfL

Now f = 0

X_{L} = 2π0L

X_{L} = 0

At zero frequency the Inductive reactance is zero and the capacitor behaves like short at 0 Hz frequency.

Ques.24. At frequencies below resonance, X_{C} > X_{L} the current _______ the source voltage

Lags

Become twice

Become half

Lead

Answer.4. Lead

Explanation:-

At a frequency below the resonant frequency, current leads the source voltage because the capacitive reactance is greater than the inductive reactance. The phase angle decreases as the frequency approach the resonant value and is 0° at resonance.

Ques.25. At frequencies above resonance, X_{L} > X_{C} and the current _______ the source voltage.

Lags

Become twice

Become half

Lead

Answer.1. Lags

Explanation:-

At frequencies above resonance, the current lags behind the source voltage, because the inductive reactance is greater than capacitive reactance. As the frequency goes higher, the phase angle approaches 90°.

Ques.26. What will be the phase difference of an RLC circuit illustrated by the impedance triangle?

37°

73°

3.7°

7.3°

Answer.1. 37°

Explanation:-

The impedance triangle when X_{C} > X_{L}

tanφ = (X_{C} − X_{L})/R

tanφ = 300/400

φ = 37°

Ques.27. Find the current in polar form of series RLC circuit when the resistance is 75 kΩ, capacitive reactance is 60 kΩ , inductive reactance is 25 kΩ and the source voltage is 10∠0°.

Ques.30. Find I, V_{R}, V_{L} and V_{C} at resonance for the circuit in Figure

I = 3.33 mA, V_{R }= 7.33 V, V_{L} = 3.33V, V_{C} = 3.33 V

I = 7.33 mA, V_{R }= 7.33 V, V_{L} = 3.33V, V_{C} = 3.33 V

I = 333 mA, V_{R }= 7.33 V, V_{L} = 3.33V, V_{C} = 3.33 V

I = 7.33 mA, V_{R }= 7.33 V, V_{L} = 7.33V, V_{C} = 7.33 V

Answer.1. I = 3.33 mA, VR = 7.33 V, VL = 3.33V, VC = 3.33 V

Explanation:-

Given

R = 1.5 kΩ

X_{C} = 1 kΩ

X_{L} = 1 kΩ

At resonance, I is maximum and equal to V_{S}/R

I = V_{S}/R = 5/1.5

I = 3.33 mA

Apply Ohm’s law to obtain the voltage magnitudes.

VR = IR = 3.33 × 2.2 = 7.33 V

V_{L} = IX_{L} = 3.33 × 1 = 3.33 V

V_{C} = IX_{C} = 3.33 × 1 = 3.33 V

All of the source voltage is dropped across the resistor. Also, V_{L} and V_{C} are equal in magnitude but opposite in phase. This causes these voltages to cancel, making the total reactive voltage zero.