Important MCQ of RL Circuit With Explanation – 2021

Ques.41. At low frequency, the Inductor in RL low pass filter acts as

  1. Open circuit
  2. Short circuit
  3. Variable Inductance
  4. None of the above

Answer.2. Short circuit

Explanation:-

The circuit input is a sinusoidal voltage with variable frequency. The output is defined as the voltage across the resistor. At low frequencies, the impedance of the inductor is very small compared with that of the resistor and the inductor effectively functions as a short circuit.

As the frequency increases, the impedance of the inductor increases resulting in a corresponding increase in the voltage drop across the inductor and a corresponding decrease in the output voltage. There is also a difference in phase angle between the input and the output voltage.

 

Ques.42. The RC low pass filter is common as compare to RL low pass filter because

  1. An inductor is a rarely used device
  2. Difficult to work with Inductive circuit
  3. The capacitor is cheaper than Inductor
  4. All of the above

Answer.3. The capacitor is cheaper than Inductor

Explanation:-

The RL low-pass filter. One can use resistors and capacitors to build either low-pass filters or high-pass filters. It is also possible instead to use resistors and inductors for these tasks.

In practical circuits, RC filters are much more common than RL filters, partly because it is easier to build an essentially ideal capacitor than to build an ideal inductor. (There is always resistance in the wires used to wind an actual inductor, and there is always capacitance between the windings.) Also, inductors are generally bulkier and more expensive than the capacitors that could be used to build comparable filters. However, one does see RL filters in some high-frequency applications.

 

Ques.43. What is the inductance of a coil whose resistance is 100? if it draws 0.55A from a 110-V, 60-Hz power line?

  1. 0.459 H
  2. 45.9 H
  3. 4.59 H
  4. 45 H

Answer.1. 0.459 H

Explanation:-

Z = V/I = 100/0.55 = 200 Ω

$\begin{array}{l} {\rm{Z = }}\sqrt {{R^2} + {X^2}_L} \\ \\ {{\rm{X}}_{\rm{L}}}{\rm{ = }}\sqrt {{Z^2} – {R^2}} \\ \\ {{\rm{X}}_{\rm{L}}}{\rm{ = }}\sqrt {{{(200)}^2} – {{(100)}^2}} \\ \\ {{\rm{X}}_{\rm{L}}}{\rm{ = }}173\Omega \end{array}$

Inductive Reactance

XL = 2πfL

L = XL /2πf

L = 173/6.28 × 60

L = 0.459 H

 

Ques.44. A 225-µH choke coil of negligible resistance is to limit the current through it to 25 mA when 40V is impressed across it. What is the frequency of the current?

  1. 1 MHz
  2. 0.2 MHz
  3. 0.1 MHz
  4. 0.5 MHz

Answer.1. 1 MHz

Explanation:-

Inductive reactance

XL = VL/IL

= 40/(25 × 10−3) = 1600 Ω

f = XL/2πfL

f = 1600/6.28 × (255 × 10−6)

f = 106 = 1 MHz

 

Ques.45. When the inductor of RL circuit is short circuit the total inductance

  1. Increase
  2. Decrease
  3. Remain same
  4. Any of the above

Answer.2. Decrease

Explanation:-

It is possible for some of the windings of coils to short together as a result of damaged insulation. This failure mode is much less likely than the open coil and is difficult to detect. Shorted windings may result in a reduction in inductance because the inductance of a coil is proportional to the square of the number of turns. A short between windings effectively reduces the number of turns, which may or may not have an adverse effect on the circuit depending on the number of turns that are shorted.

 

Ques.46. In a parallel RL circuit, an open resistor or inductor will cause the total current to _____

  1. Remain same
  2. Decrease
  3. Increase
  4. None of the above

Answer.2. Decrease

Explanation:-

Open Components in Parallel Circuits:– In a parallel RL circuit, an open resistor or inductor will cause the total current to decrease because the total impedance will increase. Obviously, the branch with the open component will have zero current.

 

Ques.47. When the resistor of the RL circuit is open, then the inductor voltage______.

  1. Increases
  2. Decreases
  3. Remains Same
  4. Zero

Answer.4. Zero

Explanation:-

Effect of an Open Resistor:- When the resistor is open, there is no current and the inductor voltage is zero.

 

Ques.48. When the inductor of the RL circuit is open, then the resistor voltage______.

  1. Increases
  2. Decreases
  3. Zero
  4. Remain same

Answer.3. Zero

Explanation:-

Effect of an Open Inductor:- The most common failure mode for inductors occurs when the winding opens as a result of excessive current or a mechanical contact failure. It is easy to see how an open coil affects the operation of a basic series RL circuit. Obviously, there is no current path; therefore, the resistor voltage is zero, and the total applied voltage appears across the inductor.

 

Ques.49. In RL circuit high pass filter the circuit ______ low frequency and ______ high frequency.

  1. Stop, Pass
  2. Pass, Stop
  3. Stop, Stop
  4. Pass, Pass

Answer.1. Stop, Pass

Explanation:-

In RL high pass filter the circuit tends to prevent lower frequency signals from appearing on the output but permits higher frequency signals to pass through from input to output.

 

Ques.50. A steady current of 20 mA is passed through a coil with an inductance of 100 mH. What is the voltage induced by the coil?

  1. 10 V
  2. 5 V
  3. 2 V
  4. 0 V

Answer.4. 0 V

Explanation:-

If the circuit is dc, the rate of change of current Δi/Δt = 0. So

${V_L} = L\frac{{\Delta i}}{{\Delta t}}$

L(0) = 0 V

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