Class 12 Alternating Current CBSE Questions & Answers

Class 12 · Alternating Current

This is Physics Class 12 Alternating Current CBSE Questions & Answers. There are 15 questions in this test with each question having around four answer choices.

Questions & Answers

1
The correct equation for a series LCR circuit excited by AC is
  • A
    \(L{{{d^2}q} \over {d{t^2}}} + 2R{{dq} \over {dt}} + {q \over C} = 0\)
  • B
    \(L{{{d^2}q} \over {d{t^2}}} + R{{dq} \over {dt}} + {q \over C} = {v_m}sin\omega t\)
    Correct
  • C
    \(L{{{d^2}q} \over {d{t^2}}} + R{{dq} \over {dt}} + 2{q \over C} = {v_m}sin\omega t\)
  • D
    \(L{{{d^2}q} \over {d{t^2}}} + R{{dq} \over {dt}} + {q \over C} = 0\)
2
The current in a series LCR circuit excited by AC of frequency \(\omega \) is in general
  • A
    imsin(\(\omega {\rm{t}}\) + 90 )
  • B
    2imsin(\(\omega {\rm{t}}\))
  • C
    imsin(\(\omega {\rm{t }} + {\rm{ }}\phi \))
    Correct
  • D
    \({{\rm{i}}_{\rm{m}}}{\rm{sin}}\omega {\rm{t}}\)
3
For a series LCR circuit the input impedance at resonance
  • A
    equals the resistance R
    Correct
  • B
    equals the resistance \({\rm{R}} + {\rm{j}}\omega {\rm{L}}\)
  • C
    equals the resistance \(\omega {\rm{L}}\)
  • D
    equals the resistance \({\rm{1}}/\omega {\rm{C}}\)
4
At resonance the current in an LCR circuit
  • A
    is zero
  • B
    is local minimum
  • C
    is minimum
  • D
    is maximum
    Correct
5
The sharpness of resonance is given by
  • A
    \({\omega _0}{\rm{L}}\) /C
  • B
    \({\omega _0}{\rm{L}}\) C
  • C
    \({\omega _0}{\rm{L}}\) /R
    Correct
  • D
    \({\omega _0}{{\rm{L}}^{\rm{2}}}\)/R
6
If V and I are rms voltage and current in an AC circuit, and \({\rm{cos}}\phi \) the power factor the power consumed by a reactive load is in general
  • A
    VI
  • B
    VI \({\rm{sin}}\phi \)
  • C
    VI \({\rm{cos}}\phi \)
    Correct
  • D
    2VI \({\rm{cos}}\phi \)
7
For a parallel LC circuit the resonant frequency is
  • A
    \({1 \over {LC}}\)
  • B
    \({1 \over {\sqrt {2LC} }}\)
  • C
    \({2 \over {\sqrt {LC} }}\)
  • D
    \({1 \over {\sqrt {LC} }}\)
    Correct
8
For a parallel ideal LC circuit at resonance the input impedance across L or C is
  • A
    zero
  • B
    \({\omega _0}{\rm{C}}\)
  • C
    infinite
    Correct
  • D
    \({\omega _0}{\rm{L}}\)
9
Transformer uses the principle of
  • A
    Induction
    Correct
  • B
    least action
  • C
    charge conservation
  • D
    conduction
10
For a transformer the ratio of output to input equals
  • A
    input turns divided by output turns
  • B
    output turns divided by twice the input turns
  • C
    output turns multiplied by input turns
  • D
    output turns divided by input turns
    Correct
11
A light bulb is rated at 50W for a 220 V supply. The resistance of the bulb, the peak voltage of the source and the rms current through the bulb are
  • A
    468 \(\Omega \), 411 V, 0.267A
  • B
    968 \(\Omega \), 350 V, 0.327A
  • C
    768 \(\Omega \), 391 V, 0.297A
  • D
    968 \(\Omega \), 311 V, 0.227A
    Correct
12
A pure inductor of 20.0 mH is connected to a source of 230 V. Inductive reactance and rms current in the circuit (if the frequency of the source is 50 Hz). are
  • A
    8.28 \(\Omega \), 36.6 A
  • B
    6.28 \(\Omega \), 36.6 A
    Correct
  • C
    6.28 \(\Omega \), 46.6 A
  • D
    9.28 \(\Omega \), 36.6 A
13
A lamp is connected in series with a capacitor and connected to an AC source. As the capacitance value is decreased.
  • A
    The lamp glows brighter
  • B
    The lamp starts turning on and off
  • C
    The lamp does not glow
  • D
    The lamp glows dimmer and dimmer
    Correct
14
25.0 \(\mu {\rm{F}}\) capacitor is connected to a 220 V, 50 Hz source. Capacitive reactance and the current (rms) in the capacitor are circuit
  • A
    127 \(\Omega \), 1.73 A
    Correct
  • B
    127 \(\Omega \) , 1.23 A
  • C
    102 \(\Omega \) , 1.03 A
  • D
    111 \(\Omega \) , 1.73 A
15
A light bulb and an open coil inductor are connected to an ac source . On replacing the open coil inductor with an iron core inductor
  • A
    The bulb glows dimmer
    Correct
  • B
    The bulb glows on and off
  • C
    The bulb glow is unaffected
  • D
    The bulb glows brighter