Question bank Class 12 Physics Chapter 13. A.C CIRCUITS
Standard:- 12th
Question Bank
Subject:- Physics
Chapter 13. A.C CIRCUITS
MCQ’S (1 Mark Each)
1) An electric current in an LC – circuit at resonance is called -
a) The wattless current
b) The displacement current
c) The idle current
d) The apparent current
Ans: a) The wattless current
2) In a series LCR circuit at resonance, the applied emf and current are -
a) Out of phase
b) Differ in phase by π/4 radian.
c) Differ in phase by π/2 radian.
d) In phase
Ans: d) In phase
3) A series LCR resonant circuit is used as
a) A potential divider circuit.
b) A radio wave transmitter
c) A source of displacement current
d) A tunning circuit in a television receiver set.
Ans: d) A tunning circuit in a television receiver set.
4) If AC voltage is applied to pure capacitor, then voltage across the capacitor
a) Leads the current by phase angle π rad.
b) Leads the current by phase π/2 rad.
c) Lags the current by phase angle π rad.
d) Lags the current by phase angle π/2 rad.
Ans: d) Lags the current by phase angle π/2 rad.
5) A parallel LC resonant circuit is used as ….
a) a tunning circuit in a television receiver set.
b) a transformer
c) a rectifier
d) a filter circuit.
Ans: d) a filter circuit
6) An electric bulb operates 10 V d.c. If this bulb is connected to an a.c. source and gives normal brightness, then peak value of the source is …….
a) 141.4 V
b) 14.14 V
c) 1.414 V
d) 0.1414 V
Ans: b) 14.14 V
7) A coil of resistance 300Ω and inductance 1.0 H is connected across an alternating voltage of frequency Hz, therefore phase difference between the voltage and current in the circuit is
a) 1800
b) 900
c) 450
d) 00
Ans : c) 450
Very Short Answer (VSA) ( 1 MARK Each )
1. Define capacitive reactance.
2. A charged 10 micro farad capacitor is connected to a 81 mH inductor. What is the angular frequency of free oscillations of the circuit? ( Ans: 1.1 X 103 rad per sec)
3. State the equation for impedance Z in an A.C. circuit.
4. In LCR series circuit, what is the condition for current resonance?
5. State any one characteristic of a parallel LC AC resonance circuit.
6. What is the relation between average current and rms current over half cycle.
7. If the peak value of an alternating emf is 15V, what is its mean value over half cycle?
( Ans: 9.548 V)
Short Answer I (SA1) ( 2 MARKS Each )
1. State the average or mean value of an alternating emf? Obtain the expression for it.
2. Explain term impedance and state the formula for it in the case of an LCR series circuit.
3. State any two characteristics of a series LCR AC resonance circuit.
4. In LCR series circuit, what is the (a) impedance and (b) reactance at current resonance?
5. A series LCR circuit has resistance 10Ω and reactance is 7√2 Ω. What is the impedance of the circuit? ( Ans: 14.07Ω )
6. A coil of resistance 10 Ω and inductance 100mH and a capacitor of variable capacitance are connected across a 20V,50Hz A.C. supply. At what capacitance will resonance occur? (Ans: 318.5μ𝐹)
7. Find the current in a circuit consisting of a coil and a capacitor in series with an A.C source of 110V (r.m.s.), 60Hz.The inductance of a coil in 0.80 H and its resistance is 50Ω.The capacitance of a capacitor is 8μF. (Ans: 𝐼 = 1.88𝐴)
8. A 0.5μF capacitor is discharged through a 10 millihenry inductor. Find the frequency of discharged. ( Ans: 2.25 × 10 𝐻𝑧)
9. What is the capacitive reactance of a capacitor of 5μF at a frequency (1) 50 Hz and (2) 20KHZ? (Ans: 636.94Ω, 1.59Ω)
Short Answer II (SA2) ( 3 MARKS Each )
1) State the rms value of an alternating current? Write the relation between the rms value and peak value of an alternating current that varies with time.
2) Explain the term inductive reactance. State its unit and dimensions.
3) Explain the term capacitive reactance. State its unit and dimensions.
4) What is the inductive reactance of a coil of inductance 10mH at a frequency (1) 50Hz (2) 1000Hz (3) 20kHz?
(Ans: 3.14Ω,62.8Ω,1256Ω)
5) An alternating emf of 230V,50Hz is connected across a pure ohmic resistance of 50Ω.
Find (1) the current (2) equations for instantaneous values of current and voltage.
(Ans: 𝐼 = 4.6𝐴 , E0 =325.27 V, I0=6.5A, 𝐼 = 6.5 𝑠𝑖𝑛 100𝜋𝑡,𝐸 = 325.27 𝑠𝑖𝑛 100𝜋𝑡)
6) A radio can tune over the frequency range of a portion of MW broadcastband( 800kHz-1200kHz). If its LC circuit has an effective inductance of 200mH, what must be the range of its variable condenser? (Ans: 88pF to 198pF)
Long Answer ( LA) ( 4 marks Each)
1) Obtain the expression for the applied emf and the effective resistance of the circuit when alternating emf is applied to an LR circuit.
2) Obtain the expression for the applied emf and the effective resistance of the circuit when alternating emf is applied to an CR circuit.
3) Obtain the expression for the resonant frequency of the LCR series circuit and explain electrical resonance in an LCR series circuit.
