NCERT Solutions for class 12 Physics | Chapter 7 - Alternating Current

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Questions
1 A transformer works on the principle of
A self induction
B electrical inertia
C mutual induction
D magnetic effect of the electrical current

Answer:mutual induction
2 Quality factor and power factor both have the dimensions of
A time
B frequency
C work
D angle

Answer:angle
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3 At resonance frequency the impedance in series LCR circuit is
A maximum
B minimum
C zero
D infinity

Answer:minimum
4 In a series LCR circuit the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. The phase difference between the applied voltage and the current in the circuit is
A 30°
B 45°
C 60°
D 0°

Answer:Lenz’s law is a consequence of the law of conservation of momentum.
5 At resonant frequency the current amplitude in series LCR circuit is
A maximum
B minimum
C zero
D infinity

Answer:maximum
6 In which of the following circuits the maximum power dissipation is observed?
A Pure capacitive circuit
B Pure inductive circuit
C Pure resistive circuit
D None of these

Answer:Pure resistive circuit
7 In a pure capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be
A remains same
B doubled
C halved
D zero

Answer:halved
8 Alternating voltage (V) is represented by the equation
A V(t) = Vm eωt
B V(t) = Vm sin ωt
C V(t) = Vm cot ωt
D V(t) = Vm tan ωt

Answer:V(t) = Vm sin ωt
9 Which of the following graphs represents the correct variation of capacitive reactance Xc with frequency v u?
A Class 12 Physics Chapter 7 Alternating Current
B Class 12 Physics Chapter 7 Alternating Current
C Class 12 Physics Chapter 7 Alternating Current
D Class 12 Physics Chapter 7 Alternating Current

Answer:Class 12 Physics Chapter 7 Alternating Current
10 For an ideal-step-down transformer, the quantity which is constant for both the coils is
A current in the coils
B voltage across the coils
C resistance of coils
D power in the coils

Answer:power in the coils
11 In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
A Only resistor
B Resistor and inductor
C Resistor and capacitor
D Only inductor

Answer:Resistor and capacitor
12 The rms value of potential difference V shown in the figure is

Class 12 Physics Chapter 7 Alternating Current

A V0/√3
B V0
C V0/√2
D V0/2

Answer:V0/√2
13 Two coils X and Y are connected in series across a 240 V, 50 Hz supply. The resistance of X is 5Ω and inductance of Y is 0.02 H. The power consumed is 3 kW and the power factor is 0.75. The impedance of the circuit is:
A 0.144 Ω
B 1.44 Ω
C 14.4 Ω
D 144 Ω

Answer:14.4 Ω
14 In pure inductive circuit current
A lags behind emf by π/2
B leads the emf by π/2
C lags behind emf by π
D leads the emf by π

Answer:lags behind emf by π/2
15 Faraday constant:
A depends on the amount of the electrolyte
B depends on the current in the electrolyte
C is a universal constant
D depends on the amount of charge passed through the electrolyte

Answer:is a universal constant
16 A rejector circuit is:
A series resonant circuit
B parallel resonant circuit
C LCR circuit
D None of these

Answer:parallel resonant circuit
17 The current through an inductor of 1H is given by I = 3t sin t. The voltage across the inductor of 1 H is:
A 3 sin t + 3 cos t
B 3 sin t + 3t cos t
C 3 cos t + t sin t
D 3t cos t + sin t

Answer:3 sin t + 3t cos t
18 The inductive reactance of an inductor coil of 1/π H at 50 Hz is:
A 50/π Ω
B π/50 Ω
C 100 Ω
D 50 Ω

Answer:100 Ω
19 In the series L-C-R circuit, the voltmeter and ammeter readings are:

Class 12 Physics Chapter 7 Alternating Current

A V = 100 Volts, I = 2A
B V = 100 Volts, I = 5A
C V = 1000 Volts, I = 2A
D V = 300 Volts, I = 1A

Answer:V = 100 Volts, I = 2A
20 The phase difference b/w the A.C. and e.m.f. π/2 Which of the following can not be the instituent of the circuit?
A LC
B L alone
C C alone
D RL

Answer:RL

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