Alternating Current

Alternating Current Class 12 Physics MCQs

1. Alternating voltage (V) is represented by the equation
(a) V(t) = V_m e^ωt
(b) V(t) = V_m sin ωt
(c) V(t) = V_m cot ωt
(d) V(t) = V_m tan ωt

2. The rms value of potential difference V shown in the figure is

3. The phase relationship between current and voltage in a pure resistive circuit is best represented by

4. In the case of an inductor
(a) voltage lags the current by \(\frac{π}{2}\)
(b) voltage leads the current by \(\frac{π}{2}\)
(c) voltage leads the current by \(\frac{π}{3}\)
(d) voltage leads the current by \(\frac{π}{4}\)

5. Which of the following graphs represents the correct variation of inductive reactance X_L with frequency u?

6. 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

7. Which of the following graphs represents the correct variation of capacitive reactance Xc with frequency v u?

8. 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

9. 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

10. In series LCR circuit, the phase angle between supply voltage and current is

11. 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°

12. At resonance frequency the impedance in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

13. At resonant frequency the current amplitude in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

14. Quality factor and power factor both have the dimensions of
(a) time
(b) frequency
(c) work
(d) angle

15. The natural frequency (ca0) of oscillations in LC circuit is given by

16. A transformer works on the principle of
(a) self induction
(b) electrical inertia
(c) mutual induction
(d) magnetic effect of the electrical current

17. 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

Question 18.
The average value of a cover a complete cycle is:
(a) l₀
(b) \(\frac{l_0}{2}\)
(c) \(\frac{l_0}{√2}\)
(d) zero

Question 19.
Power factor of an ac circuit is a measure of:
(a) virtual power
(b) power lost in the circuit
(c) mean power
(d) all the above

Question 20.
The dimensional formula of L/R is similar to that of:
(a) frequency
(b) time
(c) length
(d) none of these

Question 21.
Which of the following has dimensions different from the rest?
(a) L/R
(b) \(\frac{1}{RC}\)
(c) \(\sqrt{LC}\)
(d) RC

Question 22.
The dimensional formula of impedance is;
(a) [ML²T^-2A^-2]
(b) [ML²T^-3A^-2]
(c) [ML²T^-2A^-1]
(d) [ML²T^-2A^-3]

Question 23.
Energy dissipates in LCR circuit in :
(a) L only
(b) C only
(c) R only
(d) All of the above

Question 24.
An iron core transformer with a turns ratio of 8 : 1 has 120 V applied across the primary. The voltage across the
(a) 15 V
(b) 120 V
(c) 180 V
(d) 960 V

Question 25.
What is the ratio of inductive and capacitance reactances in an ac circuit?
(a) 1
(b) ω²L
(c) ω²LC
(d) l

Question 26.
An acceptor circuit is :
(a) series resonant circuit
(b) parallel resonant circuit
(c) LCR circuit
(d) None of these

Question 27.
A broadcast circute broadcasts at 300 m band. A condenser of capacitance 2.4 lµF is available. The value of the inductance required for resonant circuit is:
(a) 10^-4 H
(b) 10^-8 H
(c) 10^-6 H
(d) 10^-2 H

Question 28.
In series resonant circuit:
(a) reactance is zero
(b) current is zero
(c) voltage is zero
(d) None of these

Question 29.
In parallel resonant circuit:
(a) impedance is very high
(b) current is very high
(c) voltage is very high
(d) None of these

Question 30.
The no. of turns in the primary coil of a transformer is 200 and the no. of turns in the secondary is 10. If 240 V a.c. is applied to primary, the output from the secondary
(a) 6 V
(b) 12 V
(c) 24 V
(d) 48 V

Question 31.
Transformer works on the principle of:
(a) convertor
(b) invertor
(c) mutual induction
(d) self-induction

Question 32.
Which of the following effects is not shown in alternating current ?
(a) Chemical effect
(b) Magnetic effect
(c) Heating current
(d) All of these

Question 33.
An ac ammeter connected in series in an ac circuit reads 5A. The peak value of current:
(a) 5 A
(b) 5√2A
(c) \(\frac{5}{√5}\) A
(d) zero

Question 34.
The phase difference b/w the A.C. and e.m.f. \(\frac{π}{2}\) Which of the following can not be the instituent of the circuit?
(a) LC
(b) L alone
(c) C alone
(d) RL

Question 35.
The Q factor of an LCR circuit at resonance is :
(a) \(\sqrt{R^2+ω^2L^2}\)
(b) \(\sqrt{R^2+(\frac{1}{cω}})^2\)
(c) \(\frac{ω_rL}{R}\)
(d) \(\sqrt{R^2+\frac{1}{c^2ω^2}}\)

Question 36.
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

Question 37.
The natural frequency of an L -C-Circuit is equal to :
(a) \(\frac{\sqrt{LC}}{2π}\)
(b) \(\frac{1}{3π\sqrt{LC}}\)
(c) \(\frac{1}{2π}\sqrt{\frac{L}{C}}\)
(d) \(\frac{1}{2π}\sqrt{\frac{C}{L}}\)

Question 38.
In the series L-C-R circuit, the voltmeter and ammeter readings are:

(a) V = 100 Volts, I = 2A
(b) V = 100 Volts, I = 5A
(c) V = 1000 Volts, I = 2A
(d) V = 300 Volts, I = 1A

Question 39.
The band width of a series resonant circuit is :
(a) f₀/Q₀
(b) \(\frac{R}{2πL}\)
(c) \(\frac{R}{4πL}\)
(d) 4R

Question 40.
A rejector circuit is:
(a) series resonant circuit
(b) parallel resonant circuit
(c) LCR circuit
(d) None of these

Question 41.
The inductive reactance of an inductor coil of \(\frac{1}{π}\) H at 50 Hz is:
(a) \(\frac{50}{π}\) Ω
(b) \(\frac{π}{50}\) Ω
(c) 100 Ω
(d) 50 Ω

Question 42.
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

Question 43.
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

Question 44.
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

Question 45.
In the above problem, the value of the resistance of the coil Y is:
(a) 0.58 Ω
(b) 5.8 Ω
(c) 1.16 Ω
(d) 11.6 Ω

Question 46.
An electric circuit containing capacitor only suffers:
(a) equal resistance for A.C. and D.C.
(b) Resistance of A.C. > Resistance of D.C.
(c) Resistance for A.C. < Resistance of D.C.
(d) offers infinite resistance for D.C.

Question 47.
The average power dissipation in pure inductance or capacitance is:
(a) \(\frac{1}{2}\) LI²
(b) \(\frac{3}{2}\) CV²
(c) either (a) or (b)
(d) zero