CBSE Class 12 Physics Alternating Current Important Questions Worksheet Set A

1 Show that the average power though over a completer cycle in an ac circuit consisting of capacitance is zero. 

2.A lamp is connected in series with a capacitor. Predict your observations for dc and ac connections. What happens in each case if the capacitance of the capacitor is reduced?

3 A 15 μF capacitor is connected to a 22V, 50Hz source. Find the capacitive reactance and the current (rms and peak) in the circuit. If the frequency is doubled, what happens to the capacitive reactance and the current? 

4 A light bulb and an open coil inductor are connected to an ac source through a key as shown in fig.The switch is closed and after sometime, an iron rod is inserted into the interior of the inductor. The glow of the light bulb (a) increases; (b) decreases ;(c) is unchanged, as the iron rod is inserted. Give your answer with reasons. 

5 Discuss about an LCR circuit with the help of a phasor diagram with relevant equations for current and voltage  

6 What is meant by resonance? How it is achieved in case of an LCR circuit? Give the graph showing the variation of current with frequency. Can this condition be achieved in case of LR circuit? Explain

7 Give the condition at which one can allow maximum value of current through an LCR circuit. What can you say about the current for frequencies other than resonant frequency? 

8 What is Q- factor of an LCR circuit? What is the role of Q-factor in measuring the sharpness of resonance? Give an expression to find the Q-factor.

9 A resistor of 200Ω and a capacitor of 15 μF are connected in series to a 220V, 50 Hz ac source. (a) Calculate the current in the circuit; (b) Calculate the voltage (rms) across the resistor and the capacitor. Is the algebraic sum of theses voltages more than the source voltage? If yes, resolve the paradox.

10 Derive an expression to find the power in an Ac circuit. What is the importance of power factor in an LC R circuit?

11 A sinusoidal voltage of peak value 283 V and frequency 50 Hz is applied to a series LCR circuit in which R= 3Ω, L=25.48mH, and C=796μF. Find (a) the impedance of the circuit ;(b) the phase difference between the voltage across the source and the current; (c) the power dissipated in the circuit; and (d) the power factor.

12 A sinusoidal voltage of peak value 283 V and frequency 50 Hz is applied to a series LCR circuit in which R= 3Ω, L=25.48mH, and C=796μF .(a) What is the frequency of the source at which resonance occurs? (b) Calculate the impedance, the current, and the power dissipated at the resonant condition.

13 At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. If she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work?

Important Questions for NCERT Class 12 Physics Alternating Current

Question. A circuit when connected to an AC source of 12 V gives a current of 0.2 A. The same circuit when connected to a DC source of 12 V, gives a current of
0.4 A. The circuit is
(a) series LR
(b) series RC
(c) series LC
(d) series LCR

Answer: A

Question. Which of the following combinations should be selected for better tuning of an L-C-R circuit used for communication ?
(a) R = 20 W, L = 1.5 H, C = 35 mF
(b) R = 25 W, L = 2.5 H, C = 45 mF
(c) R = 15 W, L = 3.5 H, C = 30 mF
(d) R = 25 W, L = 1.5 H, C = 45 mF 

Answer: C

Question. Turn ratio in a step up transformer is 1 : 2 if a Lechlanche cell of 1.5 V is connected across the input, then the voltage across the output will be
(a) 0.1 V
(b) 1.5 V 
(c) 0.75 V
(d) zero

Answer: D

Question. In a circuit the coil of a choke : 
(a) decreases the current
(b) increases the current
(c) has high resistance to D. C. circuit
(d) no effect with the current

Answer: A

Question. In a circuit, the current lags behind the voltage by a phase difference of p /2, the circuit will contain which of the following : 
(a) only R
(b) only C
(c) R and C
(d) only L

Answer: D

Question. The coefficient of mutual inductance, when magnetic flux changes by 2 × 10–2 Wb and current changes by 0.01 A is 
(a) 8 henry
(b) 4 henry
(c) 3 henry
(d) 2 henry

Answer: D

Question. In an ideal parallel LC circuit, the capacitor is charged by connecting it to a D.C. source which is then disconnected. The current in the circuit
(a) becomes zero instantaneously 
(b) grows monotonically
(c) decays monotonically
(d) oscillates instantaneously

Answer: D

Question. A capacitor of capacitance 2μF is connected in the tank circuit of an oscillator oscillating with a frequency of 1 kHz. If the current flowing in the circuit is 2 m A, the voltage across the capacitor will be : 
(a) 0.16 V
(b) 0.32 V
(c) 79.5 V
(d) 159 V

Answer: A

Question. In an AC circuit the potential differences across an inductance and resistance joined in series are respectively 16 V and 20 V. The total potential difference of the source is 
(a) 20.0 V
(b) 25.6 V
(c) 31.9 V
(d) 53.5 V

Answer: B

Question. With the decrease of current in the primary coil from 2 amperes to zero value in 0.01s the emf generated in the secondary coil is 1000 volts. The mutual inductance of the two coils is
(a) 1.25 H
(b) 2.50 H 
(c) 5.00 H
(d) 10.00 H

Answer: C

Question. An AC source of angular frequency w is fed across a resistor R and a capacitor C in series.
The current registered is I. If now the frequency of source is changed to w/3 (but maintaining the same voltage), the current in the circuit is found to be halved. Calculate the ratio of reactance to resistance at the original frequency w 
(a) √(3/5)
(b)√(2/5)
(c) √(1/5)
(d) √(4/5)

Answer: A

Question. If an AC main supply is given to be 220 V. The average emf during a positive half cycle will be
(a) 198 V
(b) 220 V 
(c) 240 V
(d) 220 √2V

Answer: A

Question. A 40 mF capacitor is connected to a 200 V, 50 Hz ac supply. The r.m.s value of the current in the circuit is, nearly
(a) 1.7 A
(b) 2.05 A
(c) 2.5 A
(d) 25.1 A 

Answer: C

Question. A small signal voltage V(t) = V0 sinwt is applied across an ideal capacitor C
(a) Current I(t) is in phase with voltage V(t).
(b) Current I(t) leads voltage V(t) by 180°.
(c) Current I(t), lags voltage V(t) by 90°.
(d) Over a full cycle the capacitor C does not consume any energy from the voltage source.

Answer: D

Question. A coil has an inductance of 0.7 henry and is joined in series with a resistance of 220 W. When the alternating emf of 220 V at 50 Hz is applied to it then the phase through which current lags behind the applied emf and the wattless component of current in the circuit will be respectively 
(a) 30°, 1 A
(b) 45°, 0.5 A
(c) 60°, 1.5 A
(d) none of these

Answer: B

Question. In a AC circuit the voltage and current are described by V 200sin (319-Π/6) volts and i 50sin (314t+Π/6) respectively. The average power dissipated in the circuit is : 
(a) 2.5 watts
(b) 5.0 watts
(c) 10.0 watts
(d) 50.0 watts

Answer: A

Question. The ratio of mean value over half cycle to r.m.s. value of A.C. is
(a) 2 : π
(b) 2 √2 : π
(c) 2 : π
(d) 2 :1

Answer: B

Question. Which one of the following curves represents the variation of impedance (Z) with frequency f in series LCR circuit? 

Answer: C

Question. In a LCR circuit at resonance which of these will effect the current in circuit
(a) R only
(b) L and R only
(c) R and C only
(d) all L, C and R

Answer: A

Question. In a series resonant circuit, having L,C and R as its elements, the resonant current is i. The power dissipated in circuit at resonance is
(a) i²R/(ωL-1/ωC)
(b) zero
(c) i² ωL
(d) i² R.
Whereas ω is angular resonant frequency

Answer: D

Question. Two coils A and B are connected in series across a 240 V, 50 Hz supply. The resistance of A is 5 Ω and the inductance of B 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: C

Question. In an oscillating LC circuit the max. charge on the capacitor is Q. The charge on capacitor when the energy is stored qually between electric and magnetic field is
(a) Q/2
(b) Q/√3
(c) Q/√2
(d) Q

Answer: C

Question. In a region of uniform magnetic induction B = 10^–2 tesla, a circular coil of radius 30 cm and resistance π² ohm is rotated about an axis which is perpendicular to the direction of B and which forms a diameter of the coil. If the coil rotates at 200 rpm the amplitude of the alternating current induced in the coil is
(a) 4π² mA
(b) 30 mA
(c) 6 mA
(d) 200 mA

Answer: C

Question. Which of the following statement is incorrect ?
(a) In LCR series ac circuit, as the frequency of the source increases, the impedence of the circuit first decreases and then increases.
(b) If the net reactance of an LCR series ac circuit is same as its resistance, then the current lags behind the voltage by 45°.
(c) At resonance, the impedence of an ac circuit becomes purely resistive.
(d) Below resonance, voltage leads the current while above it, current leads the voltage.

Answer: D

Question. A step up transformer operates on a 230 V line and supplies a current of 2 ampere. The ratio of primary and secondary winding is 1:25 . The current in primary is
(a) 25 A
(b) 50 A
(c) 15 A
(d) 12.5 A

Answer: B

Question. An alternating voltage V = V₀ sin wt is applied across a circuit. As a result, a current I = I₀ sin (ωt – π/2) flows in it.
The power consumed per cycle is
(a) zero
(b) 0.5 V₀I₀
(c) 0.707 V₀I₀
(d) 1.414 V₀I₀

Answer: A

Question. In a pure capacitive A.C. circuit current and voltage differ in phase by
(a) 0°
(b) 45°
(c) 90°
(d) 180°

Answer: C

Question. The primary of a transformer has 400 turns while the secondary has 2000 turns. If the power output from the secondary at 1000 V is 12 kW, what is the primary voltage?
(a) 200 V
(b) 300 V
(c) 400 V
(d) 500 V

Answer: A

Question. Resonance frequency of LCR series a.c. circuit is f₀. Now the capacitance is made 4 times, then the new resonance frequency will become
(a) f₀/4
(b) 2f₀
(c) f₀
(d) f₀/2.

Answer: D

Question. The resistance of a coil for dc is in ohms. In ac, the resistance will
(a) be zero
(b) decrease
(c) increase
(d) remain same

Answer: C

Question. In an a.c. circuit, the r.m.s. value of current, I_rms is related to the peak current, I₀ by the relation
(a) I_rms = √2 I₀
(b) I_rms = πI₀
(c) I_rms = 1/π I₀
(d) I_rms =1/√2 I₀

Answer: D

Question. In an A.C. circuit, a resistance of R ohm is connected in series with an inductance L. If phase angle between voltage and current be 45°, the value of inductive reactance will be
(a) R/4
(b) R/2
(c) R
(d) cannot be found with given data

Answer: C

Question. A capacitor has capacitance C and reactance X, if capacitance and frequency become double, then reactance will be
(a) 4X
(b) X/2
(c) X/4
(d) 2X

Answer: C

Question. A steady potential difference of 10 V produces heat at a rate x in a resistor. The peak value of the alternating voltage which will produce heat at a rate x/2 in the same resistor is
(a) 5 V
(b) 5√2 V
(c) 10 V
(d) 10√2 V

Answer: C

Question. The impedance in a circuit containing a resistance of 1 Ω and an inductance of 0.1 H in series, for AC of 50 Hz, is
(a) 100 √10Ω
(b) 10 √10Ω
(c) 100Ω
(d) √10Ω

Answer: B

Question. In an A.C. circuit with phase voltage V and current I, the power dissipated is
(a) VI
(b) V²I
(c) VI²
(d) V²I²

Answer: A