MCQ of Alternating Current | Chapter 7 | Physics | Class 12 | CBSE |

MCQ of Alternating Current, Chapter 7, Physics Class 12

Question 1: In an AC circuit containing only capacitance, the current

1. Leads the voltage by 180º
2. Remains in phase with voltage
3. Leads the voltage by 90º
4. Lags the voltage by 90º

Answer: C (Leads the voltage by 90º)

Question 2: The resonant frequency of a circuit is f. If the capacitance is made 4 times the original value, then the resonant frequency will become

1. f / 2
2. 2 f
3. f
4. f / 4

Answer: A ( f / 2)

Question 3: In L-R circuit, resistance is 8 ohm and the inductive reactance is 6 ohm, then impedance is 

1. 2 ohm
2. 14 ohm
3. 4 ohm
4. 10 ohm

Answer: D ( 10 ohm)

Question 4: An inductance and a resistance are connected in series with an AC potential. In this circuit 

1. The current and the potential difference across the resistance lead the PD across the resistance lead the PD across the inductance by phase angle π / 2
2. The current and the potential difference across the resistance lag behind PD across the inductance by angle π / 2.
3. The current and the potential difference across the resistance lag behind the PD across the inductance by an angle π.
4. The PD across the resistance lags behind the PD across the inductance by an angle π / 2 but the current in the resistance leads the potential difference across inductance by π / 2. 

Answer: B ( The current and the potential difference across the resistance lag behind PD across the inductance by angle π / 2 )

Question 5: A 10 ohm resistance, 5 mH coil and 10 µF capacitor are joined in series. When a suitable frequency alternating current source is joined to this combination, the circuit resonates. If the resistance is halved, the resistance frequency 

1. Is halved
2. Is doubled
3. Remains unchanged
4. In quadrupled 

Answer: C (Remains unchanged)

Question 6: The peak value of AC voltage on a 220 V mains is

1. 240 √2 V
2. 230 √2 V
3. 220 √2 V
4. 200 √2 V

Answer: C (220 √2 V)

Question 7: Alternating current is transmitted at far off places

1. At high voltage and low current
2. At high voltage and high current
3. At low voltage and low current
4. At low voltage and high current 

Answer: A (At high voltage and low current)

Question 8: When power is drawn from secondary coil of transformer, the dynamic resistance 

1. Increases
2. Decreases
3. Remains unchanged
4. Changes electrically

Answer: A (Increases)

Question 9: Transformer is based upon the principle of 

1. Self induction
2. Mutual induction
3. Eddy current
4. None of these 

Answer: B (Mutual induction)

Question 10: When an AC voltage is applied to a L-C-R circuit, which of the following is true?

1. I and V are out of phase with each other in R
2. I and V are in L with in C, they are out of phase
3. I and V are out of phase, in both L and C
4. I and V are out of phase in L and in phase in C

Answer: C (I and V are out of phase, in both L and C)

Question 11: The turns ratio of a transformer is given as 2 : 3. If the current through the primary coil is 3 A, thus calculate the current through load resistance 

1. 1 A
2. 4.5 A
3. 2 A
4. 1.5 A 

Answer: C ( 2 A)

You may also read MCQ of Electric Charges and Fields, MCQ of Electrostatic Potential and Capacitance, MCQ of Current Electricity, MCQ of Moving Charges and Magnetism, MCQ of Magnetism and Matter, MCQ of Electromagnetic Induction for better understanding of the chapters.

Question 12: A transformer is an electrical device used for 

1. Producing direct current
2. Producing alternating current
3. Changing D.C into A.C.
4. Changing A.C. voltages 

Answer: D (Changing A.C. voltages)

Question 13: Out of the following, choose the wrong statement:

1. A transformer cannot work on d.c.
2. A transformer cannot change the frequency of a.c.
3. A transformer can produce a.c. power
4. In a transformer, when a.c. voltage is raised n times, the alternating current reduces to 1/n time

Answer: C ( A transformer can produce a.c. power)

Question 14: High voltage transmission line is preferred as 

1. Its appliances are less costly
2. Thin power cables are required
3. Idle current very low
4. Power loss is very less 

Answer: D (Power loss is very less)

Question 15: In a purely resistive a.c. circuit, the current

1. Is in phase with the e.m.f.
2. Leads the e.m.f. by a difference of π radians phase
3. Leads the e.m.f. by a phase difference of π / 2 radians
4. Lags behind the e.m.f. by a phase difference of π / 4 radians

Answer: A (Is in phase with the e.m.f. )

Question 16: In R-L-C series ac circuit, impedance cannot be increased by

1. Increasing frequency of the source
2. Decreasing frequency of the source
3. Increasing the resistance
4. Increasing the voltage of the source

Answer : D (Increasing the voltage of the source)

Question 17: Alternating current cannot be measured by DC ammeter, because

1. AC cannot pass through DC ammeter
2. AC changes direction
3. Average value of current for complete cycle is zero
4. DC ammeter will get damaged

Answer : C (Average value of current for complete cycle is zero)

Question 18: In a transformer the number of turns of primary and secondary coil are 500 and 400 respectively. If 200 V a.c. is supplied to the primary coil, then ratio of currents in primary and secondary coils is

1. 4 : 5
2. 5 : 4
3. 5 : 9
4. 9 : 5

Answer : A (4 : 5)

Question 19: A battery of 12 volt is connected to primary of a transformer with turns ratio n_s / n_p = 10. Voltage across secondary would be

1. 120 V
2. 1.2 V
3. 12 V
4. Zero

Answer : D (Zero)

Question 20: The reactance of capacitor with capacitance C is X. If both frequency and the capacitance become double then a new reactance will be

1. X
2. 2 X
3. 4 X
4. X / 4

Answer : D (X / 4)

Question 21: A power transmission line feeds input power at 2300 V to a step down transformer with its primary windings having 4000 turns, giving the output power at 230 V ,if the current in the primary of the transformer is  5 A, and its efficiency is 90%, the output current would be

1. 50 A
2. 25 A
3. 45 A
4. 20 A

Answer : C (45 A)

Question 22: 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 constituted the circuit?

1. Only resistor
2. Resistor and inductor
3. Resistor and capacitor
4. Only inductor

Answer : C (Resistor and capacitor)

Question 23: In which of the following circuits the maximum power dissipation is observed?

1. Pure capacitive circuit
2. Pure inductive circuit
3. Pure resistive circuit
4. None of these

Answer : C (Pure resistive circuit)

Question 24 : A pure resistive circuit element X when connected to an AC supply of peak voltage 200 V gives a peak current of 5 A which is in phase with the voltage. A second circuit element Y when connected to the same AC supply also gives the same value of peak current but the current lags behind by 90°. If the series combination of X and Y is connected to the same supply, what will be the rms value of the current?

1. 10 / √2 A
2. 5 / √ 2 A
3. 5 / 2 A
4. 5 A

Answer : C ( 5 / 2 A)

Question 25: At resonance frequency the impedance in series LCR circuit is

1. Maximum
2. Minimum
3. Zero
4. Infinity

Answer : B ( Minimum)

Question 26: In a pure capacitive circuit if the frequency of AC source is doubled then its capacitive reactance will be

1. Remains same
2. Doubled
3. Halved
4. Zero

Answer : C (Halved)

Question 27: The loss of energy in the form of heat in the iron core of transformer is

1. Iron loss
2. Copper loss
3. Mechanical loss
4. None of these

Answer: A (Iron loss)

Question 28: A small town with a demand of 800 kW of electric power at 220 V is situated 15 km away from an electric plant generating power at 440 V. The resistance of the two wire line carrying power is 0.5 ohm per km. The town gets power from the line through a 4000 – 220 V step down transformer at a substation in the town. The line power loss in the form of heat is

1. 400 kW
2. 600 kW
3. 300 kW
4. 800 kW

Answer : B (600 kW)

Question 29: A series L C R circuit with R = 22 ohm, L = 1.5 H and C = 40 uF is connected to a variable frequency 220 V ac supply. When the frequency of the supply equals the natural frequency of the circuit, what is the average power transferred to the circuit in one complete cycle ?

1. 2000 W
2. 2200 W
3. 2400 W
4. 2500 W

Answer : B (2200 W)

Question 30: In highly inductive load circuit, it is more dangerous when

1. We close the switch
2. Open the switch
3. Increasing the resistance
4. Decreasing the resistance

Answer: B (Open the switch)

Question 31: A transformer consisting of 300 turns in the primary and 150 turns in the secondary gives output power of 2.2 kW. If the current in the secondary coil is 10 A then the input voltage and current in the primary coil are

1. 440 V and 20 A
2. 440 V and 5 A
3. 220 V and 10 A
4. 220 V and 20 A

Answer : B (440 V and 5 A)

Question 32: An alternating voltage is connected in series with a resistance R and an inductance L .If the potential drop across the resistance is 200 V and across the inductance is 150 V ,then the applied voltage is

1. 350 V
2. 250 V
3. 500 V
4. 300 V

Answer : B (250 V)

Question 33: An ideal choke takes a current of 8 A when connected to an AC source of 100 V and 50 Hz. A pure resistor under the same condition takes a current of 10 A. If two are connected in series to an AC supply of 100 V and 40 Hz then the current in the series combination of above resistor and inductor is

1. 10 A
2. 5 A
3. 5√2 A
4. 10√2 A

Answer : C (5√2 A)

Question 34: In an L-C-R series AC circuit at resonance

1. The capacitive reactance is more than the inductive reactance
2. The capacitive reactance equals the inductive reactance
3. The capacitive reactance is less than the inductive reactance
4. The power dissipated is minimum

Answer: B (The capacitive reactance equals the inductive reactance)

Question 35: A choke is preferred to a resistance for limiting current in AC circuit because

1. Choke is cheap
2. There is no wastage of power
3. Choke is compact in size
4. Choke is good absorber of heat

Answer: B (There is no wastage of power)

Question 36: A 50 Hz AC source of 20 V is connected across R and C. The voltage across R is 12 V. The voltage across C will be

1. 8 V
2. 16 V
3. 10 V
4. Not possible to determine unless values of R and C are given

Answer: B (16 V)

Question 37: Alternating voltage V = 140 sin 50 t is applied to a resistor of 10 ohm. This voltage produces heat in the resistor in the time t. To produce the same heat in the same time required DC current is

1. 14 A
   
2. About 20 A
   
3. About 10 A
   
4. None of these

Answer: C (About 10 A)

Question 38: A virtual current of 4 A and 50 Hz flows in an AC circuit containing a coil. The power consumed in the coil is 240 W. If the virtual voltage across the coil is 100 V. Its inductance will be 

1. 1 / 3π H
2. 1 / 5π H
3. 1 / 7π H
4. 1 / 9π H

Answer: B (1 / 5π H)

 

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