# MCQs for Physics Class 12 with Answers Chapter 7 Alternating Current

Students of class 12 Physics should refer to MCQ Questions Class 12 Physics Alternating Current with answers provided here which is an important chapter in Class 12 Physics NCERT textbook. These MCQ for Class 12 Physics with Answers have been prepared based on the latest CBSE and NCERT syllabus and examination guidelines for Class 12 Physics. The following MCQs can help you to practice and get better marks in the upcoming class 12 Physics examination

## Chapter 7 Alternating Current MCQ with Answers Class 12 Physics

MCQ Questions Class 12 Physics Alternating Current provided below have been prepared by expert teachers of grade 12. These objective questions with solutions are expected to come in the upcoming Standard 12 examinations. Learn the below provided MCQ questions to get better marks in examinations.

Question. In an ac circuit, the maximum value of voltage is 423 volts. Its effective voltage is
(a) 400 volt
(b) 300 volt
(c) 323 volt
(d) 340 volt

B

Question. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?
(a) For a given power level, there is a lower current.
(b) Lower current implies less power loss.
(c) Transmission lines can be made thinner.
(d) It is easy to reduce the voltage at the receiving end using step-down transformers.

A, B, D

Question. The peak voltage of 220 V ac mains is
(a) 155.6 V
(b) 220.0 V
(c) 311 V
(d) 440 V

C

Question. If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is

B

Question. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply power to a passive load consisting of a resistance Rg and a reactance XL. For maximum power to be delivered from the generator to the load, the value of XL is equal to
(a) zero
(b) Xg
(c) – Xg
(d) Rg

C

Question. An inductive circuit have zero resistance. When ac voltage is applied across this circuit, then the current lags behind the applied voltage by an angle
(a) 30°
(b) 45°
(c) 90°
(d) 0°

C

Question. To reduce the resonant frequency in an LCR series circuit with a generator
(a) the generator frequency should be reduced.
(b) another capacitor should be added in parallel to the first.
(c) the iron core of the inductor should be removed.
(d) dielectric in the capacitor should be removed.

B

Question. In an ac circuit, the emf (ε) and the current (i) at any instant are given by ∈ = E0 sin ~t, i= I0 sin (ωt – Φ) Then average power transferred to the circuit in one complete cycle of ac is

D

Question. The reactance of a capacitance at 50 Hz is 5 Ω. If the frequency is increased to 100 Hz, the new reactance is
(a) 5 Ω
(b) 10 Ω
(c) 2.5 Ω
(d) 125 Ω

C

Question. In a pure capacitive circuit, the current
(a) lags behind the applied emf by angle π/2
(b) leads the applied emf by an angle π
(c) leads the applied emf by angle π/2
(d) and applied emf are in same phase

C

Question. The average power dissipation in pure inductance is

D

Question. In a pure inductive circuit, the current
(a) lags behind the applied emf by an angle π
(b) lags behind the applied emf by an angle π / 2
(c) leads the applied emf by an angle π / 2
(d) and applied emf are in same phase

B

Question. An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) ac source. The power dissipated in the circuit is
(a) 8 W
(b) 12 W
(c) 14.4 W
(d) 18 W

C

Question. If an LCR circuit contains L = 8 henry; C = 0.5 mF, R = 100 Ω in series. Then the resonant angular frequency will be:
(c) 600 Hz
(d) 500 Hz

B

Question. In an ac circuit, voltage V and current i are given by
V = 100 sin 100 t volt
i = 100 sin (100t + π/3) mA
The power dissipated in the circuit is
(a) 104W
(b) 10 W
(c) 2.5 W
(d) 5 W.

C

Question. When a voltage measuring device is connected to ac mains, the meter shows the steady input voltage of 220 V. This means
(a) input voltage cannot be ac voltage, but a dc voltage.
(b) maximum input voltage is 220 V.
(c) the meter reads not V but <V2> and is calibrated to read < V2 > .
(d) the pointer of the meter is stuck by some mechanical defect.

C

Question. The potential differences across the resistance, capacitance and inductance are 80 V, 40 V and 100 V respectively in an L-C-R circuit, the power factor for this circuit is
(a) 0.4
(b) 0.5
(c) 0.8
(d) 1.0

C

Question. When an ac voltage of 220 V is applied to the capacitor C
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plates is in phase with the applied voltage.
(d) power delivered to the capacitor is zero.

C, D

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

C

Question. The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is
(a) 1/√2 A
(b) √2 A
(c) 2 A
(d) 2 √2 A

A

Question. In an a.c. circuit, the r.m.s. value of current, Irms is related to the peak current, I0 by the relation

D

Question. Fleming’s left and right hand rules are used in
(a) DC motor and AC generator
(b) DC generator and AC motor
(c) DC motor and DC generator
(d) Both rules are same, any one can be used

C

Question. In a RLC circuit capacitance is changed from C to 2 C. For the resonant frequency to remain unchanged, the inductance should be changed from L to
(a) 4 L
(b) 2 L
(c) L/2
(d) L/4

C

Question. The time taken by the current to rise to 0.63 of its maximum value in a d.c. circuit containing inductance (L) and resistance (R) depends on
(a) L only
(b) R only
(c) L/R
(d) LR

C

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

C

Question. A bulb and a capacitor are connected in series to a source of alternating current. If its frequency is increased, while keeping the voltage of the source constant, then bulb will
(a) give more intense light
(b) give less intense light
(c) give light of same intensity before

A

Question. An LCR series circuit, connected to a source E, is at resonance. Then the voltage across
(a) R is zero
(b) R equals applied voltage
(c) C is zero
(d) L equals applied voltage

B

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

A

Question. In LCR circuit if resistance increases quality factor
(a) increases finitely
(b) decreases finitely
(c) remains constant
(d) None of these

B

Question. The power factor of an AC circuit having resistance (R) and inductance (L) connected in series and an angular velocity w is
(a) R/ωL
(b) R/(R2 + ω2L2)1/2
(c) ωL/R
(d) R/(R2 – ω2L2)1/2

B

Question. Which of the following will have the dimensions of time
(a) LC
(b) R/L
(c) L/R
(d) C/L

C

Question. A.C. power is transmitted from a power house at a high voltage as
(a) the rate of transmission is faster at high voltages
(b) it is more economical due to less power loss
(c) power cannot be transmitted at low voltages
(d) a precaution against theft of transmission lines

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

A

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

C

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

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

C

Question. In the circuit of Fig, the bulb will become suddenly bright if

(a) contact is made or broken
(c) contact is broken
(d) won’t become bright at all

C

Question. Energy in a current carrying coil is stored in the form of
(a) electric field
(b) magnetic field
(c) dielectric strength
(d) heat

B

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

Whereas ω is angular resonant frequency

D

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.

D

Question. The core of any transformer is laminated so as to
(a) reduce the energy loss due to eddy currents
(b) make it light weight
(c) make it robust and sturdy
(d) increase secondary voltage

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.

D

Question. An inductance L having a resistance R is connected to an alternating source of angular frequency w. The Quality factor Q of inductance is
(a) R/ ϖL
(b) (ϖL/R)2
(c) (R /ϖL)½
(d) ϖL/R

D

Question. The time constant of C–R circuit is
(a) 1/CR
(b) C/R
(c) CR
(d) R/C

C

Question. In a circuit L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45°. The value of C is

D

Question. In an A.C. circuit, the current flowing in inductance is I = 5 sin (100 t – π/2) amperes and the potential difference is V = 200 sin (100 t) volts. The power consumption is equal to
(a) 1000 watt
(b) 40 watt
(c) 20 watt
(d) Zero

D

Question. An inductance of negligible resistance whose reactance is 22 Ω at 200 Hz is connected to 200 volts, 50 Hz power line. The value of inductance is
(a) 0.0175 henry
(b) 0.175 henry
(c) 1.75 henry
(d) 17.5 henry

A

Question. In an a.c. circuit V and I are given by
V = 100 sin (100 t) volts
I =
100 sin (100 t + π/3) mA
the power dissipated in the circuit is
(a) 104 watt
(b) 10 watt
(c) 2.5 watt
(d) 5.0 watt

C

Question. In the circuit shown in fig, the resonant frequency is

(a) 75 kc/s
(b) 750 kc/s
(c) 7.5 kc/s
(d) 75 mc/s

A

Question. The r.m.s value of an a.c. of 50 Hz is 10 amp. The time taken by the alternating current in reaching from zero to maximum value and the peak value of current will be
(a) 2 × 10–2 sec and 14.14 amp
(b) 1 × 10–2 sec and 7.07 amp
(c) 5 × 10–3 sec and 7.07 amp
(d) 5 × 10–3 sec and 14.14 amp

D

Question. A step down transformer reduces 220 V to 110 V. The primary draws 5 ampere of current and secondary supplies 9 ampere. The efficiency of transformer is
(a) 20%
(b) 44%
(c) 90%
(d) 100%

C

Question. If resistance of 100Ω, and inductance of 0.5 henry and capacitance of 10 × 106 farad are connected in series through 50 Hz A.C. supply, then impedance is
(a) 1.8765 Ω
(b) 18.76 Ω
(c) 187.6 Ω
(d) 101.3 Ω

C

Question.An inductive circuit contains resistance of 10 ohms and an inductance of 2 henry. If an A.C. voltage of 120 Volts and frequency 60 Hz is applied to this circuit, the current would be nearly
(a) 0.32 Amp
(b) 0.16 Amp
(c) 0.48 Amp
(d) 0.80 Amp

B

Question. Using an A.C. voltmeter the potential difference in the electrical line in a house is read to be 234 volt. If the line frequency is known to be 50 cycles/second, the equation for the line voltage is
(a) V = 165 sin (100 π t)
(b) V = 331 sin (100 π t)
(c) V = 220 sin (100 π t)
(d) V = 440 sin (100 π t)

B

Question. An alternating current is given by i = i1 cosϖt + i2 sinϖt The rms current is given by

C

Question. Determine the rms value of the emf given by E (in volt) = 8 sin ( w t) + 6sin (2 w t)
(a) 5 √2V
(b) 7 √2V
(c) 10 V
(d) 10 √2V

A

Question. An AC voltage source has an output of V = 200sin 2pft . This source is connected to a 100 W resistor. RMS current in the resistance is
(a) 1.41 A
(b) 2.41 A
(c) 3.41 A
(d) 0.71 A

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

B

Question. In an LR circuit f = 50 Hz, L=2H, E=5 volts, R=1 W then energy stored in inductor is
(a) 50 J
(b) 25 J
(c) 100 J
(d) None of these

D

Question. In a circuit inductance L and capacitance C are connected as shown in figure. A1 and A2 are ammeters. When key K is pressed to complete the circuit, then just after closing key (K), the readings of A1 and A2 will be
(a) zero in both A1 and A2
(b) maximum in both A1 and A2
(c) zero in A1 and maximum in A2
(d) maximum in A1 and zero in A2

D

#### Fill in the Blanks

Question. The capacitive reactance limits the _______________ in a purely capacitive circuit in the same way as the resistance limits the current in a purely resistive circuit.

amplitude of the current

Question. For many purposes, it is necessary to change an alternating voltage from one to another of greater or smaller value. This is done with a device called _______________ using the principle of mutual induction.

transformer

Question. The inductive reactance is directly proportional to the inductance and to the _______________ of the circuit.

frequency

Question. In a pure inductive circuit current _______________ the voltage by a phase angle of π/2

lags

Question. The quantity ϖ0/2Δϖ D is regarded as a measure of the _______________.

sharpness of resonance

Question. The average power dissipated depends not only on the voltage and current but also on the _______________ of the phase angle φ between them.

cosine

Question. In an ac circuit, containing pure resistance, the voltage and current are in _______________ phase.

same

Question. The average power supplied to an inductor over one complete cycle is _______________.

zero

Question. In a pure capacitive circuit, the current _______________ the voltage by a phase angle of π/2

Question. The phenomenon of resistance is common among systems that have a tendency to oscillate at a particular frequency. This frequency is called the system’s _______________.