# Alternating Current MCQ for ISC Physics Class-12

Alternating Current MCQ for ISC Physics Class-12 . These MCQ  / Objective Type Questions is based on latest reduced syllabus according 2021-22 session on bifurcated pattern. Main motto of MCQ Type Question is cracking the next upcoming exam of council. Visit official website CISCE for detail information about ISC Board Class-12 Physics.

## MCQ Type Questions for ISC Physics Class-12 Alternating Current

 Board ISC Class 12th (XII) Subject Physics Ch-Name Alternating Current Syllabus on bifurcated syllabus (after reduction) Session 2021-22 Topic MCQ / Objective Type Question

### MCQ Type Questions of Alternating Current for ISC Physics Class-12

Question 1:   A transformer is employed to

(a) convert A.C. into D.C.

(b) convert D.C. into A.C.

(c) obtain a suitable A.C. voltage

(d) obtain a suitable D.C. voltage

Answer: (c) obtain a suitable A.C. voltage

Question 2: 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

Question 3:  A current of 4A flows in a coil when connected to a 12V dc source. If the same coil is connected to a 12V, 50 rad/s a.c. source, a current of 2.4A flows in the circuit. Determine the inductance of the coil.

(a) 0.08 H

(b) 0.04 H

(c) 0.02 H

(d) 1 H

Question 4:   Identify the principle behind the working of an a.c. generator.

(a) Eddy currents

(c) Lenz’s law

(d) Electromagnetic induction

Question 5:  In an LCR series a.c. circuit, the voltage across each of the components, L, C and R is 50V. The voltage across the LC combination will be

(a) 100 V

(b) 150 V

(c) 50 V

(d) 0 V

Question 6: Identify the function of a phasor from the following.

(a) Phasor is a vector quantity used to represent a sinusoidal signal

(b) Phasor is a scalar quantity used to represent a sinusoidal signal

(c) Phasor is a vector quantity used to represent a cosine signal

(d) Phasor is a scalar quantity used to represent a cosine signal

Answer: (a) Phasor is a vector quantity used to represent a sinusoidal signal

Question 7: 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

Question 8:  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°

Question 9: An a.c. generator consists of a coil of 50 turns and an area 2.5 m2 rotating at an angular speed of 60 rad s-1 in a uniform magnetic field of 0.3 T between two fixed pole pieces. What is the flux through the coil, when the current is zero?

(a) Maximum

(b) Minimum

(c) Zero

(d) Independent of current

Question 10: At resonance frequency the impedance in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

Question 11: At resonant frequency the current amplitude in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

Question 12: Quality factor and power factor both have the dimensions of

(a) time

(b) frequency

(c) work

(d) angle

Question 13: A transformer works on the principle of

(a) self induction

(b) electrical inertia

(c) mutual induction

(d) magnetic effect of the electrical current

Question 14: 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: (d) power in the coils

Question 15: The power factor in a circuit connected to an A.C. The value of power factor is

(a) unity when the circuit contains an ideal inductance only

(b) unity when the circuit contains an ideal resistance only

(c) zero when the circuit contains an ideal resistance only

(d) unity when the circuit contains an ideal capacitance only

Answer: (b) unity when the circuit contains an ideal resistance only

Question 16: In series combination of R, L and C with an A.C. source at resonance, if R = 20 ohm, then impedence Z of the combination is

(a) 20 ohm

(b) zero

(c) 10 ohm

(d) 400 ohm

Question 17: Determine the rms value of current in the circuit if a 50 Ω resistor is connected across an ac source of 200 V, 50 Hz?

(a) 4 A

(b) 200 A

(c) 50 A

(d) 1 A

Question 18: Which value of the current do you measure with an a.c. ammeter?

(a) Instantaneous current

(b) Root mean square value

(c) Electromotive force

(d) Peak current

Answer: (b) Root mean square value

Question 19:  Assertion : When the frequency of the AC source in an LCR circuit equals the resonant frequency, the reactance of the circuit is zero, and so there is no current through the inductor or the capacitor.

Reason : The net current in the inductor and capacitor is zero.

(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.

(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion

(c) Assertion is correct, reason is incorrect

(d) Assertion is incorrect, reason is correct

Answer: (d) Assertion is incorrect, reason is correct

Question 20:  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: (b) V(t) = Vm sin ωt

Question 21: In the case of an inductor

(a) voltage lags the current by π/2

(b) voltage leads the current by π/2

(c) voltage leads the current by π/3

(d) voltage leads the current by π/4

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 constitute the circuit?

(a) Only resistor

(b) Resistor and inductor

(c) Resistor and capacitor

(d) Only inductor

Question 23:  A coil of 40 henry inductance is connected in series with a resistance of 8 ohm and the combination is joined to the terminals of a 2 volt battery. The time constant of the circuit is

(a) 20 seconds

(b) 5 seconds

(c) 1/5 seconds

(d) 40 seconds

Question 24:  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 A

(b) 0.16 A

(c) 0.48 A

(d) 0.80 A

Question 25:  The transformer voltage induced in the secondary coil of a transformer is mainly due to

(a) a varying electric field

(b) a varying magnetic field

(c) the vibrations of the primary coil

(d) the iron core of the transformer

Answer: (b) a varying magnetic field

Question 26:  The loss of energy in the form of heat in the iron core of a transformer is

(a) iron loss

(b) copper loss

(c) mechanical loss

(d) None of these

Question 27:  A 100 turn coil of area 0.1 m2 rotates at half a revolution per second. It is placed in a uniform magnetic field of 0.01 T perpendicular to the axis of rotation of the coil. Calculate the maximum voltage generated in the coil?

(a) 256.33 V

(b) 89.12V

(c) 0.314 V

(d) 3.1455 V

Explanation: The maximum voltage generated in the coil,

e0 = nBAω
e0 = nBA × (2πv)
e0 = 100 × 0.01 × 0.1 × 2π × 0.5
e0 = 0.314 V

Question 28:  An a.c. generator consists of a coil of 1000 turns and cross-sectional area of 3m2, rotating at a constant angular speed of 60 rad s-1 in a uniform magnetic field 0.04 T. The resistance of the coil is 500Ω. Calculate the maximum current drawn from the generator.

(a) 2500 A

(b) 1.44 A

(c) 6.25 A

(d) 0.55 A

Explanation: e0 = nBAω
e0 = 100 × 0.04 × 3 × 60
e0 = 720 V.
The maximum current drawn = 𝑒0/𝑅=720/500

I = 1.44 A.

Question 29:  Adithya peddles a stationary bicycle the pedals of which are attached to a 500 turn coil of area 11 m2. The coil rotates at half a revolution per second and it is placed in a uniform magnetic field of 20.5 T perpendicular to the axis of rotation of the coil. What is the maximum voltage generated in the coil?

(a) 354035 V

(b) 85000V

(c) 111647 V

(d) 46464 V

Explanation: The maximum voltage generated in the coil,

e0 = nBAω
e0 = nBA × (2πv)
e0 = 500 × 20.5 × 11 × 2π × 0.5
e0 = 345035 V

Question 30: ‘X’ is a rectangular coil consisting of a large number of turns of copper wire wound over a soft iron core in an a.c. generator. Identify X.

(a) Slip ring

(b) Armature

(c) Copper brushes

(d) Field magnet

Question 31: What is the rms value of the current, if its instantaneous current value is 8 sin314 t?

(a) 2√4 A

(b) 10 A

(c) 4√2 A

(d) 50 A

Question 32: What will be the rms value of the voltage, if the sinusoidal value voltage is given as E = 100 sin 314 t applied across a resistor of resistance 15 ohms?

(a) 200 V

(b) 70.71 V

(c) 100 V

(d) 33.87 V

Explanation: E = E0 sinωt.
Erms = 𝐸0/2
Erms = 100/2
Erms = 70.71 V

Question 33: Calculate the rms voltage, if the peak value of an a.c. supply is 300 V.

(a) 561.4 V

(b) 969.2 V

(c) 625.1 V

(d) 212.1 V

Explanation: E0 = 300 V
Erms = 0.707 E0
Erms = 0.707 × 300
Erms = 212.1 V

Question 34: Find out the net power consumed over a full cycle, if a 150 Ω resistor is connected to a 350 V, 100 Hz ac supply.

(a) 2.3 W

(b) 350 W

(c) 805 W

(d) 500 W

Explanation: Irms = 𝐸𝑟𝑚𝑠𝑅.
Irms = 350150
Irms = 2.3 A.
Pav = Erms Irms
Pav = 350 × 2.3
Pav = 805 W

Question 35: Calculate the value of the peak current, if the rms value of current in an ac circuit is 50 A?

(a) 70.7 A

(b) 65 A

(c) 50 A

(d) 1 A

Explanation: Irms = 50 A.
I0 = √2 Irms
I0 = 1.414 × 50
I0 = 70.7 A
Therefore, the peak current is 70.7 A.

Question 36: What is the general expression of a sinusoidal signal?

(a) A(t) = Am sin⁡(ωt – Φ)

(b) A(t) = sin⁡(ωt + Φ)

(c) A(t) = Am sin⁡(ωt + Φ)

(d) A(t) = 2Am sin⁡(ωt + Φ)

Answer: (c) A(t) = Am sin⁡(ωt + Φ)

Question 37: What does a phasor represent?

(a) Current and resistance

(b) Current and voltage

(c) Voltage and resistance

(d) Voltage and power

Question 38: Find the true statement.

(a) Length of the phasor must be greater than the peak value of alternating voltage or alternating current

(b) When the current reaches its maximum value after emf becomes maximum, then the current is considered to be leading ahead of emf

(c) When the emf reaches its maximum value after current becomes maximum, then the emf is considered to be behind the current

(d) Phasors for voltage and current are in the same direction when the phase angle between voltage and current is zero

Answer: (d) Phasors for voltage and current are in the same direction when the phase angle between voltage and current is zero

Question 39: Which among the following varies in both magnitude and sign over a cycle?

(a) Apparent power

(b) Effective power

(c) Instantaneous power

(d) Average power

Question 40:What is the power factor in a pure inductive or capacitive circuit?

(a) -1

(b) 0

(c) 1

(d) Infinity