# Class 12 Physics MCQ – AC Generator

This set of Class 12 Physics Chapter 7 Multiple Choice Questions & Answers (MCQs) focuses on “AC Generator”.

1. 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

2. 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) 14.4 A
c) 6.25 A
d) 0.55 A

Explanation: e0 = nBAω
e0 = 1000 × 0.04 × 3 × 60
e0 = 7200 V.
The maximum current drawn = $$\frac {e_0}{R} = \frac {7200}{500}$$
I = 14.4 A.

3. 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

4. Identify the principle behind the working of an a.c. generator.
a) Eddy currents
c) Lenz’s law
d) Electromagnetic induction

Explanation: The working of an a.c. generator is based on the principle of electromagnetic induction. When a coil is rotated about an axis perpendicular to the direction of the uniform magnetic field, an induced emf is produced across it and hence current is set up in it.

5. An armature coil consists of 30 turns of wire, each of area A = 0.05 m2 and total resistance of 10 Ω. It rotates in a magnetic field of 0.15T at a constant frequency of $$\frac {140}{\pi }$$ Hz. Determine the value of maximum induced emf produced in the coil.
a) 1 V
b) 500 V
c) 63 V
d) 43 V

Explanation: e0 = nBAω.
e0 = 30 × 0.15 × 0.05 × 2π × ($$\frac {140}{\pi }$$)
e0 = 63 V.

6. In an a.c. generator, electrical energy is converted to mechanical energy by electromagnetic induction.
a) True
b) False

Explanation: When a coil is rotated about an axis perpendicular to the direction of the uniform magnetic field, an induced emf is produced across it. In an a.c. generator, mechanical energy is converted to electrical energy by electromagnetic induction.

7. ‘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

Explanation: A rectangular coil consisting of a large number of turns of copper-wound over a soft iron core is called the armature. The soft iron core is used to increase the magnetic flux.

8. 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

Explanation: The current is zero when the coil is vertical. In this position, flux through the coil is maximum. On the other hand, the current is maximum, when the coil is horizontal. So, when the current is zero, the flux through the coil is maximum.

Sanfoundry Global Education & Learning Series – Physics – Class 12.