Each of the following questions are followed by four answers. Select the correct answer in each case.

1. For inducing emf in a coil the basic requirement is that

• Flux should link the coil
• Change in flux should link the coil
• Coil should form a closed loop
• Both (b) and (c) are true.

Explanation: Since

where N is the number of turns of the coil loop and Δ Φ /Δ t is the rate of change of magnetic flux.

2. The device in which induced emf is statically induced is

Transformer

ac generator

alternator

dynamo

Explanation: When the conductor is stationary and the magnetic field is moving or changing, the emf induced is called statically induced emf. See comprehensive question 9 for details.

3. The north pole of a magnet is brought near a metallic ring as shown in the figure. The direction of induced current in the ring will be

• Anticlockwise
• Clockwise
• First anticlockwise and then clockwise
• First clockwise and then anticlockwise

Explanation: Apply Lenz’s law combined with the Right Hand Rule. According to Lenz’s Law, the ring will produce a magnetic north in face of the north pole of the magnet which is brought nearer to the ring. This will try to repel the magnet so that the magnetic flux on the ring remains constant. The flux due to the external magnet is increasing. By Right Hand Rule, the ring will have to induce current in the anti-clockwise direction so that the north of its magnetic field faces the incoming magnet and to oppose the increase in the magnetic flux on the ring.

4. What is the coefficient of mutual inductance, when the magnetic flux changes by 2 * 10^-2 Wb and change in current is 0.01 A?

2H

3H

½ H

Zero

Solution: The coefficient of mutual inductance M is given by

Put the values, we have,

5. The induced emf in a coil is proportional to

• Magnetic flux through the coil
• Rate of change of magnetic flux through the coil
• Area of the coil
• Product of magnetic flux and area of the coil

Hint: Use Faraday’s law

6. In a coil current changes from 2 to 4 A in 0.05 s. If the average induced emf is 8 V then

0.2 henry

0.1 henry

0.8 henry

0.04 henry

Solution: Formula

 Now Δ I = 4-2=2 Amp, Δ t = 0.5 s, ϵ = 8 V. Put the values,

L = 8 (0.5/2) = 4 * 0.05 = 0.2 henry

7. Which of the following quantities remain constant in step up transformer?

Current

Voltage

Power

Heat

Explanation: For any transformer V_pI_p = V_sI_s . therefore, the product of voltage and current is constant. Since product of voltage and current is power, therefore, power remains constant.

8. Step up transformer has a transformation ratio 3:2. What is the voltage in secondary, if voltage in primary is 30 V.

45 V

15 V

90 V

300 V

Solution: The transformer is step up, therefore, V_s/V_p = 3/2 ⇒ V_s = 30*3/2 = 45 V

9. Eddy current is produced when

• A metal is kept in varying magnetic field
•  A metal is kept in steady magnetic field
• A circular coil is placed in a steady magnetic field
• A current is passed through a circular coil

Hint: See comprehensive question 11.