Lenz's Law - Practice Questions & MCQ

Lenz's law-

This law gives the direction of induced emf /induced current.

According to Lenz's law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it.

And this law is based upon the law of conservation of energy.

• The induced current in a closed-loop circuit

1. When N pole of a bar magnet moves towards the coil the flux associated with loop increases and an emf is induced in it. 

 To repel the approaching north pole, the induced current is set up in the loop (if the loop is closed) in such a direction so
that the front face of the loop behaves as the north pole. Therefore induced current as seen by observer O is in an anticlockwise
direction (as shown in the figure).

2. Similarly When N pole of a bar magnet moves away from the loop as shown in the figure. 

To attract the north pole, the induced current is set up in the loop (if the loop is closed) in such a direction so that the front face of the loop behaves as the south pole. Therefore induced current as seen by observer O is in a clockwise direction.

3. Similarly When S pole of a bar magnet moves towards the loop as shown in the figure. 

To repel the approaching south pole, the induced current is set up in the loop (if the loop is closed) in such a direction so that the front face of the loop behaves as the south pole. Therefore induced current as seen by observer O is in a clockwise direction (as shown in the figure).

4. Similarly When S pole of a bar magnet moves away from the loop as shown in the figure. 

To attract the south pole, the induced current is set up in the loop (if the loop is closed) in such a direction so that the front face of the loop behaves as the north pole. Therefore induced current as seen by observer O is in an anticlockwise direction.

• If the loop is free to move the cause of induced emf in the coil can also be termed as relative motion. Therefore to oppose the relative motion between the approaching magnet and the loop, the loop will itself start moving in the direction of motion of the magnet.