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Lenz's Law - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #JEE Main

Quick Facts

  • 24 Questions around this concept.

Solve by difficulty

A uniform horizontal magnetic field B exists in the region A B C D. A rectangular loop of mass m and horizontal side l and resistance R is placed in the magnetic field as shown in figure. With what velocity should it be pushed down so that it continues to falls without acceleration?
                       

A straight-line conductor of length 0.4 m is moved with a speed of \mathrm{7 m/s } perpendicular to a magnetic field of intensity \mathrm{0.9\, wb/m^{2} }. The induced e.m.f. across the conductor is

Two identical circular loops of metal wire are lying on a table without touching each other. Loop A carries a current which increases with time. In response, the loop B :

As shown in the figure, P and Q are two coaxial conduction loops separated by some distance. When the switch S is closed, a clockwise current  I_P flows in P (as seen by E) and an induced current I_{Q_1} flows in Q. The switch remains closed for a long time. When S is opened, a current I_{Q_2} flows in Q. Then the direction I_{Q_1} and I_{Q_2} (as seen by E) are :
                         

A current-carrying wire is placed below a coil in its plane, with current flowing as shown. If the current increases

Two coils, A and B are as shown in the figure. A current starts flowing in B as shown, when A is moved towards B and stops when A stops moving. 

The current in A is counter clockwise. B is kept stationary when A moves. We can infer that:

There is a uniform magnetic field directed perpendicular and into the plane of the paper. An irregular shaped conducting loop is slowly changing into a circular loop in the plane of the paper. Then:

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Which of the following figure correctly depicts the Lenz’s law. The arrows show the movement of the labelled pole of a bar magnet into a closed circular loop and the arrows on the circle show the direction of the induced current

 

An aluminium ring B faces an electromagnet A. The current I through A can be altered:

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A metallic ring is dropped down, keeping its plane perpendicular to a constant and horizontal magnetic field. The ring enters the region of magnetic field at t = 0 and completely emerges out at t = T sec.The current in the ring varies as:

 

Concepts Covered - 1

Lenz's law

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.

 

 

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Lenz's law

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