Difficulty Level:

Lenz's law is consequence of the law of conservation of

(1) Charge

(2) Momentum

(3) Mass

(4) Energy

(4) The energy of the field increases with the magnitude of the field. Lenz’s law infers that there is an opposite field created due to increase or decrease of magnetic flux around a conductor so as to hold the law of conservation of energy.

Difficulty Level:

The magnetic flux through a circuit of resistance *R *changes by an amount $\Delta \phi $ in time $\Delta t$, Then the total quantity of electric charge *Q*, which passing during this time through any point of the circuit is given by

(1) $Q=\frac{\Delta \varphi}{\Delta t}$

(2) $Q=\frac{\Delta \varphi}{\Delta t}\times R$

(3) $Q=-\frac{\Delta \varphi}{\Delta t}+R$

(4) $Q=\frac{\Delta \varphi}{R}$

(4) We know that $e=\frac{d\varphi}{dt}$

But *e=iR* and $i=\frac{dq}{dt}\Rightarrow \frac{dq}{dt}R=\frac{d\varphi}{dt}$ ⇒ $dq=\frac{d\varphi}{R}$

Difficulty Level:

A metallic ring is attached with the wall of a room. When the north pole of a magnet is brought near to it, the induced current in the ring will be

(1) First clockwise then anticlockwise

(2) In clockwise direction

(3) In anticlockwise direction

(4) First anticlockwise then clockwise

(3) As it is seen from the magnet side induced current will be anticlockwise.

Difficulty Level:

A coil having an area *A*_{0} is placed in a magnetic field which changes from *B*_{0 }to 4*B*_{0} in a time interval *t*. The e.m.f. induced in the coil will be** **

(1) $\frac{3{A}_{0}{B}_{0}}{t}$

(2) $\frac{4{A}_{0}{B}_{0}}{t}$

(3) $\frac{3{B}_{0}}{{A}_{0}t}$

(4) $\frac{4{B}_{0}}{{A}_{0}t}$

(1) $e=-\frac{d\varphi}{dt}=\frac{-3{B}_{0}{A}_{0}}{t}$

Difficulty Level:

A copper ring is held horizontally and a bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet while it is passing through the ring is

(1) Equal to that due to gravity

(2) Less than that due to gravity

(3) More than that due to gravity

(4) Depends on the diameter of the ring and the length of the magnet

(2) When the magnet is allowed to fall vertically along the axis of loop with its north pole towards the ring. The upper face of the ring will become north pole in an attempt to oppose the approaching north pole of the magnet. Therefore the acceleration in the magnet is less than *g.*

**Note : ** If coil is broken at any point then induced *emf* will be generated in it but no induced current will flow. In this condition the coil will not oppose the motion of magnet and the magnet will fall freely with acceleration *g*. (*i.e. a = g*)

Difficulty Level:

A magnet is brought towards a coil (i) speedily (ii) slowly then the induced e.m.f./induced charge will be respectively

(1) More in first case / More in first case

(2) More in first case/Equal in both case

(3) Less in first case/More in second case

(4) Less in first case/Equal in both case

(2) The magnitude of induced e.m.f. is directly proportional to the rate of change of magnetic flux. Induced charge doesn’t depend upon time.

Difficulty Level:

As shown in the figure, a magnet is moved with a fast speed towards a coil at rest. Due to this induced electromotive force, induced current and induced charge in the coil is *E*, *I* and *Q* respectively. If the speed of the magnet is doubled, the incorrect statement is** **

(1) *E* increases

(2) *I* increases

(3) *Q* remains same

(4) *Q* increases

(4) Induced charge doesn’t depend upon the speed of magnet.

Difficulty Level:

A coil having 500 square loops each of side 10 *cm* is placed normal to a magnetic flux which increases at the rate of 1.0 *tesla/second*. The induced e.m.f. in volts is

(1) 0.1

(2) 0.5

(3) 1

(4) 5

(4) $\left|e\right|\text{\hspace{0.17em}}=N\left(\frac{\Delta B}{\Delta t}\right).A\mathrm{cos}\theta $

$=500\times 1\times {(10\times {10}^{-2})}^{2}\mathrm{cos}0=5V.$

Difficulty Level:

When a magnet is pushed in and out of a circular coil *C* connected to a very sensitive galvanometer *G* as shown in the adjoining diagram with a frequency *v*, then

(1) Constant deflection is observed in the galvanometer

(2) Visible small oscillations will be observed in the galvanometer if *v *is about 50 *Hz*

(3) Oscillations in the deflection will be observed clearly if *v* = 1 or 2 *Hz*

(4) No variation in the deflection will be seen if *v* = 1 or 2 *Hz*

(3) When frequency is high, the galvanometer will not show deflection.

Difficulty Level: