Using Lenz's Law

Q.

A conducting square frame of side ‘a’ and along straight wire carrying current I are located in the same plane a shown in the figure. The fame moves to the right with constant velocity ‘v’. The emf induced in the frame will be proportional to:

1. $\frac{1}{x^2}$

2. $\frac{1}{(2x-a{)}^2}$

3. $\frac{1}{(2x+a{)}^2}$

4. $\frac{1}{(2x-a)(2x+a)}$

Q. A current carrying loop is placed in a uniform magnetic field . The torque acting on it does not depend on

1. shape of the loop.
2. area of the loop.
3. number of turns in the loop.
4. strength of the current.

Q.

A length L of wire carries a steady current I. It is bent first to form a circular plane coil of one turn. The same length is now bent more sharply to give a double loop of smaller radius. The magnetic field at the centre caused by the same current is

[NCERT 1980

1. A quarter of its first value

2. Unaltered

3. A half of its first value

4. Four times of its first value

Q. A metal ring is held horizontally and a bar magnet is dropped through the ring with its trajectory along the axis of the ring. The acceleration of the falling magnet is

1. more than $g$
2. equal to $g$
3. less than $g$
4. Data not sufficient to predict

Q.

A coil having 500 square loops each of side 10 cm is placed normal to a magnetic field which increases at the rate of 1 T s^−1. The induced emf. is?

1. 0.1V

2. 5.0V

3. 0.5V

4. 1.0V

Q. The net charge in a current carrying wire is zero, still the magnetic field exerts a force on it. This is because a magnetic field exerts force on

1. stationatry charge
2. moving charge
3. a positive charge only
4. a negative charge only

Q.

If the magnetic field is parallel to a surface then the magnetic flux through the surface is

1. infinite

2. small but not zero

3. zero

4. large but not infinite

Q. A circular table with smooth horizontal surface is rotating at an angular speed $\omega$ about its axis. A groove is made on the surface along a radius and a small particle is gently placed inside the groove at a distance $l$ from the centre. Then the speed of the particle with respect to the table as its distance from the centre becomes $L$ is given by $v=\omega \sqrt{L^2-n{l}^2}$. The value of $n$ is

Q.

Magnetic fields at two points on the axis of a circular coil at a distance of$0.05m$ and$0.2m$ from the center are in the ratio$8:1$. The radius of coil is ______

1. $0.15m$

2. $0.2m$

3. $0.1m$

4. $1.0m$

Q. The magnetic flux linked with a coil at any instant ‘t’ is given by $\varphi =5t^3-100t+300,$ the e.m.f. induced in the coil at t = 2 second is

1. 40 V
2. 140 V
3. – 40 V
4. 300 V

Q.

A straight line conductor of length 0.4 m is moved with a speed of 7 m/s perpendiculars to a magnetic field of intensity 0.9 Wb/m².

The induced e.m.f. across the conductor is

(a) 5.04 V

(b) 25.2 V

(c) 1.26 V

(d) 2.52 V

1. $5.04V$

2. $25.2V$

3. $1.26V$

4. $2.52V$

Q. A beam of electrons is moving with constant velocity in a region having electric and magnetic fields of strength $20{\text{Vm}}^{-1}$ and $0.5\text{T}$ at right angles to the direction of motion of the electrons. What is the velocity of the electrons ?

1. $8{\text{ms}}^{-1}$
2. $5.5{\text{ms}}^{-1}$
3. $20{\text{ms}}^{-1}$
4. $40{\text{ms}}^{-1}$

Q. A thin circular ring of area $A$ is held perpendicular to a uniform magnetic field of induction $B.$ A small cut is made in the ring and a galvanometer is connected across the ends such that the total resistance of the circuit is $R.$ When the ring is suddenly squeezed to zero area, the charge flowing through the galvanometer is

1. $\frac{BR}{A}$
2. $\frac{AB}{R}$
3. $ABR$
4. $\frac{B^{2}A}{R^2}$

Q. The output (X) of the logic circuit shown in figure will be

1. $X=\stackrel{=}{A}.\stackrel{=}{B}$
2. $X=\overline{A+B}$
3. $X=A.B$
4. $X=\overline{A.B}$

Q.

The total charge induced in a conducting loop when it is moved in magnetic field depends on

1. The rate of change of magnetic flux

2. Initial magnetic flux only

3. The total change in magnetic flux

4. Final magnetic flux only

Q. A loop (circuit) is placed in uniform $\overrightarrow{B}$ & flux through the loop is given by $\varphi =4t$. Find the power supplied to the 6 volt battery.

1. 6 Watt
2. 2 Watt
3. 3 Watt
4. 5 Watt

Q. In the L-R circuit shown in the figure, the key $K$ is closed at time $t=0$, then

1. the current through $R_1$ decreases with time $t$
2. the potential drop across $L$ increases with $t$
3. the magnetic energy stored in the steady state equals $\frac{1}{2}L\frac{E^2}{R_2^2}$
4. the total current through the battery is, in the steady state, $\frac{E}{R_1}+\frac{E}{R_2}$

Q. (a) Draw a labelled diagram of an ac generator. Obtain the expression for the emf induced in the rotating coil of N turns each of cross-sectional area A, in the presence of magnetic field $\overrightarrow{B}$ .
(b) A horizontal conducting rod 10 m long extending from east to west is falling with a speed 5.0 $m{s}^{-1}$ at right angles to the horizontal component of the Earth's magnetic field, $0.3\times {10}^{-4}Wb{m}^{-2}$. Find the instantaneous value of the emf induced in the rod.

Q. An electron is constrained to move along the $y-$axis with a speed of $0.1c$ ($c$ is the speed of light) in the presence of electromagnetic wave, whose electric field is $\overrightarrow{E}=30\hat{j}\sin (1.5\times {10}^{7}t-5\times {10}^{-2}x)\text{V/m}$. The maximum magnetic force experienced by the electron will be

(given $c=3\times {10}^8{\text{ms}}^{-1}$ & electron charge $=1.6\times {10}^{-19}\text{C})$

1. $3.2\times {10}^{-18}\text{N}$
2. $2.4\times {10}^{-18}\text{N}$
3. $4.8\times {10}^{-19}\text{N}$
4. $1.6\times {10}^{-19}\text{N}$

Q.

A copper ring is held horizontally and a bar magnet is dropped through the ring with its length along the axis of the ring. Then, the acceleration of the falling magnet (neglect air resistance) will be

1. g

2. more than g

3. less than g

4. zero

Q.

The direction of the magnetic field due to a current-carrying conductor can easily be found by using left hand thumb rule.

1. True
2. False

Q. Magnetic field associated with loop is given by graphs below. [Take anticlockwise current positive]

1. 
2. 
3. 
4. 

Q. How the SI unit of magnetic field is N/Am?

Q. A metallic ring is held horizontally and a magnet is allowed to fall vertically through the ring, then the acceleration of this magnet is

1. Equal to $g$
2. More than $g$
3. Less than $g$
4. Sometimes less and sometimes more than $g$

Q. Magnetic flux passing through a coil changes with time as, $\varphi =10t^2+5t+1$ where $\varphi$ is in m wb and t in sec. then magnitude of emf at t = 5s is

1. 0.105 V
2. 0.01V
3. 0.205V
4. 1.2 V

Q. A current carrying solenoid is approaching a conducting loop as shown in the figure. The direction of induced current as observed by an observer on the other side of the loop will be

1. Anticlockwise
2. Clockwise
3. East
4. West

Q. An electron has drift velocity of $4\times {10}^{–4}\text{m/s}$. If the mobility of electron is $3\times {10}^{–2}{\text{m}}^2{\text{V}}^{–1}{\text{s}}^{–1}$, then applied electric field is -

1. $4\times {10}^{-6}{\text{Vm}}^{-1}$

2. $7.5\times {10}^{-6}{\text{Vm}}^{-1}$
3. $1.33\times {10}^{-2}{\text{Vm}}^{-1}$
4. $3.33\times {10}^{-9}{\text{Vm}}^{-1}$

Q.

Can you have negative EMF$?$

Q.

The magnitude of the earths magnetic field at a place$B_0$ and the angle of dip is$\delta$. A horizontal conductor of length$l$ lying along the magnetic North-South moves Eastwards with a velocity v, the emf induced across the conductor is

1. zero

2. $B_{0}lv\sin \delta$

3. $B_{0}lv$

4. $B_{0}lv\cos \delta$

5. $B_0\sin \delta$

Q.

In an electromagnetic induction, what is the source of electric energy that causes induced current?