Objective Physics for NEET

Abhay Kumar

Chapter 20

Magnetic Effect of Current and Magnetism - all with Video Answers

Educators

Chapter Questions

Problem 1

An electron having a charge $e$ moves with velocity $\vec{v}$ in $+x$ -direction. An electric field acts on it in $+y$ -direction. The force on the electron acts along
(a) $+z$ -direction
(b) $-z$ -direction
(c) $+y$ -direction
(d) $-y$ -direction

Narayan Hari

Narayan Hari

Numerade Educator

Problem 2

A charged particle moving in a magnetic field experiences a resultant force
(a) in the direction of the field
(b) in the direction opposite to the field
(c) in any direction perpendicular to the field
(d) in the direction perpendicular to the field as well as perpendicular to the direction of its motion

Narayan Hari

Numerade Educator

Problem 3

An electric charge in uniform motion produces
(a) an electric field only
(b) a magnetic field only
(c) both the electric and magnetic fields
(d) no such fields at all

Narayan Hari

Numerade Educator

Problem 4

When a charged particle enters in a magnetic field $\vec{B}$ in a direction perpendicular to $\vec{B}$ and moves with uniform speed $v$ inside the magnetic field, then
(a) the energy and momentum both change
(b) only energy changes, momentum does not change
(c) only momentum changes, energy does not change
(d) both energy and momentum do not change

Narayan Hari

Numerade Educator

Problem 5

A positively charged $(+q)$ particle of mass $m$ has kinetic energy $K$ enters vertically downward in a horizontal field of magnetic induction $\vec{B} .$ The acceleration of the particle is
(a) $q B \sqrt{\frac{2 K}{m}}$
(b) $\frac{q B \sqrt{2 K}}{(m)^{3 / 2}}$
(c) $\frac{2 q B}{(m)^{3 / 2}} \sqrt{2 K}$
(d) $2 q B \sqrt{\frac{2 K}{m}}$

Narayan Hari

Numerade Educator

Problem 6

An electron of charge $e$ and mass $m$ enters normally into a uniform magnetic field $B$. The radius of the circular path of the electron is
(a) $\frac{e B}{m y}$
(b) $\frac{e v}{m B}$
(c) $\frac{m v}{e B}$
(d) $\frac{m B}{e v}$

Narayan Hari

Numerade Educator

Problem 7

If proton and electron are projected at right angle to uniform magnetic field with the same kinetic energy, then
(a) the radius of the circular trajectory of proton $r_{p}$ is smaller than the radius of the circular trajectory of the electron $r_{e}$
(b) $r_{p}>r_{e}$
(c) $r_{P}=r_{e}$
(d) both the particles move in straight line

Narayan Hari

Numerade Educator

Problem 8

A charged particle $q$ is moving with a velocity $\vec{v}_{1}=2 \hat{i}$ $\mathrm{m} / \mathrm{s}$ at a point in a magnetic field $\vec{B}$ and experiences a force $\vec{F}_{1}=q(\hat{k}-2 \hat{j}) \mathrm{N}$
If the same charge moves with velocity $\vec{v}_{2}=2 \hat{j} \mathrm{~m} / \mathrm{s}$ from the same point in that magnetic field and experiences a force $\vec{F}_{2}=q(\hat{i}+2 \hat{k}) \mathrm{N}$, the magnetic induction at that point will be
(a) $\hat{i}+\frac{1}{2} \hat{j}-\frac{1}{2} \hat{k}$
(b) $-\frac{1}{2} \hat{i}+\frac{1}{2} \hat{j}+\hat{k}$
(c) $\frac{1}{2} \hat{i}-\frac{1}{2} \hat{j}+\hat{k}$
(d) $-\frac{1}{2} \hat{i}+\hat{j}+\frac{1}{2} \hat{k}$

Narayan Hari

Numerade Educator

Problem 9

A charged particle moving in a uniform magnetic field when losses $4 \%$ of its kinetic energy, the radius of curvature of its circular path
(a) decreases by $2 \%$
(b) increases by $2 \%$
(c) increases by $4 \%$
(d) decreases by $4 \%$

Narayan Hari

Numerade Educator

Problem 10

A deuteron of kinetic energy $50 \mathrm{keV}$ is describing a circular orbit of radius $0.5 \mathrm{~m}$ in a plane perpendicular to the magnetic field $\vec{B}$. The kinetic energy of the proton that describes a circular orbit of radius $0.5 \mathrm{~m}$ in the same plane with the same field $\vec{B}$ is
(a) $5 \mathrm{keV}$
(b) $10 \mathrm{keV}$
(c) $50 \mathrm{keV}$
(d) $100 \mathrm{keV}$

Narayan Hari

Numerade Educator

Problem 11

An electron moves with a speed of $2 \times 10^{5} \mathrm{~m} / \mathrm{s}$ along the $+x$ -direction in a magnetic field $\vec{B}=(\hat{i}-4 \hat{j}-3 \vec{k})$ tesla. The magnitude of the force (in newton) experienced by the electron is (the charge on electron $=1.6 \times 10^{-19} \mathrm{C}$ )
(a) $1.18 \times 10^{-13}$
(b) $1.28 \times 10^{-13}$
(c) $1.6 \times 10^{-13}$
(d) $1.72 \times 10^{-13}$

Narayan Hari

Numerade Educator

Problem 12

A proton of velocity $(3 \hat{i}+2 \hat{j}) \mathrm{m} / \mathrm{s}$ enters a field of magnetic induction $(2 \hat{j}+3 \hat{k})$ tesla. The acceleration produced in the proton is (specific charge of proton $0.96$ $\left.\times 10^{8} \mathrm{C} / \mathrm{kg}\right)$
(a) $2.8 \times 10^{8}(2 \hat{i}-3 \hat{j}) \mathrm{m} / \mathrm{s}^{2}$
(b) $2.88 \times 10^{8}(2 \hat{i}-3 \hat{j}+2 \hat{k}) \mathrm{m} / \mathrm{s}^{2}$
(c) $2.8 \times 10^{8}(2 \hat{i}+3 \hat{k}) \mathrm{m} / \mathrm{s}^{2}$
(d) $2.88 \times 10^{8}(\hat{i}-3 \hat{j}+2 \hat{k}) \mathrm{m} / \mathrm{s}^{2}$

Narayan Hari

Numerade Educator

Problem 13

A long straight horizontal wire carries a current of $1 \mathrm{~A}$ in the east to west direction. What is the magnitude and direction of the magnetic field induction due to the current $1 \mathrm{~m}$ below the conductor?
(a) $4 \pi \times 10^{-7}$ towards south
(b) $4 \pi \times 10^{-7}$ towards north
(c) $2 \pi \times 10^{-7}$ towards south
(d) $2 \pi \times 10^{-7}$ towards north

Narayan Hari

Numerade Educator

Problem 14

A wire carrying current $i$ is in the shape of a plane curve $r=f(\theta)$ where $r$ and $\theta$ are the polar co-ordinates. The magnetic field at the centre of curvature of the current carrying wire is
(a) $\frac{\mu_{o} i}{4 \pi} \int \frac{\sin \theta d \theta}{r}$
(b) $\frac{\mu_{o} i}{4 \pi} \int \frac{\sin \theta d \theta}{r^{2}}$
(c) $\frac{\mu_{o} i}{2 \pi} \int \frac{\sin \theta d \theta}{r}$
(d) $\frac{\mu_{o} i}{2 \pi} \int \frac{\sin \theta d \theta}{r^{2}}$

Narayan Hari

Numerade Educator

Problem 15

A current of $20 \mathrm{~A}$ flows through each of two parallel long wires which are $4 \mathrm{~cm}$ apart. The force exerted per unit length of each wire is
(a) $1 \times 10^{-3} \mathrm{~N}$
(b) $2 \times 10^{-3} \mathrm{~N}$
(c) $3 \times 10^{-3} \mathrm{~N}$
(d) $4 \times 10^{-3} \mathrm{~N}$

Narayan Hari

Numerade Educator

Problem 16

In hydrogen atom, the electron is making $5 \times 10^{15}$ revolutions per second. If the radius of the orbit is $0.8 \times$ $10^{-10} \mathrm{~m}$, then the magnetic field produced at the centre of the orbit is
(a) $1.57 \mathrm{~T}$
(b) $3.14 \mathrm{~T}$
(c) $4.71 \mathrm{~T}$
(d) $6.28 \mathrm{~T}$

Narayan Hari

Numerade Educator

Problem 17

A square loop of wire of edge length ' $a$ ' carries a current $i$. the value of magnetic induction at the centre of the square loop is
(a) $\frac{\mu_{a} i}{\pi a}$
(b) $\frac{2 \sqrt{2} \mu_{o} i}{\pi a}$
(c) $\frac{\mu_{o} i}{\sqrt{2} \pi a}$
(d) $\frac{2 \mu_{o} i}{\pi a}$

Narayan Hari

Numerade Educator

Problem 18

$A$ and $B$ are two conductors carrying a current $I$ in the same direction. $x$ and $y$ are the two electron beams moving in the same direction as shown in the figure. There will be
(a) repulsion between $A$ and $B$, attraction between $x$ and $y$
(b) attraction between $A$ and $B$, repulsion between $x$ and $y$
(c) repulsion between $A$ and $B$ and also $x$ and $y$
(d) attraction between $A$ and $B$ and also $x$ and $y$

Narayan Hari

Numerade Educator

Problem 19

A long wire having a semi-circular loop of radius $r$ carries a current $i$ as shown in figure. The magnetic induction at the centre $c$ due to the entire wire is
(a) $\frac{3 \mu_{o} i}{4 r}$
(b) $\frac{\mu_{o} i}{2 r}$
(c) $\frac{\mu_{a} i}{4 r}$
(d) $\frac{\mu_{a} i}{8 r}$

Narayan Hari

Numerade Educator

Problem 20

A circular coil has 100 turns and a mean diameter of $10 \mathrm{~cm}$. It carries a current of $2 \mathrm{~A}$. The strength of the magnetic field at a point on its axis at a distance $2 \mathrm{~m}$ from the centre of the coil is approximately
(a) $6.28 \times 10^{-7} \mathrm{~Wb} / \mathrm{m}^{2}$
(b) $3.14 \times 10^{-7} \mathrm{~Wb} / \mathrm{m}^{2}$
(c) $1.57 \times 10^{-7} \mathrm{~Wb} / \mathrm{m}^{2}$
(d) $10^{-7} \mathrm{~Wb} / \mathrm{m}^{2}$

Narayan Hari

Numerade Educator

Problem 21

Two circular coils $A$ and $B$ of same radius are made from wire of similar material but radius of the wire $B$ is twice that of $A$. The ratio of values of the potential difference across $A$ to that of $B$ so that the magnetic field induction at their centre may be same, will be
(a) 1
(b) 2
(c) $\frac{1}{2}$
(d) 4

Narayan Hari

Numerade Educator

Problem 22

A cell is connected across two points $A$ and $B$ of a closed circular wire. The magnetic field at the centre of the circular wire is
(a) $\frac{\mu_{o} i}{2 a}$
(b) $\frac{2 \mu_{e} i}{a}$
(c) $\frac{\mu_{o} i}{a}$
(d) zero

Narayan Hari

Numerade Educator

Problem 23

The magnetic induction at the centre $O$ of the current loop shown in the adjoining diagram is
(a) $\frac{\mu_{o} i}{4 \pi}\left(\frac{1}{a_{1}}-\frac{1}{a_{2}}\right) \theta$
(b) $\frac{\mu_{a} i}{2 \pi}\left(\frac{1}{a_{1}}-\frac{1}{a_{2}}\right) \theta$
(c) $\frac{\mu_{o} i}{4 \pi}\left(\frac{1}{a_{1}}+\frac{1}{a_{2}}\right) \theta$
(d) $\frac{\mu_{o} i}{2 \pi} \cdot \frac{\theta}{a_{2}}$

Narayan Hari

Numerade Educator

Problem 24

An infinitely long straight conductor is bent into the shape as shown in the figure. It carries a current of $i$ ampere and the radius of the circular loop is ' $a$ ' metre. The magnetic induction at the centre of the circular part is
(a) $\frac{\mu_{e} i}{4 a}(\pi-1)$
(b) $\frac{\mu_{o} i}{2 a}\left(1-\frac{1}{\pi}\right)$
(c) $\frac{\mu_{o} i}{2 a}(\pi-1)$
(d) $\frac{\mu_{o} i}{4 a}\left(1-\frac{1}{\pi}\right)$

Narayan Hari

Numerade Educator

Problem 25

A wire loop formed by joining two semi-circular wires of radii $a_{1}$ and $a_{2}$ carries a current $i$ as shown in the adjoining figure. The magnetic induction at the centre $O$ is
(a) $\frac{\mu_{o} i}{4}\left(\frac{1}{a_{1}}+\frac{1}{a_{2}}\right)$
(b) $\frac{\mu_{\theta} i}{4}\left(\frac{1}{a_{1}}-\frac{1}{a_{2}}\right)$
(c) $\frac{\mu_{o} i}{2}\left(\frac{1}{a_{1}}+\frac{1}{a_{2}}\right)$
(d) $\frac{\mu_{e} i}{2}\left(\frac{1}{a_{1}}-\frac{1}{a_{2}}\right)$

Narayan Hari

Numerade Educator

Problem 26

A circular coil of 100 turns and of diameter $20 \mathrm{~cm}$ carries a current of $1 \mathrm{~A}$. It is to be turned to an angle $\theta$ in a magnetic field of $5 \mathrm{~T}$ from a position $\theta=0^{\circ}$ to $\theta=180^{\circ}$. The magnetic moment of the coil is
(a) $\pi \mathrm{A}-\mathrm{m}^{2}$
(b) $\frac{\pi}{2} \mathrm{~A}-\mathrm{m}^{2}$
(c) $2 \pi \mathrm{A}-\mathrm{m}^{2}$
(d) $\frac{3 \pi}{8} \mathrm{~A}-\mathrm{m}^{2}$

Narayan Hari

Numerade Educator

Problem 27

Which of the following graphs correctly represents the variation of magnetic field with distance $R$ from a long straight current carrying conductor?

Narayan Hari

Numerade Educator

Problem 28

A wire $P Q R S$ shown in the figure carries a current $i$. The radius of the circular part is $r$. The magnetic field at the centre $O$ of the circular part of the wire is given by
(a) $\frac{\mu_{o} i}{2 r}$
(b) $\frac{\mu_{o} i}{8 r}$
(c) $\frac{3 \mu_{e} i}{4 r}$
(d) $\frac{3 \mu_{o} i}{8 r}$

Narayan Hari

Numerade Educator

Problem 29

An electron is projected along the axis of a circular conductor carrying some current. Electron will experience force
(a) along the axis
(b) perpendicular to the axis
(c) at an angle of $4^{\circ}$ with axis
(d) no force experienced

Narayan Hari

Numerade Educator

Problem 30

The ratio of magnetic induction on the axis of a long straight current carrying solenoid at a point on the end to that at the centre of the solenoid
(a) $1: 1$
(b) $2: 1$
(c) $1: 2$
(d) $1: \sqrt{2}$

Narayan Hari

Numerade Educator

Problem 31

A magnet of magnetic moment $10 \hat{i} \mathrm{~A}-\mathrm{m}^{2}$ is placed along $x$ -axis in a magnetic field $(\hat{i}+2 \hat{j})$ tesla. The torque acting on the magnet is
(a) $20 \mathrm{~N}-\mathrm{m}$ along $x$ -axis
(b) $10 \sqrt{5} \mathrm{~N}-\mathrm{m}$ along $z$ -axis
(c) $20 \mathrm{~N}-\mathrm{m}$ along $z$ -axis
(d) $10 \sqrt{5} \mathrm{~N}-\mathrm{m}$ along $y$ -axis

Narayan Hari

Numerade Educator

Problem 32

A long hollow metallic cylinder of radius $R$ has a current $i$ ampere. The magnetic induction $B$ away from the axis at a distance $x$ from the axis of varies as shown in
(a)
(b)
(c)
(d)

Pritesh Ranjan

Numerade Educator

Problem 33

The magnetic lines of force inside a bar magnet
(a) are from north-pole to south-pole of the magnet
(b) do not exist
(c) depend upon the area of cross-section of the bar magnet
(d) are from south-pole to north-pole of the magnet

Narayan Hari

Numerade Educator

Problem 34

A magnetic needle is kept in a non-uniform magnetic field. It experiences
(a) a torque but not a force
(b) neither a force nor a torque
(c) a force and a torque
(d) a force but not a torque

Narayan Hari

Numerade Educator

Problem 35

The material suitable for making electromagnets should have
(a) high retentivity and high coercivity
(b) low retentivity and low coercivity
(c) high retentivity and low coercivity
(d) low retentivity and high coercivity

Narayan Hari

Numerade Educator

Problem 36

Niddles $N_{1}, N_{2}$ and $N_{3}$ are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will
(a) attract $N_{1}$ and $N_{2}$ strongly, but repel $\mathrm{N}_{3}$
(b) attract $N_{1}$ strongly,$N_{2}$ weakly and repel $N_{3}$ weakly
(c) attract $N$, strongly, but repel $N_{2}$ and $N_{3}$ weakly
(d) attract all three of them

Narayan Hari

Numerade Educator

Problem 37

Three identical bar magnets are arranged such that their lengths form the three sides of an equilateral triangle as shown in the figure. The magnetic moment of each magnet is $m .$ The effective magnetic moment of the given combination is
(a) $3 m$
(b) $m$
(c) $\frac{m \sqrt{3}}{2}$
(d) zero

Narayan Hari

Numerade Educator

Problem 38

A small bar of some magnetic material is placed in an external magnetic field. The bar get magnetised such that the magnetic lines of induction so produced have been shown in the adjoining diagram. The material is a
(a) paramagnetic substance
(b) diamagnetic substance
(c) ferromagnetic substance
(d) non-magnetic substance

Narayan Hari

Numerade Educator

Problem 39

Of the following diagrams the lines of magnetic induction due to a bar magnet are given by

Narayan Hari

Numerade Educator

Problem 40

A magnetic needle suspended by a silk thread is vibrating in the earth's magnetic field. If The temperature of the needle is increased by significant amount, then
(a) the time period decreases
(b) the time period remains unchanged
(c) the time period increases
(d) the magnetic needle stops vibrating

Narayan Hari

Numerade Educator

Problem 41

At a point on the right bisector of a magnetic dipole, the magnetic field induction
(a) varies as $1 / r$
(b) varies as $1 / r^{2}$
(c) varies $1 / r^{3}$
(d) is zero

Narayan Hari

Numerade Educator

Problem 42

A ferromagnetic substance is placed in the varying magnetising field $H$. The magnetic induction $B$ is measure for various values of direct and reverse magnetising field. Following graph has been plotted for $B$ versus $H .$ Choose the any wrong statement
(a) There is a limit of direct and reverse external magnetising field at which the magnetisation and hence the magnetic induction saturates
(b) Even after removing the external magnetising field some residual magnetisation called 'retentivity' is left over the substance
(c) On increasing the reverse magnetising field, the retentivity decreases to zero for a value of magnetising field which is known as 'susceptibility'
(d) On increasing the reverse magnetising field the retentivity decreases to zero for a value of magnetising field known as 'coercivity'

Narayan Hari

Numerade Educator

Problem 43

The intensity of magnetisation $I$ is plotted against the magnetising field $H$ for different substances. the curves $A$ and $B$ shown in the adjoining figure are associated with
(a) soft iron and steel respectively
(b) steel and soft iron respectively
(c) a diamagnetic and paramagnetic substance respectively
(d) a paramagnetic and diamagnetic substance respectively

Narayan Hari

Numerade Educator

Problem 44

Which one of the following graphs represents the behaviour of magnetic susceptibility $(\chi)$ of the paramagnetic substance with the magnetising field $(H) ?$
(a)
(b)

Narayan Hari

Numerade Educator

Problem 45

At curie point a ferromagnetic material becomes
(a) diamagnetic
(b) paramagnetic
(c) strongly ferromagnetic
(d) non-magnetic

Narayan Hari

Numerade Educator

Problem 46

The variation of magnetic susceptibility $(\chi)$ with absolute temperature $(T)$ for a ferromagnetic substance is represented by the graph

Narayan Hari

Numerade Educator

Problem 47

A magnet suspended freely at the magnetic pole will show its direction
(a) vertical
(b) horizontal
(c) $45^{\circ}$ from the horizontal
(d) turn in any direction

Narayan Hari

Numerade Educator

Problem 48

If magnetic lines of force are drawn by keeping magnet vertical, then the number of neutral points will be
(a) 1
(b) 2
(c) 4
(d) 5

Narayan Hari

Numerade Educator

Problem 49

If the distance between two similar magnetic poles held $1 \mathrm{~cm}$ apart be doubled, then the force of interaction between them will be
(a) unchanged
(b) doubled
(c) halved
(d) one-fourth of the original value

Narayan Hari

Numerade Educator

Problem 50

The pole strength of a magnetic needle is $p$ and its magnetic moment is $m .$ If the needle is broken into two equal pieces, the pole strength and the magnetic moment of each piece will be respectively
(a) $p$ and $m$
(b) $\frac{p}{2}$ and $\frac{m}{2}$
(c) $p$ and $\frac{m}{2}$
(d) $\frac{p}{2}$ and $m$

Narayan Hari

Numerade Educator

Problem 51

Two magnets of exactly equal lengths have magnetic moments $m_{1}$ and $m_{2}$ respectively. What will be the effective magnetic moment, if both these magnets are placed one over the other with similar poles together?
(a) $\frac{m_{1}+m_{2}}{2}$
(b) $m_{1}+m_{2}$
(c) $2\left(m_{1}+m_{2}\right)$
(d) $\frac{m_{1}-m_{2}}{2}$

Narayan Hari

Numerade Educator

Problem 52

The total magnetic field due to the earth is $3.2 \mathrm{~A} / \mathrm{m}$, then the total magnetic induction due to the earth is nearly
(a) $3.2 \mathrm{~T}$
(b) $4 \mathrm{~T}$
(c) $3.2 \times 10^{-5} \mathrm{~T}$
(d) $4 \times 10^{-6} \mathrm{~T}$

Narayan Hari

Numerade Educator

Problem 53

The current sensitivity of a tangent galvanometer is increased if
(a) magnetic field increases
(b) number of turns in the coil increases
(c) number of turns in the coil decreases
(d) the radius of the coil increases

Narayan Hari

Numerade Educator

Problem 54

The sensitivity of a tangent galvanometer is maximum when deflection $\theta$ is equal to
(a) $0^{\circ}$
(b) $30^{\circ}$
(c) $45^{\circ}$
(d) $90^{\circ}$

Narayan Hari

Numerade Educator

Problem 55

The variation of magnetic susceptibility $(\chi)$ with absolute temperature $(T)$ for a diamagnetic substance is given by
(a)
(b)
(c)
(d)

Narayan Hari

Numerade Educator

Problem 56

At a certain place of latitude $2^{\circ}$ on the surface of earth, the angle of dip is
(a) $2^{\circ}$
(b) $4^{\circ}$
(c) $86^{\circ}$
(d) $88^{\circ}$

Narayan Hari

Numerade Educator

Problem 57

The effective magnetic moment of the system shown in the adjoining figure is
( $p$ is the pole strength and $2 a$ is the length of each side of the triangle)
(a) $2 a p$
(b) $\sqrt{2} a p$
(c) $\sqrt{3} a p$
(d) $2 \sqrt{3} a p$

Narayan Hari

Numerade Educator

Problem 58

Curie temperature is the temperature above which
(a) a ferromagnetic material becomes para-magnetic
(b) a paramagnetic material becomes diamagnetic
(c) a ferromagnetic material becomes diamagnetic
(d) a paramagnetic material becomes ferro-magnetic

Narayan Hari

Numerade Educator

Problem 59

The value of angle of dip is zero at the magnetic equator because on it
(a) $V$ and $H$ are equal
(b) The value of $V$ and $H$ is zero
(c) The value of $V$ is zero
(d) The value of $H$ is zero

Narayan Hari

Numerade Educator

Problem 60

At the magnetic poles of the earth, a compass needle will be
(a) Vertical
(b) Bent slightly
(c) Horizontal
(d) Inclined at $45^{\circ}$ to the horizontal

Narayan Hari

Numerade Educator