Tag: magnetic effects of current and magnetism

Questions Related to magnetic effects of current and magnetism

Force between two identical bar magnets whose centres are  $r meters $ apart is $4.8 N,$ when their axis are in the same line. If the separation is increased to $2r$ meters, the force between them is reduced to:

electrostatic and magnetic analogy magnetism and matter magnetic effects of current and magnetism physics

  1. 2.4 N

  2. 1.2 N

  3. 0.6 N

  4. 0.3 N

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Correct Option: B
Explanation:

$F=\dfrac{\mu _{0}m _{1}m _{2}}{4\pi d^{2}}$
$F\propto \dfrac{1}{d^{2}}$
$Fd^{2}=Constant$
$F _{1}d _{1}^{2}=F _{2}d _{2}^{2}$
$4.8\times r^2=F _{2}(2r)^2$
$F _{2}=1.2  N$

The magnetic induction at a distance d from the magnetic pole of the unknown strength m is B. If an identical pole is now placed at a distance of 2d from the first pole, the force between the two poles is          

electrostatic and magnetic analogy magnetism and matter magnetic effects of current and magnetism physics

  1. mB

  2. $\frac{mB}{2}$

  3. $\frac{mB}{4}$

  4. 2mB

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Correct Option: C
Explanation:
$B=\dfrac{\mu _{0}m}{4\pi d^{2}}$
$r=2d$
$F=\dfrac{\mu _{0}m _{1}m _{2}}{4\pi r^{2}}$
$F=\dfrac{\mu _{0}\times m\times m}{4\pi (d)^{2}}$
$F=\dfrac{\mu _{0}m^{2}}{4\pi \times 4d^{2}}$
$F=\dfrac{mB}{4}N$

The force acting on a pole of strength of 20 Am is 8x10$^{-4}$ N, when placed in a magnetic field. The magnetic induction field strength is :

electrostatic and magnetic analogy magnetism and matter magnetic effects of current and magnetism physics

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

  2. 0.4$\times $10$^{4}$ T

  3. 4 G

  4. 4$\times $10$^{-5}$ T

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Correct Option: D
Explanation:

$m=20Am$
$F=8\times10^{-4}N$
$F=Bm$
$8\times 10^{-4}=20\times B$
$B=4\times 10^{-5}T$

Two magnetic poles have their strengths in the ratio 3 : 2. They are kept at a distance of 0.6 m in air and the force of repulsion is found to be 0.06 dynes. The pole strengths are (in amp. m)

electrostatic and magnetic analogy magnetism and matter magnetic effects of current and magnetism physics

  1. 1.8, 1.2

  2. 18, 12

  3. 6, 4

  4. 0.6, 3.6

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Correct Option: A
Explanation:

$1 dyne = 10^{-5}$
$\mu _{0}=4\pi \times 10^{-7}TmA^{-1}$ 
$m _{1}=3m$
$m _{2}=3m$
$F=\dfrac{\mu _{0}m _{1}\times m _{2}}{4\pi \times r^{2}}$ r=0.6m
$0.06\times 10^{-5}=\dfrac{4\pi \times 10^{-7}\times 3m\times 2m}{4\pi \times (0.6)^{2}}$
$m^{2}=(0.6)^{2}$
$m=0.6$
$m _{1}=1.8$
$m _{2}=1.2$




A closely wound solenoid of $2000$ turns and area of cross-section $1.5\times10^{-4}\ m^{2}$ carries a current of $2.0A$. It is suspended through its centre and perpendicular to its length, allowing it to turn in a horizontal plane in a uniform magnetic field $5\times10^{-2}$ Tesla making an angle of $30^{o}$ with the axis of the solenoid. The torque on the solenoid will be-

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. $1.5\times10^{-3}\ N.m$

  2. $1.5\times10^{-2}\ N.m$

  3. $3\times10^{-2}\ N.m$

  4. $3\times10^{-3}\ N.m$

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Correct Option: D

Two bar magnet are kept together and suspended freely in earth's magnetic field. When both like poles are aligned, the time period is 6 sec. When opposite poles are aligned, the time period is 12 sec. The ratio of magnetic moments of the two magnets is :

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. $5/3$

  2. $2/1$

  3. $3/2$

  4. $3/1$

Show answer
Correct Option: A
Explanation:

$T _1= 6 = 2\pi \sqrt{\dfrac{1}{(M _1+M _2)B}}$
$T _2= 12 = 2\pi \sqrt{\dfrac{1}{(M _1+M _2)B}}$
$\therefore \dfrac{6}{12}=\dfrac{1}{2}= \sqrt{\dfrac{M- M _2}{M _1+M _2}}$ or $ M _1+M _2= 4( M _1-M _2) $
$ 3M _1= 5M _2$
$\dfrac{M _1}{M _2}=\dfrac{5}{3}$

Puneet peddles a stationary bicycle. The peddles are attached to a $100$ turn coil of area $0.10 \ m^2$. The coil rotates at half a revolution per second and it is placed in a uniform magnetic field of $0.01 \ T$ perpendicular to the axis of rotation of the coil. What is maximum voltage generated in the coil? 

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. $0.314 \ V$

  2. $0.615 \ V$

  3. $0.921 \ V$

  4. $0.084 \ V$

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Correct Option: A

A bar magnet of length $16 cm$ has a pole strength of 500 milli amp.m. The angle at which it should be placed to the direction of external magnetic field of induction $2.5$ gauss so that it may experience a torque of $\sqrt { 3 } \times{ 10 }^{ -5 }$ N.m. is

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. $\pi $

  2. $\dfrac { \pi }{ 2 } $

  3. $\dfrac { \pi }{ 3 } $

  4. $\dfrac { \pi }{ 6 } $

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Correct Option: D

A bar magnet of magnetic moment $M$ and moment of inertia I is freely suspended such that the magnetic axial line is in the direction of magnetic meridian. If the magnet is displaced by a very small angle $(\theta )$ , the angular acceleration is (Magnetic induction of earths horizontal field $ = $ $B _H$)

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. $\dfrac{MB _{H}\theta }{I}$

  2. $\dfrac{IB _{H}\theta }{M}$

  3. $\dfrac{M\theta }{IB _{H}}$

  4. $\dfrac{I\theta }{MB _{H}}$

Show answer
Correct Option: A
Explanation:

Torque acting on the magnet is  $T=MB _H sin \theta $
Since $ \theta$ is very small , $T = MB _H \theta $
$I {\alpha} =  MB _H \theta $
$ \alpha = \dfrac{MB _H \theta}{I} $

When a bar magnet is suspended freely in a uniform magnetic field, identify the correct statements:

a) The magnet experiences only couple and undergoes only rotatory motion

b) The direction of torque is along the suspension wire

c) The magnitude of torque is maximum when the magnet is normal to the field direction

physics magnetic effects of current and magnetism the bar magnet magnetic field due to bar magnet intensity of magnetic field and torque on a bar magnet

  1. only a and c are correct

  2. only a and b are correct

  3. only b and c are correct

  4. a, b, c are correct

Show answer
Correct Option: D
Explanation:

The bar magnet will only
experience a torque, It will not
experience any force
The direction of the torque will be
along the suspended wire according to
right hand thumbrule.
$\vec{z}=\vec{m}\times \vec{B}$
$=mB sin \theta .$
And its magnitude will be maximum if $\theta =90$