Tag: magnetic field due to bar magnet

Questions Related to magnetic field due to bar magnet

If a bar magnet is kept perpendicular in a magnetic field of unit magnetic induction then its magnetic moment is equal to:

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. magnetic flux

  2. torque

  3. pole strength

  4. magnetic flux density

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Correct Option: B
Explanation:

From the given question, 

$B=1T$
$\theta=90^0$
The torque acting on the bar magnet is given by
$\tau=MBsin\theta=M\times 1\times sin90^0$
$\tau=M$
The magnetic moment is equal to the torque.
The correct option is B.

The small magnets each of magnetic moment $10A-{ m }^{ 2 }$ are placed end on position 0.1m apart from their centres.The force acting between them 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. $0.6\times { 10 }^{ 7 }N$

  2. $0.06\times { 10 }^{ 7 }N$

  3. $0.6N$

  4. $0.06N$

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Correct Option: C
Explanation:

Given,


$M _1=M _2=10Am^2$


$r=0.1m$

The force acting between the magnet is given by,

$F=\dfrac{\mu _0}{4\pi }.\dfrac{6M _1M _2}{r^4}$

$F=10^{-7}\times \dfrac{6\times 10\times 10}{(0.1)^4}$

$F=0.6N$

The correct option is C.

A magnetic needle lying parallel to a magnetic field requires W unit of work to turn it through $60^0$. The torque needed to maintain the needle in this position 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. $\sqrt3W$

  2. W

  3. $(\sqrt3/2)W$

  4. 2W

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Correct Option: A
Explanation:
Work done to rotate the needle$=$Potential energy of needle$=$W
If M$=$Magnetic dipole moment of the needle and B$=$magnetic field then:
$W=MB(\cos\theta _1-\cos\theta _2)$ (where $\theta _1=0^o$ and $\theta _2=60^o$)
$\Rightarrow W=MB(1-1/2)=\dfrac{MB}{2}$ or $MB=2W\rightarrow (1)$
So restoring torque acting on the needle is given by:
$\tau =M\times B=MB\sin 60^o$(In magnitude)
$\Rightarrow \tau =\dfrac{\sqrt{3}}{2}MB$
$\Rightarrow \tau =\dfrac{\sqrt{3}}{2}2W=\sqrt{3}W$.

A bar magnet having centre O has a length of 4 cm. Point $P _1$ is in the broad side-on and $P _2$ is in the end side-on position with $OP _1=OP _2=10$ metres. The ratio of magnetic intensities H at $P _1$ and $P _2$ 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. $H _1:H _2=16:100$

  2. $H _1:H _2=1:2$

  3. $H _1:H _2=2:1$

  4. $H _1:H _2=100:16$

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Correct Option: B

A magnet of moment  $80A{m^2}$ is placed in a uniform magnetic field of induction $1.8 \times {10^{ - 5}}T$. If each pole of the magnet experiences a force of  $25 \times {10^{ - 3}}N$, the length of the magnet 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. 0.292cm

  2. 5.76cm

  3. 0.362cm

  4. 2.262cm

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Correct Option: B

Which of the following cannot be the shape of the path of a charged particle moving in a uniform magnetic field?

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. straight line

  2. parabolic

  3. circular

  4. helical

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Correct Option: C
Explanation:

It will be a circular path with radius

$r=\left( \frac { mc }{ qB }  \right) $

and time period of

$T=\left( 2\times pi\times \frac { m }{ qB }  \right) $

This you can get from study of force on moving charge in magnetic field.

This is similar to cyclotron except that, in cyclotron, there is presence of electric field in addition to magnetic field. Hence in cyclotron, the radius of circular path keeps on increasing as speed of charge increases.

A charge particle moves in a uniform magnetic field. The velocity of the particle at some instant makes right angle with the magnetic field. The path of the particle 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. A straight line

  2. A circle

  3. Any curved path

  4. Zig-zag

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Correct Option: B

A magnet is kept fixed with its length parallel to the magnetic meridian. An identical magnet is parallel to this such that its center lies on perpendicular bisector of both. If the second magnet is free to move, it will have

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. translatory motion only

  2. rotational motion only

  3. both translatory and rotational motion

  4. vibrational motion only

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Correct Option: C
Explanation:

Both translatory and rotational motion as it will exprrience both torque and force.

A current carrying wire is arranged at any angle in an uniform magnetic field, then

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 force acts on wire

  2. only torque acts on wire

  3. both

  4. none

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Correct Option: A

An infinitely long cylinder is kept parallel to an uniform magnetic field B directed along positive z axis. The direction of induced current as seen from the z axis 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. clockwise of the +ve z axis

  2. anticlockwise of the + ve z axis

  3. zero

  4. along the magnetic field

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Correct Option: C