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Questions Related to physics

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

When a charged particle moves perpendicular to a uniform magnetic field, its 

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. energy and momentum both change.

  2. energy changes but momentum remains unchanged.

  3. momentum changes but energy remains unchanged.

  4. energy and momentum both do not change.

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Correct Option: C
Explanation:

Since the force exerted by the magnetic field is perpendicular to the direction of the particle the speed of the particle cannot change but its velocity changes .

so C option is the correct answer

A proton with kinetic energy K describes a circle of radius r in a uniform magnetic. An $\alpha$- particle with kinetic energy K moving in the same magnetic field will describe a circle of radius? 

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. $\cfrac {r} {2}$

  2. r

  3. $2r$

  4. $4r$

Show answer
Correct Option: C
Explanation:

WE KNOW THAT 


charge on proton = q
charge on alpha particle = 2q

mass of proton =m 
mass of alpha particle =4m

for proton

 $ r =\dfrac{m\times v}{q\times B}$


for alpha particle

$ R = \dfrac{{4m}\times v}{2q\times B}$

$ R = \dfrac{2m\times v}{q\times B}$

$ R = 2r$

A magnetic needle is placed parallel to a magnetic field. The amount of work done in rotating the coil by an angle of 60$^0$ is W units. Then, the torque required to keep the needle in the displaced position 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. W

  2. $\sqrt{3}$W

  3. $\left( \sqrt{3} / 2 \right )$W

  4. W/2

Show answer
Correct Option: B
Explanation:

Torque at an angle $\theta  = k sin \theta $


work done to bring from zero to 60 degree $ =W = \int _0^{60}{ k sin \theta d \theta } = {\left | -k cos \theta \right |} _0^{60} = \dfrac{k}{2} $

$ \Rightarrow k = 2W $

torque at 60 degree required to keep the needle stable  $= k sin 60 = 2 W \dfrac{\sqrt 3 }{2} = \sqrt 3 W $ 

A bar magnet of moment $4Am^{2}$ is placed in a non-uniform magnetic field. If the field strength at poles are 0.2 T and 0.22 T then the maximum couple acting on it 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. 0.04Nm

  2. 0.84Nm

  3. 0.4 Nm

  4. 0.44Nm

Show answer
Correct Option: B
Explanation:

Average of the two field strengths at the poles is $\frac{0.2 +0.22}{2}=0.21$
Copuling( torque): $MB=4 \times 0.21 : Nm= 0.84:Nm$