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Quizzes (598) Questions (13441)

Questions Related to physics

Multiple choice 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

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:

  1. 0.292cm

  2. 5.76cm

  3. 0.362cm

  4. 2.262cm
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Magnetic moment M = pole strength (m) * length (L). Force on each pole F = m * B. Given M = 80, B = 1.8e-5, F = 25e-3. m = F / B = 25e-3 / 1.8e-5 = 25000 / 18 = 1388.89. L = M / m = 80 / 1388.89 = 0.0576 m = 5.76 cm.

Multiple choice 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

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

  1. straight line

  2. parabolic

  3. circular

  4. helical
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

Multiple choice 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

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

  1. A straight line

  2. A circle

  3. Any curved path

  4. Zig-zag
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When a charged particle enters a uniform magnetic field with a velocity perpendicular to the field, the magnetic force acts as a centripetal force, causing the particle to move in a circular path.

Multiple choice 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

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

  1. translatory motion only

  2. rotational motion only

  3. both translatory and rotational motion

  4. vibrational motion only
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

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

Multiple choice 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

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

  1. only force acts on wire

  2. only torque acts on wire

  3. both

  4. none
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A current-carrying wire in a magnetic field experiences a Lorentz force given by F = I(L x B). Torque is only experienced if the wire is part of a loop or has a specific geometry allowing for a moment arm relative to an axis, but a simple wire experiences a net force.

Multiple choice 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

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 

  1. clockwise of the +ve z axis

  2. anticlockwise of the + ve z axis

  3. zero

  4. along the magnetic field
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Since the cylinder is parallel to the uniform magnetic field, the magnetic flux through any cross-section perpendicular to the z-axis remains constant. According to Faraday's law, no change in flux means no induced EMF and thus no induced current.

Multiple choice 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

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

  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.
Reveal answer Fill a bubble to check yourself
C Correct answer
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

Multiple choice 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

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? 

  1. $\cfrac {r} {2}$

  2. r
  3. $2r$
  4. $4r$
Reveal answer Fill a bubble to check yourself
C Correct answer
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$

Multiple choice 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

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

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

  4. W/2
Reveal answer Fill a bubble to check yourself
B Correct answer
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 $ 

Multiple choice 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

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

  1. 0.04Nm

  2. 0.84Nm

  3. 0.4 Nm

  4. 0.44Nm
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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