Tag: intensity of magnetic field and torque on a bar magnet

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Questions Related to intensity of magnetic field and torque on a bar magnet

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

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

  1. magnetic flux

  2. torque

  3. pole strength

  4. magnetic flux density
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

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 :

  1. $0.6\times { 10 }^{ 7 }N$
  2. $0.06\times { 10 }^{ 7 }N$
  3. $0.6N$
  4. $0.06N$
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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 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 

  1. $\sqrt3W$

  2. W
  3. $(\sqrt3/2)W$

  4. 2W
Reveal answer Fill a bubble to check yourself
A Correct answer
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)
$\Rightarrow \tau =\dfrac{\sqrt{3}}{2}MB$
$\Rightarrow \tau =\dfrac{\sqrt{3}}{2}2W=\sqrt{3}W$.
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 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

  1. $H _1:H _2=16:100$
  2. $H _1:H _2=1:2$
  3. $H _1:H _2=2:1$
  4. $H _1:H _2=100:16$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For a short magnet, magnetic intensity H at broad side-on (equatorial) is H1 = M / (4 * pi * d^3) and at end side-on (axial) is H2 = 2 * M / (4 * pi * d^3). The ratio H1:H2 = 1:2.

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.