Tag: magnetism

Questions Related to magnetism

A short bar magnet with the north pole facing north forms a neutral point at P in the horizontal plane. If the magnet is rotated by $90^o$ in the horizontal plane, the net magnetic induction at $P$ is ( Horizontal component of earth's magnetic field $= B _H$):

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

  1. zero

  2. $2 B _H$

  3. $\displaystyle \dfrac{\sqrt{5}}{2} B _H$

  4. $\sqrt{5}B _H$

Show answer
Correct Option: D
Explanation:

When the north pole of short bar magnet is facing North pole of the earth, at the neutral point P, which is on equatorial line. 
$B _H = \dfrac {\mu _0 M}{4\pi d^3} = B _1$ ............(1)
When the magnet is rotated by $90^o$, the magnetic induction at P which is on axial line,
$B _H = \dfrac {\mu _0 2M}{4\pi d^3} = B _2$ ............(2)
Therefore, net magnetic induction at P is
$B _{net} = \sqrt {(B _1^2 + B _2^2)}$
$B _{net} = \sqrt {(1^2 + 2^2)}B _H = \sqrt 5 B _H$

The magnetic field strength at a point at a distance d from the centre on the axial line of a very short bar magnet of Magnetic moment $M$ is $B$. The Magnetic induction at a distance $2d$ from the centre on the equatorial line of a Magnetic Moment $8M$ will be

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

  1. $4B$

  2. $0.25B$

  3. $0.5B$

  4. $2B$

Show answer
Correct Option: C
Explanation:

$B=\frac{2\mu _0m}{4 \pi d^{3}}$

$B equatorial =\frac{\mu _0m}{4 \pi d^{3}}$

$d _1=d$
$d _2=2d$
Also,
$m _1=M$
$m _2=8M$

$B _{1}=\frac{2\mu _0m}{4 \pi d^{3}}............1$

$B _{2}=\frac{2\mu _0m}{4 \pi d^{3}} = \frac{\mu _0 8M}{4 \pi (2d)^{3}}.........2$
From 1 & 2 we get that,
$B _{equatorial} =\frac{B}{2}$

Charge is uniformly distributed in  a space. The net flux passing through the surface of an imaginary cube of side''a'' in the spaceis $\phi $ the space is 0. The net flux passing through the surface of an imaginary sphere of radius ''a''- in the space will be:

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

  1. $\phi $

  2. $\dfrac { 3 }{ 4\pi } \phi $

  3. $\dfrac {2\pi }{ 3 } \phi $

  4. $\dfrac {4\pi }{ 3 } \phi $

Show answer
Correct Option: A
Explanation:

external flux of a surface is given by : E.ds.

since, the flux through the cube would be $E\times a2 = x$

therefore for a sphere,  the flux would be $E.\times Φ a2$

which is equal to $Φ$

In null method of comparison of magnetic moments, the net magnetic field at the centre of the DMM, when null deflection is obtained is

uniform magnetic field lines of earth magnetism physics

  1. $0$

  2. $B _{H}$

  3. between $0$ and $B _{H}$

  4. above $B _{H}$

Show answer
Correct Option: B
Explanation:

In null method , two magnets are kept on either side of the arms so that the net deflection is zero . So the magnetic field due to the bar magnets gets cancelled and only earth's horizontal magnetic field is left

In both $\tan A$ and $\tan B$ positions in a Deflection Magnetometer,  the bar magnet is always placed

uniform magnetic field lines of earth magnetism physics

  1. parallel to the magnetic needle of the deflection magnetometer

  2. parallel to the aluminum pointer of the deflection magnetometer

  3. perpendicular to the aluminum pointer

  4. parallel to the arm having the wooden scale

Show answer
Correct Option: A
Explanation:

In $\tan A$ position the bar magnet is placed horizontally, parallel to the arm of the deflection magnetometer and parallel to the magnetic needle of the deflection magnetometer. 

In $\tan B$ position the bar magnet is placed horizontally, perpendicular to the arm of the deflection magnetometer and parallel to the magnetic needle of the deflection magnetometer. 

The absolute value of earths magnetic horizontal intensity is determined with the help of

uniform magnetic field lines of earth magnetism physics

  1. DMM only

  2. VMM only

  3. DMM & VMM

  4. Plane mirror

Show answer
Correct Option: C
Explanation:

The absolute value of earths magnetic horizontal intensity is determined with the help of DMM and VMM.

Among the following statements: 

A) In $\tan A$ position of deflection magnetometer $B$ and $B _{H}$ are perpendicular 

B) In $\tan B$ position of deflection magnetometer $B$ and $B _{H}$ parallel

uniform magnetic field lines of earth magnetism physics

  1. A true & B false

  2. A false & B true

  3. A and B are true

  4. A and B are false

Show answer
Correct Option: A
Explanation:

In $\tan A$ position , the arms are in east-west direction and the compass needle is in north-south direction and the bar magnet is kept parallel to the arms, perpendicular to the compass needle.

In $\tan B$ position , the arms are in north-south direction and the compass needle is in same direction , the magnet is kept in east - west direction , perpendicular to the compass needle.

The restoring couple for a magnet oscillating in the vibration magnetometer is provided by

uniform magnetic field lines of earth magnetism physics

  1. horizontal component of earths magnetic field

  2. gravity

  3. torsion in the suspended thread

  4. magnetic field of magnet

Show answer
Correct Option: A
Explanation:

When the bar magnet in vibration magnetometer is displaced from the earth's magnetic field , the torque acts on it to allign it with the earth's magnetic field . So the earth's magnetic field is  the restoring force.

Assertion (A) : In deflection magnetometer a short magnetic needle is arranged in the compass box
Reason (R) : The magnetic needle is found in the uniform magnetic field produced by earth and bar magnet
uniform magnetic field lines of earth magnetism physics

  1. Both A and R are true and R is the correct explanation of A

  2. Both A and R are true and R is not correct explanation of A

  3. A is true, But R is false

  4. A is false, But R is true

Show answer
Correct Option: A
Explanation:

In Deflection magnetometer ,the bar magnet is kept at a distance from the magnetic needle and the deflection thus observed is due to the earth's and bar magnet's magnetic field..

Deflection magnetometer is held in $\tan B$ position. A magnet placed on one of its arms produces no deflection. This implies that the axis of the magnet is

uniform magnetic field lines of earth magnetism physics

  1. in the east - west direction

  2. in the north-south direction

  3. perpendicular to the wooden bench

  4. North - East

Show answer
Correct Option: B
Explanation:

In $\tan B$ position , the arms are in the north south direction and the aluminium pointer are in east- west direction. So the compass needle is in north- south direction. For the magnetic field lines of the magnet to be ineffective then they must be in the north south direction so that they will not create a torque.

If the magnet is in east- west direction , a couple acts on the magnet and thus causes deflection.