Physics

Magnetism and Magnetic Effects

230 Questions

Magnetism and magnetic effects focus on the forces exerted by magnetic fields on moving charges and magnetic materials. Questions cover magnetic dipoles, flux density, the motion of charged particles, and electromagnetic relationships. It is a vital physics topic for government competitive exams.

Magnetic dipolesCharged particle motionMagnetic flux densityBar magnetsEarth magnetism

Magnetism and Magnetic Effects Questions

Multiple choice physics moving charges and magnetism field due to a current carrying conductor magnetic field due to a straight current carrying conductor magnetic field lines due to current

A particle of mass M and charge Q moving with velocity $\vec v$ describe a circular path of radius R when subjected to a uniform transverse magnetic field of induction B. The work done by the field when the particle completes one full circle is

  1. $\displaystyle \left ( \frac{Mv^2}{R} \right ) 2 \pi R$
  2. $zero$
  3. $BQ2 \pi R$
  4. $BQv2 \pi R$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Upon completing a full circle net displacement is 0.
Work done by the magnetic field is 0 because the net displacement caused by the magnetic field is 0.

Multiple choice physics moving charges and magnetism field due to a current carrying conductor magnetic field due to a straight current carrying conductor magnetic field lines due to current

Assertion: Magnetism is relativistic

Reason: When we move along with the charge, so that there is no motion relative to us, we find no magnetic field associated with the charge

  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
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A magnetic field is a region around some magnetic material or some moving electric charge. Within it the force of magnetism acts. Thus Magnetism is the aspect of the combined electromagnetic force. Also, it refers to the physical phenomena caused by magnets.

A magnetic field can be produced by the moving electric charge. As, the motion of any object is always relative, therefore the magnetic field will also be relativistic in nature.

As the reason is the correct explanation for the assertion

Hence option A is correct.

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

An electric dipole of momentum $3 \times {10}^{-8}\ Cm$ is placed in an electric field of $6 \times {10}^{4}\ N/C$ with is axis making an angle of $30^o$ with the field . Find the torque acting on the dipole.

  1. $ (9 \times 10^{-5} )N-m$
  2. $ (9 \times 10^{-4} )N-m$
  3. $ (9 \times 10^{-3} )N-m$
  4. $ (90 \times 10^{-8} )N-m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

We know that Torque $(\vec \tau) = \vec p \times \vec E$


$  \tau = pE\sin \theta$
$ = (3\times 10^{-8})\times ( 6\times 10^4) \sin30^0$
$ = (9 \times 10^{-4} )N-m$

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

A magnetic dipole is placed at right angles to the direction of lines of force of magnetic induction B. If it is rotated through an angle of $180^0$, then the work done is 

  1. 2 MB

  2. MB

  3. -2 MB

  4. Zero
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Work done in rotating a magnetic dipole is W = MB(cos(theta1) - cos(theta2)). Rotating from 90 degrees to 270 degrees (180 degree rotation) results in W = MB(cos(90) - cos(270)) = MB(0 - 0) = 0.

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

In a certain region of space, electric field is along z-direction throughout. The magnitude of electric field is, however, not constant but increases uniformly along the positive z-direction, at the rate of $10^5 NC^{-1}$ per metre. What is the torque experienced by a system having a total dipole moment equal to $10^{-7}$ C-m in the negative z-direction?

  1. 2 N-m

  2. 3 N-m

  3. 4 N-m

  4. Zero
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\tau =\vec P\times \vec E = PE \sin\theta $
Since, $ \theta = 180^o \rightarrow \tau =0 $

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

What will be the magnitude of torque on an electric dipole having dipole moment of  $4 \times 10 ^ { - 9 }  { cm }$  placed in a uniform electric field of intensity of  $5 \times 10 ^ { 4 } { NC } ^ { - 1 }$  making an angle  $180 ^ { \circ }$  with the field.

  1. $10 ^ { - 4 } N - m$
  2. $0$
  3. $2\times 10^{ { -4 } }{ N }-{ m }$
  4. $10 ^ { -6 } N - m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Torque is given by tau = pE sin(theta). When the dipole is at 180 degrees to the field, sin(180) = 0, resulting in zero torque.

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

If a dipole of dipole moment $\displaystyle \vec { p } $ is placed in a uniform electric field $\displaystyle \vec { E } $, then torque acting on it is given by :

  1. $\displaystyle \vec {\tau } =\vec { p } .\vec { E } $
  2. $\displaystyle \vec { \tau } =\vec { p } \times \vec { E } $
  3. $\displaystyle \vec { \tau } =\vec { p } +\vec{ E } $
  4. $\displaystyle \vec { \tau } =\vec { p } -\vec { E } $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Dipole moment of the dipole p and uniform Electric field $E$. we know that dipole moment $p = qa$ (where q is charge and a is the dipole length).And when a dipole of Dipole moment p is placed in a uniform Electric field E , the torque $\tau = Either  force \times \text{perpendicular  distance  between  the  two  forces }= qaE sin \theta$ or $\tau = pEsin \theta$ or $\tau = p \times E$

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

The torque acting on a dipole of momentum $\vec { p } $ in an electric field $\vec { E } $:

  1. $\vec { p } \times \vec { E } $
  2. $\vec { p } .\vec { E } $

  3. zero
  4. $\vec { E } \times \vec { p } $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Force acting on dipole due to electric field is given by,

$F=qE$
Torque on a dipole in an electric field is given by,
$T= F\times d = Fdsin\theta=qEdsin\theta$.     .....(i)
But dipole moment is given by,
$P=qd$    ....(ii)
By (i) and (ii) we get,
$\therefore$ $T= pEsin\theta=\overrightarrow{p}\times \overrightarrow{E}$

Multiple choice torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

What will be the magnitude of torque on an electric dipole having dipole moment of $4\times { 10 }^{ -9 }cm$ placed in a uniform electric field of intensity of $5\times { 10 }^{ 4 \,\,}N { C }^{ -1 }$ making an angle ${180}^{o}$ with the field.

  1. ${ 10 }^{ -4 }N-m$
  2. $2\times { 10 }^{ -4 }N-m$
  3. $0$ (zero)
  4. ${ 10 }^{ -6 }N-m$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$\overrightarrow { \tau  } =\overrightarrow { p } \times \overrightarrow { E } $

$\overrightarrow{\tau} \longrightarrow$torque
$[\theta \longrightarrow$angle of the dipole moment$(p)$ with the field$(E)]$
$\therefore |\tau |=pE\sin { \theta  } =pE\sin { 180° } =0$

Multiple choice physics magnetic effect of electric current oersted experiment oersted's experiment magnetic field due to a straight current carrying conductor

A compass needle placed at a distance $r$ from a short magnet in $\tan\ A$ position shown a deflection of $60^{o}$. If the distance is increased to $r(3)^{1/3}$, then the deflected of the compass needle is:

  1. $30^{o}$
  2. $60^{o}\ \times (3)^{1/3}$
  3. $60^{o}\ \times (3)^{2/3}$
  4. $90^{o}\ \times (3)^{1/3}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For a short magnet in the tan A position, the magnetic field B is proportional to 1/r^3. Since B = B_h * tan(theta), tan(theta) is proportional to 1/r^3. If r increases to r * (3)^(1/3), r^3 increases by a factor of 3, so tan(theta) decreases by a factor of 3. tan(60) = sqrt(3), so the new tan(theta) = sqrt(3)/3 = 1/sqrt(3), which corresponds to 30 degrees.

Multiple choice physics magnetic effect of electric current oersted experiment oersted's experiment magnetic field due to a straight current carrying conductor

A magnetic needle vibrates in a vertical plane parallel to the magnetic meridian about horizontal axis passing through its centre. The frequency is $\pi$. If the plane of oscillation turned about a vertical axis by ${90}^{o}$, the frequency of oscillation in vertical plane will be:

  1. $\pi$

  2. zero
  3. less than $\pi$
  4. more than $\pi$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Frequency $\pi=\dfrac{1}{2\pi}\sqrt{\dfrac{BM}I}$

On turning through angle $90°$, effective field is $V$ and $B>V$
$\implies $ new frequqncy $<\pi$ (less than $\pi)$

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

In an electormagnetic wave, the phase difference between electric field $\vec { E }$ and magnetic field $ \vec { B } $ is :

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

  4. Zero
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The electric and magnetic field components of a linearly polarized electromagnetic wave oscillate in such a way that they peak at the same time and they become zero at the same time but they point to different directions in space, separated by an angle of $90^{\circ}$.


Since there is no time difference between the peaks of the electric and magnetic oscillations the phase difference between the electric and magnetic field vectors of a linearly polarized electromagnetic wave is zero.

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

An electromagnetic wave in vacuum has the electric and magnetic field $\overset { \rightarrow  }{ E } $ and $\overset { \rightarrow  }{ B } $  which are always perpendicular to each other. If the direction of polarization is given by $\overset { \rightarrow  }{ X }  $ and that of wave propagation by $\overset { \rightarrow  }{ k } $ then:

  1. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ B } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $
  2. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ E } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ E\times } \overset { \rightarrow }{ B } $
  3. `$\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ B } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ E\times } \overset { \rightarrow }{ B } $
  4. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ E } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In an electromagnetic wave, electrical and magnetic fields are perpendicular to each other. The wave propagates in a direction perpendicular to both electric and magnetic fields as given by  $\vec E\times \vec B$.

So, the direction of propagation of the wave will be perpendicular to the to the direction of oscillation of the fields.
$\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $
And the direction of polarization must be perpendicular to the electric field and parallel to the magnetic field.
$\vec X||\vec E$.
The correct option is $(B)$.

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

Which of the following statement is false for the properties of electromagnetic waves?

  1. Both electric and magnetic field vectors attain the maxima and minima at same place and same time.

  2. The energy in electromagnetic wave is divided equally between electric and magnetic field vectors.

  3. Both electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of wave.

  4. These waves do not require any material medium for propagation.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In electromagnetic waves,

1. The electric field and magnetic field varies continuously with time and have maxima and minima at same place and at same time.
2. Both electric and magnetic field have same energy.
3. both electric and magnetic field are perpendicular to each other and perpendicular to direction of propagation.
4. These waves don't require any material medium to propagate, they can propagate in vacuum as well.
So, the false statement will be the statement given in the option $(C)$
Hence, the correct option is $(C)$

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

Magnetic induction due to a short bar magnet on its axial line is inversely proportional to cube of distance of the point.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Magnetic induction due to a short bar magnet on its axial line,

$B=\dfrac{\mu _0 M}{4\pi d^3}$
Magnetic induction due to a short bar magnet on its axial line is inversely proportional to cube of distance of the point.
$B\propto\dfrac{1}{d^3}$