Physics

Magnetism and Magnetic Effects

218 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

Biot-Savart law indicates that the moving electrons (velocity $\bar v$ ) produce a magnetic field $\bar B$ such that:

  1. $\bar B \perp \bar v$
  2. $\bar B \parallel \bar v$

  3. it obeys inverse cube law.

  4. it is along the line joining the electron and point of observation.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Magnetic field produced by charges moving with velocity $\bar v$, at a distance r is $ \bar B$ = $\left ( \dfrac{\mu _0}{4\pi } \right )$.q$\dfrac{\bar v \times \hat r}{r^2}$
Therefore $\bar B \perp \bar v$

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 vertical straight conductor carries a current vertically upwards. A point P lies to the east of it at a small distance and another point Q lies to the west at the same distance the magnetic field at P is :

  1. greater than at Q

  2. same as at Q

  3. less than at Q

  4. greater or less than at Q depending upon the strength of current
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

As per Biot Savart Law, Magnetic field at a point is inversely proportional to square of distance from the current carrying conductor. Therefore magnitude of magnetic field is same at both points P and Q, irrespective of their position from the conductor.

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

The Biot-Savart's law in vector from is:

  1. $ d\overrightarrow { B } =\dfrac { \mu _ o }{ 4\pi } \dfrac { di\left( \overrightarrow { l } \times \overrightarrow { r } \right) }{ r^ 2 } $
  2. $ d\overrightarrow { B } =\dfrac { \mu _ o }{ 4\pi } \dfrac { i\left( \overrightarrow { dl } \times \overrightarrow { r } \right) }{ r^ 2 } $
  3. $ d\overrightarrow { B } =\dfrac { \mu _ o }{ 4\pi } \dfrac { i\left( \overrightarrow { r } \times \overrightarrow { dl } \right) }{ r^ 2 } $
  4. $ d\overrightarrow { B } =\dfrac { \mu _ o }{ 4\pi } \dfrac { i\left( \overrightarrow { dl } \times \overrightarrow { r } \right) }{ r^ 3 } $
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Biot-Savart law in vector form gives the magnetic field dB due to a current element idL at position r. The correct expression has the cross product of current element vector dl and position vector r in the numerator, divided by r cubed in the denominator to make it dimensionally consistent as an inverse-square law in vector form.

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

Which of the following particles will deviate $(< \pi/2)$ maximum when they enter magnetic filed region perpendicularly with same velocity and travel same distance.

  1. $He^{}$

  2. Proton
  3. $\alpha-particle$
  4. $Li^{++}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The radius of the path is r = mv / (qB). For the same velocity and distance, the particle with the smallest radius (highest q/m ratio) will deviate the most. Li++ has a higher charge-to-mass ratio than the others.

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

Which of the following gives the value of magnitude field according to, Biot-Savart's law'

  1. $ \frac {i\triangle l sin \theta}{r^2} $
  2. $ \frac {\mu _o}{4 \pi} \frac {i \triangle l sin \theta}{r} $
  3. $ \frac {\mu _o}{4\pi} \frac {i \triangle l sin \theta}{r^2} $
  4. $ \frac {\mu _o}{4 \pi} i \triangle l sin \theta $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Biot-Savart's law states that the magnetic field dB is proportional to i*dl*sin(theta)/r^2. The constant of proportionality is mu0/4pi.

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

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 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.
$B\propto\dfrac{1}{d^3}$

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

If r be the distance of a point on the axis of a bar magnet from its centre, the magnetic field at this point is proportional to :

  1. (1/r)
  2. (1/r$^2$)
  3. (1/r$^3$)
  4. (1/r$^5$)
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For a short Bar Magnet, the magnetic induction at a point on the axix at a distance $r$ from centre is given by  the formula

$B = $   $(\dfrac{\mu _0}{4\pi} )\dfrac{2M}{r^3}$

$\Rightarrow$ $B= \dfrac{K}{r^3}$

$\Rightarrow$ $B\propto \dfrac{1}{r^3}$
Therefore, C is correct option.