WBJEE PYQs for Moving Charges and Magnetism with Solutions: Practice WBJEE Previous Year Questions

1.

A galvanometer has 40 divisions and a sensitivity of 12 μA/div. it can be converted into a voltmeter to read 3 V by connecting-

• (A) resistance nearly 6 k Ω in series
  
• (B) 6 k Ω in parallel
  
• (C) 5 Ω in series
  
• (D) 6.6 k Ω in series
  

2.

An electric current ?I? enters and leaves a uniform circular wire of radius r through dfiametrically opposite points. A particle carrying a charge q moves along the axis of the circular wire with speed v. What is the magnetic force experienced by the particle when it passes through the centre of the circle ?

• $qv\frac{\mu_{0}i}{a}$
• $qv\frac{\mu_{0}i}{2a}$
• $qv\frac{\mu_{0}i}{2\pi a}$
• Zero

3.

A particle with charge q moves with a velocity v in a direction perpendicular to the directions of uniform electric and magnetic fields, E and B respectively, which are mutually perpendicular to each other. Which one of the following gives the condition for which the particle moves undeflected in its original trajectory?

• $v =\frac{E}{B}$
• $v=\frac{B}{E}$
• $v=\sqrt{\frac{E}{B}}$
• $v=q\frac{B}{E}$

4.

A square conducting loop is placed near an infinitely long current carrying wire with one edge parallel to the wire as shown in the figure. If the current in the straight wire is suddently halved, which of the following statements will be true? "The loop will ....... "

• stay stationary
• move towards the wire
• move away from the wire
• move parallel to the wire

5.

Magnetic field intensity $H$ at the centre of a circular loop of radius $r$ carrying current $I$ is

• $\frac {R}{I} $ oersted
• $\frac {2\pi I} {r}$ oersted
• $\frac {I} {2 \pi r}$ oersted
• $\frac {2\pi r}{I}$ oersted

6.

An equilateral triangle is made by uniform wires $AB, BC, CA. A$ current $I$ enters at $A$ and leaves from the mid point of $BC$. If the lengths of each side of the triangle is $L$, the magnetic field $B$ at the centroid $O$ of the triangle is

• $\frac{\mu_{0}}{4\pi}\left(\frac{4I}{L}\right)$
• $\frac{\mu_{0}}{3\pi}\left(\frac{4I}{L}\right)$
• $\frac{\mu_{0}}{4\pi}\left(\frac{2I}{L}\right)$
• Zero

7.

The ratio of magnetic field and magnetic moment at the centre of a current carrying circular loop is $x$. When both the current and radius is doubled the ratio will be

• \(\frac{x }{ 8}\)

• \(\frac{x }{ 4}\)

• \(\frac{x }{ 2}\)

• $2x$

8.

An electron of charge $e$ and mass $m$ is moving in circular path of radius $r$ with a uniform angular speed $\omega$. Then which of the following statements are correct ?

• The equivalent current flowing in the circular path is proportional to $r^2$
• The magnetic moment due to circular current loop is independent of $m$
• The magnetic moment due to circular current loop is equal to $2e/m$ time the angular momentum of the electron
• The angular momentum of the particle is proportional to the areal velocity of electron.

9.

A bar magnet falls from rest under gravity through the center of a horizontal ring of conducting wire as shown in the figure,

Which of the following graph best represents the speed (v) vs. time(t) graph of the bar magnet?

10.

Two long parallel wires separated by $0.1\, m$ carry currents of $1\, A$ and $2\, A$ respectively in opposite directions. A third current-carrying wire parallel to both of them is placed in the same plane such that it feels no net magnetic force. It is placed at a distance of

• 0.5 m from the $1^{st}$ wire, towards the $2^{nd}$ wire.
• 0.2 m from the $1^{st}$ wire, towards the $2^{nd}$ wire.
• 0.1 m from the $1^{st}$ wire, towards the $2^{nd}$ wire.
• 0.2 m from the $1^{st}$ wire, away from the $2^{nd}$ wire.