1

### JEE Main 2019 (Online) 9th January Evening Slot

A particle having the same charge as of electron moves in a ciurcular path of radius 0.5 cm under the influence of a magnetic field 0f 0.5 T. If an electric field of 100 V/m makes it to move in a straight path, then the mass of the particle is (Given charge of electron = 1.6 $\times$ 10$-$19C)
A
9.1 $\times$ 10$-$31 kg
B
1.6 $\times$ 10$-$27 kg
C
1.6 $\times$ 10$-$19 kg
D
2.0 $\times$ 10$-$24 kg

## Explanation

Given,
radius of circular path(r) = 0.5 cm
Magnetic field (B) = 0.5 T
Electric field (E) = 100 V/m

Charge of particle (q) = 1.6$\times$10$-$19 C

As particle is moving in a circular path so,

${{m{v^2}} \over r} = qvB$

$\Rightarrow$  r = ${{mv} \over {qB}}$ . . . . . . . (1)

When electric field of 100 v/m is applied on the particle then particle is moving in the straight line. So, the net force on the particle is zero.

$\therefore$    Fnet = 0

$\Rightarrow$   Fe = Fm

$\Rightarrow$  qE = qvB

$\Rightarrow$   E = vB  . . . . .(2)

From equation (1) and (2) we get,

r = ${m \over {qB}}\left( {{E \over B}} \right)$

= ${{mE} \over {q{B^2}}}$

$\Rightarrow$   m = ${{q{B^2}r} \over E}$

= ${{1.6 \times {{10}^{ - 19}} \times {{\left( {0.5} \right)}^2} \times 0.5 \times {{10}^{ - 2}}} \over {100}}$

= 2 $\times$ 10$-$24 kg
2

### JEE Main 2019 (Online) 9th January Evening Slot

One of the two identical conducting wires of length L is bent in the form of a circular loop and the other one into a circular coil of N identical turns. If the same current is passed in both, the radio of the magnetic field at the central of the loop (BL) to that at the center of the coil (BC), i.e. ${{{B_L}} \over {{B_C}}}$ will be :
A
N
B
${1 \over N}$
C
N2
D
${1 \over {{N^2}}}$

## Explanation For loop,

L = 2$\pi$R

For coil,

L = N $\times$ 2$\pi$r

$\therefore$   2$\pi$R = N $\times$ 2$\pi$r

$\Rightarrow$  R = Nr

$\Rightarrow$  r = ${R \over N}$

We know,

BL = ${{{\mu _0}i} \over {2R}}$

and  BC = N $\times$ ${{{\mu _0}i} \over {2r}}$

$\therefore$  ${{{B_L}} \over {{B_C}}} = {{{{{\mu _0}i} \over {2R}}} \over {N \times {{{\mu _0}i} \over {2\left( {{R \over N}} \right)}}}} = {1 \over {{N^2}}}$
3

### JEE Main 2019 (Online) 10th January Morning Slot

A magnet of total magnetic moment 10-2${\widehat i}$A-m2 is placed in a time varying magnetic field, B${\widehat i}$ (co where B = 1 Tesla and $\omega$ = 0.125 rad/s. The work done for reversing the direction of the magnetic moment at t = 1 second, is -
A
0.014 J
B
0.028 J
C
0.01 J
D
0.007 J

## Explanation

Work done, W = $\left( {\Delta \overrightarrow \mu } \right).\overrightarrow B$

= 2 $\times$ 10$-$2 $\times$ 1 cos(0.125)

= 0.02 J
4

### JEE Main 2019 (Online) 10th January Morning Slot

A solid metal cube of edge length 2 cm is moving in a positive y-direction at a constant speed of 6 m/s. There is a uniform magnetic field of 0.1 T in the positive z-direction. The potential difference between the two faces of the cube perpendicular to the x-axis, is -
A
2mV
B
12 mV
C
6 mV
D
1 mV

## Explanation

Potential difference between two faces perpendicular to x-axis will be
$\ell .\left( {\overrightarrow V \times \overrightarrow B } \right) = 12$mV