1

JEE Main 2019 (Online) 11th January Morning Slot

The given graph shows variation (with distance r form centre) of :

A
Electric field of a uniformly charged sphere
B
Electric field of a uniformly charged spherical shell
C
Potential of a uniformly charged sphere
D
Potential of a uniformly charged spherical shell

Explanation

As the field inside the uniformly charged hollow sphere or spherical shell is zero, so the potential inside it is constant, whereas outside it varies inversely with distance.
2

JEE Main 2019 (Online) 11th January Morning Slot

In the circuit shown,

the switch S1 is closed at time t = 0 and the switch S2 is kept open. At some later time(t0), the switch S1 is opened and S2 is closed. The behavior of the current I as a function of time 't' is given by :
A
B
C
D

Explanation

From time t = 0 to t = t0, growth of current takes place and after that decay of current takes place.

most appropriate is (A)
3

JEE Main 2019 (Online) 11th January Morning Slot

In a Wheatstone bridge(see fig.), Resistances P and Q are approximately equal. When R = 400 $\Omega$, the bridge is balanced. On interchanging P and Q, the value of R, for balance, is 405 $\Omega$. The value of X is close to :

A
402.5 ohm
B
401.5 ohm
C
403.5 ohm
D
404.5 ohm

Explanation

For a balanced bridge,

${P \over Q} = {{400} \over X}$ ......(i)

After interchanging P and Q,

${Q \over P} = {{405} \over X}$ ......(ii)

Multiplying equation (i) and (ii), we get

X2 = 400$\times$405

$\Rightarrow$ X = $\sqrt {400 \times 405}$ = 402.5 $\Omega$
4

JEE Main 2019 (Online) 11th January Evening Slot

An electric field of 1000 V/m is applied to an electric dipole at angle of 45o. The value of electric dipole moment is 10–29 C.m. What is the potential energy of the electric dipole?
A
- 7 $\times$ 10–27 J
B
$-$ 9 $\times$ 10–20 J
C
$-$ 10 $\times$ 10–29 J
D
$-$ 20 $\times$ 10–18 J

Explanation

U = $-$ $\overrightarrow P .\overrightarrow E$

= $-$ PE cos $\theta$

= $-$ (10$-$29) (103) cos 45o

= $-$ 0.707 $\times$ 10$-$26 J

= $-$ 7 $\times$ 10$-$27 J.