# Electrostatics Mock Test-4

Questions No:1

An electron of mass me, initially at rest, moves through a certain distance in a uniform electric field in time t1. A proton of mass mp, also initially at rest, takes time t2 to move through an equal distance in this uniform electric field. Neglecting the effect of gravity, the ratio t1/t2 is nearly equal to

[A] me/mp

[B] (mp/me)1/2

[C] (me/mp)1/2

[D] (mp/me)

Questions No:2

A charged particle of mass m and charge g is released from rest in an electric field of constant magnitude E. The KE of the particle after time t is

[A] Eq2m /2t2

[B] 2E2t2/mq

[C] E2q2t2/2m

[D] Eqm/2t

Questions No:3

A particle A has a charge + q and particle B has a charge + 4g each having the same mass m. When released from rest through same potential difference the ratio of their speeds VA/VB is

[A] 2 : 1

[B] 1 : 2

[C] 1 : 4

[D] 4 : 1

Questions No:5

Three charges are placed at the vertices of an equilateral triangle of side 10 cm. Assume q1 = 1 µ C, q2 = – 2 µ C and q3 = 4µ C. Work done in separating the charges to infinity is

[A] -4.5 J

[B] 4.5 J

[C] 45 J

[D] None of these

Questions No:6

Two positive charges 12 µ C and 8 µ C are 10 cm apart. The work done in bringing them 4 cm closer is

[A] 5.8J

[B] 5.8eV

[C] 13 J

[D] 13 eV

Questions No:7

Three charges Q, (+ q) and (+ q) are placed at the vertices of an equilateral triangle. If the net electrostatic energy of the system is zero, then Q is equal to

[A] (- q/2)

[B] – q

[C] + q

[D] zero

Questions No:8

Two particles of masses m and 2m with charges 2gand gare placed in a uniform electric field E and allowed to move for same time. Find the ratio of their kinetic energies

[A] 8 : 1

[B] 4 : 1

[C] 2:1

[D] 16 : 1

Questions No:9

When the separation between two charges is increased, the electric potential energy of the charges

[A] increases

[B] decreases

[C] remains the same

[D] may increase or decrease

Questions No:10

If a positive charge is shifted from a low-potential region to a high-potential region, the electric potential energy

[A] increases

[B] decreases

[C] remains the same

[D] may increase or decrease

Questions No:11

The work done in carrying a charge of 5&mcro;C from a point A to a point B in an electric field is 10 mJ. Then potential difference (VB – VA) is

[A] + 2 kV

[B] – 2 kV

[C] + 200 kV

[D] – 200 V

Questions No:12

If E = 0, at all points of a closed surface

[A] the electric flux through the surface is zero

[B] the total charge enclosed by the surface is zero

[C] no charge resides on the surface

[D] All of the above

Questions No:13

Flux coming out from a positive unit charge placed in air, is

[A] ε 0

[B] ε0-1

[C] c) (4 π ε 0)-1

[D] d) 4 π ε 0

Questions No:14

An electric dipole is kept in non-uniform electric field. It experiences

[A] a force and a torque

[B] a force but not a torque

[C] a torque but not a force

[D] neither a force nor a torque

Questions No:15

An electric dipole of moment p is placed normal to the lines of force of electric intensity E, then the work done in deflecting it through an angle of 180 0 is

[A] pE

[B] +2pE

[C] -2pE

[D] zero

Questions No:16

Electric charges q, q, -2q are placed at the corners of an equilateral triangle ABC of side l. The magnitude of electric dipole moment of the system is

[A] ql

[B] 2ql

[C] $\sqrt{3}$ql

[D] 4ql

Questions No:17

The torque acting on a dipole of moment p in an electric field E is

[A] p . E

[B] p x E

[C] zero

[D] E X p

Questions No:18

Two opposite and equal charges 4 x 10-8 C when placed 2 X 10 -2 cm away, form a dipole. If this dipole is placed in an external electric field 4X10 8 N/C, the value of maximum torque and the work done in rotating it through 1800 will be

[A] 64 X 10-4 N-m and 64 X 10-4 J

[B] 32 X 10-4 N-m and 32 X 10-4 J

[C] 64 X 10-4 N-m and 32 X 10-4 J

[D] 32 X10-4N-mand64x10-4J

Questions No:19

If Ea be the electric field strength of a short dipole at a point on its axial line and Ee that on the equatorial line at the same distance, then

[A] Ee=2Ea

[B] Ea=2Ee

[C] Ea = Ee

[D] None of these

Questions No:20

The potential at a point due to an electric dipole will be maximum and minimum when the angles between the axis of the dipole and the line joining the point to the dipole are respectively

[A] 900 and 1800

[B] 00 and 900

[C] 900 and 00

[D] 00 and 1800

Questions No:21

A molecule with a dipole moment p is placed in an electric field of strength E. Initially the dipole is aligned parallel to the field. If the dipole is to be rotated to be anti-parallel to the field, the work required to be done by an external agency is

[A] -2pE

[B] -pE

[C] pE

[D] 2pE

Questions No:22

The electric field due to an electric dipole at a distance r from its centre in axial position is E. If the dipole is rotated through an angle of 900 about its perpendicular axis, the magnitude of electric field at the same point will be

[A] E

[B] E/4

[C] E/2

[D] 2E