Class 12 Physics 2019 Board Exam

If the net electric flux through a closed surface is zero, then we can infer
(A) no net charge is enclosed by the surface.
(B) uniform electric field exists within the surface.
(C) electric potential varies from point to point inside the surface.
(D) charge is present inside the surface.

An electric dipole placed in a non-uniform electric field can experience
(A) a force but not a torque.
(B) a torque but not a force.
(C) always a force and a torque.
(D) neither a force nor a torque.

A point charge is situated at an axial point of a small electric dipole at a large distance from it. The charge experiences a force F. If the distance of the charge is doubled, the force acting on the charge will become
(A) 2F
(B) F/2
(C) F/4
(D) F/8

Use Gauss's law to show that due to a uniformly charged spherical shell of radius R, the electric field at any point situated outside the shell at a distance r from its centre is equal to the electric field at the same point, when the entire charge on the shell were concentrated at its centre. Also plot the graph showing the variation of electric field with r, for r ≤ R and r ≥ R.
(b) Two point charges of + 1 μC and + 4 μC are kept 30 cm apart. How far from the + 1 μC charge on the line joining the two charges, will the net electric field be zero ?

Using Gauss law, derive expression for electric field due to a spherical shell of uniform charge distribution σ and radius R at a point lying at a distance x from the centre of shell, such that
(i) 0 < x < R, and
(ii) x > R.
(b) An electric field is uniform and acts along + x direction in the region of positive x. It is also uniform with the same magnitude but acts in – x direction in the region of negative x. The value of the field is E = 200 N/C for x > 0 and E = – 200 N/C for x < 0. A right circular cylinder of length 20 cm and radius 5 cm has its centre at the origin and its axis along the x-axis so that one flat face is at x = + 10 cm and the other is at x = – 10 cm.
Find :
(i) The net outward flux through the cylinder.
(ii) The net charge present inside the cylinder.

If the electric flux entering and leaving a closed surface in air are ϕ₁ and ϕ₂ respectively, the net electric charge enclosed within the surface is ___________ .

Electric flux through a spherical surface shown in the figure, is ________ .

Two small identical electric dipoles AB and CD, each of dipole moment p are kept at an angle of 120° to each other in an external electric field E pointing along the x-axis as shown in the figure. Find the
(a) dipole moment of the arrangement, and
(b) magnitude and direction of the net torque acting on it.

Two electric field lines cannot cross each other. Also, they cannot form closed loops. Give reasons.
(b) A particle of charge 2 μC and mass 1.6 g is moving with a velocity 4 i ms⁻¹. At t = 0 the particle enters in a region having an electric field E (in NC⁻¹) = 80 i + 60 j. Find the velocity of the particle at t = 5 s.

Draw a pattern of electric field lines due to two positive charges placed a distance d apart.