A negatively charged oil drop is prevented from falling under gravity by applying a vertical electric field 100 Vm^-1. If the mass of the drop is 1.6 x 10^-3  g, the number of electrons carried by the drop is (g = 10 ms^-2)

• 10¹⁸

• 10¹⁵

• 10¹²

• 10⁹

Which one of the following graphs represents the variation of electric field with distance r from the centre of a charged spherical conductor of radius R?

Two conducting spheres A and B of radius a and b respectively are at the same potential. The ratio of the surface charge densities of A and B is

• $\frac{\mathrm{b}}{\mathrm{a}}$

• $\frac{\mathrm{a}}{\mathrm{b}}$

• $\frac{{\mathrm{a}}^2}{{\mathrm{b}}^2}$

• $\frac{{\mathrm{b}}^2}{{\mathrm{a}}^2}$

Three charges 2q, -q, -q are located at the vertices of an equilateral triangle. At the circumcentre of the triangle

• the field is zero but potential is non-zero

• potential is zero and the field is infinity

• the field is non-zero but potential is zero

• both the field and potential are non-zero

Millikan's oil-drop experiment established that

• electric charge depends on velocity

• specific charge of electron is 1.76 x 10¹¹ Ckg^-1

• electron has wave nature

• electric charge is quantized

Two identical conducting spheres carrying different charges attract each other with a force F when placed in air medium at a distance d apart. The spheres are brought into contact and then taken to their original positions. Now the two spheres repel each other with a force whose magnitude is equal to that of the initial attractive force.The ratio between initial charges on the spheres is 

• $- \left(3 + \sqrt{8}\right) \mathrm{only}$

• $- 3 + \sqrt{8} \mathrm{only}$

• $- \left(3 + \sqrt{8}\right) \mathrm{or} \left(-3 + \sqrt{8}\right)$

• $+ \sqrt{3}$

Small drops of the same size are charged to V volt each. If n such drops coalesce to form a single large drop, its potential will be

• Vn

• V/n

• Vn^1/3

• Vn^2/3

An electric dipole of length 1 cm is placed with the axis making an angle of 30° to an electric field of strength 10⁴ NC^-1. If it experiences a torque of $10\sqrt{2} \mathrm{Nm}$ Nm, the potential energy of the dipole is

• 0.245 J

• 2.45 × 10^-4 J

• 24.5 × 10^-4 J

• 245.0 J

49.

Charges + 2q, + q and + q are placed at the comers A, B and C of an equilateral triangle ABC. If E is the electric field at the circumcentre 0 of the triangle, due to the charge + q, then the magnitude and direction of the resultant electric field at O is

• E along AO

• 2E along AO

• E along BO

• E along CO

In co-axial cable the material used as spacer is

• teflon (or) polyethylene

• glass or mica

• a gaseous medium

• glass