Force between two identical charges placed at a distance of r in vacuum is F. Now a slab of dielectric of dielectric constant 4 is inserted  between these two charges. If the thickness of the slab is r/2, then the force between the charges will become

• F

• $\frac{3}{5} \mathrm{F}$

• $\frac{4}{9} \mathrm{F}$

• $\frac{\mathrm{F}}{4}$

An electric dipole is placed at an angle of 30° with an electric field ofintensity 2 × 10⁵ NC^-1. It experiences a torque equal to 4 Nm. Calculate the charge on the dipole if the dipole length is 2 cm.

• 8 mC

• 4 mC

• 2 mC

• 4 μC

The plot represents the flow of current through a wire at three different times

The ratio of charges flowing through the wire at different times is

• 2 : 1 : 2

• 1 : 3 : 3

• 1 : 1 : 1

• 2 : 3 : 4

A point charge + q is placed at the midpoint of a cube of side a. The electric flux emerging from the cube is

• zero

• $\frac{3{\mathrm{qa}}^2}{{\mathrm{\epsilon }}_0}$

• $\frac{\mathrm{q}}{{\mathrm{\epsilon }}_0}$

• $\frac{{\mathrm{\epsilon }}_0}{4{\mathrm{qa}}^2}$

Two equal metal balls are charged to 10 and −20 units of electricity. Then they are brought in contact with each other and then again separated to the original distance. The ratio of magnitudes of the force between the two balls before and after contact is

• 8 : 1

• 1 : 8

• 2 : 1

• 1 : 2

A conductor having a cavity is given a positive charge. Then field strengths E_A, E_B and E_C at point A (within cavity), at B (within conductor but outside cavity) and C (near conductor) respectively will be 

• E_A = 0, E_B = 0, E_C = 0

• E_A = 0, E_B = 0, E_C ≠ 0

• E_A ≠ 0, E_B = 0, E_C ≠ 0

• E_A ≠ 0, E_B ≠ 0, E_C ≠ 0

57.

The electric potential due to a small electric dipole at a large distance r from the centre of the dipole is proportional to

• r

• $\frac{1}{\mathrm{r}}$

• $\frac{1}{{\mathrm{r}}^2}$

• $\frac{1}{{\mathrm{r}}^3}$

An electron enters uniform electric field maintained by parallel plates and of value E Vm^-1 with a velocity v ms^-1, the plates are separated by a distance d metre, then acceleration of the electron in the field is

• $\frac{\mathrm{Ee}}{\mathrm{m}}$

• $\frac{-\mathrm{Ee}}{\mathrm{m}}$

• $\frac{\mathrm{Ed}}{\mathrm{md}}$

• $\mathrm{Ee} \frac{\mathrm{d}}{\mathrm{m}}$

A charged oil drop of mass 9.75 x 10^-15 kg and charge 30 x 10^-16 C is suspended in a uniform electric field existing between two parallel plates. The field between the plates, taking (g = 10 ms^-2) is

• 3.25 V/m

• 300 V/m

• 325 V/m

• 32.5 V/m

The distance between charges 5 x 10^-11 C and − 2.7 × 10^-11 C is 0.2 m. The distance at which a third charge should be placed from 4e in order that it will not experience any force along the line joining the two charges is

• 0.44 m

• 0.65 m

• 0.556 m

• 0.350 m