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101.

Two spheres of electric charges + 2 nC and −8 nC are placed at a distance d apart. If they are allowed to touch each other, what is the new distance between them to get a repulsive force of same magnitude as before ?

d

$\frac{\mathrm{d}}{2}$

$\frac{3\mathrm{d}}{4}$

$\frac{4\mathrm{d}}{3}$

102.

Three point charges of + 2q, + 2q and − 4q are placed at the corners A, B and C of an equilateral triangle ABC of side x. The magnitude of the electric dipole moment of this system is

2qx

$3\sqrt{2}\mathrm{qx}$

3qx

$2\sqrt{3}\mathrm{qx}$

103.

64 small drops of mercury, each of radius r and charge q coalesce to form a big drop. The ratio of the surface density of charge of each small drop with of the big drop is

64 : 1

1 : 64

1 : 4

4 : 1

104.

Three point charges are placed at the corners of an equilateral triangle. Assuming only electrostatic forces are acting. Then the system

will be in equilibrium if the charges rotate about the centre of the triangle

can never be in equilibrium

will be in equilibrium if the charges have the same magnitudes but different sign

will be in equilibrium if the charges have different magnitude and different signs.

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105.

Two copper balls, each weighing 10 g are kept in air 10 cm apart. If one electron from every 10^{6} atoms is transferred from one ball to the other, the Coulomb force between them is (atomic weight of copperis 63.5)

2.0 × 10

^{4}N2.0 × 10

^{10}N2.0 × 10

^{6}N2.0 × 10

^{6}N

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