Three charges q, $$-$$q and q₀ are placed as shown in figure. The magnitude of the net force on the charge q₀ at point O is $$\left[ {k = {1 \over {(4\pi { \in _0})}}} \right]$$

A + 3.0 nC charge Q is initially at rest at a distance of r₁ = 10 cm from a + 5.0 nC charge q fixed at the origin. The charge Q is moved away from q to a new position at r₂ = 15 cm. In this process work done by the field is

Four objects W, X, Y and Z, each with charge +q are held fixed at four points of a square of side d as shown in the figure. Objects X and Z are on the midpoints of the sides of the square. The electrostatic force exerted by object W on object X is F. Then the magnitude of the force exerted by object W on Z is

Two charges 14 $$\mu$$C and $$-$$4 $$\mu$$C are placed at ($$-$$12 cm, 0, 0) and (12 cm, 0, 0) in an external electric field $$E = \left( {{B \over {{r^2}}}} \right)$$, where B = 1.2 $$\times$$ 10⁶ N/(cm²) and r is in metres. The electrostatic potential energy of the configuration is