Two positive ions, each carrying a charge , are separated by a distance . If  is the force of repulsion between the ions, the number of electrons missing from each ion will be (  being the charge of an electron) 

The electric charge required to expand a soap bubble to twice its dimension is      

Two balls of same mass and carrying equal charge are hung from a fixed support of length . At electrostatic equilibrium, assuming that angles made by each thread is small, the separation,  between the balls is proportional to :         

21. Two pith balls carrying equal charges are suspended from a common point by strings of equal length. The equilibrium separation between them is . Now the strings are rigidly clamped at half the height. The equilibrium separation between the balls now become                                                   

Two equal point charges each of  are separated by a certain distance in metres. If they are located at  and , then the electrostatic force between them is

A total charge  is broken in two parts  and  and they are placed at a distance  from each other. The maximum force of repulsion between them will occur. When

Two particles  and  having equal charges are placed at a distance  apart. A third charged particle placed on the perpendicular bisection of  at distance . The third particle experiences maximum force when - 

Two point charges placed in a medium of dielectric constant 5 are at a distance  between them, experience an electrostatic force '  '. The electrostatic force between them in vacuum at the same distance  will be-

27. In fig., two equal positive point charges  interact with a third point charge . The magnitude, as well as direction, of the net force on  is                                                       

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