A free pith-ball of 8 g carries a positive charge of 5 x ${{10}^{-8}}$C

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electricfield

#1

A free pith-ball of 8 g carries a positive charge of 5 x ${{10}^{-8}}$C. What must be the nature and magnitude of charge that should be given to a second pith-ball fixed 5 cm vertically below the former pith-ball so that the upper pith-ball is stationary?


#2

Here, charge on the pith-ball A, ${ q }{ 1 }$ = 5 x ${{10}^{-8}}$ C
Mass of the pith-ball A, ${ m }
{ 1 }$ = 8g = 8 x ${{10}^{-3}}$ Kg
The weight ${ m }{ 1 }$ g or the pith-ball A acts vertically downwards.

Let ${ q }
{ 2 }$ be charge on the pith-ball B held 5 cm below the pith ball A so that the pith-ball A remains stationary.
It can be possible only, if the charges on two pith-balls are of same signs, i.e. if charge on the pith-ball A is positive, the charge on B should also be positive. As such the force on the pith-ball A due to.B, i.e. FM will act vertically upwards.
For charge ${ q }_{ 1 }$ to remain stationary,