Write the salient features of Coulomb's law.A free pith - ball of 8 g carries a positive of ` 5 xx 10 ^(-8)C ` What must be the nature and
magnitude of charge that should be given to ta second pith- ball fixed 5 cm vertically below the former pith-ball , so that the upper pith-ball is stationary ?

Explanation

For salient features Here, charge on the pith-ball ` A,q_1= 5 xx 10 ^(-8) C `
Mass of the pith-ball `A, m_1 = 8 g = 8 xx 10 ^(-3) kg `
The weight `m_1g` of 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-ball are same signs, i.e. if charge on the pith-ball A is positive , the charges on B should also be positive . As such the force on the pith-ball A due to B, i.e. `F_(AB) ` will act vertically upwards.
For charge ` q_1` to remain stationary,
` F_(AB) = m_1 g rArr (1)/(4piin _0) . (q_1q_2)/(AB^(2)) = m_1g`
Here , ` " " AB = 5 cm = 0.05 m`
` 9 xx 10 ^(9) xx (5 xx 10 ^(-8) xx q_2)/((0.05)^(2)) =8 xx 10 ^(-3) xx 9.8`
` " " q_2 = 4.36 xx 10 ^(-7) C " " ` (positive)