Question

A pith ball a of mass 9*10^5 kg carries a charge of 5mc what must be the magnitude and sign of the charge on another ball b held 2cm directly above the pith ball a so that the ball remains stationary

Answer 1

Here, q1=5 μC=5×10−6C, q2=?

       r=2 cm=2×10−2 mm=9×10−5 kg

As,        F=mg

So,        14πε0q1q2r2=mg

or,        9×109×5×10−6 q2(2×10−2)2=9×10−5×9.8

So,          q2=7.84×10−12C