Question

Two identical pith balls, each carrying a charge q, are suspended from a common point by two strings of equal length l. Find the mass of each ball if the angle between the strings is $2\theta$ in equilibrium.

Answer 1

$T\cos \theta =mg,T\sin \theta =F_{e}then\tan \theta =\frac{f_e}{mg}{F}_e=\frac{1}{4\pi {ϵ}_0}\frac{q^2}{(2l\sin \theta {)}^2}$

$\therefore m=\frac{q^2\cot \theta }{16\pi {ϵ}_{0}g{l}^2{\sin }^2\theta }$