Answer to Question #2528 in Electricity and Magnetism for jaswinder Singh

Let m = 0.1g be the mass of each spheres,
l = 20cm be length of each thread
q be the charge of each sphere.

Since the angle between threads is 2*30 = 60, it follows that the distance between these spheres is equal to l=20cm.

There are three forces acting on each of the spheres:
1) Gravitational force, P=mg
2) Tension of thread, N
3) Coulomb's force, F= q²/l²Since the spheres do not move, the sum of these forces is zero,

P + N + F = 0

Projecting this vector equation to the horisontal axis we get that

mg tan 30 = q²/l²
Hence
q = l * √{ mg /√{3} } = 20cm * √{ 0.1g * 980 cm/s² / 1,73 } = 150,44 e.s.u