What is the charge on each ball?

Question

I got the answer to the tension part of the question. Tension = 4.78e-3 N. Now, what is the charge on each ball?

Two identical, small insulating balls are suspended by separate 0.37-m threads that are attached to a common point on the ceiling. Each ball has a mass of 4.4 x 10-4 kg. Initially the balls are uncharged and hang straight down. They are then given identical positive charges and, as a result, spread apart with an angle of 51o between the threads. Determine (a) the charge on each ball and (b) the tension in the threads.

Answer 1

By the way, this is a physics question.
First let us focus our attention on a single ball. Its weight is W = 4.4x10^-4kg * 9.8m/s^2 = 0.0043N. The 0.37-m thread attached to the ball makes 51°/2 = 25.5° with vertical. The vertical component of tension T in the thread must balance the weight of the ball: T•cos(25.5°) = W, and its horizontal component must be balanced by the charge-charge interaction: T•sin(25.5°) = W•tan(25.5°) = 0.00206N.

(a) Since each thread is 0.37m long, the ball-ball distance is:
r = 2*0.37m*sin(25.5°) = 0.32m
Let the charge on each ball be q C. We have:
q^2/ 4πεr^2 = 0.00206N
Thus: q = sqrt(4πεr^2 * 0.00206N) = 0.32*sqrt(0.00206/8.99e9) C = 1.5x10^-7 C.

(b) T = W/cos(25.5°) = 4.78x10^-3 N.