# Electric Forces

1. Apr 15, 2007

### simplicity12

1. The problem statement, all variables and given/known data
Two identical small spheres of mass 2.0 g are fastened to the ends of an insulating thread of length 0.60m. The spheres are suspended by a hook in the ceiling from the centre of the thread. The spheres are given identical electric charges and hang in static equilibrium, with an angle of 30 degrees between the string halves. Calculate the magnitude of the charge on each sphere.

2. Relevant equations
F(g)= mg
F(e)= kq(1)q(2)/r^2

3. The attempt at a solution
I found the gravitational force acting on one of spheres by F(g)=mg, where the mass is 0.002 kg and the g is 9.80 N/m, and i got 0.0196 N.
Since the system is static equilbrium. The vertical component of the tensile force is equal to the force of gravity. Next i did the F(tensile horizontal)/Fg= tan15, and i got 5.25 X 10^-3 N. The next part is what confuses me... do i need to double the horizontal tensile force since i only considered one half of the string... or do i just leave it, and then using the equation F(e)= kq^2/r^2, to find the charge? The answer in the book says it's 1.2 X 10^-7 C. Can someone please clarify this for me?