Question

Two tiny spheres of mass = 8.70 mg carry charges of equal magnitude, 72.0 nC , but opposite sign. They are tied to the same ceiling hook by light strings of length 0.530 m . When a horizontal uniform electric field E that is directed to the left is turned on, the spheres hang at rest with the angle between the strings equal to 50 in the following figure. (note that q1 is positive and it is the left once) http://session.masteringphysics.com/problemAsset/1331112/7/21-82.jpg What is the magnitude E of the field?

Answer 1

Draw a diagram to show the forces which are acting.
Draw a force triangle.
Evaluate E.

The attachment shows you a way.