Two 2.0-cm-diameter insulating spheres have a 6.60 cm space between them. One sphere is charged to + 76.0 nC , the other to - 30.0 nC . Part A What is the electric field strength at the midpoint between the two spheres? Express your answer with the appropriate units.

Explanation:

Since the two charged bodies are symmetric, we can calculate the electric field taking both of them as point charges.

This can be easily seen if we use Gauss's law,

We take a larger sphere of radius, say r, as the Gaussian surface. Then the electric field due to the charged sphere at a distance r from it's center is given by,

which is the same as that of a point charge.

In our problem the charges being of opposite signs, the electric field will add up. Therefore,

where, = distance between the center of one sphere to the midpoint (between the 2 spheres)