Two 2.0-cm-diameter insulating spheres have a 6.70cm space between them. One sphere is charged to + 95.0nC , the other to – 85.0nC .
What is the electric field strength at the midpoint between the two spheres?
Should be able to replace each sphere by a point charge located at the center. So the distance at midpoint is (6.70 cm)/2 + 1 cm (radius) = 4.35 cm, which is 0.0435 meters. The field from the positive 95 nC is k*Q/r² = (9 x 10^9 N*m²/C²)(95 x 10^-9 C)/(0.0435 m)² = 451843 N/C.
The field from the -85 nC = (9 x 10^9 N*m²/C²)(-85 x 10^-9 C)/(0.0435 m)² = – 404281. The signs mean the field is away from the positive and toward the negative, so they add. The total field is 856124 N/C away from the + charge toward the – charge.