Calculating Electric Field Strength: Van de Graaff Problem Solution

[Ryo124]

A van de Graaff generator puts negative charge on a metal sphere.

Suppose the radius of the sphere is a = 6.9 cm, and the charge on the sphere is Q = -1.0×10-8 C. Determine the electric field strength at a point 1.0 cm from the surface of the sphere (outside the sphere).


I've done this problem over and over and am not getting the correct answer.
I've used E = F/q and made F = kq/r^2
...from there, I don't know what to do. Please help. Thanks

 

[Meir Achuz]

It's E that equals kq/r^2, and be carefl about what r means here.