Van de Graaff Problem: Solve for Electric Field Strength

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A Van de Graaff generator applies a negative charge of Q = -1.0×10^-8 C to a metal sphere with a radius of 6.9 cm. To find the electric field strength at a point 1.0 cm from the sphere's surface, the total distance from the center to the point must be calculated as r = 0.069 m + 0.01 m. The electric field E can be determined using the formula E = kQ/r^2, where k is Coulomb's constant. This approach treats the charge as if it were concentrated at the sphere's center, simplifying the calculation. Following these steps will yield the correct electric field strength.
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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
 
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I think they are aking you to use a test charge (like an electron) and find the force on it at r (F = kq'q/r^2). Then find the field E at this point.
 
step-by-step please

OK, but I am still unsure what to do.. Do you add .069m and .01m for r?? Can someone "walk" me through this step-by-step PLEASE.
 
The field outside the sphere is the same as thought the entire charge were concentrated at a point at its center. So r would be 0.069 + 0.01.
 
Got it. Thanks.
 
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