# Homework Help: Van de Graff Problem

1. Oct 7, 2007

### 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

2. Oct 7, 2007

### dlgoff

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.

3. Oct 7, 2007

### Ryo124

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.

4. Oct 7, 2007

### dlgoff

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.

5. Oct 7, 2007

### Ryo124

Got it. Thanks.