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**1. Homework Statement**

A 2.40 -mm-diameter glass sphere has a charge of + 1.70 nC.

What speed does an electron need to orbit the sphere 1.40 mm above the surface?

**2. Homework Equations**

Electric Field of Sphere: E=k*Q/(r^2)

q*E=M*v^2/r

**3. The Attempt at a Solution**

It seems pretty straight forward but I must be doing something wrong with the r values. Could someone tell me what the radius is considered. For the electric field is it the radius of sphere + the distance away or what and same for the other equation. Ive tried pretty much all the combos I can think of and still am getting it wrong so not sure where Im going wrong.