Radii of eqiuipotential surfaces of a charged metal sphere

[RoosterCancer]

Homework Statement

A metal sphere of radius r0 = 0.29 m carries a charge Q = 0.90 µC. Equipotential surfaces are to be drawn for 100 V intervals outside the sphere. Determine the radius r of the following equipotentials from the surface.
a) first:___m
b) tenth:___m
c) 100th:_____m

Homework Equations

V=kQ/r --> r=kQ/V

The Attempt at a Solution

I've tried plugging the numbers from the problem in the equation:

r=(8.99 x 10^9 NM^2/C^2)*(0.9 x 10^-6 C)/(100V) = 80.91m

and it's incorrect. I assume the radius of the sphere itself comes into play, but I'm not sure where. Can anyone help?

 

[SammyS]

Homework Statement

A metal sphere of radius r0 = 0.29 m carries a charge Q = 0.90 µC. Equipotential surfaces are to be drawn for 100 V intervals outside the sphere. Determine the radius r of the following equipotentials from the surface.
a) first:___m
b) tenth:___m
c) 100th:_____m

Homework Equations

V=kQ/r --> r=kQ/V

The Attempt at a Solution

I've tried plugging the numbers from the problem in the equation:

r=(8.99 x 10^9 NM^2/C^2)*(0.9 x 10^-6 C)/(100V) = 80.91m

and it's incorrect. I assume the radius of the sphere itself comes into play, but I'm not sure where. Can anyone help?

What is the potential at the surface of the ball ?

 

[RoosterCancer]

I was able to find the potential of the ball:

V=(9 x10^9NM^2/C^2)*(0.9 x10^-6 C)/0.29m = 27931.03V

and was able to answer part a, by adding 100 V to the ball's potential and using the formula. So, for b, I added 1000 V to the potential and for c I added 10000 V to the potential, but the results were incorrect.