Find the value of the potential at the following distances

[Clement]

Homework Statement

A total electric charge of 2.00 nC is distributed uniformly over the surface of a metal sphere with a radius of 20.0 cm. If the potential is zero at a point at infinity, find the value of the potential at the following distances from the center of the sphere.
(a) 48.0 cm
(b) 20.0 cm
(c) 12.0 cm

Homework Equations

V=(kQ)/r

The Attempt at a Solution

I got part a no problem, having difficulty with b and c
for b, when r approaches infinity, shouldn't the potential approach infinity? but infinity was not the right answer.
for c, when r is enclosed in the sphere, isn't the potential always going to be 0?

thanks

 

[gabbagabbahey]

Homework Equations

V=(kQ)/r

Hmmm... this is the potential due to a point charge isn't it...why do you think this is also true for the uniformly charged spherical surface?

for b, when r approaches infinity, shouldn't the potential approach infinity? but infinity was not the right answer.

$$\frac{1}{\infty}=0\neq\infty$$

for c, when r is enclosed in the sphere, isn't the potential always going to be 0?

Why would you say this?...When in doubt, go back to the mathematical definition of electrostatic potential...

 

[Clement]

got it, thanks!