# Electric Field strength of sphere

Hi everyone

I just need some help with this question. Its a simple one I'm pretty sure.

## Homework Statement

A solid sphere of radius 3 cm is made from a conductor and has a charge of
+ 4 uC placed upon it. What is the electric field strength 1 mm from the
centre of the sphere?

## Homework Equations

I think this is relevant equation to this.

E = Q/(4pi eo r^2)

## The Attempt at a Solution

Q is (4 X 10^-6)/ ( 4 * pi * (8.8 * 10^-12) * (r^2) )

I am a bit confused about r. Is r just the radius in meter which will be 0.03m or do I have to include 1mm from the center in that case it would be 0.014m?

Please help.

Thanks

## Answers and Replies

well if the charge was placed on the outer surface then
no electric field inside at any point
due to electrostatic shielding
in case it is placed inside, then r is the distance of the straight line joining the charge and the point at which to calculate the electric field
again that point must lie inside in the case