Electric Field strength of sphere

[james6008]

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

 

[clavin]

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