Calculating Electric Field Strength of a Charged Metal Sphere

[Matticus9]

Homework Statement

A metal sphere with a diameter of 10 cm has a charge distribution of 1.09085 x 10¹⁸ electrons/cm² on its surface. What is the electric field strength at a distance of 25 cm from the surface of the sphere? (Answer: 5.5 x 10¹² N/C)

Homework Equations

I wasn't given any equations. Only ones I know are
E= F/q= (kqQ/r²)/q
E= k x Q/r²

The Attempt at a Solution

E= (k x Q)/r²= 9.0 x 10⁹ x Q/ 25 cm²= ?

 

[gneill]

Hi Matticus9, Welcome to Physics Forums.

Your expression for E is not well thought out. The problem statement says that the distance of 25 cm is from the surface of the sphere, not its center. The distance in the formula should be the distance from the center.

What can you do to find the total charge Q on the surface of the sphere? You might need to dig into your geometry knowledge to add some equations to your relevant equation list.

 

[Matticus9]

I still don't understand.