Electric field at a distance greater than the radius of sphere

1. Apr 3, 2013

rocapp

1. The problem statement, all variables and given/known data

A solid ball of radius rb has a uniform charge density ρ.

What is the magnitude of the electric field E(r) at a distance r>r_b from the center of the ball?

2. Relevant equations

E(r) = (Q/ε)*1/Area

3. The attempt at a solution

If Q=ρrb

and Area = (4/3)πr3,

then E(r) = $\frac{ρrb}{ε*(4/3)*πr3}$

However,
the correct answer is in the image attached.
E(r)=
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 3, 2013

barryj

Area is not (4/3) pi r^3

Also, Idon't think q = prb

3. Apr 3, 2013

rocapp

Oh. I get it now. Thanks!