Electric field at points from a solid copper sphere

[Tui]

Homework Statement

2. A solid copper sphere of radius 15.0 cm carries a charge of 40 nC. Find the electric field (i)
12.0 cm, (ii) 17.0 cm and (iii) 75 cm from the centre of the sphere. (iv) Explain how your
answers would change if the sphere were hollow.

Homework Equations

The Attempt at a Solution

So I'm really just wanting to know if what I've done is correct and if not could you point my in the right direction? Thanks

(i). Since R>r and all charge resides in the surface then Q = 0. Since Q = 0 so does the electric field at 12cm from the centre.

(ii). E = Q/4(pi)(E0).r^2
= (40x10^-9) / (4(pi)*(8.85*10^-12)*.17^2))'
= 12445.4

(iii) same thing with part two but I got 639 instead

Not sure about iv so any help there would also be appreciated. Thanks!

 

[rude man]

Use Gauss's theorem for part iv also!
Rest is OK.