Why Is My Electric Field Calculation for a Conducting Shell Incorrect?

[starving_student]

Consider a thin spherical shell of radius 14.5 cm with a total charge of +39.1 micro coulombs distributed uniformly on its surface. (Take radially outward as the positive direction.)
(b) Find the electric field 39.6 cm from the center of the charge distribution.

Round your answer to three significant figures.

...shouldn't the electric field at this point be given by E = k * (q / r ^ 2), where q is 3.91 x 10 ^ -6 C and r is 0.396 m? Same as a point charge right?
It wants the answer in megaNewtons / Coulomb; I give 2.24 MN/C and it's wrong? Why?

Thanks for any input.

 

[Physics Monkey]

Your formula for the field is certainly correct since the charge is distributed uniformly on the shell and you're looking outside the shell. The only thing I can see is that in your post you say you used the value q = 3.91*10^-6 C when in fact 39.1 micro coulombs would be q = 39.1*10^-6 C. Maybe this is just a typo, or maybe that's your problem.