What is the electric flux through the top surface of the cube?

[jincy34]

Homework Statement

A 3 nC point charge is at the center of 5 m x 5 m x 5 m cube. What is the electric flux through the top surface of the cube?

Homework Equations

E=kq/r^2
φ = E.A

The Attempt at a Solution

E = (8.99*109)(3*10-9) / 2.52 = 4.3152 N/C

φ = E.A = (4.3152)(5*5) = 107.88
But, it is wrong. Can someone please tell me what I am doing wrong?

 

[blochwave]

You assume a constant electric field over the whole top surface

The equation E=kq/r^2 where r is, I assume you put in 5, only the electric field at the center of the top. Every other point on the top is going to have a different r and electric field, it's not constant.

Edit: actually for r it looks like you used...sqrt(2.52)? I'm not sure where that came from