Horizontal component of an electric field at origin

1. Sep 1, 2013

kirax105strike

1. The problem statement, all variables and given/known data
Calculating the horizontal component of an electric field at the origin of a semi circle. The image link is: http://imgur.com/SXYs4Rp

3. The attempt at a solution

I've tried using the formula k∏(Q)/r (8.98755*10^9)∏(-7.85*10^-6) / .113 = -1.961 * 10^6.
Can anyone see what I am doing wrong?

2. Sep 1, 2013

Staff: Mentor

Where does your formula come from? It looks like a wrong application of the formula for a point-charge. Certainly your charge is not a point.
How did you get r? This is not the same as the length of the semi-circle.

3. Sep 1, 2013

kirax105strike

The formula was from an example in the book where they used the length of the curve as r but I am guessing I have the wrong formula.