Electric field strength at the center of semicircle

[need_aca_help]

Homework Statement

A uniformly charged insulating rod of length 12.0 cm is bent into the shape of a semicircle as shown in the figure below. The rod has a total charge of −7.50 µC.

Find the magnitude of the electric field at O, the centre of the semicircle.

Homework Equations

E = 2kq / pi r²
(Acquired on the internet)

The Attempt at a Solution

E = 2(9X10⁹)(-7.5X10^-6) / pi (0.06)²
E = -135000 / 0.01130973355
E = -11936620.73

magnitude: 11936620 N/C

Got it wrong... I don't know why...

 

[BvU]

You want the magnitude of a vector $\vec E$. Contributions from different places on the rod point in different directions, so the sum vector is not as big as you calculate now.
If the ring were not half but complete, $|\vec E|$ would be ...?

 

[need_aca_help]

You want the magnitude of a vector $\vec E$. Contributions from different places on the rod point in different directions, so the sum vector is not as big as you calculate now.
If the ring were not half but complete, $|\vec E|$ would be ...?

Less...?

 

[BvU]

Zilch!

Orientate with e.g hyperphysics or http://en.wikipedia.org/wiki/Electric_field

 

[dauto]

You used the wrong radius.