A wire, of length $L(=20 cm) $ is bent into a semi- circular arc. If the two equal halves , of the arc, were each to be uniformly chraged with charges [ $\pm Q, |Q|=10^{3}\; \in_0$ coulomb where $\in_0$ is the permittivity (in SI units)of free space] the net electric field at the center O of the semi circular arc would be :
