A thin non-conducting ring of radius r has a linear charge density \lambda=\lambda_0 \cos ϕ, where \lambda_0 is a constant and ϕ is the azimuthal angle. The magnitude of the electric field strength at the centre of the ring is :

{1/{4\pi\epsilon_0}} {\lambda_0/r}

{1/{2\pi\epsilon_0}} {\lambda_0/r}