An infinite nonconducting sheet at x=0 carries a uniform surface charge density $\sigma$. A thin rod of length 2L has a uniform linear charge density $\lambda$ on one half and -$\lambda$on the other half. The rod is hinged at its mid point C at x=2 L and lies in the XOZ plane making an angle $\alpha$ with the positive X-axis as shown in figure. The torque $\overrightarrow{\tau }$ experienced by the rod is $\frac{4\sigma \lambda L^2}{k{ϵ}_0}\widehat{j}\sin \alpha$. The value of k is