Two identical conducting spheres with negligible volume have 2.1nC and -0.1nC charges, respectively. They are brought into contact and then separated by a distance of 0.5m.  The electrostatic force acting between the spheres is…………… $\times {10}^{-9}\mathrm{N}$ [Given: $4{\mathrm{\pi \epsilon }}_0=\frac{1}{9\times {10}^9}\mathrm{SI} \mathrm{units}$]