A spherical shell of radius  $R=1.5cm$ has a charge $q=20\mu C$  uniformly distributed over it. The force exerted by one half over the other half is

Electrostatic Pressure$\left(P\right)=\frac{{\sigma }^2}{2{\in }_0}$

Force(F)$=PA=P\pi R^2$

                                         $$\begin{gathered} =\frac{{\sigma }^2}{2{\in }_0}\left(\pi R^2\right) \left(\sigma =\frac{Q}{A}=\frac{Q}{4\pi R^2}\right) \\ =\left(\frac{Q^2}{16{\pi }^2{R}^4}\right)\left(\frac{\pi R^2}{2{\in }_0}\right) \\ F=\frac{Q^2}{32\pi {\in }_0{R}^2} \\ =\frac{{\left(20\times {10}^{-6}\right)}^2\times 9\times {10}^9}{8\times {\left(1.5\times {10}^{-2}\right)}^2} \\ =2000N \end{gathered}$$