A force of 10   N acts on a charged particle placed between two plates of a charged capacitor. If one…

Electric field due to each plate, $E_1=E_2=\frac{\sigma }{2{\epsilon }_0}=\frac{{\displaystyle Q}}{2A{\epsilon }_0}$

Net electric field between the plates, $E_{\mathrm{net}}=E_1+E_2=\frac{Q}{A{\epsilon }_0}$

Force on charged particle between the plates, $F_1=q{E}_{net}=\frac{qQ}{A{\epsilon }_0}=10 \mathrm{N}$

Now, in second case, the net electric field, $E=\frac{\sigma }{2{\epsilon }_0} =\frac{Q}{2A{\epsilon }_0}=5 \mathrm{N}$

Force on charged particle, $F_2=qE=\frac{qQ}{2A{\epsilon }_0}=5 \mathrm{N}$