$\mathrm{b} :$ length of one side of plates

$C = \frac{{{ \in _0}A}}{{{\ell _0}}} = \frac{{{ \in _0}bx}}{{{\ell _0}}}$                     $\frac{{dC}}{{dx}} = \frac{{{ \in _0}b}}{{{\ell _0}}}$

$F = \frac{1}{2}\frac{{{Q^2}{\ell _0}}}{{e \in _0^2{b^2}{x^2}}}\frac{{{ \in _0}b}}{{{\ell _0}}}$

${\rm{F}} = \left( {\frac{1}{2}\frac{{{{\rm{Q}}^2}{\ell _0}}}{{{ \in _0}{\rm{b}}}}} \right)\frac{1}{{{{\rm{x}}^2}}}$