Let \(d\) be the distance between the plates. Then the capacitor is \(c=\frac{\frac{d_{0} A}{d}}{d}\)
Energy stored in a capacitor,
$$
\mathrm{U}=\frac{q^{2}}{2 \mathrm{C}}=\frac{q^{2} \cdot d}{2 \varepsilon_{0} \mathrm{~A}}
$$
Energy magnitude of the force is,
$$
\begin{aligned}
|\mathrm{F}| &=\frac{d \mathrm{U}}{d x}=\frac{d}{d x}\left[\frac{q^{2} x}{2 \varepsilon_{0} \mathrm{~A}}\right] \quad[x=d] \\
&=\frac{q^{2}}{2 \varepsilon_{0} \mathrm{~A}}\left[\frac{d}{d x}(x)\right] \\
\mathrm{F} &=\frac{q^{2}}{2 \varepsilon_{0} \mathrm{~A}}
\end{aligned}
$$
