Four electric charges \(+\mathrm{q},+\mathrm{q},-\mathrm{q}\) and \(-\mathrm{q}\) are placed in order at the corners of a square of side \(2 \mathrm{~L}\). The electric potential at point midway between the two positive charges is

\(\begin{aligned}
& \mathrm{DP}=\sqrt{5} \mathrm{~L}=\mathrm{CP} \\
& \mathrm{AP}=\mathrm{BP}=\mathrm{L}
\end{aligned}\)
Potential at point \(\mathrm{P}\) due to the four charges
\(\begin{aligned}
& \mathrm{V}=\frac{1}{4 \pi \varepsilon_0}\left(\frac{\mathrm{q}}{\mathrm{L}}+\frac{\mathrm{q}}{\mathrm{L}}-\frac{\mathrm{q}}{\sqrt{5} \mathrm{~L}}-\frac{\mathrm{q}}{\sqrt{5} \mathrm{~L}}\right) \\
& =\frac{1}{4 \pi \varepsilon_0}\left(\frac{2 \mathrm{q}}{\mathrm{L}}-\frac{2 \mathrm{q}}{\sqrt{5} \mathrm{~L}}\right) \\
& =\frac{1}{4 \pi \varepsilon_0} \cdot \frac{2 \mathrm{q}}{\mathrm{L}}\left(1-\frac{1}{\sqrt{5}}\right)
\end{aligned}\)