\(PR=20cm\)
\(\mathrm{~V}_{\mathrm{PQ}}=\frac{1}{4} \times \mathrm{R}_{\mathrm{PR}}\)
\(\ell_{\min }(\mathrm{PQ})=\frac{1}{4} \times 20\)
\(=5 \mathrm{~cm}\)
$\text { ( } \mathrm{h}=6.6 \times 10^{-34} \mathrm{Js}, \mathrm{e}=1.6 \times10^{-19}\mathrm{C}$આપેલ છે.