Solution:
Solution
$$\displaystyle{U}_{{{i}}}={\left(\frac{{{2}{Q}{q}}}{{{4}\pi\epsilon_{{{0}}}{\left({a}\right)}}}\right)},{U}_{{{f}}}=\frac{{{Q}{q}}}{{{4}\pi\epsilon_{{{0}}}}}{\left[\frac{{{1}}}{{{a}+{x}}}+\frac{{{1}}}{{{a}-{x}}}\right]}$$
?=πr2Eo = 3.14 × 0.0004 × 100 = 0.125Nm2/C
$$\displaystyle{U}_{{{i}}}-{U}_{{{f}}}=\frac{{{\left({Q}{q}{x}^{{{2}}}\right)}}}{{{2}\pi{\epsilon_{{{0}}}^{{{3}}}}}}$$
[for x<<a,x2 can be neglected in comparison to a2 in denominator]
