(a) Here, q = 4 × 10−7C,
r = 9cm = 9 × 10−2m, V = ?
\(V = \frac q{4\pi \in_0 r}\)
\(= 9\times 10^9\frac{(4\times10^{-7})}{9\times 10^{-2}}\)
\(= 4\times 10^4 \) volt
(b) \(W = q \times v\)
\(= (2 \times 10^{-9}) \times 4 \times 10^4\)
\(= 8 \times 10^{-5}J\)
The answer does not depend upon the path along which the charge is brought because electrostatic forces are conservative forces.