
\(V_{net} = V_1 + V_2\)
\(= -\frac1{4\pi \epsilon_0} \frac q{r + a} + \frac 1{4\pi \epsilon_0} \frac q{r - a}\)
\(= \frac q{4\pi \epsilon_0}\left(\frac 1{r-a} - \frac 1{r + a}\right)\)
\(= \frac 1{4\pi \epsilon_0} q\left(\frac{r + a- r + a}{r^2 -a^2}\right)\)
\(= \frac 1{4\pi \epsilon_0} \frac{q.2a}{r^2 - a^2}\)
\(\because p = q.2 a\)
\(= \frac 1{4\pi \epsilon_0} \frac{p}{r^2 - a^2}\)
For r > a
\(v = \frac 1{4 \pi \epsilon_0} \frac p{r^2}\)