(a) Consider an electric dipole consisting of two point charges –q and +q separated by a small distance $AB = 2a$ with centre at $O$,
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As shown in figure, on equatorial line, the resultant electric field $\mathbf{E}$ of $\mathbf{E}_{1}$ and $\mathbf{E}_{2}\ $is parallel to the axis of the dipole but opposite to the direction of the dipole moment p as it is directed from negative charge to positive charge.