Equation 23. 12, [E_{y}=k frac{q_{mathrm{p}}}{y^{2}}left[left(1-frac{d}{2 y} ight)^{-2}-left(1+frac{d}{2 y} ight)^{-2} ight]] was derived for the case (y>d /

Equation 23. 12,

\[E_{y}=k \frac{q_{\mathrm{p}}}{y^{2}}\left[\left(1-\frac{d}{2 y}\right)^{-2}-\left(1+\frac{d}{2 y}\right)^{-2}\right]\]

was derived for the case \(y>d / 2\). Fxplain why this equation holds for the case \(y<-d / 2\).