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Linear Quadrupole [12 marks]

The arrangement of charges shown is referred to as a linear quadrupole.

$\begin{figure} \begin{center} \epsfysize 1.5in \epsfbox{PS/linquad.ps} \end{center}\end{figure}$

1.: [4 marks] Draw either the equipotentials or the electric field lines around the quadrupole.
I have drawn $\fbox{\rule[-.05cm]{0cm}{.35cm}\hspace{.35cm}}$ equipotentials $\fbox{\rule[-.05cm]{0cm}{.35cm}\hspace{.35cm}}$ electric field lines.
2.: [4 marks] Find the total electrostatic energy of this system of charges.
3.: [4 marks] Show that the electrostatic potential at a point (x,0) on the x axis drops off approximately as x^-3 for $x \gg d$ .