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\begin{figure}\centerline{ \psfig{file=problems/prob_2_2.eps,height=2.5in} }\end{figure}

Charges of $+q$ are located at the two bottom corners of an equilateral triangle with sides of length $a$. A charge of $-2q$ is at that top corner. This arrangement of charge can be considered two dipoles oriented at $60^\circ$ with respect to one another.

a) Find the electric field (magnitude and direction) at an arbitrary point on the y-axis above/outside the triangle.

b) What are the first two surviving terms in the binomial-theorem-derived series for the electric field evaluated far from the charges, i.e. - for $y >> a$2?

Robert G. Brown 2003-02-09