7  POTENTIAL ENERGY OF A SYSTEM OF CHARGES
Consider first the simple case of two charges q1and q2 with position vector
r1 and r2 relative to some origin Let us calculate the work done
(externally) in building up this configuration This means that we consider
the charges q1 and q2 initially at infinity and determine the work done by
an external agency to bring the charges to the given locations Suppose,
first the charge q1 is brought from infinity to the point r1