FIGURE 2 15
Solution
(a) Since the work done depends on the final arrangement of the
charges, and not on how they are put together, we calculate work
needed for one way of putting the charges at A, B, C and D Suppose,
first the charge +q is brought to A, and then the charges –q, +q, and
–q are brought to B, C and D, respectively The total work needed can
be calculated in steps:
(i)
Work needed to bring charge +q to A when no charge is present
elsewhere: this is zero