Question

How much work is needed to decreases the distance between a +15 µC charge

and a -20 µC charge from 1 m to 0.25 m?

Answer 1

The work done is equal to change in potential energy of system

$$W=\Delta E_p=k\frac{q_1q_2}{r_2}-k\frac{q_1q_2}{r_1}$$

$$W=9*10^9*15*10^{-6}*(-20*10^{-6})\\ \times\left(\frac{1}{0.25}-\frac{1}{1.00}\right)=-8.1\:\rm J$$