We can convince ourselves that the sign is correct by examining the case when r. is bigger than r. and the charges q and q, are of opposite sign and so attract each other. To separate the charges further requires us to do work on q, so that W is positive in (2). If q and q, are the same sign, the repulsive coulomb force would tend to separate the charges further and perform work on qt. For force equilibrium, we would have to exert a force opposite to the direction of motion so that W is negative.