If q1 and q2 are of the same sign (either both positive or both
negative), F21 is along ˆr 21, which denotes repulsion, as it should be for
like charges If q1 and q2 are of opposite signs, F21 is along – ɵr 21(= ɵr 12),
which denotes attraction, as expected for unlike charges Thus, we do
not have to write separate equations for the cases of like and unlike
charges Equation (1