Two point charges of +2 micro coulomb and +6 micro coulomb repel each other with a force of 12 N . If each is given an additional charge of -4 micro coulomb, then what will be new force?

We have, Initial charges are 2 × 10-6 C and 6 × 10-6 C. Suppose they are separated by distance ‘x’. Now,

(9 × 109)×(2 × 10-6)×(6 × 10-6)/x2 = 12

=> x2 = 0.009

Now, the new charges are [(2 – 4) × 10-6 C] and [(6 – 4) × 10-6 C] or (-2 × 10-6 C) and (2 × 10-6 C). The new force between the two is,

F = (9 × 109)×(-2 × 10-6)×(2 × 10-6)/x2

=> F =  (9 × 109)×(-2 × 10-6)×(2 × 10-6)/0.009

=> F = -4 N

The negative sign implies that the force is attractive.