Charges separation

Question

Two small conducting spheres of equal radius have charges +10uc  and -20uc respectively and placed at a distance R from  each other experience  force F1. If they ate brought in contact and seperated  to the same distance they experience  force F2. The ratio of F1/F2.        a) 1:8  b) -8:1 c) 1:2 d) -2:1 .

Answer 1

By the question, the force of attraction

F1=-(k(10*10^-6x20*10^-6))/R^2, where k is Coulomb's constant

As the spheres have got same radius, their capacities are same. When they are brought in contact the net charge (-20+10)uc=-10uc will be equally distributed and each will get -5uc So when they are kept at R distance apart the force of repulsion will be

F2= ,(k*(-)5*10^-6*(-)5*10^-6)/R^2

Hence the ratio

F1/F2= -8:1 which is option (b)