Q.

A small uncharged conducting sphere is placed in contact with an identical sphere but having 4×108 C charge and then removed to a distance such that the force of repulsion between them is 9×103 N. The distance between them is  (Take 14πε0 as 9×109 in SI units)          [2025]

1 2 cm  
2 3 cm  
3 4 cm  
4 1 cm  

Ans.

(1)

F=k(Q2)(Q2)r2

9×103=9×109×(4×108)×4×1084×r2

r2=9×109×16×10164×9×103=4×104

r=2×102 m=2 cm