CBSE Class 12 Physics NCERT Solutions Chapter – 1 Electric Charges and Fields 1: What is the force between two small charged spheres having charges of  2 × 10 − 7 C 2 × 10 − 7 C  and  3 × 10 − 7 C 3 × 10 − 7 C  placed 30 cm apart in air? Ans:  Repulsive force of magnitude  6 × 10 − 3 N 6 × 10 − 3 N Charge on the first sphere,  q 1 = 2 × 10 − 7 C q 1 = 2 × 10 − 7 C Charge on the second sphere,  q 2 = 3 × 10 − 7 C q 2 = 3 × 10 − 7 C Distance between the spheres, r = 30 cm = 0.3 m Electrostatic force between the spheres is given by the relation, F = k q 1 q 2 r 2 F = k q 1 q 2 r 2 Where, k =  1 4 π ε 0 1 4 π ε 0 1 4 π ϵ 0 = 9 × 10 9 N m 2 C 2 1 4 π ϵ 0 = 9 × 10 9 N m 2 C 2 F = 9 × 10 9 × 2 × 10 − 7 × 3 × 10 − 7 ( 0.3 ) 2 = 6 × 10 − 3 N F = 9 × 10 9 × 2 × 10 − 7 × 3 × 10 − 7 ( 0.3 ) 2 = 6 × 10 − 3 N Hence, force between the two small charged spheres is  6 × 10 − 3 N 6 × 10 − 3 N . The charges are of same nature. Hence, force between t...