Question

The small identical conducting spheres have charges of 2.0 × 10⁻⁹ C and − 0.5 × 10⁻⁹ respectively. When they are placed 4 cm apart, what is the force between them ? If they are brought into contact and then separated by 4 cm, what is the force between them ? 

Answer 1

F = 9 × 10⁻⁹ Q₁ Q₂/d²

= 9 × 10⁻⁹ × (− 0.5 × 10⁻⁹ )/0.042

= − 56.25 × 10⁻⁷ N.

When two identical spheres are brought into contact with each other and then separated, each gets half of the total charge. Hence,

Q₁ = Q₂ = [2 × 10⁻⁹ + (− 0.5 × 10⁻⁹)/2] 

= 0.75 × 10⁻⁹C

When they are separated by 4cm, 

F = 9 × 10⁻⁹ × (0.75 × 10⁻⁹)²/0.042 = 0.316 × 10⁻⁵N