(a) What is the Distance Between the Two Spheres? (b) What is the Force On the Second Exercise Chapter 1 Electric

Question 2: The electrostatic force on a small sphere of charge 0.4 µC due to another small  sphere of charge − 0.8 µC in air is 0.2 N. (a) What is the distance between the two spheres? (b) What is the force on the second sphere due to the first?
Solution:
(a) What is the distance between the two spheres?
Given that
Charge on first small sphere,     q₁ =     0.4 µC =   0.4 × 10⁻⁶ C
Charge on second small sphere, q₂ = − 0.8 µC = − 0.8 × 10⁻⁶ C
(1µC = 10 ^–6C    so multiply by 10^-6   there)
The electrostatic force on a small sphere, F = 0.2 N

Apply Coulomb’s law

q₁ = first charge
q₂ = Second charge
r = distance between both charge
ε₀= permittivity of free space
1/4πε₀  = 9 × 109 Nm2C-2
Plug the values we get

The distance between the two spheres is 0.12 m.

(b) What is the force on the second sphere due to the first?
Both charge will attract each other with the same force. Hence, attraction force on the second charged sphere due to the first charged sphere is 0.2 N.