The electrostatic force on a small sphere of charge `0.4 muC` due to another small sphere of charge `-0.8 muC` in air is 0.2 N.
What is the force on the second sphere due to the first?

Explanation

Electrostatic force between two charges obeys the Newton's third law.
i.e., force on `q_(1)` due to `q_(2)=` force on `q_(2)` due to `q_(1)`
`f_(12)=f_(21)=0.2 N`