Question

What is F21 -- the force between q2 and q1?

Answer 1

Answer:

Namely Q1 will repel Q2 with a force of let's say F21 and similarly Q2 will repel Q1 with a force of F12. So F21 is the force on Q2 due to Q1 and F12 is the force on Q1 due to Q2. Of course from Newton's third law, from action-reaction principle, these forces are equal in magnitude.